Super Jet compression numbers and proper dome size

[2XLR8]

There you go. Without reinventing the wheel all you need to do is reduce the ccs by 4 and maybe take .005 off the squish band.

 

[Pablo]

Domes are 33's. So to restate the begining question: what compression "should" I get with my current setup. I have not measured deck height or its relationship to exhaust port but can do that. My current setup is: 760BB flat top
Riva 85mm pistons
Dominator sleeves
Dasa porting
ADA head with 33cc domes
Squish is 0.040 avg. Compress is now 150.
I'm not running a base gasket. Cylinder is mated to cases with 1211 )Threebond
I was expecting to find a higher compression.

 

[SUPERTUNE]

Domes are 33's. So to restate the begining question: what compression "should" I get with my current setup. I have not measured deck height or its relationship to exhaust port but can do that. My current setup is: 760BB flat top
Riva 85mm pistons
Dominator sleeves
Dasa porting
ADA head with 33cc domes
Squish is 0.040 avg. Compress is now 150.
I'm not running a base gasket. Cylinder is mated to cases with 1211 )Threebond
I was expecting to find a higher compression.

If you assume... then your just guessing!

OK, NOW YOU NEED TO PAY ATTENTION!

Let's figure compression ratio with YOUR engine setup with these # you have here.
All we can do is get close, but not exact.
We don't have a F.V. (Fill Volume) of your combustion chamber with the engine assembled @ TDC.
Let's say the piston is about -.020 down deck height at TDC.
Using a Riva piston which is a 0* degree flattop, no dome at all.
Engine is 771.7 cc displacement, which is 385.8 per cylinder.
If we figure the piston is -.020 down (.50 mm) this would be 2.8 cc's
Add 2.8 cc's to the 33 cc domes this total is 35.8 cc for a calculated F.V.
385.8 cc displacement + 35.8 F.V. = 421.6.
421.6 divided by 35.8 FV = 11.7

This is the approximately your full stroke calculated compression ratio.
11.7 is very close to stock compression ratio of a Yamaha 701.
PSI compression you will have will be dependent on several factors like exhaust port height, how fast the engine turns over doing the compression check, how much oil residue is on the piston rings and bore and brand and quality of compression gauge.

In a performance build like yours, the compression ratio is very low.
I would like you to see at least 13:1 for a start point and I like closer to 14:1 for freestyle applications (not prolonged wide open throttle) on premium pump fuel, 92-93* octane.

If you used 30cc domes this would give you 12.7 compression ratio.
If you used 28cc domes this would give you 13.5 compression ratio.

So how close was I just guessing to start with YAK 8628L15 (28cc) dome?
Pretty close...
If staying with using ADA brand head/domes, have them custom cut to fit the squish angle to match the Riva pistons by having the dome machined @ 2* degrees and not like the YAK series domes that are 4* degrees.
# ZERO covered this in his replies.

 

[superjetkev]

dude if you got 150 even in both holes, and it runs good RIDE IT!!!!

but thats just me, summer is too short to sit on the x and type in hopes to get 5 more hp.
thats what winter is for!

but on a serious note, this stuff is all good to know, and will help you the next time you build a motor ect. all great info from reliable sources.
good luck and hope you get er sorted out

 

[lerott]

Old thread but just for those who use the search fonction :
Dasa cast cylinder 7 port stock stroke non powervalve riva pistons @ 85mm, I have used 27cc custom cut dome 2°, squish 1.2mm, compression was 13-13.5kg, runs great, my bad I've sold this engine :-(