These heads are typically crafted from high quality billet aluminum, and often have more water jacket volume than the stock part. Many are called “O” ring type heads because they utilize a large rubber “O” ring, instead of the stock steel head gasket, for sealing between the cylinder and the cylinder head (as is done on the smaller Sea Doo engines). (Please note that our technicians are not permitted to discuss the specification information of our cylinder head modifications or cooling upgrades).Ībout “O” Ring Heads – There are many aftermarket heads available for the 951 engines. Our cooling upgrade completely eliminates this air pocketing. 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. The stock 951 head casting can “land lock” air pockets in the water jacketing around the combustion chambers. Included in our head modification is a cooling system upgrade. This staggered arrangement helps to equalize the operating temperatures of the two cylinders, and helps eliminate detonation. That is, the front and rear cylinder head domes are cut to slightly different compression ratios. he Sleeper head mod is unique to those done by other shops in the sense that it has a “staggered” compression ratio. Our testing showed that very strong overall acceleration can easily be accomplished without big increases in compression ratios.Īll of our 951 head modifications include altering the squish angle of the stock head. However discussing various 951 compression ratios in terms of indicated psi is (in our experience) a total waste of time.Īll this said, the compression ratio increase of our 951 kits is “very” conservative. All this said, “relative” indicated compression measurements might be a useful diagnostic tool on an otherwise unchanged 951 motor. On the 951s, we have seen cylinder heads of several cc’s difference show indicated numbers only a few psi apart, while the accurate difference would have been 20 – 30 psi. If a very high compression head is installed, a compression gauge will not be capable of showing an accurate measurement of the increase because the extra load of compression significantly slows the starter cranking speed. However the starter in the 951s has a lot more work to do than any other pwc starter. For pwc’s with smaller displacement individual cylinders (which in this case is “all” pwc’s), it’s not too difficult for the starter to maintain a relatively narrow range of starter cranking speeds over a very wide range of indicated compression numbers. The primary reason for this is the many variables that affect electric starter cranking speed.Īccurate comparative indicated compression measurements must be taken at cranking rpm speeds that are as close together as possible. It the case of the 951 Sea Doo, indicated compression measurement is even more worthless than it has been on earlier pwc’s. As the “Compression” document on our website points out, measuring compression in this way is, at best, a “ball park” measurement with limited meaning. The use of compression gauges (to establish a measurement in “psi”) has been a very popular way for owners to compare compression. How 4-Strokes Compare to 1990s 2-StrokesĬompression – Increasing compression ratio is always a very popular first modification, and the 951’s respond well to “subtle” increases in this area.Competition – High Performance PWCs for 1998.Carburetor Fine Tuning Guide – Advanced.Yamaha Wave Blaster 2 – Performance Mods.Yamaha 1200 GPR-XL-XLL – Recreational Mods.Yamaha 1100-1200 – Sleeper Kits for Triples.Yamaha Super Jet 845cc – Big Bore Hammer Mods.Yamaha 633-701 – Recreational Engine Mods.Sea Doo Jet Boat – High Performance Mods.Sea Doo 951 GTX Di – High Performance Mods.Kawasaki STXR 1200 & Ultra 1350 – Single Pipe Engine Kits.Kawasaki STX 1100 Di – High Performance Mods.Kawasaki STX 1100 – Big Bore Hammer Kit.Kawasaki 900-1100 – Sleeper Kits for Triples.Kawasaki SXR 800 – 849cc IJSBA Stock Class Upgrade.Kawasaki SXR 800 – High Performance Mods.Kawasaki SXR 800 & SXI 750 – 849cc Big Bore Mods.Kawasaki JS 550 – High Performance Mods.Kawasaki JS 550 – Engine Modification Kits.Kawasaki 300 – Sleeper Engine and Pump Mods.
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