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increasing compression ratio
https://forums.tipec.net/viewtopic.php?f=25&t=9392		 Page 1 of 2
Author:  deLUX [ Sun Sep 05, 2010 7:18 pm ]
Post subject:  increasing compression ratio
From what I've found my 85.5 2.5na has a compression ratio of 9.5:1, but in '88 Porsche increased the compression ratio to 10.9:1 - I was wondering if I did the same I could increase the HP of my 2.5 to nearer the 200hp mark which is my target goal if I keep this motor NA.

Besides skimming the head are there any Porsche or OEM equivalent pistons I could use to achieve this that you know of?
Can I achieve this by simply fitting Turbo pistons when the cam belts etc are done over the winter?
Author:  t3rra [ Sun Sep 05, 2010 7:53 pm ]
Post subject:  Re: increasing compression ratio
Only from what iv read, there really isn't much in the way off gains from n/a engine. I think these came tuned rather then de tuned from the factory. 88 would ethir be 16v or 2.7 But then most off the race cars in keeping with the road going cArs allways seem 2 have twice the out put. Think most off the extra came from turbos.
Author:  jameso [ Sun Sep 05, 2010 9:03 pm ]
Post subject:  Re: increasing compression ratio
I was under the impression that only the US spec 944 and the 924s had the lower compression pistons at 9.5:1 and 150bhp. I suppose you could get a boroscope and look in the cylinder, the high compression piston should have a flat top with two cutouts for valve clearance.

10.9:1 2.7
10.6:1 2.5

Turbo pistons would drop the compression ratio, but I suppose you could go forced induction then.
Author:  t3rra [ Sun Sep 05, 2010 9:44 pm ]
Post subject:  Re: increasing compression ratio
I'm not 100 pectent. Check tmw. But I'm sure the us car were slightly lower in power, this was 2 meet emission requirements, I think it was done with the timing, us spec car tdc Is slightly different 2 euro. I'm sure that what I read in the peter Morgan book. But it was christmiss time. Not sure if it sold yet but on us eBay there's a super charged manfold system 4 an S. Asking 5000 dollars 2nd hand. 220 bhp wheels? Not sure if wheels is what are car are spec at but still not much more then a s2 if it is. It was only used 100 miles as fear the bottom end off the engine would last with it's use.

Seems like a Lott off money 4 not a lot off power gain, ls swap would be better bang 4 buck. But then if u take more pleasure in ur over creation?
Author:  t3rra [ Sun Sep 05, 2010 10:07 pm ]
Post subject:  Re: increasing compression ratio
Ok, 82 944 euro 163 bhp torqure 151 compression 10:6:1
Us 143bhp 137 torque compression 9:5:1. ( lead free fuel )


Also us is heavier by 87 kg

89 2.7ltr compression is increased 10:9:1 however the bhp increase 165bhp. That 2 extra
Torque 166 but later at 4200 revs in stead off 151 3000
Author:  J.J. [ Mon Sep 06, 2010 8:56 pm ]
Post subject:  Re: increasing compression ratio
deLUX wrote:
From what I've found my 85.5 2.5na has a compression ratio of 9.5:1, but in '88 Porsche increased the compression ratio to 10.9:1 - I was wondering if I did the same I could increase the HP of my 2.5 to nearer the 200hp mark which is my target goal if I keep this motor NA.

Besides skimming the head are there any Porsche or OEM equivalent pistons I could use to achieve this that you know of?
Can I achieve this by simply fitting Turbo pistons when the cam belts etc are done over the winter?


If it's just done 200K miles you will need a rebuild + a tuned head + a cam to get near the 200. If you jack up the compression you will likely just knock the bottom end out of it. You could soon spend 3K & have little to show for it. If you haven't already put it on the dyno & see what you got. If your well down it would be cheaper to find a fresh engine.
Author:  deLUX [ Tue Dec 21, 2010 7:14 pm ]
Post subject:  Re: increasing compression ratio
Right it's the middle of winter and my brains whirring on upgrades for my lovely NA. Ive discovered that down under theyre racing 944na's with a 'limited to' 11.5:1 compression ratio.

So how much do I get shaved off the head to get compression up to 11.5:1 - which seems the tried & tested cheapest route to more power?

Anybody know what the standard thickness of the head gasket is? The thinnest I've found aftermarket so far is 1.1mm. Using a thin one will also increase the CR.
Author:  944 Man [ Tue Dec 21, 2010 10:38 pm ]
Post subject:  Re: increasing compression ratio
If you skim the cylinder then you lose the cam timing. Youll need to find a vernier cam wheel to get it back.
Author:  deLUX [ Wed Dec 22, 2010 9:10 am ]
Post subject:  Re: increasing compression ratio
Ok - I presume I can have that fitted when I do the cam belt?

How much to skim?
Author:  barks944 [ Wed Dec 22, 2010 12:48 pm ]
Post subject:  Re: increasing compression ratio
Yes change the cam wheel to a vernier when you change the cam belt. If you want signifiacntly more power u certainly need a new cam profile! The 944's cam profile is very soft.
Author:  peanut [ Sun Dec 26, 2010 9:14 am ]
Post subject:  Re: increasing compression ratio
bluddyhell :roll: swopping a used S2 engine in and chipping it is surely the way to go ?. :roll:
Author:  Lee 944 [ Sun Dec 26, 2010 1:30 pm ]
Post subject:  Re: increasing compression ratio
I think you are right Peanut, however as someone who should have let go of there 944 by now I can understand deLUX's desire to keep what he has and improve. I suppose with the engine at the age it is, removing and doing a top end rebuild won't do it any harm. The bottom end will eventually die if it is thrashed, and as J.J. found out, the clutch won't be far behind.

My advice to deLUX, go for it, it's your car and I know how how you feel. However, if it is just power you want, sell it and buy a turbo ( I know you won't because you feel the same as me and my 944, which is why I bought a 928 too ).

So back to the question in hand, does anyone know how much to skim off the head?
Author:  peanut [ Sun Dec 26, 2010 2:19 pm ]
Post subject:  Re: increasing compression ratio
yep i know where you are coming from lee. Sometimes you just have to go through it :lol:
i spent most of the 70's trying to wring a few extra horse-power out of early MGB's :roll: Wouldn't have swopped my B's for anything at the time!

Knowing what I know now I'd advise looking for speed elsewhere than cubic inches first . Exhaust the cheap and simple sources first like

1. higher octane fuel
2. Improved inlet breathing (air filters etc)
3. Improved exhaust
4. Improved fuel delivery
5. improved ignition
6. Chip the ECU ( you'll need to upgrade the chip socket first)
7. Improve Aerodynamics and reduce drag ( lower car, upgrade front and rear spoilers, upgrade suspension, put a few more psi in the tyres etc )
8. reduce sprung and unsprung weight

If you just took 100 lbs out of the car and pumped the tyres harder and serviced the ignition components you'd probably gain 10-15 bhp without even touching the engine.

Look first to reducing the things that are making a demand on the engine afterall you wouldn't run a marathon in wellington boots ,wearing a mask and a heavy overcoat would you ?
Author:  deLUX [ Mon Dec 27, 2010 11:48 am ]
Post subject:  Re: increasing compression ratio
it's not just about power or you'd be correct in simply saying "buy a turbo'd 944" or putting an 3l engine in - I guess it's about the journey and having something unique whilst learning something on the way. I have another project car too that's destined to be a track toy that can be thrashed so I'm not planning on doing that with my 944 (except to drive it round the 'Ring this year).

I love my '44 and seeing as tho it's going to need belts, front oil seal & clutch this year I want to get the most out of it that I can for kicks and personalisation. I'd be very happy getting this 25 year old up to 200bhp. I'm not disappointed with the car at all - which is partly why I want to 'tune' it for my own enjoyment as an ongoing project.

I already run it solely on Tesco99 and that has made a difference IMHO, the air filter is a nice new panel one, the exhaust is still relatively new and I changed the ignition components (bar the coil) this past year - so my '44 runs well. I plan to uprate the coil and I'm thinking about an ECU upgrade like the Emerald which has been suggested before but I'm waiting to see what Barks pulls off first.

The other thing to consider is one of those distributor replacement ignition thingys - thoughts?

As for a new cam - I'm looking into that too - any recommendations?

So how much to skim when the heads off or will a thinner gasket do the trick???
Author:  J.J. [ Mon Dec 27, 2010 1:38 pm ]
Post subject:  Re: increasing compression ratio
Lee 944 wrote:
I think you are right Peanut, however as someone who should have let go of there 944 by now I can understand deLUX's desire to keep what he has and improve. I suppose with the engine at the age it is, removing and doing a top end rebuild won't do it any harm. The bottom end will eventually die if it is thrashed, and as J.J. found out, the clutch won't be far behind.

My advice to deLUX, go for it, it's your car and I know how how you feel. However, if it is just power you want, sell it and buy a turbo ( I know you won't because you feel the same as me and my 944, which is why I bought a 928 too ).

So back to the question in hand, does anyone know how much to skim off the head?



Not so. My stock clutch was in good shape. 6mm left on the plate. It just couldn't take the torque.
Cams it has to be Jon Miledge. Hes the only one to have dyno'd the results. He even tested the competition. Some of which lost power despite the makers claims!
Financially it make no sense. For what it costs to make a 2.5 n/a go you could fit an LS26 V6 & have some change. But it wouldn't sound like a '44.
Head gaskets you need a thicker turbo one or it will go every two minutes. Again don't bother if your not rebuilding the bottom end or it will just kill it. As to fitting a second hand 3 liter. Somebody on here tried that & it made less power & torque than my 2.5. Condition is everything. Enough said. :)
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