Compression calculation

[MoparMadman]

I currently have a mild 440 in my car with 9:1 compression. This number was calculated many years ago when the motor was assembled. What I do recall is the compressed head gasket thickness was .025 and 213 heads that were milled and cc’d to 88cc’s to make the 9:1 compression with the P4529429 .030 pistons.

I’m planning on replacing the existing 213 heads with a set of TF 240 aluminum heads with a 78cc combustion chamber. How do I go about calculating my new compression with the smaller combustion chambers? Thanks

 

[beanhead]

To be more accurate you also need to know how far down the pistons are sitting below the deck at TDC. Easy enough to check once you pull the heads, but if you're trying to guesstimate before then you'll have to play around with the numbers you have on one of those handy-dandy online calculators, like this one: Compression Ratio Calculator - Wallace Racing
All else being equal I'd expect an increase roughly somewhere around 3/4 of a point, or maybe a little less if you won't be using a thin gasket like you have now.

 

[MoparMadman]

Ya…I don’t recall how deep into the bores the pistons are. I’m hoping to get an idea of the new compression with 10cc’s of less combustion chamber before I spring for the heads. I’ll check out that calculator and see if I can figure it out

 

[beanhead]

Ya…I don’t recall how deep into the bores the pistons are. I’m hoping to get an idea of the new compression with 10cc’s of less combustion chamber before I spring for the heads. I’ll check out that calculator and see if I can figure it out

 

[MoparMadman]

I just found the piston specs and they are .091 in the bores. I used the calculator you provided and I should have approximately 9.6:1 with the smaller 78cc combustion chambers. Thanks

 

[gtxrt]

I just found the piston specs and they are .091 in the bores. I used the calculator you provided and I should have approximately 9.6:1 with the smaller 78cc combustion chambers. Thanks

do the pistons have valve relieves. must be a custom head gasket at .025 never see one

 

[gkent]

The depth in the hole doesn't mean **** if you already know (or think) the CR is 9:1. With a .030 over piston a decrease of 10cc's is equal to ONE compression point. So 10:1 when you're done.

 

[MoparMadman]

The depth in the hole doesn't mean **** if you already know (or think) the CR is 9:1. With a .030 over piston a decrease of 10cc's is equal to ONE compression point. So 10:1 when you're done.

I just found the depth at .091 and according to the calculator, it’s worth .6 of a point.

 

[WileE]

The most accurate method would be to cc the combustion chambers.

How do I go about calculating my new compression with the smaller combustion chambers? Thanks

 

[MoparMadman]

do the pistons have valve relieves. must be a custom head gasket at .025 never see one

No valve reliefs. It’s an old discontinued Mopar Performance steel shim head gasket

 

[gkent]

The guy's running a 9:1 compression motor and you want him to go through all the bullshit of cc'ing **** ?!?!

Here's my advice ... do the swap and when you put it back together use .020 head gaskets. Honestly, nothing you do will put you in the "that's too much compression" zone with those pistons!

 

[MoparMadman]

The guy's running a 9:1 compression motor and you want him to go through all the bullshit of cc'ing **** ?!?!

Here's my advice ... do the swap and when you put it back together use .020 head gaskets. Honestly, nothing you do will put you in the "that's too much compression" zone with those pistons!

Thanks for the advice but I’d rather not just guess.

 

[gtxrt]

i have it at 8.85 now and 9.59 with 78cc heads

 

[MoparMadman]

The most accurate method would be to cc the combustion chambers.

The chambers in the existing heads were cc’d at 88 and the new heads will be 78

 

[MoparMadman]

i have it at 8.85 now and 9.59 with 78cc heads

Exactly what I came up with. 9.6:1 is perfect for my application

 

[WileE]

The guy's running a 9:1 compression motor and you want him to go through all the bullshit of cc'ing **** ?!?!

Here's my advice ... do the swap and when you put it back together use .020 head gaskets. Honestly, nothing you do will put you in the "that's too much compression" zone with those pistons!

I'm not telling him to do anything, dipshit. I'm just saying what the most accurate method would be.

 

[gtxrt]

Exactly what I came up with. 9.6:1 is perfect for my application

if you use the .020 gasket 9.69 not sure how a alum head seals with steel gaskets

 

[MoparMadman]

if you use the .020 gasket 9.69 not sure how a alum head seals with steel gaskets

The car is in storage till spring so I have some time to research some options. I’m thinking about upgrading the existing hydraulic flat tappet to a lumpier hydraulic roller at the same time.