the right compression ratio

[duane_koda]

im in the process of building a 360 for my 74 duster. just curious as to what the highest compression ratio would be with standard iron heads opposed to what it would be with aluminum heads

[duane_koda]

to clear things up i meant highest compression on pump gas.

[freak007]

it really depends on your camshaft as well as head material,but the generally agreed maximum ratio is 9-9.25:1 for iron heads and 93 octane,10-10.25 for aluminum with 93 octane...

[kamstra]

yeah, I'd agree close to that at a max of 9.5 to 1

[451Mopar]

The maximum compression you can run with pump gas will depend alot on the cylinder heads. Stock, open chamber iron heads will not like alot of compression, where closed chamber "high swirl" heads are better. Heads made of aluminum also can handle more compression because they dissipate heat quicker. If you build a "quench" type engine where the piston comes in near contact with the head, this will also allow slightly more compression.
Then there is the question of the cam size. Larger cams close the intake valve later so there is less compression pressure.

In general if you run a small cam with less than 210 @ 0.050" duration you will want at least 9.0:1 compression, but possably less than 9.5:1 depending on heads, and quench design.
With a cam in the 220-230 @ 0.050" duration range you would want to shoot for 9.5:1 compression on the low end (Iron heads, smaller cam), but with the 230 duration cam, aluminum heads, and proper quench you could easely run 10:1 compression or more.
If you go with an even bigger cam, in the 240-250 @ 0.050" duration you would want 10.5:1 compression or more.

One thing to keep in mind is the closer you get to the maximum compression for a given configuration, your tuning (ignition curve and fuel) need to be fine tuned much better (to prevent detonation) than if you run a slightly lower compression ratio.

[duane_koda]

hey thanks for the info! i think i am gonna go with the 9.5 to 1.

[dave571]

Good point on the "quench" type. My piston to head clearance is about as tight as any safe recommendation. For that matter, the mopar engine book says .060" min and piston will hit at .050". Mine is at .054(a little more on some holes, there is factory variance/ build tolerances to contend with) Kb silvolite says they like am assembled quench of .040. I guess they should hook up with the guys who wrote the mopar engine book.

In any case...
9.9:1 with iron heads.

No ping, no problem, on premium gas. I haven't tried regular yet.

[451Mopar]

I think most of you already know I'm running 11.0:1 on pump gas.
Aluminum heads, a big cam and alot of altitude
I forgot to mention, you can run more compression at higher altitude.

[Mr.Mopar]

I have 10.1:1 with iron heads and 268 advertised duration cam and no ping at 13 degrees BTDC with premium pump gas.

[Dart]

You guys forgot the difference the pistons make. The Hyperutectic pistons allow yo to run about .5 higher compression because of their heat characteristics. They are lighter and are a good choice for a 360. Aluminum heads certainly make a difference bacause of their characteristics also. You would want at least 10 to 1 before thinking about an aluminum head.