I ordered the 440 kit from IMS with supposely a 11:1 compression piston. But the piston has no dome and doesn't appear to be any taller than stock. Don't understand how this will raise my compression.

Was everyone's piston like this? Care to explain how this raises my compression if you know?

 

The simple fact that the bore is larger will raise the compression ratio, you're compressing more volume into the same space.

Don't know about the 440 kit, but my 87mm 416 piston @ 10.8:1 requires a slight dome.

 

hey, i did my 440 kit and i noticed the same thing???? my buddie just put a 11:1 piston in a stock bore and his is a high dome. mine supposed to be 11:1 also.

 

Like I said, the larger bore doesn't require as much of a dome. If you think about it, it all makes sense.

What are you doing when you increase the bore? You are increasing volume of the cylinder. This means more air/fuel mix will be drawn in on every stroke. You have not changed the size of the combustion chamber in the cylinder head - so, you now have more air/fuel mix being squeezed into the same amount of space. Hence, higher compression.

Using that line of thought, you can see why a stock bore (85mm) piston will have a taller dome than a 440 (89mm) piston to attain the same compression ratio.

 

Rice burning.... look for a thread calling your name...

 

atc250r - what you say sorta makes sense and it probably right. But while the bore of the piston is bigger, so is the bore of the cylinder, so it seems that the volume of the combustion chamber would increase just as the size of the piston. So the 440 piston would be moving more air but the cylinder would have more room. But I am sure your right, I just can't figure it out.

jfboy - what thread you referring too?

 

my bad my computer crashed before I posted.... check back in 10min...

BTW: I just went for a ride in my sol..... Man I love that manifold but I cant wait for my internals/programming to get my midrange back and more top end [img]i/expressions/face-icon-small-tongue.gif[/img]

 

You've got to remember what you're doing with the mixture - you're squishing it into the recessed area in the head (the combustion chamber) - This area does not change when you bore the cylinder.

So in effect, you are intaking more air/fuel - because the cylinder is larger - then when the piston returns to TDC (top dead center), that extra air/fuel is squished into the same area it was before (the combustion chamber in the head).

You're better off with a flat top piston anyway, if it gives you the compression ratio you desire.

 

i got it now,,,i was thinking to much about the little area the piston doesnt travel on the top of the sleeve. Makes sense now i think about it. Yeah knew flat top pistons ignites the fuel/air better. Thanks for helping me think straight.