Other School me on Displacment + Compression = Octane

[Quinc]

My 701cc engine with ADA head running 35cc domes @ 185psi needs 91 octane.

I had that cylinder bored to 753cc and using 760 38cc domes @ 180psi needs 98 octane.

Question is why does a larger cc engine with less compression need higher octane?

 

[High Speed Industries]

I think it has something to do with bore diameter.
My dirtbike is a 250 (much smaller bore than my superjet) and it's running 255psi (not a typo) and straight 100 octane with no issues at all. If that was a ski you would need like 150 octane or something.

I'm very interested to see if bore diameter plays a part. Hopefully we find out soon.

 

[wanderer1]

Are you using the ADA dome guide for fuel requirements? Just curious if the guide for the 760 domes refers to a 62T cylinder with higher port timing?

 

[Big Kahuna]

something is off on that. I ran 33cc domes on an 84mm big bore if I am not mistaken. 180 psi.

 

[OkaucheeMustard]

The total amount of volume that is occupied by an air/fuel mixture, the compression, and the valve/port and ignition timing would all be calculated into some equation that would make most of our heads hurt. But those would be the variables.

 

[Quinc]

Are you using the ADA dome guide for fuel requirements? Just curious if the guide for the 760 domes refers to a 62T cylinder with higher port timing?

Correct going off the ADA website.
760 vs 701

 

[wanderer1]

If I remember correctly you sent your cylinder to Chuck at CPT? You should consult him since he is doing the work and can explain the differences between static/running compression in relationship to exhaust port height. Then, as mentioned before,ignition timing advance comes into play as well as fuel quality-oct, RON etc.

 

[Big Kahuna]

That chart is based on the 760 motor, not a big bore 61x. Talk to your builder

 

[DAG]

I like squishy

 

[Quinc]

If I remember correctly you sent your cylinder to Chuck at CPT? You should consult him since he is doing the work and can explain the differences between static/running compression in relationship to exhaust port height. Then, as mentioned before,ignition timing advance comes into play as well as fuel quality-oct, RON etc.

He is on vacation so I was hoping to get an answer on here.

 

[Big Kahuna]

forget answers here, we have our motors, our setups, they are not yours. Let your builder tell you what to run, or run what he has already recommended.

 

[wanderer1]

Not going to get an educated answer on here without knowing exactly what he did to the cylinder or you plans on timing/ pipe etc. Be patient and talk to Chuck. Most guys run crap fuel at high compression and get by for awhile then want to blame someone else for a failure. Just wait for Chuck unless you are at the gas station right now.

 

[Quinc]

forget answers here, we have our motors, our setups, they are not yours. Let your builder tell you what to run, or run what he has already recommended.

I agree. But I am still curious as to why a 760 needs higher octane fuel with less compression.

 

[wanderer1]

Its not the "760" part that ADA is saying requires the higher octane fuel. Until you talk to Chuck, displacement doesnt have a correlation with octane requirement. Quit focusing on displacement and read about what I posted in first reply or wait for Chuck.

 

[Lumpy]

You also have to take into consideration the water migration patterns when figuring what octane fuel is required

 

[Balduccimoto]

Nothing to do with displacement. You're missing the key difference between a 701 and a 760 and thats port timing. The 760 has a higher exhaust port height which means better efficiency at high RPM. The motor works harder and at a higher RPM, this generates more heat than a 701. Thus the need of a larger dome on the 760 to control possible detonation on pump gas.

I really dislike ADA's dome size guide. It only gives you half the story and is misleading because it misses compression ratio. I would never want to base octane requirement based off a compression gauge. A compression gauge should be only used to check the health of your motor. If your engine builder is on vacation and you're waiting to figure what domes you need, I would be patient. He has more experience with the set up than anyone, no need to reinvent the wheel.

 

[Quinc]

Its not the "760" part that ADA is saying requires the higher octane fuel. Until you talk to Chuck, displacement doesnt have a correlation with octane requirement. Quit focusing on displacement and read about what I posted in first reply or wait for Chuck.

My question isn't about my personal setup.

 

[Big Kahuna]

I agree. But I am still curious as to why a 760 needs higher octane fuel with less compression.

Port timing is different own the 760 cylinders than a 701 60 1X cylinder. Ever look at a 1200 triple compression numbers 155 pounds and above can put you into high compression race gas areas

 

[kevbo]

Go back to the basics. Octane is based on tempreture and pressure. Larger volume equals more heat which requires higher octane for a given pressure. Smaller bores can run higher combustion pressure than larger ones.

 

[Vumad]

My 701cc engine with ADA head running 35cc domes @ 185psi needs 91 octane.

I had that cylinder bored to 753cc and using 760 38cc domes @ 180psi needs 98 octane.

Question is why does a larger cc engine with less compression need higher octane?

You can manage over advanced timing or aggressive porting with higher octane fuels. The gp1200 for example runs only 125 psi from the factory and must high octane at just 150psi. It's that way to manage timing changes due to crank torsion. You could go down in octane if you take the same motor and raise the port timing, or need to go up of you increase the displacement without changing anything else.

There are a lot of factors effecting octane needs. Squish, port timing, crank torsion, displacement, done size, ignition timing, rpm.

Displacement does not mean high octane. It's how all of the components work together.