with a flat top? yeah.

 

First, you have to know the deck height/how far in the hole the piston sits to calculate your comp ratio.

I'm running a 383, 4.030 x 3.75. My Total Engine Airflow heads have been milled .020 and IIRC, that yeilds a 54cc chamber. I have -13cc pistons in a 383 with a .010 deck and a .029 head gasket, I calculate my Static compression to be 10.9:1. In my setup, a -5cc piston would yeild 12.7:1. MAybe not such a good idea on pump gas. So beware of "just use this" recommendations without everybody knowing the whole story on your motor's build.

More important is to calculate your dynamic compression ratio, which takes your cam into account. Obviously, the timing of when your intake valve opens/closes sets the limit on the amount of compression, and where in the stroke that you are actually compressing the air/fuel in the cylinder. With my cam, my dynamic compression is around 8.75:1. But I could substantially change my DCR, which is what really matters, without touching anything that's been discussed so far in this thread, just by changing the cam. FWIW, a DCR of 8.5-9 is considered optimum for street/strip use on pump gas by many people, assuming everything else is working right. In general, the greater the cam duration, the higher Static Compression Ratio you can live with. But if you're not running a big cam in your stroker, really high SCR could give you issues.

Get all the exact info on your rod length, cam timing, piston height below deck at TDC, combustion chamber volume (including head milling), etc and go to this site for a great DCR calculator. Don't guess.....know.

http://www.empirenet.com/pkelley2/DynamicCR.html

 

As an example from above:

My cam is 226/234 at .050. Not a long duration cam. So the net result is that I end up with a higher Dynamic comp ratio than if I had a bigger cam, all else being equal. A bigger cam closes the intake valve later, giving you less compression on the upstroke.

What I'm saying is that you should really factor in your cam.

 

My "just use this" recomendation is pretty much bullet proof, since it's my job to design in the DCR of the LE setups.

Lloyd makes his heads for 383's 58cc, and even with a zero deck block and a .040" gasket you get 11.8:1 and that's not even pushing it with the cams that we put in the strokers.

Jchevy, just e-mail me if you want to know all your things like DCR etc, I can set you up with all the figures you need.

Bret

 

Bret, I'm sure you know your stuff, so I'm not trying to start an argument.

But prior to my post, the cam he's going to use has never even been brought into the discussion about CR, much less the concept of DCR. It's a major factor, but I haven't seen anywhere in the thread where it's been discussed. We don't know future cam specs and you're assuming results based on your own "we design the setups" info.

What if he goes for a shorter duration cam than you use and doesn't use one of yours? I'm just sayin', better that he self educate as much as possible for best results in the long haul, rather than just doing XYZ because somebody said to. Don't you think?

 

Yeah your right man.... I think he has one of ours, and even the small one should be OK. If not we can always hook him up with the right one.

Bret

 

I have not decided if I want to run another custom cam yet, or just use a GM847 I havent really looked into it much. I didnt plan on doing this stroker right away, but I found a good local deal. Im gunna sell the -16cc pistons and pick up some -5s. Might as well do it right the first time.

 

Yep do it right the first time is the key.

Bret

 

Hey brett, how will that gm847 work in a 383? Will it be pretty good? Could you maybe PM me the specs of one of your cams you think would work well? Emissions is not a concern and I would like the car to be drivable on the street. I dont have to drive it all the time, but it still sees the street a lot. The heads I am using are LE1 heads with larger valves. I have the flow numbers some place if you need those. Probably dont want to rev past 6500 rpm, and I want good TQ numbers. My main goal with this motor is to make monster TQ. Would really like to be around 420 tq at the wheels. Is that doable? Do i need to go to a solid roller?

Also, do you own a shop? Could we work out some kind of an exchange for these -16cc pistons? Im going to talk to the guy that im buying this stuff from and see if he still has all the receipts so maybe I can try to exchange it where he bought them from.

 

PM me we can talk about the cams.

I can't take back pistons that I didn't sell for exchange since I don't use the -16cc stuff

Bret