924Board.org

[Peter_in_AU]

thought you might be interested...

The question has often come up before "why not just put Euro pistons into a 931 and make it non-interference?" or "what would happen if I put a 931 head on my 924?".

Well, after rather unscientifically measuring the volume of the 931 head chamber and the volume in the piston chamber of an old Euro 924 9.3:1 piston and plugging the numbers into the Dyno2000 compression calculator I have an approximate answer.

You would end up with a static compression ratio of about 6.7:1. Probably useful if you were running 2 bar of boost but it would be a bit "soft" with no boost

[dwak]

Peter,
There are some out there speaking about using the higher compr. pistons in the turbo setting which means a lot less boost ,I guess. I suppose there is an optimum boost to compression ratio ratio.

[Peter_in_AU]

with good programable engine management the trend for non-race cars seems to be towards higher compression and less boost so you get good off-boost performance. Basically the "build a good NA engine then turbo it" approach.

The magazines love to write about cars running 28psi that are dead below 4500rpm but something like that would not be a lot of fun to drive to work during peak-hour.

[dwak]

Ok that makes sense but what about " let's put euro pistons in a 931 and make it non-interference" statement. Why would pistons that are not dished be less likely to get whacked? Is it the actual valve travel in the turbo head that causes the fatal interference? I would rest a lot easier if I knew a practical remedy for piston smashitus.
dwak

[Peter_in_AU]

actually if you look at a 931 head and a Euro 924 piston you'll see that the answer is that if you put euro NA pistons into a 931 you'd end up with an interference engine.

The problem is the 931 valve sizes. On a 924 there is a gap of about 5mm between the outer edge of the valves and the bore. On the 924 the pistons have valve reliefs cut into them the outer edge of which is about 3mm from the outer edge of the piston. On the 931 head with the larger valves there is a gap of probably less than 2mm between the outer edge of the valves and the bore. If you cut valve reliefs into 931 pistons the edge of the relief would probably be less than 1mm from the side of the piston which would not be strong enough to reliably contain the combustion resulting in little bits of metal bouncing around in your combustion chamber.

Seems that the answer to the 931 interference question is that there is no answer. The cylinder wall is not designed to form part of the combustion chamber so pistons that have valve reliefs that go right to the edge of the piston won't work. Theoretically you could install 924 valves into a 931 head by making the seats smaller then you'd have enough clearance to grind valve reliefs into the pistons but reducing valve size sort of goes against current thinking about generating power.

Of course if Porsche had started with a clean slate when designing the 931 head they would have angled the valves and used more normal looking pistons with reliefs cut in them. Pity they didn't, that would mean that the 931 would have a twin cam head

[Cbass]

[Peter_in_AU]

measure carefully. The volumes I came up with were done in a very unscientific manner with a syringe and water but they are probably reasonably close. Still, I'm not sure I'd bet your motor on it.

The D-Prod NA engines apparently used the 931 head. The standard quote is that they used "very strange" pistons to get high compression. What "very strange" means is a mystery.

Your theory seems sound, just get it checked by someone who does this for money before you order your pistons.

[fielderd]

Hey guys, i was just wandering what the maximum compression was that can be run before problems start eg pinging/pinking. i've heard its around 12/13 :1.

If that is true then can i run 0.4 bar boost on a 9.3:1 compression ratio

1.4 x 9.3 = 13.

Is this correct?

[Cbass]

That depends on a lot of things. A lot of things.

First, the condition of your motor. If it is old and worn, you might blow it up.

Second, how well you have built the motor. If everything is studded, you stand a better chance of not blowing up your motor.

Third, what level of fuel you will run. If you use 87 octane, and have a motor that is not very well put together.... You get the picture.

My advice if you plan to drive the car, is rebuild the engine before you start putting boost through it.

[Peter_in_AU]

the theoretical maximum compression of pump fuel is apparently way higher than you'd think. I've heard numbers of 16:1 for 97 octane.

The reality is very different and depends on lots of stuff - charge temps, ignition advance, piston crown design, chamber design, air/fuel ratio, spark placement, day of week etc, etc.

0.4 bar into 9.3:1 is probably very conservative BUT there's only one way to be sure, stick it on a dyno load it up and listen.

[Rick MacLaren]

You can plop a turbo on an NA can and expect performance, but do not expect reliability.

[Cbass]

BOOOM!

[fielderd]

well i was hoping to s/charge it with intercooler as well. have recently rebuilt motor, new bearings, rings, valve's done etc. so it should be pretty solid. Its an xj motor, 125 hp.

I always run 98 octane or higher fuel so tell us what you think

[Cbass]

If you run low boost with a good intercooler, it should be fine. You should be able to run a full bar of boost, but don't do it unless you build the motor as strong as you can.

Something nobody has brought up in while. Get a air/water intercooler. They are compact, you can mount them anywhere, and it is fairly simple to tie them into the radiator.

[fielderd]

haven't had any experience with an air/water intercooler. Im guessing air runs through tubes which are surrounded by water? . Or water is injected into air to cool it down. Please enlighten me.
david