Compression question.

SJ636

SJ636

Location
Victoria, BC
Heres the issue. Im running a 701 (62T bottom, 61x top) freestyle ported by judge motorsports. Last season I had greater head on with 35cc domes. I was running 145 psi and spun a rod bearing at the end of season... Needle bearing made it up to the top end and damaged one piston (cyl are still good) so I just rebuilt the engine with a rebuilt SBT crank, WSM 81mm pistons and rings. Since the cyl never got damaged I just gave them a hone and im still within the .045 clearance range from piston to cyl wall. Now this leads me to my questions. After rebuilding I decided to run 33cc domes to bump the compression up. Everything is together now and once again im still at 145 psi in each cyl. Does that seem low compression for 33cc domes even with porting done? How come with 35cc dome I ran same compression?
 
vitaly

vitaly

Анархия - мать порядка!
Location
NY/NJ
SJ636 said:
Everything is together now and once again im still at 145 psi in each cyl. Does that seem low compression for 33cc domes even with porting done? How come with 35cc dome I ran same compression?
Click to expand...
Sounds about right. Just think about it.
 
nightrider05

nightrider05

Location
St.Louis Missouri
I dunno..all of your numbers sound low.

It seems like compression is usually 10-15 pounds lower on ported cylinders....35cc domes usually put out around 180-185 psi...so even taking off 15 psi you should have still been well above 145. And now with the smaller domes it should be even higher than that.
 
SJ636

SJ636

Location
Victoria, BC
New pistons and rings. WSM platnium pistons and rings... Thats what I dont get. Why is my compression so low? Last season with 35cc domes I was running 145 psi. The ski ran great none the less but it doesn't seem right. Now with the 33cc domes, new pistons and rings im still at 145 psi.
 
MileHighJSC

MileHighJSC

Location
Great North West
Is the Air temp the same as last year when you got the 145psi? How about the humidity? Have you run the motor long enough to seat the rings? Are you using the same comp gauge? Are you using the same brand/thickness head and base gasket? Are you using the same brand/model pistons?
 
Matt_E

Matt_E

steals hub caps from cars
Site Supporter
Location
at peace
You need to break in the engine and let the rings seat.
Then check your compression again. It will go up.

Also, your gauge may not be accurate and might have deteriorated some. I have a compression gauge that read lower and lower over the years.
 
SJ636

SJ636

Location
Victoria, BC
Matt_E said:
You need to break in the engine and let the rings seat.
Then check your compression again. It will go up.

Also, your gauge may not be accurate and might have deteriorated some. I have a compression gauge that read lower and lower over the years.
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No I have not ran the engine yet. I do think after the rings are seated comp might go up a bit... but not enough to where I want to be. Last season when I put it together it read 145 psi, I broke it in and checked it several times, and it still came up as 145 psi. The guage is new and I just used it on a dirt bike that read 185 psi so I dont think its the guage.
 
SJ636

SJ636

Location
Victoria, BC
Kawi550sx said:
Is the Air temp the same as last year when you got the 145psi? How about the humidity? Have you run the motor long enough to seat the rings? Are you using the same comp gauge? Are you using the same brand/thickness head and base gasket? Are you using the same brand/model pistons?
Click to expand...

Kawi550sx said:
Yes, are you at the same elev? You may also want or need to cc your domes, just because they are stamped 33 or 35 does not make it so.
Click to expand...

Air temp/humidity is generally the same, its all done in a heated shop so it shouldnt be far off. This is a diff comp guage tho, a new one I picked up (that I tested on another engine too and it worked fine) same head gaskets but different base gasket. The gasket I used last year was .020 thou I believe, and this time im using a base gasket from the WSM top end kit. I never did check the thickness of it.. I kinda assumed it was a standard .020 thou gasket. Last year I bought the cyl from godoco on the x, he had pistons with it that I used. This time im running new WSM pistons. Again I have not ran the engine yet, or seated the rings but do you think comp will go up that much? Cuz it should be in the 180 range rght?
 
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