Flat top piston ?'s....

[LethalInjection]

As far as piston design goes, what affects the compression ratio of flat top pistons.. I know valve relief volume is a factor, but what is the difference in say... The 8.5:1 and 10:1 trickflow pistons.. I noticed that they have the low comp pistons listed with a larger compression height.. I would think this would lead to a high compression ratio since it looks as though that would put the piston a little higher toward the head at TDC.. WTF gives...? amnit:

 

[Daggar]

I don't understand what there is left to tell you. It looks as if you've already anwered your own question.

OF COURSE higher compression pistons lead to higher compression ratios... that's the whole point.

Edit: Perhaps you're not taking the rod length and stroke length into consideration?

 

[tjm73]

Are you talking about the difference between a 1.600" and 1.610" compression height? .010"?

 

[LethalInjection]

Yeah, I was asking how they got a 8.5:1 C\R and 10:1 C\R out of a flat top piston when the lower compression piston has a longer compression height.. Neither pistons have domes, so where'd the 1.5 point bump in compression come from.. Stroke, rod, and bore are all the same..

Compression height is the distance from the wrist pin bore to the top of the piston right? So.. If there's more material in between the wrist pin and the top of the piston, wouldn't that mean that the piston is a little further up in the cylinder bore at TDC, which *should* yield a higher compression correct..? If so, then how does the 8.5:1 piston have a taller compression height yet lower compression?

 

[tjm73]

Yeah, I was asking how they got a 8.5:1 C\R and 10:1 C\R out of a flat top piston when the lower compression piston has a longer compression height.. Neither pistons have domes, so where'd the 1.5 point bump in compression come from.. Stroke, rod, and bore are all the same..

Compression height is the distance from the wrist pin bore to the top of the piston right? So.. If there's more material in between the wrist pin and the top of the piston, wouldn't that mean that the piston is a little further up in the cylinder bore at TDC, which *should* yield a higher compression correct..? If so, then how does the 8.5:1 piston have a taller compression height yet lower compression?

.010" of an inch is more acedemic than a signifigant impact on C/R. The difference in C/R in those pistons coems from the dish volume. On the 10:1 it'll be eyebrow cuts for valve clearence and on the 8.5:1 it'll be a dished top.

The .010" difference in piston placement in the bore means probably less than .5 of a single C/R point on two otherwise identical pistons.

 

[nosmatt]

it has a dish.

i have held them in my hands.


same with the probes

the compression height was becuase to get the dish shaped right, the compression was too low, thus they changed where they drilled the pin hole, this brings the piston up to deck, and maybe slightly out depending on too many factors to list.

you cannot just dig a small hole in a piston, they are "reverse dome" and if that lowers the compression too much, they have to make it up somewhere.

 

[LethalInjection]

Ok cool.. Thanks.. I knew it had to be something.. For some reason I was under the impression that they were both flat tops with reliefs and it wasn't making much sense to me..