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New 931 engine - need some advice

 
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Lau Christophersen  



Joined: 17 May 2013
Posts: 99
Location: Denmark/Copenhagen

PostPosted: Sat Nov 21, 2015 8:31 am    Post subject: New 931 engine - need some advice Reply with quote

I need some help/advice for my new 931 engine.
The plan as it is now:
Rebuild the 931 engine, but with 924 N/A pistons and mill the block down but how much?

I used Ideolas compression ratio calculator, and if i did it right i get the following:
Mill the block by 1mm: cr = 8.8:1
Mill the block by 1.5mm: cr = 9.1:1
Mill the block by 2mm: cr = 9.6:1

Another thing is the head gasket, that will change the cr-ratio too, but is the original head gasket strong enough?

The plan for the turbo:

I want to make the exhaust pipe/manifold like the 951 and in 3" stainless from turbo and back, so the turbo will end up on the drivers side of the engine. But which turbo should i choose, any recommendations?

The plan is to drive with EFI, i will buy a VEMS setup from Raceboy and use some 440cc injectors.

Another thought was to remake the intake manifold so the throttle body will be mounted forward - pointing toward the FMIC. Should i look for another throttle body when i want to redesign the intake manifold or just stick to the original one?

Many questions, i hope you have some thoughts you want to share with me, the biggest problem in my head right now is the boost VS. CR-ratio.

Low boost and high CR or high boost low CR? or something in the middle?

Maybe i have to point out that it is just a funny sunday driver, it is not build for the track at this moment.
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fiat22turbo  



Joined: 18 Jan 2006
Posts: 4040
Location: Portland, OR

PostPosted: Sat Nov 21, 2015 8:48 am    Post subject: Reply with quote

I wouldn't mount the turbo under the intake, your intake temps will suffer for it, the 951 does as well.

Be aware that you can run into flow issues to the front cylinders if you run a front mounted throttle body and a poorly designed plenum.

The stock intake is actually pretty well designed other than the plenum being slightly small and the throttle body is in a pretty inconvenient location for your use.

I would be careful of how the air flows into the plenum. I've heard good things about dual-plenum solutions along with angled plenums that place the throttle body entrance far enough away from the runner inlets to help balance the flow rates.

You should also look at ensuring your cooling system is up to the demands as well. Perhaps look at an electric water pump and reverse cooling the motor to balance the cooling across the cylinders (3 & 4 tend to get the hottest)

Good luck!
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Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose)
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Lau Christophersen  



Joined: 17 May 2013
Posts: 99
Location: Denmark/Copenhagen

PostPosted: Sat Nov 21, 2015 9:06 am    Post subject: Reply with quote

My idea was something like this: http://www.944sverige.com/forum/viewtopic.php?f=17&t=8747&start=75
There are some pictures of a modified intake manifold in the link.
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daniel  



Joined: 18 Jun 2009
Posts: 613
Location: Sydney, Australia

PostPosted: Sat Nov 21, 2015 4:39 pm    Post subject: Reply with quote

Best way to increase compression is to use a cometic head gasket.
To answer your question, the standard head gasket has stainless steel fire rings - you won't break it easily
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Over the top of skyline, total brake failure.... hit the wall at over 200 kp/h at the dipper, so anyone who has to brake for the esses is a pussy.
1977.5 Race Car, CAMS Group S Spec
1989 944 Cabriolet
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Carrera RSR  



Joined: 08 Jan 2010
Posts: 1590
Location: Somerset, UK

PostPosted: Sat Nov 21, 2015 8:18 pm    Post subject: Reply with quote

Two options for me. Follow the path Raceboy has already proven in his engines in his cars or buy the parts and the specs as proven by Mittelmotor. Both have reliable results over 350bhp. Other routes could like Dutchpug or indeed Joakims. If I built another engine, I would follow a proven model. I prefer high boost for fun. High compression, low boost is more drive able for a daily but less fun.
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1980 931 - forged pistons, Piper cam, CGT turbo, 951 ducted FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife LSD
Now www.924board.org/viewtopic.php?t=34690
Then www.924board.org/viewtopic.php?t=31252


Last edited by Carrera RSR on Sat Nov 21, 2015 8:23 pm; edited 1 time in total
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Carrera RSR  



Joined: 08 Jan 2010
Posts: 1590
Location: Somerset, UK

PostPosted: Sat Nov 21, 2015 8:21 pm    Post subject: Reply with quote

daniel wrote:
Best way to increase compression is to use a cometic head gasket.
To answer your question, the standard head gasket has stainless steel fire rings - you won't break it easily


Bust two stock head gaskets in two years. Now gone to MLS and survived 9+ months so far.
_________________
1980 931 - forged pistons, Piper cam, CGT turbo, 951 ducted FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife LSD
Now www.924board.org/viewtopic.php?t=34690
Then www.924board.org/viewtopic.php?t=31252
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