Engine performance

[333pg333]

Peter, in your estimation, how quick in a straight line is your car compared to a GT3? Have you been on the track with one in close quarters to get a good impression? I'm just curious as your car looks actually very rapid from the video, but it's sometimes misleading depending on points of reference on the track and other cars and how fast they're being driven. I have to say that based on recent experience where my car is up with the GT3 I would be surprised if yours is, only based on the build spec and boost that you're currently running, but it does look pretty darn quick from what I've seen. []

 

[DivineE]

ORIGINAL: Peter Empson

That's a funny looking alternator.

Looks seriously mean on the vids though, I wasn't expecting it to be that quick at all having seen supercharged M3's before. They're not usually that much faster without MAJOR work. I'm guessing most of the speed comes from a much improved power to weight ratio?

 

[Peter Empson]

ORIGINAL: 333pg333
Peter, in your estimation, how quick in a straight line is your car compared to a GT3? Have you been on the track with one in close quarters to get a good impression?

I don't think I've been on track with a GT3 post SC, but my guess would be that it isn't that quick (I'd be losing a reasonable amount out of corners anyway, as I still don't have a LSD). There's more power to come however, and more weight to be removed too []

 

[peanut]

ORIGINAL: Peter Empson

more weight to be removed too []

you on a diet peter ?[]

 

[Peter Empson]

ORIGINAL: peanut

you on a diet peter ?[]

No chance, beer is much too nice.
S'pose I could always use the angle grinder to cut off a couple of spare limbs off instead of removing steel from the car...

 

[944Turbo]

The guide I have to installing the LS V8 suggests using an na 5th gear - it gives a theoretical top speed of 200mph and presumably will improve the consumption with lower revs at the motorway limit.

the smaller frontal area relative to the Monaro should also improve the economy,

Tony

 

[sawood12]

Theoretical top speeds are irrelevant i'm afraid. The factor that determines the top speed of any car is the power available to overcome the aerodynamic drag at those speeds. I've read that to hit 200mph in a 944 you probably need in excess of around 500 bhp - the bhp required to achieve a given top speed not having a linear relationship. Gearing doesn't get around the fundamental laws of aerodynamics, but only allows you to get the best out of what you've got. But the improved consumption at lower speeds still holds.

When matching gearing to engine characteristics you've got to have a clear objective in mine. If you want economy you gear highly such than you exploit the torque available at low RPM's. If you want good top speeds at the expense of economy you want lower gearing to exploit power at high RPM's. You can't have both, that is why most manufacturers select non-linear gear ratio's so you can have economy at cruising speeds in top gear but good acceleration in the lower gears at the expense of economy.

 

[944Turbo]

erm yes thats why I said theoretical, though with the LS series 500BHP or more is easily achievable - my point was ref fuel consumption with a V8 in the front.

The torque / power charecteristics are going to be different with a 6.0V8 compared to a 2.5Turbo.

Tony

guilty of replying to points earlier in the thread without quoting

 

[bazhart]

Just a few minor points to add to generally accurate and good advice in all the posts.

Firstly many supercharger conversions do not change static geometric C/R.

It is torque not power that accelerates a mass. The torque produced is a function of the pressure pushing down on the piston which in turn is higher if the initial temperature and pressure (which are related) is higher before ignition. This then means that the higher the Compression pressure (within detonation limits) before ignition - the higher the resulting push on the piston - the higher the torque that results and the greater the acceleration.

Superchargers deliver best at lower revs so it can be possible to keep original C/R pistons since the upper speed inlet charge may not be higher - only the bottom end - which may not be too much for the engine to manage and for the std pistons to run without expanding so much they seize.

However if the supercharger is big and does improve top end breathing the engine should also have lower static compression pistons with greater taper fitted to run reliably.

The S2 piston is not ideal because it doesn't expect high revs power - mainly torque - but the 968 piston is more suitable and could be substituted. A higher static compression ratio piston would achieve much the same result but throughout the rev range.

Unless you have acces to change gearing ratios and final drive ratios - usually getting more torque or a wider torque spread will make a car faster and more driveable than just improving the top end bhp and revs. This is because with more revs the car is geared higher than before and the rev band needs to be biggere than it was before because the rev drop between gears is a function of the peak revs and the ratio (i.e. increases with higher revs).

Higher revs can be beneficial if the overall gearing can be lowered to increase the rear wheel torque that results (easy on a motorcycle with sprockets but not so easy in a car).

Fitting a turbo 5th gear to an S2 gearbox lowers 5th and therefore can be beneficial for all but top speed.

Turbocharged engines usually improve top end output but mid range torque loses out and unless the gearing is suitable can negate a lot of benefits except straight line performance.

As many have pointed out - acceleration - put simply - but quite correctly - is generally proportional to torque/mass or of you like torque/weight.

So reducing weight can be just a beneficial as increasing torque - and easier and cheaper with no real reliability or development issues.

Increasing bhp is a more often followed route but it must increase the average torque in between the revs from changing gear to peak revs before changing to the next gear and the overall gearing resulting must also suit the application (track or road etc). Just raising the revs and with it peak bhp - while good for bragging rights - does not usually provide the improvement in actual performance often expected.

Baz

 

[DivineE]

How can I calculate the rev drop between two gears? I'd like to install the n/a longer 5th gear in my new turbo to reduce consumption on long motorway runs and give me a higher top speed but it would annoy me massively if it dropped off boost every time I changed from 4th to 5th! I have a graph I can look at if I can work out where 5th gear will come in.

 

[Tam Lin]

ORIGINAL: DivineE

How can I calculate the rev drop between two gears? I'd like to install the n/a longer 5th gear in my new turbo to reduce consumption on long motorway runs and give me a higher top speed but it would annoy me massively if it dropped off boost every time I changed from 4th to 5th! I have a graph I can look at if I can work out where 5th gear will come in.

You could plug in the data into the Spreadsheet here . Not my work, credit to original poster etc etc

 

[Peter Empson]

ORIGINAL: bazhart

Superchargers deliver best at lower revs

Surely that depends upon the type of SC? The ones commonly used on the 944/968 conversions are centrifugal SC's (often, but quite incorrectly IMO termed as belt driven turbo chargers). These make comparitively low boost at low revs and maximum at the redline. The plus with this design is partly the ease of packaging but also it doesn't overly stress the drive train and minimises traction issues.

I've noted your understandable concerns over the pistons, but I'm curious to know whether you've seen this failure occur on an SC'd S2? I've spoken to, and know of quite a few owners of SC'd S2s and none that I'm aware of have had any problems even with high mileage and regular track use in countries far hotter than the UK.

 

[bazhart]

Hi Ben - I think you are looking at the opposite way round to make a bigger or taller 5th gear?

If you look at the standard graph with revs on the Y axis and speed on the X axis (as in most handbooks) all you have to do then is to drop a vertical line from your maximum revs you choose to change gear at and where is crosses the next gear slopping line read back across to the revs you start off in the next gear and this also therefore gives the power band.

You will see that it reduces as speed increases because with more rolling and wind resistance you need to be more in the real power band to pull through the resistance quickly.

To see what different gear ratios would do - look at the revs you decide to do the calculations at (say 6500 in 5th for arguments sake), at which revs the speed on a 944 turbo is about 172mph and when you change from 4th to 5th the revs drop to about 5,200 so the power band needed is 1,300 rpm.

Turbo 5th gear is 0.829 and an S2 5th is 0.778. The larger the ratio as a number the lower speed the car reaches - so if you divide by one and mutiply by the other - i.e. 172 / 0.778 and then multiply the answer by 0.829 the result is 183 which is the new top speed. If you look now at where the vertical line from the 4th gear crosses the new 5th gear line and read back to the revs the answer is about 4,800 rpm so the new power band needed is 1,700 rpm and you need good power/torque from 4800.

The reverse for racing by fitting the turbo 5th gear to an S2 gearbox is more difficult to compare because you are also then using a different crown wheel and pinion ratio although the revs drops will be the same.

An S2 in 5th @ 6500 rpm pulls about 154mph and the rev drop from 4th when changing up is about 1,600 rpm. Fitting the turbo 5th gives 154 / 0.829 * 0.778 = 144.5 and the rev drop is now reduced to 1,300 rpm (what it originally was in the turbo gearbox) but the lower crown wheel and pinion ration means all 5 gears now transmit 15% more torque even though the engine is unchanged and the top speed is more useable for most short circuits. The ratios are also very close to 1st to 5th in a 968 box so can be a good compromise for all but very long circuits where a 968 would never use 6th anyway.

You can play around with the graphs because the actual speed scale on the graphs can be changed to suit any scenario and the shape and relative positions between the gears will remain the same - only the speeds in the gears will change or the top revs at which you set the gear change point. However the "rev drop" is a proportion of the revs - so for example if changing from 4th to 5th @ 6500 rpm dropped 1,300 rpm - if (for the sake of demonstrating the point) you could rev the same engine to 13,000 rpm - the revs drop would be the same proportion (20%) but this would now be 2,600 rpm illustrating why revving up an engine without altering gear rations requires an engine that not only revs higher but also - at the same time - gives and even bigger power band - and you have to be very clever to do both.

As you know only too well - due to massive pressure of work - I have rushed this so please excuse any minor calculation errors.

Baz

 

[333pg333]

The stock 944 turbo gearbox is good for over 160mph Ben. Try it with the crown wheel/pinion from an S2 and then you'd know about short final drives.
You could explore the 968 box as they should have more options for gears from 911s I would suspect?

 

[DivineE]

ORIGINAL: bazhart

If you look now at where the vertical line from the 4th gear crosses the new 5th gear line and read back to the revs the answer is about 4,800 rpm so the new power band needed is 1,700 rpm and you need good power/torque from 4800.

Ha ha! Thank you. That couldn't be more perfect! I have a flat line of peak torque starting at exactly 4800rpm continuing through to the change point.[]

 

[TTM]

I would say running the turbo gearbox with the NA fifth gear would require a fair bit more power than 350bhp/500N.m (guesstimate of my 3.0 8V at ~14.5psi) to keep the engine on the safe side of active security in terms of responsiveness when driving at seriously high speeds. You don't want the engine to feel "detached" from your right foot like the two separated ends of an accordion, like on an original 250bhp turbo, when cruising at 150mph.
To me the original turbo gearbox suits the performance of my engine nicely, it feels as polished and refined as a fully original factory car, and the ratios might even be a bit too short for an engine way more powerful/torquey such as a prepped GM V8.

As far as I have looked, a much more powerful engine would work well a 944 turbo gearbox with 6th ratio added on top. Sadly there is no such thing and the only way to get close easily is to use a 968 gearbox and install taller gears from the Audi A8 FWD diesel gearbox (£££££)
For reference, ratio-wise the 968 turbo S gearbox is a 6 gear blend between the 968 NA gearbox and the 944 turbo gearbox, but would be too short for performance nearing 400bhp/500lbs.ft - my 951 with the original gearbox is already about 2 sec faster than a 968TS on 62-124mph.

 

[333pg333]

There are other alternatives but then it starts becoming customised work. I agree with Thom, just stick to the original 951 ratios. How many miles are you going to log on an open Motorway at just cruising speed anyway?? []

 

[bazhart]

Still too busy to take time over this today as well - sorry - but if we take 6000 revs as our peak the following speeds are available in each type of gearbox. Figures approximate.

Std S2 32, 54, 79, 107, 142 (rounded off to no decimal places).
Std 944 Turbo 37, 63, 93, 126, 157
Std 968 36, 57, 80, 102, 125, 146
S2 with 944 T 5th 32, 54, 79, 107, 133
Turbo with S2 5th 37, 63, 93, 126, 167

If you want to adjust for diferent peak revs just divide any speed above by 6000 and multiply by your new revs. i.e. for 6500 revs, in the last example as per Ben's interest 167*6500/6000 = 181 or for 7000 rpm = 195 and so on.

As 1st is not used for track racing (too low) the comparison between the 2nd to 5th of a 968 and the S2 gearbox with the turbo 5th is really close with the benefit of the availability of an LSD.

The same result as an S2 with a turbo 5th can be obtained with a turbo gearbox running an S2 crown wheel and pinion.

Since 6th is rarely used in a 968 for track racing, the 944 turbo and S2 gearbox - still offer a cost effective compromise and of course the 5th gear change is on the end of the gearbox behind the end cover and relatively easy to change even if you are scared to take the insides of the gearbox apart and no special tools needed. Mixing the 5th gear provides several options and in Ben's case it also offers the opposite alternative to a shorter racing 5th by creating a taller 5th gear by swapping the S2 and turbo 5th the other way around.

To lock the two gears to undo the retaining bolt just fold a cotton rag and jam between the teeth. You then need circlip pliers and a punch to remove the dowel in the selector fork. You may need a two legged puller to pull off the simple gear held in place with the bolt.

Different wheel rim sizes and tyre profiles will alter the figures slightly but not the principle.

Baz

 

[DivineE]

Many thanks for the info. I was rather hoping 200mph would be 'technically possible' so I could see how close it would get but I'm not revving it to over 7000rpm under that kind of load. That small increase will just make it a little more comfortable at my usual cruising speeds. I do actually do a lot of motorway driving.

Thanks for the description also, it does sound very simple but I think I'd still rather bring the car down and pay Hartech to change the 5th gear the first time. I'd be a mighty shame to ruin a good LSD box learning what can go wrong. If I don't like the longer ratio I can always change it back.