Yamaha 633-701 – Recreational Engine Mods

ABOUT HIGH PERFORMANCE PWC’s AND THE NEW REFORMULATED FUELS – For the last 7 years, Group K has offered two lines of pump gas engine modification kits known as “Sleeper Kits” and “92 octane Hammer Kits”. The Sleeper kits are an affordable group of modifications to stock components only. The 92 Hammer kits were designed to be the highest horsepower combination, of modifications and bolt on parts, that could run reliably on 92 octane pump gas. All our Sleeper kits are still safe to run on reformulated 92 octane. However the high performance “unfriendliness” of the new reformulated fuels is so profound that we have had to increase the octane mandate for the Hammer kits to a minimum of 96 octane.

Part of the reason for the increased octane need is the nature of the modern personal watercraft themselves. During the days of stand up pwc’s, it was rare for anyone to ride “full throttle-full rpm” for extended periods of time. However, the later model (sit down) machines handle so well that extended “full rpm” operation has become commonplace. We test our kits with this very heavy use in mind. Initially we were able to avoid the 96 octane mandate by using stock rev limiters and milder compression. However, the progressively increasing output of new machines and the decreased quality of 92 octane pump gas has brought about this unavoidable situation.

There have certainly been many isolated situations where owners have “gotten away” with running pump gas in machines that we might consider as 96 or 110 octane formats…good for them. However these exceptional situations will become more and more rare. We have no idea how the other aftermarket performance shops will be dealing with this issue, but we do know they cannot ignore it.

Octane Options – Not many folks want to drive a long distance to spend $5.00 a gallon on race fuel for their recreational pwc. However anyone that has paid an expensive bill for the repair of a seized piston has considered it. The repair cost of a seized piston is the financial basis we will use to look at the cost effectiveness of various octane options. The average repair bill for a seized piston is about $350 for a twin cylinder pwc.

By far the most cost effective octane option is a 50/50 mix of avgas. A gallon of 100-octane avgas costs about 50 cents more per gallon than 92 octane unleaded. Based on a 10 gallon tank of gas, you could buy 140 tanks full of 50/50 100 octane avgas/92 pump gas for the same money it costs to repair one seized piston.

Another option is a 25% mix of 110-octane race gas with 92 pump. With race gas at $5 per gallon, you could buy 28 tanks of this mix to equal the repair of a seized piston.

Lots of owners ride in areas where they need to buy fuel at outlets on the water. These folks are more interested in octane “boosters” that they can carry along. Unfortunately no octane booster can turn 92 octane into 96. At best, 3 oz of booster per gallon can normally make 87 into 92, or 92 into 93.5 – 94 (the increase offered by booster becomes less as base octane gets higher). Using more than 3oz per gallon offers no further increases. In addition, this small increase comes at an expensive $1 per gallon (the 25% mix of race gas costs $1.25 per gallon).

Design Weaknesses – The Yamaha engines have a few inherent design weaknesses. Modifying your Yamaha with a keen awareness of these weaknesses is what permits the construction of an effective yet reliable modification package. If these weaknesses are ignored, engine life and reliability can begin to suffer immediately.

Head Gasket – All Group K kits are intended to be used with the stock .040″ head gasket. There are many aftermarket alternatives to this gasket, i.e. .020″ copper gaskets, and “O” ringed heads, etc. It has been our experience that the primary cause of most head gasket leaks is low spots on the cylinder deck surface. To address this problem, all Group K kits include cylinder decking to assure a totally flat deck. While all of the after market gasket replacements may be capable of maintaining a seal, none of them can resolve a leak caused by an irregular cylinder deck surface.

Air Leaks – The lower end of your engine should be totally air tight up to 8 psi. New engines pass this test with no problems. However engines with older deteriorated crank seals or brittle intake gaskets can allow outside air to leak into the lower end during operation. This “air leak” can make the fuel mixture lean enough, at high rpms, to cause a piston seizure. Air leaks, not engine modifications, are by far the number one cause of piston seizures on the Yamaha twin motors.

Driver Coupler – The stock drive couplers and rubber cushion, mounted at the back of the motor, can work well on a stock machine. However these parts may have difficulty standing up to the added torque loads of the Hammer 96 engines (particularly in the Wave Raider hulls). As a safety measure, we recommend a set of “billet” drive couplers for modified machines that will frequently be run at full rpm and full load.

Top End Modification – All Group K kits include cylinder porting. In the past, cylinder porting has received an undeserved reputation as a modification that causes a loss of reliability and engine life. If the porting is performed by someone with very little Yamaha experience, that may be true. The porting mods used in all of our kits are the result of many months of performance and wear evaluation testing. We confidently claim that Group K cylinder porting will yield a wider power band and longer piston life than any other porting modification available anywhere.

All Group K modified cylinders and heads are typically prepared as matched pairs to assure that you have the proper squish clearance and compression ratio for the quality of gasoline that you intend to use. These matched cylinder and head specifications play the key role in determining the amount of overall power that the engine can make, as well as the quality of fuel that you must use.

The port layout of the stock 633cc Yamaha cylinders is by far the mildest in the entire watercraft industry. For this reason, the 633cc engines gain huge overall power margins from cylinder porting. The 701cc engines have a more advanced standard port layout, however the power gains made by cylinder porting are still very significant. In both cases, the most advanced port layouts used in our kits have a negligible affect on long term piston or ring life.

Piston Clearance – It’s common for new 701 cylinders to have as much as .005″ of piston to cylinder wall clearance. (set-up clearance is .003″ – .0035″ ; wear limit is .007″). On any racing or high output recreational packages, we recommend set-up clearances of no more than .004″. Engines intended for moderate recreational use can easily live with .004″ – .005″ set-up. Piston clearance inspection is available on request, and cylinder boring (using pistons furnished by you or us) is available in all oversizes. Since Yamaha only offers only two oversize pistons, boring beyond second over will mandate the use of Wiseco pistons. In any case, all ported cylinders are honed before return shipment.

Big Bore Conversions for 633’s – The availability of the newer 701cc top end has moved many 633cc owners to consider a big bore upgrade. There are two common ways to accomplish the 633 big bore. They are as follows:

Installing the 701cc Top End “Spending with the Big Boys” – The 633 cases need to be bored slightly in order to accommodate the larger sleeves of the 701cc cylinder. With this done, the entire 701 top end will bolt directly on. From a performance stand point, the stock 701 top end will run much stronger than a stock 633 top end. However the stock 701 conversion (@ $800 parts & labor) is no more reliable, and very questionably faster, than a 633 Sleeper kit (at $499). A fully modified 701 top end ($499 added to the $800) is much faster overall than the 633 Sleeper, and just as reliable. The modified 701 top ends generate “very” strong middle and high range power. Since the 701 cylinder has more available port area, and thereby more high output potential, a 701 cylinder on 633 cases (at about $1300) is a popular (and competitive) combination used by many riders.

Big Boring the 633 Top End to 701cc “An Affordable Alternative” – The Group K 633 Big Bore Kit offers performance very close to that of the 701 Sleeper. Simply boring the 633 cylinder to 701 works very poorly because the 633 port windows are far to small to effective feed the 701 bores. For this reason, cylinder porting, head modification, carb upgrading, and cooling system upgrading are absolutely mandatory parts of this big bore conversion. With all this done, your 633 big bore will blow off stock 701s, and be within a few percent of modified 701’s. We consider the 633 Big Bore (in both Sleeper and 96 Hammer formats) to be the most reliable of the 701cc high performance setups we construct. The total cost difference between a 633 Big Bore Hammer 96 and a 701cc Hammer 96 is about $600. The difference in overall power is about 1.5 mph of peak speed ability. If you’re a racer, that’s everything. If your not a racer, 1.5 mph is questionably worth an extra $600.

Big Bore Conversions for the 701s – 753cc-781cc Big Bore Kits – Group K offers two different Big Bore modifications for the 701 engines, based on the cylinder that you have. The first, and most affordable, is the Raider Cylinder Bore Only. The 701 Raider cylinder (which bolts onto SJ/Blaster cases with no case boring) can be bored to 753cc using the existing stock sleeves. While the base sleeve areas do become somewhat thinner, the are easily strong enough for high output long term use. Boring this cylinder leaves the port windows very undersized for a 753cc. That’s why cylinder porting will be a mandatory part of all Group K big bore kits.

We will fit 84mm bores to Wiseco pistons because they typically run .002″ – .003″ under the size of the stock Yamaha 84mm pistons. In the event that this original bore becomes slightly worn or scored, the Group K big bore cylinder can be honed slightly to a perfect clearance fit on the slightly larger Yamaha piston. After that, the .010″ and .020″ Yamaha piston can still be used as additional overbore sizes. This amount of over bore allowance would easily last most owners a couple of seasons.

The second type of big bore is the Sleeve and Bore, which must be done on the SJ/Blaster (61x coded) 701 cylinders. These cylinders do not have enough base sleeve thickness to safely accommodate an 84mm piston (753cc). For these cylinders, Group K installs a thicker wall sleeve (made to our specifications) that can safely accommodate pistons up to 85.5mm (781cc). Top case boring is required to accommodate these larger sleeves. This same “thicker wall” sleeve installation will be available for customers who desire to bore their Raider cylinders to the 85.5mm diameter (boring the Raider cases will be necessary).

Case Porting – Case porting by itself doesn’t offer a giant overall power increase on the 701 engines. However, this is a modification that can make a noticeable improvement in overall power. This increase is very desirable to many owners because it comes without any loss of reliability at all. The case porting is an “all gain – no lose” modification that should be done on any Big Bore Hammer. (note: cylinder is required with top crankcase half to do case porting)

Exhaust Systems – We recommend that a stock waterbox silencer be used on all the Sleeper and Hammer kits. Contrary to common belief, the stock Yamaha waterboxes yield excellent overall power levels in all high output applications. Unlike most after market waterboxes, the stock waterboxes can offer quiet sound levels along with strong power even when water is diverted away (water diverters are a part of every Group K engine kit). Even the 701cc Hammer 96 runs best with the stock waterbox. We have tested numerous aftermarket waterbox replacements and found none that significantly increased overall power.

All Group K Hammer kits are intended to utilize either the “Factory Pipe” brand (on the 701cc engines) or Coffman aftermarket exhaust pipe (on the 633 & 633 Big Bore engines). We do not currently recommend any other pipes for our Yamaha engines.

For all the dual carb Yamaha 701 engine formats, we have had the best overall results with the Factory Pipe Products exhaust systems. Owners looking for an exceptional increase in acceleration should also get the optional ECWI (electronically controlled water injection) system from Factory Pipe. It’s important to understand that while this system greatly improves power from 3500 – 6800 rpm, it makes no change in peak rpm or peak water speed abilities.