I've been running 111 octane for a while, but the cost is high. I'm beginning to think I don't even need much over 95 or so octane. My static compression is 11.4:1 with aluminum heads. I've just mixed 93 octane with equal amounts of 111 to give me a 102 or so octane on my double duty limited street/strip engine. The car ran like always with no spark knock whatsoever even at high RPM with the diluted gas.
According to the dynamic compression ratio calculator on the Kieth Black piston website, my effective dynamic compression ratio is 8.829:1
Is there a formula for finding the octane requirement based on cam overlap, dynamic compression ratio, etc.?
How much compression are you guys getting away with on pump gas?

 

I'm running just a hair under 11:1 compression on my 350 (10.95 iirc) with cast iron heads and a dynamic compression almost right on 8.5... i don't recall the exact number now. I had to back off the timing slightly (8 initial, 33 total), but it runs just fine on 91 even with the timing a bit higher than that. The trick to keeping a high compression motor happy with low octane like this is to keep the motor cool. Mine runs right around 190* and the fan turns on around 200* and its just happy like that.

 

I have no problems with 91 octane and 11:1 with aluminum heads and a small cam. How big is you cam?

 

The current cam is a solid roller with 256 degrees on the intake at .050 and 268 degrees on the exhaust. It's not the correct cam for this motor, but it will have to work for now. Lift is .644/.643.
I don't see the need for spending big bucks on gas if I don't have to. I might just blend my own. 50/50 (1 to 1) mix of 93 and 111 octane is 102 (runs good on this), so next I might try a 5 to1 mix (5 gallons 93 to 1 gallon 111). This should make 96 octane and should cost ALOT less than 6 gallons of race fuel! I'll prolly just have to keep experimenting. I was just hoping for an equation to use as a short cut so I wouldn't have to go through all this testing.

 

You only need enough octane to eliminate detonation. If your engine doesn't detonate with 93 octane, increasing the octane level won't give you any more power. The BTU energy of gasoline is all the same no matter what the octane level is.

A higher octane slows the burn rate of the fuel and prevents it from self detonating from high compression or boost. Too much octane can produce less power because of the slower burn rate if in too low of a compression engine.

 

That is another great reason for lowering the octane incrementally until I encounter detonation, then going back up two or so points. I really hate to make these "cocktails", but I guess there is no equation, formula, or hard-fast rule set in stone, so I'll be forced to keep going. Quench distance and camshaft overlap play important roles in how low of octane an engine can tolerate too, or so I am told. My gasket thickness is .039 and deck clearance is .008, so am I to assume a .047 quench distance? My seat to seat valve timing overlap is 78.5 degrees which is at the upper end of what you would want for a street/strip engine's timing overlap. I was thinking that it was just enough overlap to put me somewhere near pump gas octane. I just started off high and now I'm incrementally lowering it to find the "sweet spot" between power and detonation since there is no formula to follow.
I already know it can tolerate 102 octane, so I'm getting off cheaper already. If it still runs fine with 96 octane, I will try straight 93 from the pump. The whole reason for all of this, is that I just removed the iron race heads and installed aluminum race heads and a camshaft with more overlap in an attempt to not only gain more power, but also to be able to lower the octane rating of the fuel to save some dough. The engine always ran on 111 before, so now I'm able to lower the octane and save money. Question is...How low of octane will i get away with?

 

jesse, you'll know when the motor pops that you went a little too low.

 

OH, Man! Don't scare me like that! It should just rattle a bit under light to medium loads if I've gone too low I'm hoping.

 

I did'nt see a formula for getting a off the hip octane formula, but HERE is a pretty good page of calculators for lots of fun stuff.

How much total timing are you running? have you experimented going up/down with timing? my slug was built with 12.5:1 pistons but overall compression came in at 11.3:1. I'm able to run 91 ocatane all day weather cruising or racing, but I'm also a touch lower on timing than some guys as mine liked 35* total as opposed to most guys thinking they "just have" to run 40* total.

Keep experimenting since this is one area a math formula will get you close, but your overall combination of parts and how they were installed as well as your carb/timing settings and air conditions where your at will all factor in so no matter what even with a "calculated number" it's still going to be trail and error per application. I did'nt hesitate a bit prior to break in, I wanted this thing to run pump gas so by gawd the first tank of fuel prior to firing was 91 octane and no matter the condition it never pinged, and even after tear down and plug checks never showed signs of pre-ignition. I did run 110 on race day for the "cool race car smell" though...warming the tranny or maintaining temps in the lanes was'nt as cool with nasty smelling pump junk...actually a few buddies would make fun of me for running smelly azz pump gas hence the race day fuel even though it ran it's fastest passes ever on 91.

 

Thanks for your input IHI. By looking at your sig, you and I have quite similar engine combos except mine's a 406. I'm not sure how accurate my timing tab is, but I do have timing marks on the balancer. According to the balancer, with the 111 it was really responsive at 42 degrees ignition timing. I was shocked to see that it wanted so much timing, but like I said, the timing tab may not be accurate. But then again, maybe it needed the extra timing to start the flame front sooner because of the slower burn rate of the higher octane fuel. At any rate, I went ahead and lowered the ignition timing down to a reasonable 36 degrees just before adjusting octane. I have just diluted the 102 with 5 more gallons of 93. That should get me close to 96 octane. If no detonation is found, then I will finish burning this tank off and the next tank will be pure 93. As much as I drive the car, it may take a while to empty this tank out though.

 

LOL, if yours is like mine it wont take long to empty averaging 2mpg LOL!! Ya just gotta lean on it more than usual to make it suck the gas out

 

I'll definitely enjoy doing that, LOL. I'm hoping for better than 2 MPG, LOL! I'm back on my working rotation until Fri, but hopefully by next weekend I can post some good results. I'll try to make it a point to keep track of the mileage too just out of curiosity.

 

My 2mpg are kinda of tainted numbers, it's a 30 minute drive to the track, I made 5 or 6 passes then drove approx 3 miles to the gas station to fill up before treking home just to be sure I had enough fuel since my autometer fuel guage quit working at that time did'nt know how much fuel i had on board. Mathed out at 2mpg with the miles I had on the odometer from leaving home LOL!!

I'm sure in reality it was at least 8-10mpg with normal driving, but we all know you dont build motors like these for fuel savings LOL!!

 

Yeah, I know. But it would be interesting to find out. Maybe I'll try to drive it to work one day this week (80 mile round trip) weather pemitting. Hope this rain clears up!

 

Well, I was able to run straight 93 octane, but I had to reduce timing back to 32 degrees to stop detonation. The smell was horrible. Went to the local track for testing and found that the engine didn't like the 93. It would only run 111 MPH. I then added 2 gal of 111 to the 13 gal of 93 to give a full 15 gal. of 95 octane (that didn't smell so bad). I was then able to bring the timing up to 38 which yielded a best of 118 MPH. So far, including the racing, I've put 179 miles on 14 gal. of "home mixed" 95 octane which is roughly 13 MPG give or take. The trans was not getting 3rd at WOT, so the ET was all over the place (have to lift the throttle to get 3rd ), therefore I just kept tuning for MPH in the 1/4 since this is a good indicator of power. Looks like for best results, I'll have to add 2 gal. race fuel to each tank of 93. That's not nearly as hard on the wallet as straight race fuel!