Given a constant c.r. is it better to adjust via the combustion chamber or the piston? Basically is a domed piston and big combustion chamber better or a small chamber and a dished piston? Another question. I know that the dcr of a motor is what really detemines whether it will run on pump gas, not the scr. What would happen if one was to have a 13.5:1 scr and use the cam events to achieve a dcr of ~8.5. would this be feasible? Would there be any advantages or disadvantages to doing this?

 

It's best to have the smaller combustion chamber and more dish. If you wanted a motor with compression like you described above, you would need a huge cam to drop the DCR that much. In doing that, you'll probablly mess up the combination of parts, but I guess you can do whatever you want. I don't see where it would physically hurt anything.

 

You would lose A LOT of power. Unlike SCR, DCR takes everything on your motor into account. It is the true representation of how much power you'll make (Assuming your ignition and exhaust components aren't restricting it). The higher the DCR, the more power, plain and simple. ANY mod that generates more airflow or more RPM, the result is higher DCRs. That's the entire point.

By tuning the engine to not make crap for DCRs, you'd be making it perrform worse than stock, even if your SCR is sky high. It's like boost, lower compression and more boost is better than high compression and less boost. The goal is to get as much air/fuel in there are possible, and raising your DCR with a nice cam, heads, intake, etc does just that.

Advantages: Cheaper, more reliable
Dissadvantages: A HUGE loss in power, even when compared to stock IMO.

 

Domed pistons screw with flame propagation.

 

The reason I was asking is that I already have the pistons for my build, and I want to keep it a pump gas motor, albeit right on the ragged edge. I am going to be trying to pick up parts here and there, maybe used, and I just wanted to know if I could make up for a smaller combustion chamber with a bigger cam to drop my dcr back to pump gas levels. It doesn't seem like a good idea so back to square one I guess. Thanks for the insight guys.

 

The Intake Valve closing point dictates DCR, and that's it. Mods that generate airflow, such as heads and intake, don't change your DCR or create more DCR.


With the givin engine scenario he posted, he would still be making a pump gas limit DCR at 8.5. Performance would still be excellent and much better than stock. The down fall to his situation is that he would have to run a huge cam with a very late IVC in order to get the DCR he needs, which would make for bad driveability down low and a late power band. Sure you want as much air/fuel in the mix as possible, but the heads and intake still don't change DCR.

To the poster, if you already have pistons, get heads that will suit the SCR you want. The whole big chamber small chamber does not make THAT much of a diffrence in a street/strip application.