Piston volume

[Ishodu]

Does anyone know or has anyone calculated the volume of there pistons take up in the combustion chamber? I am working on a set here and after molding the top of them and measuring the volume I found that it would reduce the volume of the combustion chamber .6 cc there is a .095" dome on it but there are also large valve reliefs cut in them.
I am working on figuring out the CR of the built engine I have here. And what I have figured out so far just seems really high for the amount the piston reduces the CC volume but I do know the heads have been decked how much I am not sure yet.

 

[Ishodu]

After many many measurements with volumes and depth mics calipers etc. I have learned that I have a 3.2L that has .014" shaved off the heads that will give me 11.47:1 comp. With out shaved heads it would have given 11:1.

 

[firebat45]

Don't forget static compression ratio and dynamic compression ratio are very different things.

 

[pjtoledo]

did you include the volume between the piston and wall above the top ring to the piston lip?

 

[Ishodu]

did you include the volume between the piston and wall above the top ring to the piston lip?

No I didn't but that volume does not change though out the compression cycle so it won't change the end result numbers.

 

[pjtoledo]

No I didn't but that volume does not change though out the compression cycle so it won't change the end result numbers.

ah, but it will change the end result, but not by much.
an example would be 500cc swept volume with 50cc as the total volume for the combustion chamber/valve reliefs/gasket thickness/deck height.
so 500+50 divided by 50 = 11.0cr

( actually, it's total volume at BDC divided by total volume at TDC )

now include the volume above the top ring.

.03mm bore/piston gap X 6mm height X 92mm bore X 3.1415 = 52mm cubed.
thats .052cc volume above the top ring.

that changes the calculates to:
500 + 50 + .052 divided by 50 + .052 = 10.9896 cr.

obviously on a street engine that ain't nothin' . but if you were designing a super hi-tech race engine running 60 psi of boost that's worth taking note of.


damn, now I got a headache from thinkin'



Perry

 

[Ishodu]

Actually it will add more volume to the cylinder stroke. And add the same amount of volume to the compressed combustion chamber. Am I wrong or not enough beer in me?

 

[pjtoledo]

Actually it will add more volume to the cylinder stroke. And add the same amount of volume to the compressed combustion chamber. Am I wrong or not enough beer in me?

thats what I said, it will add .052cc to each, the total volume at BDC, and the total compressed volume at TDC.
But a compression ratio is a ratio between the big volume and the small volume. if you add an identical amount to each, it changes the ratio between them.

consider the beers to buddies ratio as an example:
there are 3 beers in the fridge, there are 2 buddies at the table, so you have a ratio of 3 beers to 2 buddies, or 1.5 beers per buddy.

now add one to each,
there are now 4 beers in the fridge and 3 buddies at the table.
the beers to buddies ratio is now 4 to 3, or 1.33 beers per buddy



moral of the story, (get more beer) for real precise CR calculation, you have to count all the tiny places volume can hide inside the cylinder/chamber.


by the way, I have a Ford Motorsport SVO catalog open right now. the tech info section has a nice diagram of where all the nasty little CRs and volumes hide.

Labatts or Molsons?


Perry

 

[Wakeboy1337]

*walks into thread*
*Glances at the posts*
*Steps out quietly*

 

[Ishodu]

Yes when you put it that way its not a bad thing. We could also measure the volume of air that fits between the spark plug threads and the electrode. Then remove the volume of the ground pin.
BTW the only beer I had today were freencold ones. Maybe someday I can try a freenyours But maybe I owe ya a freenmine.

 

[pjtoledo]

Yes when you put it that way its not a bad thing. We could also measure the volume of air that fits between the spark plug threads and the electrode. Then remove the volume of the ground pin.
BTW the only beer I had today were freencold ones. Maybe someday I can try a freenyours But maybe I owe ya a freenmine.

I think something was lost in the translation on that one. (hopefully)


I'm sure guys that design really exotic engines do take all the little crevices into account. CCing with the plug in the head will catch them.

 

[pjtoledo]

I've had my afternoon nap now, are you ready for round II ?

 

[Ishodu]

Have you figured out those brands of beer yet?

 

[pjtoledo]

Have you figured out those brands of beer yet?

nope, still loosing something in the translation

I'm no expert about beer, put me down for maybe 3 beers in the last 5 years. Never acquired a taste for it.