Boost 951 compression? Add HP?

[SabrToothSqrl]

I'm considering raising the compression on my 951 engine to get more performance out of it.

Other than having to run 91 or 93 octane, what are the down sides?

Will I get more power?
I run a Solas Concord 15/20 and hit 6,800 RPM w/a plastic pump on my 02 XP.

Factory prop spins at 7k WOT. RPM limiter is 7,200 stock.

I'd like to spin the solas at 7k or 7200 WOT.

Will higher compression alone allow this? Will the carbs need adjusted?

What issues / gains am I truly looking at?

thanks.

Factory prop tops out around 60-61MPH, solas tops out at 64/65, even with the lower RPMS. (GPS verified).

The solas seems to get me out of the hole quicker as well.

 

[Minnetonka4me]

I would prob recommend putting flame arrestors and a rejet kit along with the head. On 951s you really want to put one way valves on the back of the hull b/c the flame arrestors are so close to the bottom of the hull.

 

[SabrToothSqrl]

Will not using the stock intake allow any more of a chance of water ingestion? I like to submarine my ski and don't want to suck water in...

The Racing Handbook says to shave the head, which I don't really want to... 14:1 is a crazy high compression ratio though, stock is only 6.1 to one!

attached is the racing handbook for the 951...

lists all sorts of mods...View attachment 1999 SeaDoo Racing Handbook.pdf

 

[Dr Honda]

Will not using the stock intake allow any more of a chance of water ingestion? I like to submarine my ski and don't want to suck water in...

Yep... I dont' like aftermarket filters unless you are going to a fully Mod'ed engine.

The best bang for your buck is a factory pipe, and their water injection system.

Raising the compression on a 951 will melt piston crowns.

Unfortunately... the 951 is already on the edge of reliability, and performance. No other PWC company pulled 135 HP out of 950cc's. Polaris, Yamaha, and Kawi all pushed up to +1200cc's (and 3 cyl's) to make that kind of power.

 

[SabrToothSqrl]

the issue I have is that when I bought the ski, it had a shaved head. I believe this is what allowed it to shoot to 7K INSTANTLY from any RPM. I didn't know this, and ran lower than required octane, and the engine gave up. I then put on a factory head, lost that compression, and now the ski runs great, strong, fast, but not AS fast as it once did... I've spent years chasing that HP, and now I think I found it... question is... is it worth it?

I like the idea of the factory pipe, but you lose the oil tank and thus have to premix...

 

[Spimothy Leary]

Will not using the stock intake allow any more of a chance of water ingestion? I like to submarine my ski and don't want to suck water in...

that is correct ! IMO the stock intake is much more "secure" from water ingestion. i really don't know how well those outerwears work, they worry me but it sure is easier to tune a carb that has F/A's on it, than to remove that dam contraption !

 

[SabrToothSqrl]

after re-reading this page:

http://www.groupk.com/sd951y2k.html

that's the exact mods to the original head (that I STUPIDLY threw out in anger)...

that head was what made my ski so much faster...


" All of our 951 head modifications include altering the squish angle of the stock head. The Sleeper head mod is unique to those done by other shops in the sense that it has a “staggered” compression ratio. That is, the front and rear cylinder head domes are cut to slightly different compression ratios. This staggered arrangement helps to equalize the operating temperatures of the two cylinders, and helps eliminate detonation.

Included in our head modification is a cooling system upgrade. The stock 951 head casting can “land lock” air pockets in the water jacketing around the combustion chambers. If this air pocketing happens during high-speed operation (on a modified motor), it can cause localized over heating that can easily lead to detonation. Our cooling upgrade completely eliminates this air pocketing. (Please note that our technicians are not permitted to discuss the specification information of our cylinder head modifications or cooling upgrades)."

that 'cooling' upgrade was actually two holes in the head (one in each dome) with outlets for water hooked up to the existing water exit hose... nothing that special, but it may help cool the domes.

so, could that head be the only thing between me and more HP? (and some higher octane fuel)? or was it that head and HP that caused the engine to last only one season..? choices choices.