littlezac

Would 10.5:1 compression ratio be to high for a daily driver. I'm putting a 350 in my car and was wondering what compression ratio to run.

Apeiron

It's about the upper limit of compression for pump gas on aluminum heads. Certainly too high for cast iron and pump gas.

Air_Adam

Generally speaking, yes, thats correct... but it really depends on what cam you use. I've known of engines with only 10:1 CR with aluminum heads, but with a lame stock cam knocking and detonating like crazy... but I've also seen a few with over 11:1 CR with aluminum heads, and the right cam, and they run like a top on pump premium.

What cam do you plan to use?

JesasaurusRex

iTrader: (3)

11.32 : 1 static on 91 octane pump

littlezac

The cam i plan on using is the comp cams xe256h. Just out of curiousity, what is the highest compression ratio i could run with cast iron heads.

firebird45331

I have untouched 416 heads on a 350 with a comp magnum 270 and i don't get a bit of detonation. I run premium unleaded with an octane boost and I figured my compression to be around 10.47:1. Runs pretty strong too.

JesasaurusRex

iTrader: (3)

static compression means nothing, its dynamic compression that you should be worried about, if your cooling system is up to the task and you keep it easy on the timing you can get away with more than you thought possible. I could just throw out a number to you like 10:1 but it might be higher or lower depends on the quench, cam timing, cooling, ignition timing theres lots of factors.

Air_Adam

I'd say the CR is probably too high for a relatively small cam like that, but that CR would work great with something like a XE274H or a XE284H.

I have the XE274H and I'll be running about 10:1 CR with it with a set of aluminum heads. A friend of mine ran it in a 355 with aluminum heads and 10.5 CR and it worked great for him too.

Supervisor42

I'll weigh in on this one. It is more complicated than just numbers. How much C/R you can run depends on many factors. I'll try to relate the variables in order of prominence: Combustion chamber shape, piston to head quench distance, camshaft duration&overlap&LSA, ignition timing curve, cooling system competency, fuel mixture control, transmission shift control, rear axle gears. Any one of these variables can make 9.5 to 1 and 93 octane unworkable let alone in combination.