I'm confused...stroker motors and compression...
 

I was under the impression that, keeping all else equal, replacing the stock crank with a stroker crank would increase the compression ratio, since the piston move up and down further in the bore. Doesn't this make the ratio of the cylinder volume at TDC to cylinder volume at BDC greater? Or is my definition of "compression ratio" wrong?

Re: I'm confused...stroker motors and compression...
 

IF you replaced the stock crank with a stroker and kept all things equal, you are correct that the piston travel would be greatly increased and increase the compression ratio. The thing is, in most cases, if you replaced the crank only and used stock rods and pistons, the pistons would (at least try) to push the heads off the motor since your stack is so long.

The main parts of the stroker kit are crank, rods, and pistons. With the new pistons you either specify the compression ratio you want to end up with based on cylinder heads volume or you decide how far you want the piston to come out of the deck. Stock is something like, .003"? You can just get a crank and then you have to pick out some pistons at least, and most get new rods as well.

While a stroker crank makes the stack up larger, usually the pistons are much stronger because thier forged, but they're also thinner if you look at them from the side. This means they have a shorter compression height (height from the wrist pin to the top of the piston).

So while you're right, everyone who builds a stroker replaces the pistons with shorter ones to get an acceptable compression ratio.

J.

Re: I'm confused...stroker motors and compression...
 

OK, I think I understand. By getting shorter pistons or rods you can still get the longer stroke and retain stock compression.

The longer stroke brings more reactants (air/fuel) into the combustion chamber and that's where the power increase comes from, right. For some reason I had the impression that the power increase had a lot more to do with compression than it really does.

So I could build a stroker motor with stock compression if I wanted to put a small amount of boost to it later on?

Re: I'm confused...stroker motors and compression...
 

Lower than stock, stock, or higher than stock. All depends on the rod length and piston height. Most folks go with a longer than stock rod, and a much shorter piston. 6.125" are a popular length.

J.