Yep, these guys won't listen. There's a price to be paid for high compression. Heck, My friend's 1970 Olds 4-4-2 with a 455 had 10.25:1 compression, and he had to back the timing off so much it wasn't funny...
On the opposite end-- the 68 Merc I just sold had a 10.25 to 1 comp ratio 390 with stock C8AE-H heads, stock pistons, Crane 272 degree Energizer cam ( .523 lift, don't recall the LSA but was likely something like a 108-112) It was fine on 89 with 12 degrees initial timing. My 331 stroker has a 10.4 to 1 ratio, Ford Z303, Canfield 57 cc heads and likes nothing but 93 octane fuel. It all depends on the cam and the timing.
Yeah, I know a long duration cam bleeds-off a lot of cylinder pressure. 'Guess it really depends on the combo of components....
And the shape of the combustion chamber, and what material the heads made of. Old mopar hemis can run close to 11:1 on pump gas, but the wedge heads won't run much over 9.5:1 without problems. And aluminum heads disipate heat faster than iron, so you have to bump the compression up to compensate. But you're totally right, it depends on the package, and the cam is probably the most important part.
The arguement about aluminum heads drawing off more heat only applies til the cooling system comes up to the temp that the T-Stat keeps it at. Once it reaches operating temps, the aluminums are no different than iron in this regard. As I demonstrated above, aluminum's are no more or less sensitive to the comp ratio than iron. The cam profile and timing have more to do with what ratio and octane you can get by with.
Not quite true, althogh i did simplify it alot. There is always a temprature gradient in the material between the combustion chamber and the coolant passage. Since aluminum conducts heat better that iron, there is less of a gradient. This of course depends on the specific design of the head, and i am no expert with ford heads, so what i said may have been misleading in this context. With small block mopar heads, you can usally run a full point more compression with aluminum. But they tend to have alot of material between the chamber and jacket, so of course more of a gradient.
I've run aluminum and iron heads with the same cooling system and there's no difference in heat dissipation. With the aluminum heads, there would be a tendency for the engine to make the system hotter but there's not. Once the engine is at operating temperature the difference between the two materials goes away.
T.L--- i have 10:1 compression on my 302 and run all grades of gas with no problems. i am using edelbrock aluminum heads and have no ping or knock. it all depends on the build of the motor. i had good parts and a good builder. so far my car hasn't made it to the strip. it has logged over 3000 street miles tho going to shows.
I vote for 10:1, there is nothing like the sound of a engine with a mild cam (slight lope)and higher compression . Alot of the factory cars of the 60's and very early 70's had high compression (some were 11:1), as long as you set them up correctly (timing,fuel mixture,cam) they ran fine.
10 if you have aluminum heads 9 for iron..................Tammy's car has 10/1 with aluminum no problems. My new car has 11.5 with iron and I have no data yet other than it runs/idles great and the little drive around the block have yelded no information.
I've been running 10.5:1 compression in mine since 2003 when I built it. It makes around 300hp & gets about 12mpg w/ 3.55 gears & a little 2000 stall. 93 octane works great, it's all I run anyway in all my vehicles. It's a 302 w/ flat tops & 289 heads w/ big valves, etc. You're gonna need to up the compression to compensate for the smaller displacement!!!
