When should consider twin plugs 1978 sc engine

[brooklynporsche]

When do you need twin plugs. And what is the most practical twin plug setup.
Thanks

[RWebb]

the best idea would be to do a search and read the threads paying particular attention to Steve W's comments

it is not an easy answer & depends on Comp. ratio, fuel quality, type of induction and other factors

note you said "need" - not "will enjoy" -- different questions

964 style 12 wire distr. is the cheapest way to do it.

[Steve@Rennsport]

You need twin-ignition when the compression ratio is over 9.7:1 and/or when your local octane is 91 and you wish to use non-factory CR's,....

There are several ways to do this but I prefer using a distributor and two ignition units & coils.

[SpeedracerIndy]

Originally Posted by Steve@Rennsport

You need twin-ignition when the compression ratio is over 9.7:1 and/or when your local octane is 91 and you wish to use non-factory CR's,....

There are several ways to do this but I prefer using a distributor and two ignition units & coils.

My ROW has 9.8:1 compression from the factory and I've always run 93 octane in it. Are you saying that I can't run 91 octane? I never have, but I always wonder if I'll be in the situation where there is no 93 and I am out of gas and am stuck with 91. I thought it was safe to use 91 but it just doesn't have enough safety margin.

[Steve@Rennsport]

Originally Posted by SpeedracerIndy

My ROW has 9.8:1 compression from the factory and I've always run 93 octane in it. Are you saying that I can't run 91 octane? I never have, but I always wonder if I'll be in the situation where there is no 93 and I am out of gas and am stuck with 91. I thought it was safe to use 91 but it just doesn't have enough safety margin.

Remember that Porsche intended their ROW cars to use 98 RON (its in the owner manual) but 93 R+M/2 is close.

9.8:1 and 93 are generally OK, depending on outside air temperatures and usage. I'd be using 50/50 race gas for track days, if you do that. 91 octane fuel will cause an expensive problem at that CR, especially with the normal carbon buildup these engines accumulate over time.

91 would be safe in cool weather but if you happen get stuck with that stuff during the summer, its wise to carry some "emergency-only elixir" for those very rare occasions. Such a thing would be a combination of Toluene & Zylene but these sorts of measures are really when there are no other alternatives and refueling the car with 93 ASAP is strongly recommended.

[Flieger]

Alright, I will just open up my toolbox and measure out a few cc's of Toluene and add a dash of Zylene...

Sounds like German rocket fuel from WW2- "T"stoff,"Z"stoff now those terms make sense.

I think "C"stoff was the oxidizer for the Sodium Permangenate. It was H2O2 Hydrogen Peroxide so pure that it dissolved/oxidized (burned) off all the skin on the arms, back, and face of an ME-163 Komet pilot after a fuel leak on takeoff or maybe during refueling.

[psalt]

My ROW has 9.8:1 compression from the factory and I've always run 93 octane in it. Are you saying that I can't run 91 octane? I never have, but I always wonder if I'll be in the situation where there is no 93 and I am out of gas and am stuck with 91. I thought it was safe to use 91 but it just doesn't have enough safety margin.


The factory's "safety margin" is based on the highest load at the highest operating temperature. All paranoia aside, the first thing to understand about octane and detonation is that it is only an issue at high cylinder pressures. It does not occur at idle, or at part throttle cruise with 16 in Hg of manifold vacuum. The probability of detonation is highest at WOT at the peak torque rpm and drops off rapidly with part throttle operation. The idea that you would ever be "stranded by 87 octane" is not supported by the reality of road car driving, in which 85% of operation occurs below 35% load .