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Topic Review (Newest First)
05-27-2005 02:54 PM
pmeisel The Duratec engine in the Lincoln LS is 10.5 from the factory, and although they recommend 91 octane I get away with 89 usually.

The variable cam timing seems to allow a little more compression.... the non VCT motors stop at 10.0 C/R
05-26-2005 07:47 AM
454C10 Whatever you do. Get a good set of aftermarket or vortec heads. Factory heads suck (except for the vortex heads). Don't waste your time with 305 heads. You will loss 50hp with stock heads!

Dart Iron Eagle's or Trick Flows are pretty cheap. AFR's are the best. Vortecs are the best deal but require some machining and a vortec intake, vortec valve covers, and vortec rockers.
05-25-2005 10:27 PM
Thanks guys for the many responses

Big thanks to everyone that answered. This was a discussion at my job with some guys that were not really aware of the disadvantages of running a high compression (11.5) engine on pump gas. Which leads to the (IMO) crazy combination of using chevy 305 heads on a 350 block along with a peformance cam shaft of some kind in a late 80's chevy suburban of all things. IMO I didn't think the combination would be successful for hauling groceries, bass boats, etc. I'm not certain though because the engine rebuild shop that has agreed to do the build has not objected to the build and have said that they have the neccessary components to get the job done. Thanks again
05-25-2005 09:33 PM
4 Jaw Chuck This has been asked a million times, there is no right answer...too many variables.

Stay under 11:1 and you will be fine.
05-25-2005 09:23 PM
NDNslicks4me That's right Max. It's better to err on the safe side. That said they do have a couple of high compression fuel injected aluminum headed V8s out there.... and even some 12.5:1 motorcycle engines(ie 250 Ninja)....

But there are some many variables in this equation like vehicle weight, gears, stall, transmission, fuel, piston & combustion chamber shape & smoothness, cylinder bore, plug temp, intake design, Cam Profile, temperture, load, humidity, alltitude, driving style....{the list goes on and on} that there is no clear cut answer.

As was stated 8.5 can ping and i've personally rode in a 13:1 BBC that ran on pump gas. ( it had a 700"+ solid roller cam & aluminum heads). Personally I would have NEVER drove it like that but it was to prove a point. When we pulled it apart it showed signs of leakage around the head gasket surely from detonation.

And to back up ll77 I've seen it before, 220PSI in a 400 sbc (Lunati "412 stroker" kit with their 515" lift cam and double hump heads) ran great on pump gas. And i've seen dish top 406's ping like there was no tomorrow.

Be real, play safe. Keep it {well}under 10.5:1 with iron and 11.2:1 with aluminum unless you are a pro.
05-25-2005 09:21 PM
Lonestar You don't need AV fuel to go 13:1, what you do need is a thorough understanding as to what causes detonation and how to keep your combustion temperatures from "hot spotting" on the piston.

I run almost 13:1 on 93 octane with no detonation. Operating compression is what you need to gauge your set-up on. Don't fall into the trap of using only the popular manufactured your own, it's not that difficult but it does take time and understanding. The bad part about designing your own parts is that if you are unable to actually make them yourself it will cost you an arm and a leg (and your first born).
05-25-2005 09:10 PM
Max Keith
Av Gas 105

Our local airport used to be pretty nice about selling us Av Gas as long as it was pumped into a barrel, however, like many of the airports now, they wont sell you gas unless its going into an airplane.
Running Av Gas will require richening up your jets 2-3 sizes due to the lighter specific gravity of Av Gas Vs pump gas.
Av Gas weighs 6 lbs per gallon where pump gas weighs 6 1/2.
BTW, used to run 12.5:1 comp. with conventional spark plugs in my stock car engines, running 101/105 Av Gas.
05-25-2005 09:05 PM
bigblocksrule If you have access to Av gas ( 115/145 ) , you can go up to 13:1 with retracted tip spark plugs and have a normally aspiraterd, carbuerated engine and use a distributor to fire them.
05-25-2005 08:57 PM
Max Keith
compression ratios

I grant that there is always someone that gets away with running higher compression etc, and a lot of that has to do with cam profile.
But doesnt it make you wonder, that if gasoline for high compression engines was readily available, the auto manufacturers would be making a lot more engines with 10.5 to 11.5:1 compressions just as they did in the 60's.
Since Im a poor boy, and cant afford to experiment and take chances, Ill settle for some nominal losses in hp and know that my engines will survive.
05-25-2005 07:26 PM
Rick WI
Originally Posted by Rhansen
Assuming the timing is not excessively retarded that pretty much blows that theory.
Build your next motor with a ton of compression and pump gas and maybe you'll be the 2nd in a million to get away with it. Not doubting lluciano77 but we see detonation on iron headed motors at 10:1 a lot, and that is with 93. 11:1 with aluminum heads such as AFR 195's is not easy either with pump 93. Boarderline on the edge.
05-25-2005 06:39 PM
Originally Posted by lluciano77
I am at 220 psi with iron heads and 91 octane.
Assuming the timing is not excessively retarded that pretty much blows that theory.
05-25-2005 05:45 PM
Originally Posted by Rhansen
Years ago Car Craft claimed that the practical limit (with some exceptions) for pump gas (93 octane) was 180 psi (on a compression test).
Anybody that knows their cranking pressure want to verify this?
I am at 220 psi with iron heads and 91 octane.
05-25-2005 04:38 PM
454C10 Man! That depends on a bunch of things! But in the case of an old V8, the cam profile is the biggest player.

You should be calculating the dynamic compression, which also uses the cam specs.

I have a 11:0 sbc with a 292H comp cam in a light car and a 5spd with a 3.90 gear and have not problems with 93 octane and 38 degrees of timing.

My 70 1/2 c10 truck with a 454 (8.75:1), with a mild rv cam, 3.08 gear, and automatic will detonate if I run my timing up too high (over 38).

In general, a sbc and handle more compression ratio than a bbc. And, a lot of little import engines can burn 87 octane fuel with 10:0 cr. So you should be more motor specific.

So in a sbc with a mild cam (under 210 degrees duration at 0.050) 9:0 should be about max.
a cam between 211 to 219, 9.5:1 cr
a cam between 220 and 230, 10:1 cr
a cam between 230 and 240, 10.5:1 cr
a cam between 241 and 250 11:1 cr

This also depends on car weight and gear. More weight and less gear (more load) would need less compression.
05-25-2005 12:46 PM
Max Keith
comp ratios

Originally Posted by jimbobneutral
I don't know.. My car has 10.1:1 on 91 octane wich is available at all gas stations,and I have no probs with it. My cam is a decent size for street being .49 lift on both intake and exhaust with a 236,230 duration. Runs nice, loud but nice.I forgot, steel heads to.
Im not saying that 9.5:1 is an absolute ceiling, but rather about the upper ends of safe limits with most engines. Most can get away with running 10:1, or even 11:1, with steel heads on 91 or 93octane, however, there are a lot of places in the country where 91 and 93 octane gas just isnt available. Thats why I suggest a compression ratio of 9.5:1, which generally will peform quite well with 89 octane gas. Another factor is that going over 9.5:1 compression usually requires more than flat top pistons, unless you have small chamber heads, and domes add to the cost of the engine, as well as the cost of the higher octane gas. What little there is to be gained in a street engine going 1/2 a ratio higher, to me just isnt worth the added expense, along with maybe having to travel out of your way to get the higher octane fuel. BTW, I have a 351w with twisted wedge heads running a 230/235 degree, .510/.515 lift cam, and I run 9.5:1 compression, on 87 octane, and my engine was dynoed at 398 HP at 5500 RPM. I cant complain.
05-25-2005 11:51 AM
jimbobneutral I don't know.. My car has 10.1:1 on 91 octane wich is available at all gas stations,and I have no probs with it. My cam is a decent size for street being .49 lift on both intake and exhaust with a 236,230 duration. Runs nice, loud but nice.I forgot, steel heads to.
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