Piston Dome Volume
 

Do pistons shrink? I lost 3 cc’s in volume.

The J&E pistons are advertised as having a dome volume of 35.2 but my measurements come up with 32.4.

http://forums.pelicanparts.com/uploa...1693157956.jpg

Should the sealed space above the top ring up to the piston shoulder that is between the edge of the piston and the cylinder wall be included in the measurement?

Edit: Ian answered this: all the space above the top ring is included in the measurement of the fluid, but is not considered when measuring the open volume of the cylinder.

Piston shoulder to piston top is 8.7 mm, cylinder radius is 49mm. The empty space in 8.7 mm of height in the cylinder is 65.62 CC. Add the piston dome and I was able to add 33.2 CC’s of fluid.

65.62 - 33.2 = 32.42 CC Dome. Edited for Update: 66 - 31.5 = 34.5

No leaks, no bubbles.

http://forums.pelicanparts.com/uploa...1693097532.jpg

Dialing in a bit more, I found a .5mm difference measuring the piston. So now I’m at 33 CC dome volume. This is still 2.7 CC’s less than 35.2 J&E claimed specs.

http://forums.pelicanparts.com/uploa...1693165574.jpg
http://forums.pelicanparts.com/uploa...1693165574.jpg

My first thought on reading your post was the volume above the top ring around the top of the piston. I see you commented on it, but I can't tell how you accounted for it from your description. It should be a smallish but not insignificant volume, probably less than a cc based on some quick back of napkin math.

If case you didn't account for that, that finds close to 1 cc. You're looking for 3. Are you using a burette to measure your fluid?

That "dead space" of which you speak is needed for expansion of the piston when it gets hot. That'll account for some (most?) of the volume. The diameter of the piston above the top ring is smaller than the measurement at the skirt.

If you put a smear of grease around the top edge of the piston before you measure the volume, your fluid won't fall into the dead space and the measured dome should be higher.

Thanks for the helpful comments! Yes, I’m using burrete. I haven’t filled in the space with grease but that could be the difference. I’ve narrowed down my measurements and technique and am closer.

I’m getting repeated measurements within 31.8 - 32.4 of liquid from the burette.

The empty cylinder volume is 66 CC’s.

Update: I’m at 34.5 CC’s for dome volume. So pretty close to what J&E specs the dome at.

Piston Dome Volume Measurement………..
 

Were you using a plexiglas top cover to fill the void space in the cylinder for your dome volume measurement?

Tony

Hi Tony, yes I had the plexiglass cover. Also used a spot of red grease so I could tell it was contacting with the piston top. I measured several times and am getting repeatable numbers at 34.5 dome volume now. Much better than before.

Shane





http://forums.pelicanparts.com/uploa...1693233150.jpg
http://forums.pelicanparts.com/uploa...1693233150.jpg

I disregard all info except what I come up with. Use your dome measurements.

JE dome volume specs are occasionally awry. Trust your measurements and adjust your C/R with the deck clearance via selected cylinder base seals. You can go much closer than .039". Measure piston to head with solder too.

The space above the top ringland is usually filled with grease if that’s how your sealing the piston in the cylinder. This is called the crevice volume. It’s usually left unaccounted for. Here’s the equation to calculate that extra volume if you used grease to seal the rings.

((Bore diameter - piston top diameter))x bore circumference x top ring land depth

Make your measurements metric and the sum will be how much volume in cc’s surrounds the top of the piston that is usually covered in grease. Add this number to your dome volume measurement.

I had a conversation with Ian (ICARP). He helped me to understand that the calculation of compression ratio must include all volume above the top ring;

The space up to the piston shoulder that is between the edge of the piston and the cylinder wall must be included in compression ratio calculation as part of the combustion chamber.

The space should not be filled with grease, it isn’t dead space.

A separate equation isn’t needed in addition to add the space to the cylinder volume used to calculate compression ratio.

When calculating compression ratio, the empty space of the cylinder is determined based upon piston dome height. The piston dome height is measured from the piston shoulder to top of the piston.

The area filled with fluid from the burette must account for the entire volume of the compression chamber above the sealed top ring.

This is a little hard to wrap your head around because the space above the top ring up to the piston shoulder isn’t included in the piston dome height measurement. But, it must be included as compression chamber area, as are the valve reliefs on top of the pistons.

Wether the reliefs are above or below the measured area of height doesn’t matter, the area must be included as part of the measured negative volume (fluid CC’s from the burette).

I sincerely welcome any feedback that confirms or contradicts this information.

On a 98mm piston with a 5.10 mm distance from top ring to lip, the volumme in the void is .5 cc. Bob

How do you seal the top ring without grease?? Seems like now matter how little you use it’s still going to fill the crevice

I too wanna see how you can get the rings to seal long enough to get an accurate reading without grease or vaseline, especially if you're using alcohol as the fluid.

I used to race in classes where, if the compression ratio was even 0.01 high, you'd get thrown out. I think I have a pretty good grasp on how it's checked.

As you've seen first hand, adding that dead space will give you a lower than actual result. In racing, you'll get thrown out. On the street, you'll be more likely to run into detonation.

Check it however you want, I won't lose sleep over it.

I just lightly coated the ring and put a small dab on the ring gap. Pushed the piston top up above the deck, put waded up paper under the piston to hold it up and then pushed it down with the plexiglass seal. This sticks the ring to the landing to seal it. I used mystery oil to measure and it didn’t leak. There really isn’t much space there, 0.7 CC’s.

Thanks Bob!

I truly appreciate your wisdom and experience, I just think that not adding the “dead space” would be like not calculating for, or filling in the valve relief space.

I’ve also read several definitions of the combustion chamber and every one of them defines the area as including all of the space above the top ring.

Yes, deck clearance without base gaskets is 0.021”, I’m going to add enough base gasket and tiny layer of sealant to get it to 0.039”.

I’m not shooting for specific CR, but think I’m in the best place for single plug with 91 gas. I think 0.021” deck is just a little to tight. Plus I think that the CR would be to high for single plug.

Thanks for this comment. That makes sense. SmileWavy

I like it! Will try, have some to measure come Monday - some euro 95mm Mahles