onlyone07

I've been discussing with several people about my compression ratio of the engine I plan on building for my car.

My plans so far consist of:
stock crank
h-beams
and mahle pistons

The thing that concerns me is that the pistons I'm getting are going to have a compression ratio of 10.6:1 with a 64cc head. My heads have 54cc chambers.

I used a google search for compression calculators and came up with was 11:1. I used 4.03 as the cylinder bore, stock stroke, 5cc dish, .025 in the hole(stock correct??), and a .039 head gasket.

The car is a daily driver and I don't want the compression to be anything more than the 11:1 that it is right now. Can anyone help confirm that my calculations are correct or help me decide the proper head gasket that I may need for this combination?

TIA,
Wade

Mindgame

I calculate 11.57:1 static.

Paints a vague picture of actual cylinder pressure. Do a search for "dynamic compression" and factor in your camshaft. Dynamic is what really matters.

-Mindgame

sssalah

With reverse cooling these engines are more forgiving when it comes to higher compressions. With a larger camshaft your intake valve will be closed while the piston is higher which bleeds pressure and reduces chance for detonation. That's what dynamic compression is about.

onlyone07

I can't seem to find a consistant calculator online. I'm getting a BAD-*** deal on these pistons and I'd like to make them work for what I want to do. Does anyone know of a site that has a known good calculator on it? Or does anyone want to calculate what the LT4 cam would be with it?

I'd really like to stay away from using race gas on nitrous, which is what I think I'd have to do if I went with the 11.5:1.

Does anyone have any suggestions?

Mindgame

I come up with 8.7:1 dynamic which is right where you want to be. No race gas necessary.

A "search" will turn up the why... if you're interested in that sort of thing.

-Mindgame

onlyone07

That 8.7 dynamic compression was with what numbers? I'd like to know.

As far as doing the search. I've searched it several times the past few days and damn near wore out the function. I may not be looking at the right threads for it, though.

The last thing I want to come across as, is one of the n00bs who wants everyone to find something out for them, or give them info. I've tried to look stuff up, but nothing "that" helpful.

Thanks for you help, Mindgame.

Mindgame

I wrote a little program to calculate DCR along with other things, although I believe you can find one online. A Google search of "dynamic compression calculator" is bound to turn up something.

I used all the variables you provided to calculate static, I then used the LT4 Hotcam specs to find the dynamic stroke length at IVC. Then it's just simple 1st year algebraic expressions to arrive at a dynamic compression ratio.

-Mindgame