Jean

Not really, a few were not, among others the 3.6 993GT2 EVO, but I would think that this is due to regulations rather than functionality.

Race engines have access to good fuel anyway and they have an optimized tune for a very particular RPM and boost range, unlike a street/track car that has to fulfill multiple driving needs and fuel ratings.

PorschePhD

Well, yes you are correct. Where rules did not restrict the use they had them. I should have been more specific

One thing I don't see anyone mentioning is counter boring the stud holes and fitting a stepped washer. This stops the head from collapsing inwards towards the head stud. Typically the hardness is much less the older the heads become.

LA964RS

Cool thread....very informative. I'd just like to have 500-550 single plug...think I can accomplish that w/3.6...I live in Ca...and I run mixed gas ALL THE TIME....I do have a program for .8bar in case I get stuck in BFE....but I've never used it.

PorschePhD

On a stock motor??

LA964RS

PHd...don't think you can make +500 on a stock motor.... Unless maybe you hook up a nitrous kit.

PorschePhD

Can't you be banned for using the word nitrous on this board.

Bradford

Interesting comment Stephen. I am running a single plug (twin plug is coming) FVD 3.8 with internals and regularly see 1.5 bar with 100 octane No KABOOM yet

PorschePhD

100 octane is the key, you didn’t say 91 and I am almost certain you are not sustaining 1.5 rather peaking then dropping off on the top end, yes?

Bradford

You are correct. Peaking yes, sustaining no.

sealbeach1

I know building a 3.8 ltr motor will produce much better torque. But does the twin plug gain much in the performance or is it real value in making the motor more reliable?

What kind of HP numbers should I expect from the 3.8 ltr with proper tuning and K24 turbos?

David

ca993twin

I believe its pretty straightforward math... the 3.8 is approx 5.5% larger than the 3.6. If you can get the 3.8 to "breathe" as efficiently as the 3.6, then you should expect approximately a 5.5% HP boost and a 5.5% increase in torque. So, if you have a 500HP 3.6, and increase the capacity to 3.8 with sufficient changes to insure that the 3.8 gets the required improved air flow, you should end up with about 527.5 HP. I think its just that simple.

Jean

ca993twin is right on, I would only clarify that this is the maximum incremental peak torque that the engine can see ( 5% ) over a 3.6 ltr. at the same boost levels. This is a function of BMEP.

Whereas the area under the curve will normally be less than 5% bigger across the board since the 5% would be assuming optimum tuning and conditions.

I don't think anyone has documented scientifically the improvement in performance of twin plug vs. single plug on the exact same engine. If your single spark is not enough to burn efficiently all the fuel in the combustion process, then a twin plug will bring you improvement by burning that wasted fuel, if you have a 100% efficient combustion (which is never the case) then you would not improve. The shape of the piston dome is somewhat a hindrance on the 993TT engine for a good combustion. This is the amateur's explanation

m42racer

Jean,

You have touched on something that has bothered me for years. I had an RSR back when they were raced in the US. It always seemed to me that the twin plug never gained any extra HP from been there. It did allow the fuel trapped by the Piston dome ( high dome for high CR) to get burned and lower the detonation effect, which in turn allowed more timing. This effect stopped or lowered the detonation, but I never did feel it gained any power. I remember discussing this with Neil at the time and he hated the twin plug in those heads. In fact we redesigned the Pistons and finally found a dome configuration that worked well, allowed for the higher CR and good combustion. I think this one thing gave us the ability to gain +400HP after struggling with 390"s with the Porsche Pistons. I do remember the Ignition timing far lower with those Pistons and the engine making over 400HP. WE also ran CDI Ignition and that did make a huge difference. I must have gone thro at least 6 sets of heads over the time I had that car. Cracking was very common in thise RSR heads.
The Pistons in an NA engine have a huge dome different from a Turbo Piston. The Turbo Piston been flatter. Less obstruction than the NA engine and lower static CR. So why do these engines require that second plug? Maybe its old school engineering taking over. There is probably another solution here, but who has the time and money to find out.

But it is something that has always puzzled me.

Jean

M42racer

First, you are of course right about the piston shape on the TT engines unlike the N/A engines. Here is a picture of the pistons on a 993GT2 Race twin plug engine.



Your experiments are very interesting but I am afraid that I cannot comment on the results.There is a lot of literature out there about the benefits of twin sparks in engine combustion, namely in large bore engines where spark travel can be a bit too slow at higher RPMs for maximized HP output. Also, a single plug would force a richer air/fuel mix, which in turn is detrimental to perofrmance I believe..

The fact that our spark plug is not centered does not help either. The new engines have a centered spark plug.

ScottMellor

The other day I had to disable one spark bank and swap ecu's to get the car smogged.
I pulled the switch on one bank and the car ran like absolute crap. I thought I had a bunch of fouled plugs or something. I swapped banks and the car ran sweet as a nut.
My tech told me that I had probably been running on the exhaust-side bank at first, and that the piston dome was preventing a clean burn as it was trying to propogate the flame kernal in the opposite direction.