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Martijnus · Joined: 29 Dec 2006 Posts: 2019 Location: Netherlands

I've bought two 931 heads from a forum member. Haven't got them yet, but that doesn't matter.

I'm now running:
- 2.0 NA engine
- Decked engine block (1mm), which raises CR to 10.2:1 (in my calculations)
- MSII EFI
- ITB's 40mm inner diameter
- stock exhaust system

What I'll add before I'll reveal my turbo plans:
- EDIS functioning on MSII
- LPI (liquid petroleum gas) injection additional to my gasoline injection, MS controlled.

The plan is to rebuild at least one of the two 931 heads and eventually put this head on my engine.
Mods I plan to do are:
- Replace valve seats to cope with the LPI injection
- Polish exhaust and combustion chamber for reduced heat absorption
- See if there's anything I can improve on the ports, but I don't want to do alot in case it has negative effect.

After that I have to choose a turbo.
My CR will be around 7.5:1 when I dump the 931 head on my current engine with euro NA pistons. Thats the same as the 931 S1, which put out 170hp with 0.8bar boost.

My goal is not power, but efficiency (ok and a little bit more torque/power), I don't want a thirsty daily driver and I think a turbo can improve MPG.

I'm thinking of a small turbo which spools very early and doesn't put out that much boost. max 200 hp on the rear wheels, but 170 should be more than enough.
The CR is one thing I'm worried about. if a 931 puts out 170 hp with 7.5:1 CR, no intercooler and 0.8 bar boost, I should obtain 170hp with 8.0:1 or 8.5:1 with say 0.5 bar boost, which means I can use a smaller turbo, which spools low rpm.

New pistons are an option...but $$$ is a problem. The dish in the turbo pistons can increase efficiency a lot iirc, that's why I don't understand the install of flattops (euro pistons would be in between....).

whats the CR of a 931 S2? I'm in doubt between 8:1 and 8.5:1 (cgt?)

There will be a intercooler too of course, and I'll have to fabricate both intake and exhaust manifolds to fit the turbo and the ITB's.

Haven't read 'maximum boost' yet, and I haven't got any measurements of the 931 heads but that's my plan...

Of course running on LPI with the turbo.
Any thoughts on this? Major downsides? (why put in a lot of money and effort when you can put out 200+ and still want +-170hp... for instance

)
_________________
"Rule: Turbo's make torque, and torque makes fun." (C. Bell)

924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment

Lizard · Posted: Sat Feb 28, 2009 3:40 am Post subject:

the smaller turbo will yeild less lag, but will not be as efficent and will generate more heat.

as to a turbo helping MPG, this is a very false concept. You will get worse gas mileage with the turbo.
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3 928s,

Martijnus · Joined: 29 Dec 2006 Posts: 2019 Location: Netherlands

flosho · Posted: Sat Feb 28, 2009 4:16 am Post subject:

9xx · Joined: 18 Jun 2006 Posts: 627 Location: Jarvenpaa, Finland

morghen · Joined: 21 Jan 2005 Posts: 9095 Location: Romania

yup, we're talking ROW here..US ones are just detuned.

the S2 has 8.5 CR and runs 0.6 - 0.7 bar, my turbo puts out just 0.6 bar and it is in perfect condition..recently rebuilt.

What are you doing with the other head(the 931 cyl head

)? i'd be interested in buying it...i want to do a bit of headwork myself but it would be much easyer if i had a spare head to work on while i can still drive my car.
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Supercharger and EFI kits
https://www.the924.com

Martijnus · Joined: 29 Dec 2006 Posts: 2019 Location: Netherlands

Lizard · Posted: Sat Feb 28, 2009 1:19 pm Post subject:

It wont be significantly less MPG, but a couple.

The reason being is that the turbo will always be moving alittle more air in there as normal. more air, more fuel.
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3 928s,

RC · Joined: 25 Mar 2007 Posts: 2637 Location: Australia

No intention of getting bogged down here with your hypothetical CR calcs, only will say though that if you dont accurately know the actual volumes you may as well be pulling rabbits out of a hat. Hopefully you measured your piston/cylinder volume when you built your motor and decked the block.

931 chamber volume is supposed to be 21.5cc but any previous valve work or milling over the last 30 or so years will alter that. So suggest you CC them when you have them.

Gasket thickness will play a large part too. No idea where the 1.25mm (0.51") figure came from that is suggested in other threads since all stock ones I`ve measured are 1.6 - 1.7mm. Slip some feeler gauges under your head to verify.

Heres some better online calculators:

http://www.csgnetwork.com/compcalc.html

http://www.not2fast.com/turbo/compression/compression.shtml

With 9.3:1 Euro pistons at +1.0 (87.5) and a deck height of 4.1mm (- 0.5mm from the last time?) the volume measures 44cc. Irrelevant to your build but may just give you something to go on if you have no other data.

Will be amazed if you manage to increase MPG with a turbo.

Raceboy · Joined: 01 Mar 2004 Posts: 2327 Location: Estonia, Europe

No turbo will improve MPG.

And actually the smaller turbo you use, the worse MPG you get. Why? Because smaller turbo has more restriction to the exaust and it spins earlier resulting boost very quickly. With bigger turbine you'll get your target hp (say 200?) and turbo is basically used just for the peak hp, not for ultimate torque. MPG will be BETTER because on normal cruise situation turbine creates less backpressure and is less burden to the engine.

Your MPG will be worse compared to NA with whatever turbo you use, only it will be less worse with bigger turbine (as for compressor, it doesn't matter much on cruise).
_________________
'83 924 2.6 16v Turbo, 470hp
'67 911 2.4S hotrod
'90 944 S2 Cabriolet
'78 924 Carrera GT replica
'84 928 S, sold
'91 944 S2, sold
'82 924S/931 "Gulf", sold

'84 924, turbocharged, sold.
http://www.facebook.com/vemsporsche

Martijnus · Joined: 29 Dec 2006 Posts: 2019 Location: Netherlands

Lizard · Posted: Sun Mar 01, 2009 4:35 am Post subject:

Martijnus,

I am afraid that you arent really knowing what you are wanting at this point in time.

Get and read max boost by corcky bell it will be a great starting point.

However to pick what size of turbo you want.
You need a HP goal,
you then need to determine how many lbs of air/hr it will take for the engine used to achieve that.
Then you need to read the compressor maps and chart out roughly where that would put you on the compressor map. Once that is done you have your turbo.
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3 928s,

ic932 · Joined: 11 Feb 2005 Posts: 1104 Location: UK

I used to get approx 20mpg out of a well tuned but stock 932 which was my "partial" daily driver (if that makes sense). My other daily driver was a BMW325i (touring). Both had similar power outputs (170-80BHP.) The BMW had the better fuel economy which is no surprise.

I'm suprised that with modern EFI/turbo-charging that it isn't possible to better the 931's MPG....Is this the consensus view here?

Martijnus · Joined: 29 Dec 2006 Posts: 2019 Location: Netherlands

Raceboy · Joined: 01 Mar 2004 Posts: 2327 Location: Estonia, Europe

I didn't say EFI isn't improvement over K-Jet, I said that when adding turbo to existing EFI engine, you will get worse MPG.
And obviously if you add turbo to K-Jet, same story.

After my efi and turbocharger install to white 924, MPG is roughly the same as it was with NA form and old K-Jet, maybe a little bit worse (better on the highway, around 7-8l/100 km/h and worse in the city due to lots of acceleration).
_________________
'83 924 2.6 16v Turbo, 470hp
'67 911 2.4S hotrod
'90 944 S2 Cabriolet
'78 924 Carrera GT replica
'84 928 S, sold
'91 944 S2, sold
'82 924S/931 "Gulf", sold

'84 924, turbocharged, sold.
http://www.facebook.com/vemsporsche