Engine build progress
- Thread starter TTM
- Start date
TTM
Well-known member
Small garages aren't necessarily a bad thing, tools don't get lost as easily! I think the timing I am running now is right at the border of safety considering how fast the turbo spools and the quality of the fuel I use. There is a separate map with less advance which I was running with the airbox and there is also the possibility to reduce timing even further with the FQS, but both solutions made the car feel less responsive. The engine has done 9k miles now, it is beginning to feel really crisp and I like it better this way at 14.5psi than with less timing at 17+psi which gave it a bit of a lazy turbo-diesel feel, even if I could then get 17psi at 2700rpm. A better ignition may help getting rid of the knock by burning really all the fuel really at the right time...
Colder plugs might help, depends whats causing the knock. Did you get quicker spool up with less ignition advance? I am building a new 944 DME product at the moment. I am building a complete replacement for the digital board in the DME. It will have a modern microprocessor with usb tuning and as many features as I can implement. I can implement all sorts of features as I will be writing the code from scratch. I am wondering if it is worth adding a feature to retard the ignition when at WOT at certain RPMs to get the turbo to spool as early as possible. I guess with retarted ignition more energy is left over to spool the turbo. So say we know the turbo can spool up at best by 2700rpm when the DME detects WOT it can retard ignition a certain ammount when it gets to say 2600rpm. Although maybe this can just be mapped as normal.
sawood12
New member
The way to spool up the turbo quickly is to lean out the mixture to increase heat. It is heat that drives the turbo, not flow. If you retard ignition your combustion temps will be cooler, and volume of combustion gases will be less therefore less energy in the exhaust flow and you'll actually hamper turbo spool up. Also when you want to go faster the thing that actually accelerates the engine is advancing the ignition so at WOT when you want full acceleration you want ignition to be as advanced as possible and throw as much fuel at it as possible to ensure you don't run lean and keep combustion temps down, so retarding ignition at WOT will again hamper acceleration.
Thanks for the info Scott. The engine is out of your old car btw, got the head off and pleased to report it has excellent bores. You can just catch your fingernail on the wear mark at the top of the stroke, not bad for 135K! The question is what to do now! Ive been tempted by building a 2.8 stroker, can't decide on what turbo to go with either. About the only thing Ive settled on is building my own injection system which should help keep the cost down.
sawood12
New member
Sounds good. Will watch with great interest. I still have great memories of her. I knew she was a good one. The specialist I took it to for servicing always commented on how 'tight' the car felt, and he's seen a fair few 944's in his time. I would love another 944 but fear that I would not be able to find a car as good as that one so when I do dip my toe back into Porsche ownership I think I might go for something else, or if I do go for another 944 go for something that has been majorly tinkered with e.g. 3 ltr sleeved engine or v8.
TTM
Well-known member
Spent most of the weekend at the wheel, starting all over from scratch (=blank map) and it seems I got the knock under control, after filling from 4 different petrol stations in order to add uncertainty on fuel quality. I never mentioned it before but John at Vitesse had told me several months ago about a "hidden function" that allows to add a significant factor in the fueling loop. That was when I was running overly rich at part throttle. Before anyone asks about this function, I'm afraid you will have to purchase a Vitesse kit to find out! Now, after hair-splitting adjustements I wasn't able to spot knock anymore, either with the boost solenoid inactive (=wastegate spring, 15psi) or if I set it to 18psi with a flat boost ramp. I think I can feel the software retarding ignition advance when running 18psi instead of 15psi - the engine has more "anger" and feels more reactive and swift at 15psi than at 18psi, at which it feels like a real tractor. Gobs of torque, but not quite a "coherent" power level. I wish I had access to the ignition map. With regards to the lean spike, and considering the fuel map I now established, I am pretty sure that no knock appears at the moment of the lean spike. Sly is building another of his "Knock module interface" for me, so I will soon be able to log knock with the Zeitronix and spot exactly under which conditions knock takes place. I think the oil cooling needs to be improved, as after several hard accelerations the oil pressure needle shows a shade over 4 bar, while when I drive sedately it shows a shade below 5 bar, as was the case with the 2.5. These 104mm blocks need more than the standard turbo oil cooler! [
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HI Sawood, having built a torquey 3 litre 944 turbo (reported in the UK in 911 and Porsche World recently), I am now building a more ambitious 3 litre with 16 valve 968 head (to acomplish negative overlap if and when when needed), nimonic exhaust valves (although the std ones seem non magnetic at the ends?), and various other specially engineered features including using a 2.5 block (to have more coolant channel depth) and Nikasil alloy plated liners fitted to increase the bore and support the tops against machined out parts of the old block. Special pistons are currently in manufacture to suit. However I am an "old engineer" good with designing, making and assembling things in metal but poor at computerised stand alone management systems - which I will have to allow my younger staff to sort out and operate (I think it would frustrate me too much). I am learning a lot about turbos and think I understand quite a lot of it - but your comment about the performance of the turbo relying on temperature - I did not follow - unless by stating that you imply that higher temperatures also increases pressure and therefore flow rate (or I suppose you could view that as the volume you could pass in a given timescale). I imagined it was the flow of gas that spins up the turbo through a combination of mass flow hitting the blade angle and pressure drop accross it increasing the effect - and increasing temperature would increase pressure and through that a potential increase in volume - therefore also effectively increase the flow rate but I cannot see how temperature alone is the governing factor - but I am very willing to learn if there is another phenomenon going on. I am planning bigger exhausts to prevent back pressure restricting breathing but realise that should also reduce exhaust gas temperatures (since temperature, pressure and volume are directly or inversley mutually dependent and directly related - increase one and you increase the other - or with volume it works the other way - more volume - less pressure and temperature. While this project is mainly to enable my staff to learn more about stand alone systems before we embark on working tuning up say a Cayman or similar in the future - my job is to mechanically build something that will be strong enough and well enough engineered to achieve those objectives reliably and so - while I thought I understood most of what is neccessary to achieve that - I am always keen to learn anything at all - from anyone - that I didn't know or misundserstood. I am not therefore in any way trying to put you on the spot but rather picking your brains - as it is prefectly clear to me that there are a lot of people (particularly in the USA) who have far greater knowledge of turbocharging this engine than me. regards. Baz
Heres my take on it... Temperature is important for pressure and along with flow it tells you how much kinetic energy is in the gas. Flow and pressure are the result of the same type of energy except flow is directed motion and pressure is random. Volume in the crossover is constant so and increase in temp causes an increase in pressure. Both flow and pressure transfer energy to the turbo. Flow is much better than temperature for this though as the energy is directed at the turbine blades rather than randomly exerting pressure. But we have temperature and flow so we might as well make the best of both. The reason you get a pressure drop post turbo is that the flow and temperature have been reduced due to the kinetic energy being transferred to the turbine blades and that the cross section of the downpipe is greater than that of the crossover pipe thus volume has changed. Increasing the exhaust volume might not reduce the gas temperature, maybe just reduce the pressure as the volume has increased. Pressure is simply the effect of the random kinetic motion of a gas at temperature within a volume. You cant change temperature without adding or removing energy from the gas, thus simply changing volume will only change pressure not temperature.
sawood12
New member
Sort of, except the pressure and flow are intrinsically linked and are not explicit so are both mutually important. A turbine extracts energy primarily from the temperature of the gas stream rather than it's speed (i.e. it is not a windmill). That is why fuel is added as the energy you are actually exploiting is the chemical energy locked up in the fuel that is liberated by burning it. The turbine is shaped so that it first throttles the area to slow the flow right down - a perfectly efficient turbo will have zero flow at its inlet (impossible to achieve of course). This increases the pressure at the turbine inlet. The turbine is then shaped so that the gas is allowed to expand through it, and as it expands it increases in flow i.e. it accelerates. It is this acceleration of the flow that generates the force that turns the turbine (force = mass x acceleration). It does this by turning the accelerating flow round a corner - so it is a bit like a garden sprinkler in this respect. So the power generated by the turbo is proportional to the pressure drop across it and the acceleration of the gas through it. To increase one you need to increase the other. So given this, to increase the spool up speed of a turbo you need to increase the pressure at its inlet (the pressure drives the expansion which drives the acceleration of the flow which drives the turbine) there are two ways to increase the pressure - increase the gas temperature i.e. lean out the AFR (for a short time) to increase temps which is the only option with a fixed geometry turbine, or to throttle the turbine more to slow the flow down and increase the inlet pressure - this is what a variable inlet turbine does (as installed on a 997 turbo) which closes up the throat area of the turbine at low rpm to maintain turbine speed.
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