Ok, here's what I'd like to know:

What octane rating must be ran with the following compression ratios?:

9.0:1

9.5:1

10.0:1

Thanks!

It depends on the cam. The intake closing is the major factor. Follow the link in my sig to the Dymanic CR page. With a big enough cam you can run pump gas in a 12:1 engine.

Does 'big' mean that it has a longer duration for the intake being open (so pressure can not be built up as much)?

Thats it...problem is you are talking static compression when dynamic needs to be considered. Dynamic is the pressure it actually sees. If a motor can blow off compression or heat you can run lower octane.

i ran 87 octane with a true 11.7 to 1 compression ratio on my bbc drove it on the streets alot and got 11 mpg out of it on the highway with 3.70 gears............but of course i had a 4500 flash stall and a solid cam with 262/273 @ .050 duration...............

Originally posted by ddeennis:
i ran 87 octane with a true 11.7 to 1 compression ratio on my bbc drove it on the streets alot and got 11 mpg out of it on the highway with 3.70 gears............but of course i had a 4500 flash stall and a solid cam with 262/273 @ .050 duration............... So is this because you have a cam that is bleeding off enough pressure, that you're able to run 87? I'm guessing the DCR would normally be high with a higher SCR unless you're bleeding off pressure..do you run aluminum heads, tight quench, etc?

Or the corollary to what Pat said...

With an eff. combustion chamber, you can burn low octane gas in a high CR. motor!!

Mr Fueling proved that with his work truck BB custom heads being sold running like 11 to 1 ON 87 OCTANE in a work truck Duallie..

pdq67

I run 11.1:1 on 89 ethanol all over the place, but im running alot of cam.

This is very interesting...what I would like to know from the guys running 11:1 or 12:1 engines with big cams is how is your bottom end torque? Does the higher compression produce good low end torque even with a long duration cam? Or would you be better off with a 10:1 engine and smaller cam. Just curious? I bet a 12:1 with a big cam sounds awesome!

11.2:1 CR, 288/296 hydraulic cam, 93 octane, no issues. The 4-speed and 4.10's help alot.

I run 10.59:1 CR with 210# of cranking pressure on 92/93 octane no problem.

Chris reminded me,.....I have 230# cranking compression also.

Junkyard,
Like I mentioned in my other reply. I ran 87 octane without pinging on a 9.73:1 C.R. I had 268/280, 224/230 @.050 comp cam. other factors like quench, head swirl design make it better or worse. when I used to research, I heard a lot of talk about 10:1 being the max on 92 gas, but after my experience, I don't believe it. great to have this site because of all the good feedback from real experiences.

Yes I agree this site does have some of the best tech info.

Now what the heck is quench?

After some reading it seems like it's primarly cylinder pressure that determines what octane to to use with a certain compression ratio....that is true right?

I read on one site (and I think it was kinda already briefed here) that if the cam has enuff overlap it will lower the cylinder pressure and therefore can use a lower octane fuel.

Now what I'm wondering is how do you determine what the cam overlap is?

Also how do you determine cylinder pressure?

OK first, the quench area is the flat part of the cylinder head that comes close to hitting the piston. The idea behind "quench effect" is that the closer you get the piston to this flat part of the head the more tubulence you cause in the chamber. This helps prevent hot spots in the chamber from prematurely lighting off the charge (detonation). Ever try lighting up a smoke in the wind? Same basic idea . . . (probably over-simplified it a bit).

Ideal quench height (the distance between the quench area of the head and the piston) is typically right about .040" in a street engine --- much tighter than that and you could have a little high RPM collision between the head and piston, much wider and the quench effect is reduced. I run .034" personally, much tighter than that is REALLY pushing it IMO.

Now on the subject of octane vs. compression. I run 11.3:1 compression on pump 93 octane with a 252* @ .050 duration cam, 294* seat duration. Cranking pressure is right about 215 psi and my useable power band is 2500 - 6500 RPM.

A higher static compression ratio DEOS help make up some of the low RPM power that is lost with a bigger cam, but only to a point. You still need to match your cam and compression ratio with the RPM band that want to operate in. In other words, more compression is not a cure-all for too much cam and more cam is not a cure-all for too much compression.

That being said, if you set up your quench correctly then properly match your cam and compression ratio you CAN run a higher static compression ratio on pump gas. Just don't go this route expecting a killer pump gas ride if the rest of your car isn't set up for the cam's power band. And there are other factors that affect octane requirements too, like air inlet temp, coolant temp, ignition timing curve, and probably most important is combustion chamber design.

The short answer . . . it depends. The best compression ratio and cam for your ride depends on what gears and stall you are going to run.

Originally posted by Junkyard Dawg:
Yes I agree this site does have some of the best tech info.

Now what the heck is quench?

After some reading it seems like it's primarly cylinder pressure that determines what octane to to use with a certain compression ratio....that is true right?

I read on one site (and I think it was kinda already briefed here) that if the cam has enuff overlap it will lower the cylinder pressure and therefore can use a lower octane fuel.

Now what I'm wondering is how do you determine what the cam overlap is?

Also how do you determine cylinder pressure? Overlap has only a minor effect on cylinder pressure (as a byproduct of LSA). The intake closing point is the major factor. That is, early closing of the intake (a shorter duration cam) traps more A/F mixture in the chamber. Later closing (longer duration) traps less and reduces cylinder pressure. Follow the link in my sig for how to calculate overlap. Take a look at the Dynamic CR page while you're there. Cylinder pressure is difficult to calculate since the variable are very dynamic and change with rpm, throttle position, cam timing, and ignition timing along with other factors.

Check out the June Hot Rod Magazine, page 20, where their race 572 is tested with both race gas and pump gas.