Engine Builders: Highest compression w/93 octane?
 

I'm (re)building up a 3.4L twin plug based on a 3.2 Motronic. Specs are 98mm bore, 964 cams, JE pistons.

93 octane is highest currently available now that "Sunoco 94" has become "Sunoco 93".

I am considering changing the compression ratio to compensate for the available octane but I really don't want to machine my pistons if I don't have to. Mixing in some race gas is not an option except at the track.

I know people get away with 9.8:1 single plug on 92 octane and the factory 964 ran 10.4:1 with twin plugs.

What's the highest safe compression I can get away with my twin plug motor using 93 octane? How about 92?

Thanks in advance,
Chris

EDIT: The subject should read "Highest" not "Highed". My spelling sins are uncorrectable when they are in the subject line. I really do know how to spell, reelly I duu. :D

M64 (factory 3.6) ran 11.3 (not 10.4) with twin plugs, BUT also had knock sensors and smart engine managment, that probably would retard the ignition if knock was detected (this is how it's usually done, but I don't know the engine managment well enough to state with 100% confidence)

what are your pistons?

Chris; I think the other variable that needs to be considered is how retarded are you going to run the ignition? You might be able to get away with 9.9:1, but would have to significantly retard the ignition. In fact a better compromise might (or might not) be to reduce the CR and retard the ignition less.

I'm still hoping to try that experiment on a dyno with my "worst case" 2.0E with it's 9.9:1 pistons. I just don't see an available day coming up here in the next month or two.

Another theory is that I suspect that I'd get less pinging on 93 octane if I were to change to S cams with a longer duration since they would reduce the static compression of the engine.

Yes the factory compression figure for the 964 motor is 11.3.
However, when you actually measure it, you get 10.4.
Other Porsche motors have the same discrepancy.
For example:
A US 3.2 is ~9.2:1 not 9.5:1.
A 3.3 Turbo is 6.5:1 not 7:1.

-Chris

I think you missed the part where I mention that my motor is twin plug. 9.9 would be easy with T.P.
-Chris

Oh yeah -- there it is "Twin Plug". :rolleyes:

The same question still applies, what's the better trade-off? More CR and less ignition advance or vice versa.

Well that's interesting. Where can I read about this? What is the 9.3:1 SC really?

Thanks!

Well, you're going to have to have a custom Motronic chip made anyways (right?). If so, then I would build the engine as you want it, and then remap the chip to retard the timing if you find the engine is pinging.

In general, I wouldn't recommend running your 3.4 engine with the stock Motronic chip. It sounds like too many engine changes, and it may not run correctly. At the bare minimum, you won't get as much power out of it, and with the increased displacement, you'll probably run a little lean...

-Wayne

No question there. I had a custom chip from Cyntex which was totally unsuitable. The advance was way off for example. I think I'll have Steve Wong make the next one, write my own or some combination of the two.
-Chris

That's a really good question John. I wonder if it is even possible to retard the ignition enough to avoid pinging with a high compression motor? Anyone know?

If you could, you could run a high compression motor on pump gas but be able to put in race fuel and switch programs to take advantage of it. The tradeoff might be OK even if the retarded high compression motor in "Clark Kent mode" (*) made less horsepower than a normal compression motor.
-Chris

* "Clark Kent mode" You saw it here first! :D

Build the best engine you can, and then remap the chip to accomodate for the gas. This is where an aftermarket Engine Management System is ideal - you can modify the programming without having to make a new chip each time...

-Wayne

That's what my Honda Odyseey does. It looses about 5 or 10 HP when the ignition retards because of low octane fuel. I'll have to pull the manual to get the specifics.

Absolutely, wholeheartedly agree!!!! Nothing like having the capability of a file for intended octanes and driving intentions (economy, power). Also, the flexibility to modify an intake, exhaust, etc. and the ease to optimize the map for the modification...ahhhhh.

I'd still like to hear from someone with actual 911 motor experience on this but the "drift" this thread is taking is also very useful.

Let me put the "comp vs. ign timing" question another way. Would you bet your own motor on the supposition that you can retard the spark enough to be safe on pump gas? Show of hands please? :D Right now I'm running 1.23mm or 1.0mm of and deck height (can't remember) which corresponds to 10.4 or 10.6:1 respectively.
-Chris

Nope and Chris learn how to spell would you?:D

Dean

Chris, I think you're fine running 10.3:1 CR on 93 octane. Euro Carreras are about the same CR and have been running for almost 20 years with 98 RON, about equal to 93 CLC octane. Porsche admitted that you can run 95 RON (about U.S. 90 octane) on the Euros when the supply of 98 became a scarcity in Europe. AFAIK, the only supply of 98 RON in Europe is Shell Optimax. Stock Euros are mapped at around 20 degrees btdc at wot. The consensus among Carrera chip tuners in Europe, is that you can dial the chip timing in these things up to 27 degrees on single plug reliably with 98 RON. This also coincides similarly with what FVD and RUF program in their chips for the Euros. With twin plug, I'm sure you can run less timing and make more power, giving you an even better margin. Plus I know you have the capability to author your own chip :p Believe it or not, when Andial used to do a lot of these conversions with twin plug, they just used what basically was the stock Clubsport chip with 23 degrees timing.

If you need to do a quick and dirty dial down of the ignition timing, you could always turn the fuel quality switch to postion 4 to provide an overall 2.79 degrees reduction in ignition timing, giving you the equavalent of around 3 octane points of margin - also known as factory option M240, for cars in countries with inferior fuel. This is recently what some of the UK Carrera owners have found set in their DMEs, and setting it back to position 0 helped uncover new found power.

But most important if to make sure your air fuel ratio is correct. Use a wideband and make absolutely sure. We don't want reenact what happened to your motor last time. :eek:

EDit: just reread that you are running 10.6 cr. Just want to add that most Euros really measure out at 10.0-1:1 cr.

Thanks for the info Steve. A friend did a 3.2 Motronic twin plug a couple of years ago about the same time I did mine. Andial told him to use the stock chip which really surprised me.
-Chris

Yes, shows you how detuned the stock chips are when Andial tells you you can run twin plug with the stock chip timing, and run a 3.4 displacement increase. You may be able to do this with the 28 pin chips, but I personally would not run a stock 24 pin chip with a 3.4 - would get lean in certain regions. The 28 pin chips run so rich at wot for a 3.2, just cutting back on fuel here raises the hp considerably.

He was running a 3.2 so fueling wouldn't have been quite as bad as if he'd been running a 3.4. However, he was running an early 24 pin Euro box. As an aside, his motor went lean and blew up at the same track, we think for the same reason as mine - bad tank venting.
-Chris