PSIand octane

[tz375]

It's an old subject and there are more variables than I want to think about, but does anyone have any useful charts of Octane levels versus cold cranking pressures - just as a starting point or at least some rule of thumb.

The old GT has a quiescent chamber so I expect it to need a higher octane fuel than a motor with good quench/squish.

The reason I ask is that the cranking pressure on my latest motor is a little higher than I want and I'm not sure I'll get away with it. I can hardly kick the thing over so I know I need to eat my Wheaties, but that isn't my concern right now. Eventually it will have a spacer plate and that will most likely drop the PSI level back to something safer as long as I keep the inefficient pipes.

Any ideas? Anyone?

[ja-moo]

What is the static compression? The Kawis (crappy chambers and no squish and huge clearance) run 71 mm bore and 155 psi comp stock. 91 octane needed 25* full advance.

[tz375]

Hi John. How about 170 - 175 psi? or maybe 165psi kicking it over.

My last high comp GT was running around 150 and I ran that on 91 Octane just to be safe at 22 degrees, but I didn't take the time to test it to see what it would stand before I sold it.

Stock timing is 24 degrees BTW, and they tend to pink on street gas that they sell now, even with all that ethanol to boost octane levels on stock jetting which is probably lean for E10.

[dinogt]

Logicaly it is your exaust pipe which does the compression.
Don't used E10, it is dangerous for your motor.
Don't over 20° after 8000rpm
91 octane is not enough
What is your squish ?
What is your compression (exaust opening in degres if you want)?
What is your ignition ?
Is your exaust big and short or thin and long ?

[ja-moo]

Hi John. How about 170 - 175 psi? or maybe 165psi kicking it over.

My last high comp GT was running around 150 and I ran that on 91 Octane just to be safe at 22 degrees, but I didn't take the time to test it to see what it would stand before I sold it.

Stock timing is 24 degrees BTW, and they tend to pink on street gas that they sell now, even with all that ethanol to boost octane levels on stock jetting which is probably lean for E10.

You are in race gas territory. You might want to adjust, as the more compression, the more resistance to higher rpms. I just redid a motor that had that high of compression, it reved to the moon after dropping the comp.......

[tz375]

Thanks John. That was what I was getting to.

The issue here is that this wasn't the set of barrels I wanted to use, but the ported set just didn't work out yet. I was planning to use that head on the motor with wild ports and a spacer plate for even wilder porting, but I thought I would get a baseline dyno reading just to know where I started from. And that's when I thought I'd better check where the compression ended up.

The plan for this motor is that it will be a low end stump puller and will never rev past about 7k with a 3 into 1 pipe - but when I dyno test it with Jemcos it may be too much effective compression. The barrels I wanted to use are early style L with short intake duration. This set of barrels are more or less stock (but slightly short) that I had available.

I can always pick up a drum of race gas to stay safe until I slip the spacer in. I don't need to test every possible permutation of carbs, pipes and porting just because it's possible to do so.

[ja-moo]

Thanks John. That was what I was getting to.

The issue here is that this wasn't the set of barrels I wanted to use, but the ported set just didn't work out yet. I was planning to use that head on the motor with wild ports and a spacer plate for even wilder porting, but I thought I would get a baseline dyno reading just to know where I started from. And that's when I thought I'd better check where the compression ended up.

The plan for this motor is that it will be a low end stump puller and will never rev past about 7k with a 3 into 1 pipe - but when I dyno test it with Jemcos it may be too much effective compression. The barrels I wanted to use are early style L with short intake duration. This set of barrels are more or less stock (but slightly short) that I had available.

I can always pick up a drum of race gas to stay safe until I slip the spacer in. I don't need to test every possible permutation of carbs, pipes and porting just because it's possible to do so.

That's a good plan!