3 litre 944 Turbo on track
- Thread starter Diver944
- Start date
Probably you would only need to limit torque in the lower gears with a more powerful engine. Gear based boost control would sort that out, under dry conditions at least! And as Hartech has been saying the rate at which torque can be applied needs to take into account the weight transfer of the car, so you should design the onset of boost to allow for the weight transfer of the car to take effect. That would make a big difference to 944's with engines like this.
Bob
New member
He still hasn't mentioned any cost on the PH thread, but I am hoping it would be under £5000 for a 3 litre rebuild, hopefully less if you supplied a donor 968/S2 engine
How much would you have to spend and what bolt ons would be required to get a 2.5 car into the mid 300's and what boost level would you need to run at?
AndrewS
Member
Removing an engine from a 944 Turbo is 10 hours (as is refitting it), that's an estimated £1,200 + VAT in labour alone for removal and refit. Add to that the work required to change the displacement to 3.0 L (machining, crank, rods, pistons, gaskets) + new clutch kit (makes sense with engine out), belts and water pump etc. As the turbo is exposed, makes sense to refurbish that also. When an engine is removed, the wiring loom is often fried and breaks up. Studs break and other things collapse after being disturbed after 20 years of heat cycling and it can start to add up to £5,000 very quickly.
When I built my 2.8, it cost a lot more than £5,000 and that was 10 years ago! ;-)
I do very much like Barry's approach; it's probably much the same cost as a typical engine rebuild with all the additional benefits described by Paul earlier in the thread.
Regards,
Andrew
When I built my 2.8, it cost a lot more than £5,000 and that was 10 years ago! ;-)
I do very much like Barry's approach; it's probably much the same cost as a typical engine rebuild with all the additional benefits described by Paul earlier in the thread.
Regards,
Andrew
AndrewS
Member
With respect to getting a 2.5 Turbo to make a genuine and useable 350bhp, you would need to address the following:
Turbo: ideally a number 10 hotside (high boost for longer with less drop off)
Remove the exhaust restrictions
Increase the injector size
Implement a MAF or MAP system for monitoring the air being ingested into the engine
Dual Port Wastegate and electronic boost control
Cost of the above would be £2500 to £5000 depending on approach, labour costs and parts used.
Regards,
Andrew
Turbo: ideally a number 10 hotside (high boost for longer with less drop off)
Remove the exhaust restrictions
Increase the injector size
Implement a MAF or MAP system for monitoring the air being ingested into the engine
Dual Port Wastegate and electronic boost control
Cost of the above would be £2500 to £5000 depending on approach, labour costs and parts used.
Regards,
Andrew
Barry Hart
New member
Hi Guys, over the last few years I have been posting on different web sites - and it becomes quite a time consuming exercise - but despite being a member of this club for man years, I think this is my first posting.
I thought I ought to write something because you are all discussing things about my car without my involvement - so here goes.
Firstly Paul - can you post the power/torque graphs on here (I didn't see any -perhaps I missed).
There is a lot posted on piston heads and 968/uk already but this engine was built for several reasons. 1st because we thought this was how Porsche built the 968S and wanted to test the theory, secondly because we have training sessions at work and had got to turbo charged engines, had the bits lying around and decided to build it, thirdly because I believe that torque is more important than bhp.
Much argument about this last point and I know for any given revs the torque and bhp are directly linked - however - power is effectively torque * revs, yet acceleration is prortional to torque. I wanted to prove that torque was important and this car has done so.
Although bigger time areas provide the best bmep at high revs they also compress the power band and if taken too far need time to settle the gas flow and produce high outputs which an accelerating car does not have. People forget that @ 6000 revs the valves are open only half the time they are open @ 3000 revs - so it is more difficult to fill the cylinders at higher revs.
Anyway - to answer a few of the questions you have raised.
The liners are or will be Nicasil on Cast Aluminium. Our engine is a 944 turbo fitted with a 968 block, 2.7 head, 968 pistons with lowered compression, a 968 clutch plate and an LSD fitted with the widest plates available.
It runs everything else standard (including the boost manifold pressure etc) and produces 350 lbs fit torque and 300 bhp in a very smooth delivery that does not seem to spin up the back wheels but is very quick indeed.
We recognise the pitfalls of making it more powerful and didn't intend that anyway. We will be developing a more powerful example in the future but meanwhile will offer this kit at around £5000 + Vat (if built from a 944 turbo).
We think we can fit the liners to a 2.5 engine so could probably turn a 2.5 into a 3 litre although you would need a crankshaft etc.
The car is easy to drive and would make a superb road car.
I will be happy to answer any other questions you may have although I might not be able to reply straight away.
Baz
I thought I ought to write something because you are all discussing things about my car without my involvement - so here goes.
Firstly Paul - can you post the power/torque graphs on here (I didn't see any -perhaps I missed).
There is a lot posted on piston heads and 968/uk already but this engine was built for several reasons. 1st because we thought this was how Porsche built the 968S and wanted to test the theory, secondly because we have training sessions at work and had got to turbo charged engines, had the bits lying around and decided to build it, thirdly because I believe that torque is more important than bhp.
Much argument about this last point and I know for any given revs the torque and bhp are directly linked - however - power is effectively torque * revs, yet acceleration is prortional to torque. I wanted to prove that torque was important and this car has done so.
Although bigger time areas provide the best bmep at high revs they also compress the power band and if taken too far need time to settle the gas flow and produce high outputs which an accelerating car does not have. People forget that @ 6000 revs the valves are open only half the time they are open @ 3000 revs - so it is more difficult to fill the cylinders at higher revs.
Anyway - to answer a few of the questions you have raised.
The liners are or will be Nicasil on Cast Aluminium. Our engine is a 944 turbo fitted with a 968 block, 2.7 head, 968 pistons with lowered compression, a 968 clutch plate and an LSD fitted with the widest plates available.
It runs everything else standard (including the boost manifold pressure etc) and produces 350 lbs fit torque and 300 bhp in a very smooth delivery that does not seem to spin up the back wheels but is very quick indeed.
We recognise the pitfalls of making it more powerful and didn't intend that anyway. We will be developing a more powerful example in the future but meanwhile will offer this kit at around £5000 + Vat (if built from a 944 turbo).
We think we can fit the liners to a 2.5 engine so could probably turn a 2.5 into a 3 litre although you would need a crankshaft etc.
The car is easy to drive and would make a superb road car.
I will be happy to answer any other questions you may have although I might not be able to reply straight away.
Baz
Diver944
Active member
Finally we manage to tempt you onto the biggest and best UK 944 forum [
]You (and your technical knowledge) will be very welcome Barry and we already have a few other Independant Specialists registered. In fairness to our other Indies, I need to point out that your membership name should not be advertising your business in any way (rules is rules [8|] ) so it would be helpful if we could change it to 'Barry Hart'.
Here's the link to the graphs posted on the other threads:
I didn't get a chance to render any of the video this weekend, but I have had a look through the tape. We managed to catch some footage of the MX5 spinning
just after the rest of us slowed down because of oil flags. The sound is very quiet so it appears a little uninspiring. We should have removed the boot carpet to get a bit more noise like this:http://www.youtube.com/watch?v=mjI-lc9YNfs
sawood12
New member
Barry, it is refreshing to hear your take on things. I too have often felt that Torque is what is really important but all you seem to get on forums is talk of big bhp. I guess motorsport is to blame where due to capacity and other constraints forces the engineers to use higher revs in search of more power out of necessity rather than choice or preference. This build seems like an ideal cost effective way to get decent well behaved engine at a reasonably low level of tune, therefore should be reliable?? It is this sort of thing that could get me back into a 944 sooner than I had planned!
I briefly read the PH thread and seem to remember there was some talk about concerns over using 968 pistons as they are not designed for use in a turbocharged engine. Is this a real concern or just a minor detail that doesn't, or shouldn't be of consequence??
I briefly read the PH thread and seem to remember there was some talk about concerns over using 968 pistons as they are not designed for use in a turbocharged engine. Is this a real concern or just a minor detail that doesn't, or shouldn't be of consequence??
sawood12
New member
Yep, power is the rate of Work, so you either have a high revving 'weak' engine vs. a low revving 'strong' engine both developing the same power, but one will be nicer to drive than the other!
The stronger engine can push higher gear ratios, therefore more distance travelled per RPM leading to faster accellaration. The weaker engine needs to rev alot higher to get that level of accellaration whilst pushing lower gears and you lose time working the gearbox and fighting against all the inefficiencies of a 4 stroke engine at higher rpm's.
The stronger engine can push higher gear ratios, therefore more distance travelled per RPM leading to faster accellaration. The weaker engine needs to rev alot higher to get that level of accellaration whilst pushing lower gears and you lose time working the gearbox and fighting against all the inefficiencies of a 4 stroke engine at higher rpm's.
Neil Haughey
New member
+ more friction losses unless of course the high revving engine is smaller/fewer cylinders etc to compensate.
More torque is equivalent to reducing weight in many ways. Before we stripped down the S2 my brother drove it with about 120 Kg removed and he was amazed how lively it felt, the slightest touch of the throttle launched the car down the road. Its very much the torque one feels when we talk about fast or powerful cars (perversely), I guess this is why I have found many cars to be ultimately a bit of a let down because despite the great headline figures all the torque is damped down trying to shift these tank weight cars around these days. Conversely this is why all you boosty boys love your cars so much.
More torque is equivalent to reducing weight in many ways. Before we stripped down the S2 my brother drove it with about 120 Kg removed and he was amazed how lively it felt, the slightest touch of the throttle launched the car down the road. Its very much the torque one feels when we talk about fast or powerful cars (perversely), I guess this is why I have found many cars to be ultimately a bit of a let down because despite the great headline figures all the torque is damped down trying to shift these tank weight cars around these days. Conversely this is why all you boosty boys love your cars so much.
TTM
Well-known member
For what it's worth this sort of build has been done in Germany for more than ten years as in the "early" days of 3.0 turbo engine builds specific pistons were hard to come by, at least at a reasonable price.
The graphs show pretty good figures, wondering by how much the CR was reduced on the pistons.
That a 3.0 can make 350bhp with a K26/8 at stock boost pressure is most impressive - congrats!
The graphs show pretty good figures, wondering by how much the CR was reduced on the pistons.
That a 3.0 can make 350bhp with a K26/8 at stock boost pressure is most impressive - congrats!
Barry Hart
New member
Thanks for the encouraging comments. We used a C/R of 8.5/1.
I have cobbled together this reply from some of the other forum responses (so a bit disjointed but covers lots of issues).
Can I just say that throughout my work with engines I have had to work most things out for myself (often because there was no one else up there where we were working at the time or what others said didn't work or didn't make sense). I believe that this process develops the brain and enables those with the ability to expand its potential rather like finding a way for a computer to make its own internal programming more sophisticated and more powerful as it solves more problems. Because of this I used to surprise people by my willingness to discuss my latest ideas "" but I did so because I believed that by describing something new that I had worked out "" it actually prevents my competitors from going through that learning curve and eventually restricts their own thinking power.
Often I have worked things out that later I found others had also done "" often before me or at the same time. Sure enough people in general assume you have found out and copied the other person "" and try to discredit your work by stating that so and so did it years before and implying that you just copied them - but this is in my experience never the case. People who can work things out do not copy others ideas (and would not be too interested in what anyone else thought or did before) and anyway at any one time people are on the same wavelength. So just because someone else may have been thought the same process and solved the same problems doesn't in my view discredit the ability of anyone else working the same things out for themselves at a later date. I find others who have also achieved actual success also understand this "" it seems only those that never actually made anything cannot wait to point out you were not the first "" just to bring you down a peg or two.
While I feel confident in my knowledge of naturally aspirated engines my experience of tuning turbos is relatively minimal and so now I am "hooked" by the experience I am interested in understanding the processes and problems and trying to work things out for myself. Just because others may have built 3 litre turbos in Germany 10 years ago doesn't mean I knew that and what we did was entirely because we thought it out for ourselves. We know that (and to be fair until the results were proven recently there were many others pointing out that it was the wrong approach and wouldn't work) and frankly I don't care what others may conclude.
I was extremely encouraged to see the results of my theory about torque etc working and also that the rear wheels can drive high torque if the acceleration is smooth enough and the extra weight loading from the torque reaction can then sustain grip "" and this has encouraged me to build a much more powerful version with a good torque spread. Acceleration is a strange concept and only ever an average between 2 speeds "" the rate of change of acceleration being an interesting problem and almost certainly behind the lack of grip some turbos experience under sudden heavy acceleration - but it will be interesting to compare actual speed differences with the power or torque available and this engine allows for this "" as it is possible to change up at different rev points and establish which part of the graph gave the best results (as Paul F thought it made little difference changing up @ 5K for example). So I am going to buy a more expensive road dyno to enable us to record full acceleration runs and use that to experiment with different power characteristics and finally see or prove once and for all whichever power band/torque delivery is quickest (to establish a curve we seek to replicate with more power/torque).
I agree that at any given revs the torque and power are directly linked "" so from that you could argue that they are the same thing "" however "" as someone rightly pointed out "" you can reach a high bhp figure at high revs by using an engine with large breathing, or inlet chargers, but these always result in lower power/torque at lower revs as the large system is unstable at lower charge rates.
Gear boxes force you to drive the car from the revs you find when you change gear to the peak revs where you change again and so your maximum acceleration will come from the maximum overall output you can obtain throughout that revs range. If you raise the peak revs and do not change the overall gear ratio "" then you have a bigger rev gap to drive through when you change up "" so your higher revving engine must also have a bigger power band to be as quick and this is usually the opposite of the result you obtain.
F1 engines work because although their bmep is not staggering (because the higher the revs the shorter the time the various valves are open to flow the air and exhaust) but bmep is proportional to torque and it is the rear wheel torque that accelerates the car "" so if they didn't reduce the overall gearing they would have a F1 car geared for 700 mph "" but by reducing that gearing by about 300% they triple the rear wheel torque from a more conventional gearing (for a normally revving engine) and the result is good rear wheel torque again.
Most of us are stuck with our gearing (apart from perhaps an S2 gearbox in a turbo giving a small torque improvement) and the straights on most race tracks are too short to reach high top speeds - so it can in my view be beneficial when converting a road car to a track car to explore lowering the overall peak revs and exploit the extra time valves are open at lower revs and therefore work in a more beneficial area for greater bmep and torque and this is what we built this engine to explore with great results.
Furthermore when dyno testing such revvy engines with big turbos or ports etc there is a few seconds in which they output is relatively low and rises as the unstable gas flow (caused by the big ports or time areas) settles down "" finally reaching impressive figures. Unfortunately during acceleration there is no time at all when you are stable at one rev and the whole process is dynamic, so such large breathing engines do not reach that bhp figure in a transient state (as driving in a race say) whereas engines with slightly lower breathing are stable sooner and produce maximum bmep (or torque) lower down and with a wider range "" so when people like me promote torque and others promote power and others say they are the same "" what we are all arguing about is the shape of the power or torque curve we are going to use when we change gear. Concentrating on the best average torque throughout that range will always be achieved by a slightly more torquey engine than concentrating on a steeper shape to the torque curve resulting in a bigger maximum bhp reading at high revs but a lower average torque in the rev range and a less stable output in transient state of accelerating.
We used this idea to provide an inexpensive torquey and driveable car with excellent torque and acceleration. However we could go further and intend to develop a new engine to keep the bottom end torque and improve torque higher up "" to maintain traction and get the best all round result.
Developing the existing 944 turbo would result in spiralling costs and eventual limitations so at the moment I think it is best to keep the existing 3 litre turbo as it is and offer it as a kit when I have worked out the costs.
We will also be making liners to repair all 944's or alter their capacity and exploring the best pistons etc (none of which will be possible quickly).
I have had a few Eureka moments during my work with engines (perhaps the most relevant when I realised about the link between specific time areas and BMEP and that it was compression pressure that was the key and a variable compression ratio engine was the answer "" normally aspirated). Of course a turbo is just that "" by providing big changes to the pressure differentials it improves the compression pressure outside the scope of normally aspirated engines and as such varies the true C/R. The problem seems to me to be that while you can force more air into the engine by using a bigger turbo "" you cannot force it out of the exhaust so easily and using a large exhaust and big turbo then results in it being top endy as the surge of acceleration makes the cars less driveable.
It makes no difference if I eventually discover that others have come up with the same idea (and it was in 1933 that I know of a variable compression engine first being built "" but I didn't know why or what the designer was trying to achieve until perhaps 30 years ago).
I have now had another Eureka moment of how to solve the problem of building a turbocharged engine to achieve the same bottom end torque as our turbo and a flatter torque curve to provide around 450bhp while keeping that lovely bottom end "" but to experiment and build it I need to start with another engine. I will probably find that someone else has done it before as well "" but for me it will be brand new territory and very exciting and satisfying.
As you can see I have well and truly got the bug again and am looking forward to testing this new theory in public.
I think that racing is the proof of the theory (and have had too many race wins with my ideas and engines to be excluded from that "club") and I would intend eventually to put the results to the test on the track.
To experiment with settings I will need a cylinder head that has the capacity to flow a lot of air and exhaust at high revs, and to be able to easily vary the cam timing and overlap (perhaps even in operation at different revs) and the 968 engine is better at both than the 8 valve 944 turbo and seems a more sensible starting point.
We intend to use the existing car to test the inlet and outlet manifold pressures (already fitting the equipment to establish our view that the exhaust is choking the engine) to prove the need to vary overlap. But a single camshaft does not lend itself to allow easy adjustments to overlap and so I do think a DOHC engine would give more alternatives to optimise performance. I would also like to be able to change overlap with revs (which the 968 variocam could allow me to do) and so a 968 car (or a 968 head on the 944 turbo with external control) may be necessary to discover more "" easily. As someone else said "" this also would lend itself to eventually further develop more modern 6 cyl engines as they have the same system on board.
My guess is that while you can pump in loads of air to the engine to raise the bmep, you are not also pumping it out again "" so the exhaust pressure can rise above the inlet pressure and any overlap can therefore stunt the incoming charge. Of course you can then raise the inlet pressure to overcome this but in so doing you end up with a laggy drive and this limits the amount of torque you can transmit and drive quickly. It would seem that as inlet pressure rises, it would be a good idea to move to negative overlap (might be wrong but can test this with a twin cam head "" too expensive to make loads of cams to test this theory with a single cam). Negative overlap reduces time area so then you possibly need to increase lift if you want to avoid the easy solution of a bigger turbo with all its downsides.
We have a chassis here ready to be modified and just need a 968 engine to progress the plan.
Baz
I have cobbled together this reply from some of the other forum responses (so a bit disjointed but covers lots of issues).
Can I just say that throughout my work with engines I have had to work most things out for myself (often because there was no one else up there where we were working at the time or what others said didn't work or didn't make sense). I believe that this process develops the brain and enables those with the ability to expand its potential rather like finding a way for a computer to make its own internal programming more sophisticated and more powerful as it solves more problems. Because of this I used to surprise people by my willingness to discuss my latest ideas "" but I did so because I believed that by describing something new that I had worked out "" it actually prevents my competitors from going through that learning curve and eventually restricts their own thinking power.
Often I have worked things out that later I found others had also done "" often before me or at the same time. Sure enough people in general assume you have found out and copied the other person "" and try to discredit your work by stating that so and so did it years before and implying that you just copied them - but this is in my experience never the case. People who can work things out do not copy others ideas (and would not be too interested in what anyone else thought or did before) and anyway at any one time people are on the same wavelength. So just because someone else may have been thought the same process and solved the same problems doesn't in my view discredit the ability of anyone else working the same things out for themselves at a later date. I find others who have also achieved actual success also understand this "" it seems only those that never actually made anything cannot wait to point out you were not the first "" just to bring you down a peg or two.
While I feel confident in my knowledge of naturally aspirated engines my experience of tuning turbos is relatively minimal and so now I am "hooked" by the experience I am interested in understanding the processes and problems and trying to work things out for myself. Just because others may have built 3 litre turbos in Germany 10 years ago doesn't mean I knew that and what we did was entirely because we thought it out for ourselves. We know that (and to be fair until the results were proven recently there were many others pointing out that it was the wrong approach and wouldn't work) and frankly I don't care what others may conclude.
I was extremely encouraged to see the results of my theory about torque etc working and also that the rear wheels can drive high torque if the acceleration is smooth enough and the extra weight loading from the torque reaction can then sustain grip "" and this has encouraged me to build a much more powerful version with a good torque spread. Acceleration is a strange concept and only ever an average between 2 speeds "" the rate of change of acceleration being an interesting problem and almost certainly behind the lack of grip some turbos experience under sudden heavy acceleration - but it will be interesting to compare actual speed differences with the power or torque available and this engine allows for this "" as it is possible to change up at different rev points and establish which part of the graph gave the best results (as Paul F thought it made little difference changing up @ 5K for example). So I am going to buy a more expensive road dyno to enable us to record full acceleration runs and use that to experiment with different power characteristics and finally see or prove once and for all whichever power band/torque delivery is quickest (to establish a curve we seek to replicate with more power/torque).
I agree that at any given revs the torque and power are directly linked "" so from that you could argue that they are the same thing "" however "" as someone rightly pointed out "" you can reach a high bhp figure at high revs by using an engine with large breathing, or inlet chargers, but these always result in lower power/torque at lower revs as the large system is unstable at lower charge rates.
Gear boxes force you to drive the car from the revs you find when you change gear to the peak revs where you change again and so your maximum acceleration will come from the maximum overall output you can obtain throughout that revs range. If you raise the peak revs and do not change the overall gear ratio "" then you have a bigger rev gap to drive through when you change up "" so your higher revving engine must also have a bigger power band to be as quick and this is usually the opposite of the result you obtain.
F1 engines work because although their bmep is not staggering (because the higher the revs the shorter the time the various valves are open to flow the air and exhaust) but bmep is proportional to torque and it is the rear wheel torque that accelerates the car "" so if they didn't reduce the overall gearing they would have a F1 car geared for 700 mph "" but by reducing that gearing by about 300% they triple the rear wheel torque from a more conventional gearing (for a normally revving engine) and the result is good rear wheel torque again.
Most of us are stuck with our gearing (apart from perhaps an S2 gearbox in a turbo giving a small torque improvement) and the straights on most race tracks are too short to reach high top speeds - so it can in my view be beneficial when converting a road car to a track car to explore lowering the overall peak revs and exploit the extra time valves are open at lower revs and therefore work in a more beneficial area for greater bmep and torque and this is what we built this engine to explore with great results.
Furthermore when dyno testing such revvy engines with big turbos or ports etc there is a few seconds in which they output is relatively low and rises as the unstable gas flow (caused by the big ports or time areas) settles down "" finally reaching impressive figures. Unfortunately during acceleration there is no time at all when you are stable at one rev and the whole process is dynamic, so such large breathing engines do not reach that bhp figure in a transient state (as driving in a race say) whereas engines with slightly lower breathing are stable sooner and produce maximum bmep (or torque) lower down and with a wider range "" so when people like me promote torque and others promote power and others say they are the same "" what we are all arguing about is the shape of the power or torque curve we are going to use when we change gear. Concentrating on the best average torque throughout that range will always be achieved by a slightly more torquey engine than concentrating on a steeper shape to the torque curve resulting in a bigger maximum bhp reading at high revs but a lower average torque in the rev range and a less stable output in transient state of accelerating.
We used this idea to provide an inexpensive torquey and driveable car with excellent torque and acceleration. However we could go further and intend to develop a new engine to keep the bottom end torque and improve torque higher up "" to maintain traction and get the best all round result.
Developing the existing 944 turbo would result in spiralling costs and eventual limitations so at the moment I think it is best to keep the existing 3 litre turbo as it is and offer it as a kit when I have worked out the costs.
We will also be making liners to repair all 944's or alter their capacity and exploring the best pistons etc (none of which will be possible quickly).
I have had a few Eureka moments during my work with engines (perhaps the most relevant when I realised about the link between specific time areas and BMEP and that it was compression pressure that was the key and a variable compression ratio engine was the answer "" normally aspirated). Of course a turbo is just that "" by providing big changes to the pressure differentials it improves the compression pressure outside the scope of normally aspirated engines and as such varies the true C/R. The problem seems to me to be that while you can force more air into the engine by using a bigger turbo "" you cannot force it out of the exhaust so easily and using a large exhaust and big turbo then results in it being top endy as the surge of acceleration makes the cars less driveable.
It makes no difference if I eventually discover that others have come up with the same idea (and it was in 1933 that I know of a variable compression engine first being built "" but I didn't know why or what the designer was trying to achieve until perhaps 30 years ago).
I have now had another Eureka moment of how to solve the problem of building a turbocharged engine to achieve the same bottom end torque as our turbo and a flatter torque curve to provide around 450bhp while keeping that lovely bottom end "" but to experiment and build it I need to start with another engine. I will probably find that someone else has done it before as well "" but for me it will be brand new territory and very exciting and satisfying.
As you can see I have well and truly got the bug again and am looking forward to testing this new theory in public.
I think that racing is the proof of the theory (and have had too many race wins with my ideas and engines to be excluded from that "club") and I would intend eventually to put the results to the test on the track.
To experiment with settings I will need a cylinder head that has the capacity to flow a lot of air and exhaust at high revs, and to be able to easily vary the cam timing and overlap (perhaps even in operation at different revs) and the 968 engine is better at both than the 8 valve 944 turbo and seems a more sensible starting point.
We intend to use the existing car to test the inlet and outlet manifold pressures (already fitting the equipment to establish our view that the exhaust is choking the engine) to prove the need to vary overlap. But a single camshaft does not lend itself to allow easy adjustments to overlap and so I do think a DOHC engine would give more alternatives to optimise performance. I would also like to be able to change overlap with revs (which the 968 variocam could allow me to do) and so a 968 car (or a 968 head on the 944 turbo with external control) may be necessary to discover more "" easily. As someone else said "" this also would lend itself to eventually further develop more modern 6 cyl engines as they have the same system on board.
My guess is that while you can pump in loads of air to the engine to raise the bmep, you are not also pumping it out again "" so the exhaust pressure can rise above the inlet pressure and any overlap can therefore stunt the incoming charge. Of course you can then raise the inlet pressure to overcome this but in so doing you end up with a laggy drive and this limits the amount of torque you can transmit and drive quickly. It would seem that as inlet pressure rises, it would be a good idea to move to negative overlap (might be wrong but can test this with a twin cam head "" too expensive to make loads of cams to test this theory with a single cam). Negative overlap reduces time area so then you possibly need to increase lift if you want to avoid the easy solution of a bigger turbo with all its downsides.
We have a chassis here ready to be modified and just need a 968 engine to progress the plan.
Baz
Barry Hart
New member
Sory about the rules - how do I change the name from Hartech to B Hart - advice needed please.
Baz
Baz
Barry Hart
New member
That's OK TTM. The C/R is perhaps one of the most missunderstood issues - which those that know little about engines I think imagine reflects what goes on inside the engine (not in any way inferring you are one of them).
The thing that makes the engine quick is the pressure pushing down on the piston and this in turn - few people realise - is greater if the pressure before ignition is greater - right up until detonation which must be avoided.
If the engine cylinder was 100 % full of air (mixed with fuel) then the volumetric efficiency is called 100% and then if the C/R was say 10/1 you could estimate that the pressure before ignition on compression would be round about 10 times ambient (not exactly but I am using this to explain something so bear with me).
True if you then fitted H/C pistons (say 12/1) then the pressure before ignition would be even higher.
Now the result of higher compression pressure before ignition is a much higher pressure after ignition and therefore a harder push down on the piston and more torque.
Engines only manage to produce high volumetric efficiency in a small rev band and everywhere else there is less air trapped so whatever C/R is in use - the compression pressure will be lower.
The C/R must be pre-set so that at the most efficient trapped revs/pressure it is never so high to initiate pre-ignition and everywhere else in the rev range the pressure is correspondingly lower and so therefore is the pressure on the piston and the torque.
Now torque is proportional to bmep (the average pressure acting on the piston to produce that bhp/torque) so wherever the maximum trapped pressure is available so is the maximum torque.
You can alter the revs where this maximum pressure is by tuning or by forcing air into the engine you can increase it - but you must then lower the geometric C/R to compensate - or it would be too high and detonate. So when a Turbo is built with lower C/R pistons the compression pressure is just the same as a naturally aspirated engine when they are working at maximum efficiency
Unfortunately when the turbo is not very effective you are left with the similar cylinder filling to a normally aspirated engine but running with the lower C/R you fitted to make it work when the turbo is filling the cylinders and therefore not very powerful and because the difference in the two (on turbo cylinder pressure to off turbo cylinder pressure) is greater in a turbo - so the rate of change (or torque) when it comes into the turbo range is higher - giving a thrilling push but often breaking traction and upsetting the geometry.
Because the amount of time the valves are open reduces with revs - a normally aspirated engine loses cylinder filling (or torque) as the revs rise (because there is less time to fill the cylinders) but since power is proportional to torque * revs the power continues to rise.
The cylinder fills up because there is greater pressure outside the cylinder than inside it and a turbo increases this pressure difference so improves cylinder filling especially at higher revs - but it can also do so at lower revs if it is relatively small compared to the engine capacity and this improves mid range rather than peak bhp (as we have done by increasing the capacity but keeping the turbo the same size).
The increase in performance we achieved is therefore not "because we lowered the compression ratio" but because we increased the capacity and we had to lower the compression ratio of the 968 pistons because otherwise they would be too high. When running with the extra cylinder filling of the turbo - the actual true compression pressure above the piston would be the same or similar. This is born out by the fact that we achieved 20% more bhp from and engine 20% larger - but the benefit was much more torque/bmep lower down where the engine is more efficient now that the air flow is higher than it was originally designed to manage at higher revs.
Pumping more air in is great but getting it out is more of a problem as the back pressure reduce the pressure difference again on overlap and is the Achiles heel of a turbo. It is this area we are working on for our next engine.
Baz
The thing that makes the engine quick is the pressure pushing down on the piston and this in turn - few people realise - is greater if the pressure before ignition is greater - right up until detonation which must be avoided.
If the engine cylinder was 100 % full of air (mixed with fuel) then the volumetric efficiency is called 100% and then if the C/R was say 10/1 you could estimate that the pressure before ignition on compression would be round about 10 times ambient (not exactly but I am using this to explain something so bear with me).
True if you then fitted H/C pistons (say 12/1) then the pressure before ignition would be even higher.
Now the result of higher compression pressure before ignition is a much higher pressure after ignition and therefore a harder push down on the piston and more torque.
Engines only manage to produce high volumetric efficiency in a small rev band and everywhere else there is less air trapped so whatever C/R is in use - the compression pressure will be lower.
The C/R must be pre-set so that at the most efficient trapped revs/pressure it is never so high to initiate pre-ignition and everywhere else in the rev range the pressure is correspondingly lower and so therefore is the pressure on the piston and the torque.
Now torque is proportional to bmep (the average pressure acting on the piston to produce that bhp/torque) so wherever the maximum trapped pressure is available so is the maximum torque.
You can alter the revs where this maximum pressure is by tuning or by forcing air into the engine you can increase it - but you must then lower the geometric C/R to compensate - or it would be too high and detonate. So when a Turbo is built with lower C/R pistons the compression pressure is just the same as a naturally aspirated engine when they are working at maximum efficiency
Unfortunately when the turbo is not very effective you are left with the similar cylinder filling to a normally aspirated engine but running with the lower C/R you fitted to make it work when the turbo is filling the cylinders and therefore not very powerful and because the difference in the two (on turbo cylinder pressure to off turbo cylinder pressure) is greater in a turbo - so the rate of change (or torque) when it comes into the turbo range is higher - giving a thrilling push but often breaking traction and upsetting the geometry.
Because the amount of time the valves are open reduces with revs - a normally aspirated engine loses cylinder filling (or torque) as the revs rise (because there is less time to fill the cylinders) but since power is proportional to torque * revs the power continues to rise.
The cylinder fills up because there is greater pressure outside the cylinder than inside it and a turbo increases this pressure difference so improves cylinder filling especially at higher revs - but it can also do so at lower revs if it is relatively small compared to the engine capacity and this improves mid range rather than peak bhp (as we have done by increasing the capacity but keeping the turbo the same size).
The increase in performance we achieved is therefore not "because we lowered the compression ratio" but because we increased the capacity and we had to lower the compression ratio of the 968 pistons because otherwise they would be too high. When running with the extra cylinder filling of the turbo - the actual true compression pressure above the piston would be the same or similar. This is born out by the fact that we achieved 20% more bhp from and engine 20% larger - but the benefit was much more torque/bmep lower down where the engine is more efficient now that the air flow is higher than it was originally designed to manage at higher revs.
Pumping more air in is great but getting it out is more of a problem as the back pressure reduce the pressure difference again on overlap and is the Achiles heel of a turbo. It is this area we are working on for our next engine.
Baz
So with a turbo you can use the boost pressure to increase the CR and compensate for decreases in volumetric efficiency. This is where RPM boost control comes in then? You should be able to create a pretty flat torque curve doing this. So you could increase the area under your power curve doing this without creating a very high torque at low rpms and stressing the internals of the engine?
Edit: It is also harder to produce torque at high revs right? Similar to how its harder to exert a force when pusing a a moving car than a stationary one. It must be harder to exert a torque on a fast moving piston than a slow moving one. So as well as losses in volumetric efficiency you would have this effect to contend with?
Edit: It is also harder to produce torque at high revs right? Similar to how its harder to exert a force when pusing a a moving car than a stationary one. It must be harder to exert a torque on a fast moving piston than a slow moving one. So as well as losses in volumetric efficiency you would have this effect to contend with?
Barry Hart
New member
Yes if you could increase boost with revs you could theoretically overcome the reduction in time area and maintain the same true Compression pressure (not increase it because you would already be set just below detonation anyway) - but to do this you need a big turbo and inlet pipes and exhaust - all of which reduce potential to also obtain good bottom end torque.
If your analogy about pushing the car refers to the difficulty keeping up with it to push it hard then I suppose there is some correlation but in reality the air is already passing up the inlet pipe with inertia and so only needs a small boost to speed up and overcome the slight reduction in time area.
Baz
If your analogy about pushing the car refers to the difficulty keeping up with it to push it hard then I suppose there is some correlation but in reality the air is already passing up the inlet pipe with inertia and so only needs a small boost to speed up and overcome the slight reduction in time area.
Baz
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