Spark plugs and compression

O.K. this question is about a Porsche engine in a Porsche car. 1983 SC motor with early style SSI and PMO carbs. I live at sea level (at the beach in the Pacific Northwest) and I want to know if anyone has any experince with increasing there compression ratio without adding plugs. I want to run pump gas but how high can I go before I need dual plugs and then how high can I go. Talked to the guys at Car Quip in Boulder CO and they were no help they live to high in the mountains. I am going to add much larger cams and new pistions but I want some advice on how much compression I can get away with. My thought is to go from 9.3 stock to 10.3 or 10.5 and much deeper valve pockest in the pistons.

Randy Jones
1971 911(It was born an E but it's a Heinz 57 now)

I believe Bruce Anderson has said 9.5:1 is about the practical limit with today's pump gas, and that the old 'S' pistons that run 9.8:1 are a bit too high. He has had plenty of experience with modified and custom engines, so that sounds like good advice to me!

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Warren Hall
1973 911S Targa

Thanks Warren, any idea how high I can go with twin plug, whats the 993 11 something to one.

RLJ

In answer to the how high can you go: I don't know, but do know that the computer controlled engines have knock sensors to alter the ignition to avoid detonation. This is not likely an option with your setup.

I have a question about your setup though:

What are the PMO carbs like? Cost, ease of conversion, etc?

Thanks very much

Cam

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Cameron Baudinet
1975 911S

Check these guy's out http://www.rennsportsystems.com/~porsche/ They are in your neck of the woods.

RLJ,
I have a 78SC. The pistons and jugs were swapped out to increase the displacement to 3.2L, and the compression ratio to 9.6, or 9.8 I can't remember which. Under normal engine temps the car runs great with no pinging, but after hard driving, especially on hot days, and when the oil temp gets up to about 195-210 degrees, the engine will start pinging at higher RPM's when getting on the throttle. I'm not monitoring head temp so oil temp is all I can go by. I always use 93 octane gas. As I see it my options are:
Retard timing (which makes it run crappy until hot), add octane boost (haven't tried it yet), change the advance curve on the distributor (again runs crappy until hot at high RPM but unchanged at low RPM), somehow keep those heads from getting to that temp, or going to a lower CR piston, or never drive m-y c ar * h a r - d...
Excuse me I was laughing a little on that last one

Just my experience,
Leon


[This message has been edited by leon (edited 07-28-2000).]

Leon,

You might want to try backing off your ignition advance at 6000 rpm about 2 degrees at a time, until the wide-open throttle pinging stops. Then, you will have an acceptable timing for the summer-mix gasoline! Of course, when the winter-mix gas arrives, you will have the gremlin reappear, but I suspect you do less winter driving, anyway.

I'm not sure you would have to replace your pistons to get the compression down to the 9.3 to 9.5 'comfort zone' withe lousy gas they are pushing these days. You might be able to mill a small amount off the crown of the pistons, and maybe 'space-up' the cylinders 0.010" to 0.020" at the same time to adjust the compression downwards a bit ... it doesn't sound like you are over the practical limit by much at all!

The dual-plug solution is a nice idea, but unbelieveably expensive ... not counting the head work and assembly costs, the distributor ALONE, is $3-$4000 from Andial, not to mention another CDI-unit and wiring modifications!!!

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Warren Hall
1973 911S Targa

You guys are funny. Cummins Diesel is next.

Anyway, here's a though: Bruce Anderson points out that pre-ignition and detonation are different things. Detonation, says Bruce, is an explosion (a back-draft if you will) caused by extreme pressures and temperatures developing during the movement of the wall of fire across the combustion chamber. Bruce's description is very interesting.

Preignition is when something in your combustion chamber is just so hot that it ignites the gas before the spark has a chance. Where this is happening, the thing that gets too hot HAS TO BE something sharp or small. Spark plug electrodes, usually. Or a shard of metal protruding from a cylinder or piston. A bigger surface (like the piston face) cannot get hot enough.

The moral of the story: Maybe a "cooler" spark plug would make a difference. If it does not, then this may tell you that you have "detonation" which is more expensive to fix (lower compression, increase octane, twin plugs).

FWIW.

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'83 SC

Warren,
When talking about backing off the timing @ 6000 RPM do you mean changing the advance curve by replacing the springs in the distributor? If not, how do you do it. My distributor has no vacuum connection.

I went w/ forged 9.5:1 pistons on my 2.2 rebuild, which was "safer" than going to 2.2S (9.8:1) pistons for a street car that has to run on our MBTE SoCal gas formulas and in moderate heat (sometimes 100+) too.

Anyways, the factory dual-plug parts sound out of this world price wise. You can also go to crankfire, like the Electromotive rig. Anybody know what those cost in comparison?

My God, what a mess.

Gee this is nice everyone typing about something fun and no sniping!

First off the PMO carbs are still in the box. I bought them used from this web sites classified section. buying used Webers scared me, too much of a chance to getting someone elses porblems, these were taken off a car when the owner had a change of status I was told they had about 5 hours on them and they look very clean. I paid $1400,I will still need manifolds $135,air cleaners$135 and linkage. New they are $1800 It has been suggested to me by an engine builder to buy the tallest manifold I can get.

Rather than spending the bucks for more CC's I think I want to go to twin plug and crank fire spark. The Electromotive unit is about $1500, what is cool about it is you can tune it with your laptop. My engine builder has a dyno so he can tune everything before bolting it in. Electromotive has a web site worth checking out. The carbs, new cams,new pistons and maybe twin plug crank fire is my next project. I just took my car down off of the jack stands yeaterday I am putting it back together after a bare metal paint job.

About 9 years ago.I built a TT 300zx(91) to about 500HP at the crank and 425 at the wheels. Tested on a dyno.Turbos ,mass air,intercoolers, injectors etc etc.I used an eletromitive unit. I cound NOT keep the thing dialed in.I set it up about 5 times with my wrench.we had a guy from Pantara sport in Ca. a dist. for them and I could not keep it going.Just my experence.This was 9 years ago so I don't know if things changed or it was me??The dist. was good enough to buy it back from me. Could have been the set up on the car I don't know. Just my .02. By the by if the unit works you can tune it in under load while driving! Very nice! good luck!
BTW 88'911