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

[beanhead]

Hey nice ride! Your around my area. A lot depends on your camshaft as F4R/T alluded to and if it was once a high-compression build, you may have a longer-duration cam in there? That said...in the hot months (May-Oct) you may well need to cut pump 91 with some 110. I know that stuff's expensive but I'd rather be safe personally. Octane booster products will do nothing but turn your plugs funny colors.
Can't get any race-grade fuel at pumps anymore like we used to a few years back, which I'm sure your aware of...head over to the Red Triangle yard on Chestnut just south of Jensen, they sell VP in quantities from large drums down to 5-gallon cans.

 

[Jerry Hall]

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.

I was told early on at the previous airport I couldn't drive to the pump, but they were like 50 feet away then. The place now is less than 50 feet from the closest gate. I now have a 40 gallon rolling container and pump that easily goes in the gate.

 

[diesel_lv]

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)

That airport had a sign at the pumps 2 yrs ago saying "No gas cans, pump into vehicles Only".

 

[Darter6]

"No gas cans, pump into vehicles Only".

Because of Lake Mead, we have several sources because of boats. One will not let you pump into vehicle, but cans are ok.It is at a boat storage yard.
Some of the boat docks have non-ethanol and leaded.
A few local gas stations have 100 unleaded at $8.00 a gallon.

 

[streetmachine]

Because of Lake Mead, we have several sources because of boats. One will not let you pump into vehicle, but cans are ok.It is at a boat storage yard.
Some of the boat docks have non-ethanol and leaded.
A few local gas stations have 100 unleaded at $8.00 a gallon.

I don't want to take over here but, since it's the same topic what does everyone think I should be running octane with my small block? Here's my specifications 408 Magnum Stroker. It has 10.5.1 compression, aluminum heads, Hughes custom grind roller cam (228/236@ .050 and .550/.565 on a 108 LSA), Performer RPM Air Gap Intake, MSD Ignition, Holley Street Avenger 770cfm, 1.7 Crower Rockers and Lifters, 160F Robert Shaw thermostat. I have read some debate that the camshaft makes a difference in the octane you can run where a lopey cam bleeds off more cylinder pressure? I'm still learning so, I'm still seeing what the answer would be. All I can get nearby is either 91 or 93 octane.

 

[33 IMP]

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.

I just advised the op to use a can, so somebody can't report him for using leaded gas in a street car. He is in California, after all.

 

[6PKRTSE]

Both my Charger and Challenger are 10.5:1 pump gas engines that I run on 93. Even with 93 my Charger will occasionally spark knock from some of the off brand big name station's with iron heads. My Challenger is a aluminum headed Hemi no problems with it. I also run VP NO2 when on the spray.

 

[33 IMP]

The cars I got away with iron heads/10 1/2+ cr all had 260° at .050, or more, a solid flat tappet purple shaft, and two solid rollers. One was also very light, under 2400 lbs, with me in it.

 

[Budnicks]

If it doesn't detonate or ping like hell
91 oct. (93 if you can get it) wil do OK
I know it's not ideal
where it could hurt it is under a load
or when you have tall freeway gears, part throttle
then apply more throttle under a load, it will detonate

IMHO
unless you are over about 11.4:1 static compression
or you run hotter like 200*f & at part throttle
especially under a load a lot,
(like going up hills part throttle/with a load, with freeway gears,
like 3.23:1 (to about 3.54:1) or lower numerically)

'All iron engines' these older OE style engines have a tendency
to run much hotter in the combustion chambers
the lead in the old fuels was (in layman's terms)
to cool the exhaust valve when it was contacting the seats

(most modern cylinder heads, especially Alum., have hard seat now,
intended for use with unleaded fuels)

Ideally, if you can get it to run at like 165*-185* max temps
it would be fine on 91/93 octane

have a good cooling system, a good shroud is required
a clean cooling system, with the correct ratio of water to antifreeze
(you can use a product like {Royal Purple} Purple Ice, or {another}
Wetter than Water... it's a/are coolant conditioner/s, makes the coolant perform
stick to the components better, helps make it more efficient or better)

Oxygenated fuels, added alcohol/methanol
or E85 if you can find it (need to set the carbs up for it)
will help it run far cooler in the intake charge & combustion chambers

there was a great Engine Masters just the other night
on MotorTrend Tv on "Octane, does it make power"
& it's uses, needs or making power
they used an old school 327 cheby,
but it's just for demonstrations/cheap

there's also waterless coolants, sort of expensive
but in some cases works very well

the other thing is if the compression is too high
including cam valve overlap & static compression
you could go to a thicker head gasket (like 0.050" compressed)
to drop some (adds volume) of the combustion chambers compression
or get a cam that 'bleeds off' the compression some too
for a street driver, it won't do much to affect performance

 

[Cranky]

Bought a 71 340 Cuda in 86 that was supposed to be 10.5 (rebuilt engine) and it ran pretty dang good with a mild cam but once the engine went over 180, it would ping but just one gallon of 100LL av gas on top of a full tank took care of that. Owned that car for 16 years and it was still running good when I sold it. Dumb me. Anyways, never did to a compression check on it to see what the cranking PSI was.....again, dumb me.

 

[Bee1971]

I have been pumping and selling 100LL for the better part of 35 years

The airport stopped us from selling to the open public many many moons ago. Another words if it didn’t have a tail number and wings

Just like I can’t drive the company plow truck that is driven both inside and outside the airport grounds and is licensed thru the State Of WI inside the Airport gate and fill it up with 87 octane that is intended for non - licensed tugs and heaters and fuel trucks and de - icers that are not used on public roads


In some states the issue is highway motor fuel tax. If you fill the 5 gal. can at the airport with 100 LL you are not paying highway fuel tax

The Airport and the State usually has an issue with that

Now the ma and pa FBOs or self serv in the bush , well

 

[diesel_lv]

Because of Lake Mead, we have several sources because of boats. One will not let you pump into vehicle, but cans are ok.It is at a boat storage yard.
Some of the boat docks have non-ethanol and leaded.
A few local gas stations have 100 unleaded at $8.00 a gallon.

I was replying to 33 IMP as he uses the airport by my house. I also use the airport but not in 2 yrs for my motorcycles and turbo jet boat.

 

[ykf7b0]

My .030 over bore 440 with 915 heads and with 10.62 to 1 compression has never pinged with 93 octane gasoline. I have mistakenly put 87 octane in and never pinged but there was 93 still in the tank. As mentioned previously, your cam profile has a lot do with what you can get away with as far as fuel. I believe lots of overlap bleeds some compression off.

 

[OzzyOzB]

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)

We happen to have a small (unincorporated) community airport near me, that can probably hook me up. (P.S. Cast Iron Heads for those who asked)
Thanks for the tip!
-Cheers

 

[OzzyOzB]

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.

I'm with you on that one. I'm sure I must've missed part of the explanation, because now that you point it out, that totally doesn't make any sense at all. hmmmm.....

 

[OzzyOzB]

Bought a 71 340 Cuda in 86 that was supposed to be 10.5 (rebuilt engine) and it ran pretty dang good with a mild cam but once the engine went over 180, it would ping but just one gallon of 100LL av gas on top of a full tank took care of that. Owned that car for 16 years and it was still running good when I sold it. Dumb me. Anyways, never did to a compression check on it to see what the cranking PSI was.....again, dumb me.

Cool. Hey, everyone here probably wishes they also had a '71 Cuda, (and has car's they wish they didn't sell) but at least you got to drive one for 16 years! That's 16 years of Super Cool Awesomeness!!!!. -Cheers

 

[Darter6]

airport by my house.

Yeah,our local airport has a fuel truck and pumps. Once you could get Avgas but not anymore.
Must have proof of plane ownership.Used to get my neighbor but they moved away.

 

[Hemirunner]

A lot of variables here. First, a wedge and a hemi are two different animals when it comes to detonation resistance. Second, intake valve closing point (which varies greatly depending on camshaft design) will affect the dynamic compression ratio and that has a huge impact on what octane fuel the engine will tolerate. An engine can safely run 93 octane at 12:1 CR if the combination is designed to do so. Or it might detonate on 93 at 10:1 CR if the DCR is too high. At this point, you really need to know a lot more about that particular engine before you can make that fuel determination.

 

[ckessel]

Ozzy, look for any of the Indian Casino's in your area. A friend of mine hauls fuel from Vegas that only goes to the Casino's. Its straight gas plus 10% ethanol. It does not contain any of the CARB additives that wreak havoc on our old cars. His company goes from San Diego county, his and my old stomping grounds, to the north end of the state hauling fuel to the Casino's.

 

[SlinktRR]

Hemirunner makes a great point. Get your engine specs and cam card together and go to Wallace racing calculators to determine dynamic compression ratio. Cam makes a big difference. 8.0 dynamic compression is typically safe on 93. Also your cylinder pressure, I'm around 160 and it runs good at 10:1 static comp but the closer you get to 200psi the more on the edge you will be.