compression how high?
 

how can i go on a single plug set up 79sc 3.0 on 93 ocatane

A little confused by the question. Octane rating doesn't change one's compression. Also per the owner's manual for my '83 for fuel ".....Regular, 91 RON (87 CLC on U.S. fuel pumps), gasoline" So 93 octane is not needed. Though the compression ratio is 9.3 : 1, which I always considered "high compression ratio", which would require 91 or higher octane, Porsche says to use 87. Would there be something to be gained by using 93....that's debatable and any gain, is it worth the significant cost difference between 87 and 93 octane?

Here is CT, premium gas is north of $4 a gallon and for a 21 gallon tank........

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can i use the je 10.1 piston with single plug?

I' ve read that 9.8:1 is the sweet spot for 91octane. I'm sure someone with more direct experience will chime in.

Sounds like you have a fun project in front of you! Enjoy...I'm jealous. :)

Nossir, you can't do that safely, especially with CIS FI.

Keep it below 9.5:1 with CIS.

the static compression ratio is not all that significant of a number first of all. Meaning, you're not going to see massive improvements with a small jump (and no other changes) from 9.3 to 10.0. It takes large changes to net a meaningful difference..and as you make those large changes, other components become the weak links in the chain. Cams become less than ideally profiled, rev limits become too low, and so on

The ability to run higher static compression is directly related to your ability to control the mapping.

Steve,

Curiosity question. When you say "with CIS" does that mean that a higher compression setup on a 3.0 demands that you would need to either go carbs or some aftermarket EFI? Understand that it would entail a lot of other stuff -- like I said, I'm just curious.

Without regard for the required fuel and other mechanical upgrades, increasing compression ratio, even a little, improves volumetric efficiency which improves power as well as fuel mileage (all things being equal).

As you might have sensed from the previous posts, there are other factors that need to be addressed with incrementally higher C.R. and/or help/increase the resultant torque and HP improvements while also maintaining long service life. All these come at additional cost. In the end, it depends on your engine budget and what you want to achieve.

Sherwood

CIS engines run quite lean and that translates into high cylinder head and EGT's, compared to carbureted and MFI engines which run richer. Such things reduce the threshold of detonation in air-cooled engines so one needs to be conservative and build in margins for hot weather and high loads.

Good Engine Management on the other hand, permits very fine control of fueling and timing that allows more compression & ignition timing for improved power and throttle response.

Steve, do you think there is anything to be gained by running 91-93 octane in a stock SC?

Not Steve, but my '78 with EFI and 8.5:1 compression runs great on regular unleaded.

I understand it does not serve any purpose and can lead to delayed ignition because of the US 79 SC's low compression ratio. These engine are deisned to work optimal with regular gasoline.

The Euro '83 SC runs 9.8:1 with CIS BUT it has the larger runners and a different fuel head (more fuel) on its CIS than the US Spec 9.3:1. Different dizzy too so it is much more than just a compression bump.
I'd take Steve's advice unless you were going to get the Euro CIS/runners etc. or some other induction method.
Or, just source a 930-10 motor.

Ain't that the truth, I was shocked to see the track mileage 2.7MFI and 3.0 carb guys are getting while the 3.0 cis guys can't get enough fuel through their engines

JMHO naturally, but I always use the best fuel I can buy for an air-cooled engine.

Remember, these cars were designed and calibrated for the fuel chemistries of the day and things are now VERY different.

I've seen a lot of broken rings (due to detonation damage) from engines that were, according to their owners, "running great!". Never rely on one's ears as the "knock sensors" unless you have a substantial bank account.

Some informative reading on this subject is here:
Gasoline, Timing & Twin-ignition | Rennsport Systems | Porsche Performance Systems and Support

its not that there is anything to be gained, its a matter of protecting your engine if you have higher compression. if you run hi compression and lower octane fuel, the timing will have to be backed off from optimum.

i think the OP's real question was a great one and he is starting at the right place in that he is trying to build a motor around specific fuel, 93 octane.
once you figure out the fuel you want to run, then figure out the compression, cams and timing.

I'm pushing it in Az as I've a 930-10 with 9.8 to 1 and the only gas avail is 91 OCT. It also gets to a balmy 110.
I've done what I can to negate this by using an Octane booster (ACES IV) and by installing a later Carrera oil radiator/block off/fan unit in place of the trombone....of course the cooler plugs and proper timing. The fan is switched in the cockpit so I control when/where not the thermostat.
I've also fattened the CO to 2.5-3 ish with a better exhaust (SSI's).
The engine runs very cool (194-200) even in 100 degree plus temps and the max I've seen is 210 when running hard (redlining it).
I'm putting an AFR in next week so I can watch my ratio under load.
This approach is presently working.

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i got 40 webers, headers. plan on goin 964cam or some dc style cam , i like to spin the motor up, in other words THRASH IT