What Octane Level should I need for 10.5 to 1 Compression?

[OzzyOzB]

SO, I recently bought a 1969 Superbee with a 426 Hemi, but it turns out the engine was once set up at 13:1 Compression. However, it has since been sleeved so that it's now 10.5 to 1.
I'm still working on getting the car all back up to speed (mechanically), but my question is "Does anyone here know (or have a history of trying to run) what Octane Level I'll probably need to run it?" Back when I though it was 10.1/4 to 1, I was optimistic that I "might" get away with 91 Octane pump gas, but now I'm not so sure that's even in the cards. Also, as a side note, are certain brands of 91 Octane closer to actually being 91 Octane than other brands? Thanks for any insight you guys can provide. -Cheers
(P.S. that's not me, ...it's my 14 year old kid, salivating.)

 

[monaco66coupe]

In 1969 mobil premium was 98-102 octane,pink and would evaporate off your skin in seconds,,,,,and the oder was hard to beat. regular was 95,,,however today the composition of fuel is totally different, and the engines have been modified to run on crap 91-94. how good? depends on the setup.

The good news is my 69 340 is 10:5 and with the modern cam it runs very well. however it runs a lot better if I add 3-4 gallons of 110 racing fuel in it!

 

[OzzyOzB]

In 1969 mobil premium was 98-102 octane,pink and would evaporate off your skin in seconds,,,,,and the oder was hard to beat. regular was 95,,,however today the composition of fuel is totally different, and the engines have been modified to run on crap 91-94. how good? depends on the setup.

The good news is my 69 340 is 10:5 and with the modern cam it runs very well. however it runs a lot better if I add 3-4 gallons of 110 racing fuel in it!

Well, Given the Price of Octane Boost ($20/can) maybe I should look into the cost of adding a few gallons of racing fuel instead. LOL ;)

 

[SteveSS]

It gets even weirder when you're at 7,500 above sea level like me.

 

[diesel_lv]

Cast iron or aluminum heads? Even aluminum have a tough time w 91 in extreme heat. Iron heads, don't even think about it.

 

[Jerry Hall]

I never tried anything less than 93 octane. Mine are 10.25 stock. To run per the service manual, you might get by with 93. I mix 93 octane 50/50 with 100 octane LL aviation fuel, so I can advance timing and performance tune.
You haven't stated what cam, pistons and other specifications you have, so tuning will be trial and error until you get the combination that works for you.

 

[Photon440]

In 1969 mobil premium was 98-102 octane,pink and would evaporate off your skin in seconds,,,,,and the oder was hard to beat. regular was 95,,,however today the composition of fuel is totally different, and the engines have been modified to run on crap 91-94. how good? depends on the setup.

The good news is my 69 340 is 10:5 and with the modern cam it runs very well. however it runs a lot better if I add 3-4 gallons of 110 racing fuel in it!

Just like horsepower measurements looked higher in the 1960s because they were measured differently, gasoline octane was also measured differently compared to now. That 95 RON octane gas in 1969 would be about the same as today's 91 R+M/2 octane rating. Europe and much of the rest of the world still uses the old method so it looks like they have better fuel, but it's a number game.

 

[Kern Dog]

SO, I recently bought a 1969 Superbee with a 426 Hemi, but it turns out the engine was once set up at 13:1 Compression. However, it has since been sleeved so that it's now 10.5 to 1.

I don't understand how an engine that has "been sleeved" can have lower compression.
I was running at 10.94 to 1 with aluminum heads in my 440/493 and had to run 104 octane to be free of detonation. I changed to thicker head gaskets which brought the compression down to 10.07 to 1. I can run 91 octane without knocking even in 110 degree heat. I'd think that a Hemi could tolerate a bit higher compression given the better head design.

 

[66Satellite47]

OP, I also don't know how sleeving the motor decreased the CR from 13 to 10.5. But I presume you have iron heads, I'd try mixing 100 LL AV gas, maybe 1 to 3 or 4 with your premium.
Edit: I'm presuming your local AV gas is cheaper than your local 110 race gas.

 

[hunt2elk]

My opinion? You are still going to struggle at times with 93. I have been dropping the compression ratio down to 9.7:1 on my motors. This has been the solution for no detonation while using the pure gas 91 that is available by me.

 

[Outlawd]

VP110 is 8.49/gal around here. I buy a barrel every spring. I figure thats cheap compared to rebuilding my motor plus it smells awesome!

 

[F4R/T]

I have a 11:0 and 10:5 engines , 93 plus and race gas , lead additive for the one no problems could I get more out of them yes but I like compression .. The cam and heads make a huge difference so what are the specs of the rest of the engine. I try to get as much timing as I can within reason.

 

[65Fury440]

I have 11.25 cr with AL heads and run 90 octane non ethanol with no detonation * BUT*
The engine doesn't go over 180 degrees
Don't hog on it in a high gear
I have a Progression Ignition distributor and drop total timing back to 34 degrees on street with crap gas.
Its much better to run good gas when available, or a gallon or 2 of 110 as stated above.

 

[Darter6]

My guess. When the engine was rebuilt and sleeved the builder dropped the compression with new pistons/head gaskets.
My 472 Hemi is at 9.6 to 1 and has aluminum heads. It is said Iron heads hold more heat thus detonation is most likey with pump gas at 91.
West Coasters aren't as fortunate to have 93. Add a few gallons of race fuel 100 or more and you will be fine.

 

[33 IMP]

I have gotten away with 10.5-11 to one with iron heads, on West coast cat-pee 91, BUT... I had big camshafts, and barely streetable torque converters. Now that I have access to 100LL avgas, I run 50/50, cause I can (and AZ avgas is cheaper that Cali 91).
If the op has access to avgas, that's what I would recommend. Otherwise, a couple gallons of 110 added to 91, as others have said. Save the hemi, don't detonate!.
(Look for a tiny local airport with a self serve credit card pump, and bring gas cans. DONT put it straight into your bee)

 

[dadsbee]

 

[33 IMP]

Yeah, but Dad, that's YOUR airport, nobody gonna report you for doing that!

 

[dadsbee]

I can do the same at our local airport as well, but of course I know the manager. At 2 bucks a liter they don't really care who or how they sell it...

 

[Jerry Hall]

I can do the same at our local airport as well, but of course I know the manager. At 2 bucks a liter they don't really care who or how they sell it...

The airport I get mine at, told me they didn't care who bought it, and I could tell anyone interested. The airport I previously used told me just the opposite, but it changed from private ownership to a county owned facility at the same time the pumps were upgraded. Plus they moved the pumps to where it would be a 1/4 mile hike to fill any containers.

 

[dadsbee]

Ours we can just throw our 4 ways on and drive right across the apron to the pumps. Gate to the hangars and field is always open in the daytime.