95mm JE piston dome volume

[jpnovak]

I CC'd my pistons last night (and again tonight) and measured what I think is too small of a dome volume. I am looking for some comparison numbers. If you have build notes from a set of 3.0L 95mm 9.5:1 (nominal) pistons please post your dome volumes. If you measured the volume above the piston that works too as long as I know the piston depth for the measurement.

I measured a volume above the piston to be 66.65 ml at a depth of 14.56mm. At 95mm this gives me a calculated dome volume of 36.59 ml.

The problem is that this dome volume leaves me a compression ratio of 8.7:1. I am having a hard time believing it can be this far off.

[MBruns]

Jamie, what engine are you working on, also what stroke,deck hgt.,head vol.,etc.
I will look at some of the files I have and help

Mike Bruns JBRacing.com

[jpnovak]

MIke, Thank you for your help.

I am working from memory. my notes are at home.

I am rebuilding a 1978 930/03 3.0L
70.4mm stroke SC crank
Stock heads drilled for twin plug. Head volume 93.4ml
deck height measured at 0.97mm
95mm bore.

It would be great if I only have an error in calculation. The measurements have been repeated several times with identical results. To verify my technique I measured a set of 98mm Mahle RSR pistons. These should be 10.3:1 on the smaller chamber RSR heads (according to Bruce Anderson's book). I measure the dome and calculated 10.1:1 using my head volume.

[MBruns]

Jamie, with that info I get 8.74 to 1 figuring in about .8 cc for the vol. above the top ring, to get 9.5 you will need a dome vol. of 42.5 cc, sometimes the off the shelf JE pistons give you a deep valve pocket and some times a larger dia. pocket for oversize valves that you may not need, check your valve to piston clear. both for depth and dia. and see if you are giving up a bunch of vol. its roughly 6cc too much.

Mike Bruns

[jpnovak]

My numbers agree with yours. I am measuring on the bench. As soon as I get head studs in place I will measure the valve-piston clearance. I can tell the valve pockets are extremely deep. I was not worried about valve clearance in the least bit based upon their depth.

[jpnovak]

Can someone measure the depth of a JE valve pocket? I want to check to see if mine have been cut deeper.

At least I did not suggest to fill the pockets with JB weld.

[tom1394racing]

Jaime

For reference...I measured a dome volume of 41 cc on my 95mm Mahle 10.5 RSR pistons during my 3.0 RSR rebuild.

[JohnJL]

Jaime,
I don't have my old calculations anymore but you are on-track in checking those carefully. My JE Piston 95mm 10:1 pistons ended up with over 11:1 compression when I got them. I couldnt believe it so I had the engine shop check and it was correct. Same heads, plugging, crank and engine as yours.

[JohnJL]

Also, if you are twin-plugging why did you stay down at 9.5:1? Forced induction plans down the road?

[YTNUKLR]

Looks like you are correct with your numbers.

36.3cc piston dome vol
95x70.4mm 3.0L
0.97mm deck
93.4cc head vol
__________________
yields 8.77:1 by my calc's

9.5:1 will require a 42cc piston dome, all else unchanged
10.0:1 will require a 45cc dome, all else unchanged
10.5:1 will require a 48cc piston dome, all else unchanged

[kenikh]

Originally Posted by JohnJL

Also, if you are twin-plugging why did you stay down at 9.5:1? Forced induction plans down the road?

9.5:1 is very appropriate for twin plugging, especially on a 95mm bore. Ask Mr. Weiner and he'll read you the gospel.

[JohnJL]

Hi Kenik,
Yes, Steve and I discussed back then. I'm a customer and appreciate his guidance.

I understood the "rule of thumb" to be single=plug was fine up to around 10:1 on good fuel. In any case I twin-plug as a matter of course now on all builds. Was just interested why only 9:1 if he was already twin-plugging. No biggie anyway.

[jpnovak]

Everyone has asked the million dollar question. Actually, its a thousand dollar question because that is what new pistons should cost.

I twin plugged the heads since I was anticipating the compression to be higher than the 9.5:1 stated JE value. It seems that every set of JE pistons that I have seen posted measured CR values came out higher than expected. John, Yours are a good example.

I admit I am somewhat perplexed that the CR measured so much lower than what I expected. I admit that I picked them up off the board. They had been assembled in an engine but never run. This might be why. For a single plug motor they should come out about 8.9:1 given the volume gain from eliminating the second plug. I measured this last night on a spare, stock 3.0 SC head. They would be perfect for single plug application where decent cams are spec'd.

I too am a believer in twin plug above 9.5:1 unless its a small bore engine on race gas. Of course, a 2.0L might be an exception due to valve angles. I am also a believer in what Steve W. has to say about engine design and performance. He types, I read, comprehend and commit to the gray matter knowledge base.

[JohnJL]

Well Jaime, maybe you can turn lemons into lemonade and boost that baby ;-)

[jpnovak]

I would be lying if I said I have not thought about it. Maybe I will keep them on the shelf for that project one day. Its all planned between the ears. though the expected power increase would cause my transmission and CV axles to scream for mercy.

[cnavarro]

The dome volume calculations that JE uses to determine the compression ratio is based off a 90cc head with a .040" deck. That's not to say that every JE is that way, just the off the shelf ones. Same goes with the valve pockets. Typically, they put .185" deep intake and .135" deep exhaust pockets, which usually are good for up to say GE100s, but sometimes we take them out even to .200" x .150" deep. The shelf pistons usually are cut for stock valves too - we typically cut the lower compression JEs for stock valves and put larger pockets to accomodate the more common big valve setups on the higher compression pistons. Furthermore, depending on the job, I might even fudge the CR calcs using a .050" deck, to provide a little added dome volume, just in case the heads end up larger than the 90cc running average we use. If there is any doubt about a particular piston, call up JE and have them pull up the specs on the piston - they can give you the dome volume as well as other specs on the piston, since not every 9.5:1 JE may be the same (that's why their custom).

[jpnovak]

Charles, Thank you for the reply. I measured the depth of my valve pockets to find 0.274" and 0.235 for intake and exhaust, respectively. This is likely the reason for the lower dome volume and resulting compression loss.