I was going to ask this in another thread but didn't want to hijack it .

There was talk about running a certain compression with aluminum heads as to not lose performance.

Right now I'm running aluminum heads and I did go with the thicker head gasket, for security reasons, and I believe my compression is around 9:1 , I forget the exact # but I know it's not over 9.5:1.

I just know this car has lots of power, and I don't see how my low compression could be hurting me, granted more would = more HP, but is having around 9:1 hurting me with these heads?

 

The general rule of thumb i believe is that you can run a full point of compression higher then a similar iron head setup. This is due to the greater heat dissipating properties of aluminum. So you will have pretty low compression. But the good news is that running a relatively small cam like the XE-268 you are, will give a decent DCR. The DCR is much more important then SCR so your in decent shape. I would say that you are ready for a blower now, though.

 

If you're willing to use the required octane fuel, then
a higher compression ratio would make more power and torque
than what you have now.
On 92 octane gas a 10:1 compression ratio would be noticable better than 9:1.
The thickness of a headgasket really has not much to do with
durability. Has more to do with proper surface machining,
installation, control of detonation, preignition and operation temerature extremes.
If you're going to get a headgasket problem it will be because
of one of these factors, Thick or thin.

If you wish to go with 87 octane gas, then you're better off with
the lower compression ratio you have now.

You're certainly much farther ahead overall than when you started. (with stock heads).

The difference in octane tolerance (or power) in the real world between cast iron head and alumium heads is not exactly 1 compression ratio. More like a half a cr. The exact maximum compression that your motor will tolerate (without detonation) using a certain octane fuel is not absolutely knowable but, it won't be near as high as some
"dynamic compression " theorists would lead you to believe.
That being said, just remember, having just a little too much compression will reduce power output ( detonation) and cost you money (engine damage) much more than having just a little less than absolute. So ya need to use discression in the real world.

So, simply put, There is room for improvement.

 

Thanks, I just thought someone said aluminum heads required more compression than iron heads.

I don't really care if I had to put in 93 octane, if it meant more power, I just don't have the pistons to allow that high compression right now

 

Hey! Gives ya a good excuse to start on a beauti new
flat top pistoned 0 decked 355 or 383. Hell, go big or go home.
build a 10:1 compresion 406 SBC and bolt your alumium heads on.

Then , HOLD ON Newt, we're headed for the Alfalfa!!!

 

LOL, just give me your credit card # and I'll get started right away

Damn college and owing money

 

So are you happy with the heads Mark?
I have been following what you are doing as we are in the same situations with our under performing 350's. I went with the iron Pro's. and am looking forward to see the results. Also went with an EX274 cam, RPM airgap intake, and doing the auto to manual swap right now too. Should be like a whole new car.

 

Oh, the car hauls now.

 

this kinda has very little to do with this, but i gotta ask what will probably be a really stupid question to you guys.

Does boring an engine .060 over increase or decrease compression? I know it creates more suction, but it also creates more room for it to build up in, and if you're using the same old heads then it really shouldn't suck in all that much more anyway.

thanks

 

Boreing a motor .060" over would increase the compression ratio.
All else being the same.

You can try it your self here:
Compression ratio Calc

 

thx