Porsche Enthusiasts Club Forum

I would imagine that the iron sleeves could possibly help stabilise the cylinder although as this is really just an insert, maybe not. The issue we’ve often had with the open bores is with the headgasket splitting or tearing around the compression ring. This has been put down to the open bores shifting at high boost and cylinder pressures. Are the Cossies an open deck or solid block Tim? We’ve always assumed the 3L block should be more resistant to the cylinders moving due to the ‘siamesed’ bores. Most people that have modified these cars over the years have started with the 2.5L block so perhaps not enough general information has trickled down. Then there are deck plates as another form of keeping the cylinders totally stable. Not many have gone for this option. Hoping to be able to speak of sustained headgasket life on the new motor partially due to this addition. It will be interesting following your progress Tim. Be interested to know how much ign timing you’re cranking in on your higher boost pulls?

A friend down here went through a very lengthy and expensive process of installing a twin scroll turbo setup with twin crossovers/twin wastegates etc...ran into backpressure issues. This started when the motor was a 2.5L and has since 'grown' to a 3L. I believe the problems have been resolved and he is very happy with that aspect of his motor.

Do you have more details on your crossover pipe modification? At what boost level do you think it becomes a restriction?

Have you thought about keeping 2 split-pulse pipes right up to the turbine housing, and running a twin-scroll T3 turbine housing? It would possibly clean up the flow and maybe help spool-up.[/quote]

HI Wes, I start by using a standard 944 turbo crossover pipe, and put a back pressure gauge on it from the Y section. Straight away it was obvious it was going to be restrictive. I change the pipe from the Y section for slightly bigger bore, but then at 7500 rpm, 582hp and 22psi boost we still had 33 psi back pressure. I have now change the pipe again for a 2.5 inch bore pipe and waiting to get it back on the dyno. If it still as back pressure I will then have to go up on exhaust housing/exhaust wheel size.

I did consider using twin scroll, but the crossover pipe would and been a task to make. Plus I also spoke to our local Honeywell rep who said that the advantages of twin scroll on a four cylinder engine would be better but minimal. Twin scroll turbo's work better on six cylinder plus engines.

Cheers Tim

Hi Patrick, The cosworth block starts off as closed deck, but over the years of development and the increase in power things have changed. When we went over 9 to 1 comp/ratio on the WRC rally engines, the cast iron bores split under the pressure. So we ended up boring out the complete bore and inserting a 1/4 inch thick steel liner, semi wet liner. As to my engine, whether fitting a liner as helped rigidity I would not like to say. I think the 1.9 rod ratio will help with piston to bore wall thrust pressures.

Cheer Tim

Hi Tim, thanks for the info on the crossover. Agree that the split-pulse/twin-scroll is more of an advantage on 6 cylinders.

I've bought a large(ish) Holset turbo for my 924 Turbo 2.0L project so I'm currently trying to work out a new exhaust manifold/crossover for it and move the turbo position, whilst hopefully keeping lag to a minimum.
I'm not hoping for boost super-low in the rev range but I still need a useable power band.

Those WRC rally engines sound interesting - how much boost with 9:1 CR will split the bores on one of those? I bet you have some stories to tell about those I always loved the WRC technology.

Hi Wes, I'm afraid getting a turbo that comes on boost low down in the rev range but then performs higher in the rev range without back pressure is near impossible, it always seems to be a compromise between the two. I'm having the problem at the moment.

The WRC engines are a nice piece of work. The latest one's we run at 10.2 to 1 and we run near 40psi of boost low down, seeing 525/50 lbs ft of torque but then as the rev's and power rise we drop the boost down, so at 6000/6500 we only have 7 or 8 psi of boost. The reason we have to do this is, because the turbo has to run a 34mm restrictor and has the rev's rise the turbo can not get enough air through the restrictor to sustain high boost. It will start to cavitate and over speed, destroying the turbo. The engine is run on race fuel.

Cheer Tim

Hi Tim,

I run a 3.0 8V engine with a GTX3076R and with the standard crossover pipe I get a backpressure/boost ratio varying between 1.0 and 1.2. In fact it will settle to 1.0 once I reach peak boost at ~3500rpm and won't creep to 1.2 until nearing 7k rpm. I have peak boost set at "only" 18 psi, with a compression ratio of around 8.0.

I am not sure I understand the logic behind increasing the bores of the crossover without increasing along the turbine size, which seems to be the bottleneck here. The way I see it, and I could be completely wrong, is that you may make slightly less backpressure during the spool up phase (and consequently harm turbo spool) but ultimately still reach the same peak level of backpressure, if perhaps a bit later in the RPM range? That would be, of course, without affecting cam timing or the timing map.

33/22 makes a ratio of 1.5 - would you consider this to be too high?
At what rpm do you see 15 psi of boost when simulating a dyno load equivalent to the 4th gear a 944T gearbox?

Thanks for sharing on your interesting engine build and looking forward to reading more on your progress.

Cheers,
Thom

Tim, do you have any dyno charts and ign maps? Be interested to see these if possible.

Hi Patrick, The engine should be going back on the dyno in the next two weeks. Once its done I will post the dyno sheets and I'll let you know what ignition I'm running. At what mapping points do you want to know the ignition?

Cheers Tim

Tim, here is a copy of my Ign map on a 2.5L 8v without many modifications. All stock internals, intake, extractors. Just a bigger turbo (GT3076 .82) and a fairly big cam, downpipe and exhaust. This running a Motec M400 Ecu and E85 fuel at somewhere peaking towards 1.5 bar momentarily...flattening out to about 1.35 bar. So lots of restrictions and we squeezed 365whp on a Dyno Dynamics wheel dyno which are generally regarded as being pretty conservative.

I got my manifold back in the week after being crackle black, so I have been in to work today and fitted it back on the engine. Here are some pictures of the manifold and the turbo in its finished position.





This one shows inside the manifold and how I have made the trumpet part of the back plate.

















Hopefully back on dyno for next weekend.

That looks stunning!

I can look at modified engines and turbos all day long, especially this one.

It does look good. I hope the cross over pipe works out. Which turbo did you finally settle on?

I popped up to see Tim's engine this week and it's a very nice build. I dread to think what it would have cost at normal labour rates though . The block isn't much different to the usual sleeved rebuild but the head/cams/inlet/crossover/turbo/engine management/ etc are things that I doubt any other road going 944/68 is running in the UK. It will be a monster .

Is the final spec all sorted now?