Compression on 951 Carbed Engine

[TLH101]

What should compression be on a brand new, never fired, rebuilt 951carb engine? I figure this would be a good test to see which of my 4 compression gauges is closest to accurate.

 

[garrettb]

Mine was around 120-125. They say it increases after break-in, but I never did get around to checking mine again after I had broken it in.

 

[TLH101]

Thanks, that's what I suspected, but first hand info is always better.

 

[Ankeneyou]

135 psi is perfect on a 951

 

[MacAddict]

Yeah... proper break in is best before trying to get good compression readings. Be sure to have good battery and hold throttle wide open. 951 should be no less than 135... 150 is optimal. I personally am a huge fan of Mototunes hard break in procedure Used it on 3 different engines now with awesome results... very little blow-by.... recently did a top to bottom rebuild of 717 and 951 Seadoos with stock heads, no milling, and getting a solid 150psi on both holes 951 and 145psi on 717 with SnapOn and Mac tools gauges. But I measured my squish gap BEFORE putting things together with gaskets, then added thickness of gaskets, to make sure it was going to be tight. Worked well....

 

[mikidymac]

150 is not correct for a stock 951. A stock 951 should be 135 max.

 

[lloyd54123]

I've always heard 150 but I'm also running a 580. Are you able to explain why it has lower compression? Strictly curiosity.

 

[snikwad003]

I've always heard 150 but I'm also running a 580. Are you able to explain why it has lower compression? Strictly curiosity.

Someone more smarter will need to confirm but I think it's to prevent detonation for a lower minimum octane need

 

[mikidymac]

It's just the way the 951 is.

All the 787 and smaller engines run 150 psi for perfect compression.

 

[Coastiejoe]

I will second that,, simply the way the 951 is built,,,

 

[lloyd54123]

Ok thanks guys!

 

[MacAddict]

150 is not correct for a stock 951. A stock 951 should be 135 max

I will second that,, simply the way the 951 is built,,,

So pure stocker 135psi? But modified 951, 150psi is an acceptable magic number? My sources/friends, which include the guy that built the green carbon fiber XPL 4tec frankenstein, have had me chasing the 150 mark, which I have been able to achieve with: (SnapOn and MacTools testers)

• stock head (base gasket adjusted based on raw squish clearance)
• cylinders bored 1over
• Wossner pistons
• 97.5 white pipe
• DI waterbox
• stock BNi carbs rejetted
• 2.3 needle/seats
• PROKs + outerwears
• RAVE solenoid delete
• waterbox RAVE delete

I run higher octane E0 with K100MG fuel treatment, as I have sworn off ethanol in my toys... She's been running like a raped ape after following MotoMan's hard break in....

All hail the great pumpkin! :thumbsup:

 

[Ankeneyou]

Yup, 135 was the stock psi from the factory. I assume you've got tighter squish to get up to 150.

I've seen 951s running well with just over 100 psi but that seems to be the basement figure.

I assume this is Todd from the FB group based on engine color?

 

[mikidymac]

So pure stocker 135psi? But modified 951, 150psi is an acceptable magic number? My sources/friends, which include the guy that built the green carbon fiber XPL 4tec frankenstein, have had me chasing the 150 mark, which I have been able to achieve with: (SnapOn and MacTools testers)

• stock head (base gasket adjusted based on raw squish clearance)
• cylinders bored 1over
• Wossner pistons
• 97.5 white pipe
• DI waterbox
• stock BNi carbs rejetted
• 2.3 needle/seats
• PROKs + outerwears
• RAVE solenoid delete
• waterbox RAVE delete

I run higher octane E0 with K100MG fuel treatment, as I have sworn off ethanol in my toys... She's been running like a raped ape after following MotoMan's hard break in....

All hail the great pumpkin! :thumbsup:

View attachment 38622

Yes, we are talking a stock 951 per the OP. 135 psi is the goal and limit on a correctly rebuilt stocker.

On a modded ski you can go as high as you like. Or at least until it goes boom.

 

[MacAddict]

I assume this is Todd from the FB group based on engine color?

That would be me!

 

[mikidymac]

That would be me!

View attachment 38624

Nice looking ski, LOL. I have the same year. Love it.

 

[Ankeneyou]

Haha, I win the door prize.

 

[MacAddict]

Nice looking ski, LOL. I have the same year. Love it.

She looks quite different now though.... LOL But yes, HELLA fun ski, especially surf jumping....