Yamaha Super Jet 845cc – Big Bore Hammer Mods

Overview

In 2008, the IJSBA permitted the big boring of two-stroke Super-stock racing-class stand-up machines to 850cc. In addition, in 2007 the IJSBA has allowed the use “any modification level” of “any homologated pwc engine” in a Freestyle machine. Since that 2007 freestyle ruling there has been a flurry of huge displacement custom engines being built for pro freestyle. While their power potential is great, so is their $5000- $8000 price tags.

Back in the “real-world”, a growing number of free-style competitors, and recreational-riders are looking for a reliable and pump gas safe setup that can make pro level free-style grunt, without the pro-level maintenance or astronomical price tag.

The Group K 845cc Freestyle Big Bore was developed specifically for these freestyle enthusiasts. When developing this package, we considered it very important to specifically define the engine packages goals. Of course sheer horsepower is an important goal … but not “the” most important. The most important priority for a good freestyle setup is to have the quickest possible engine acceleration (aka response), along with a power delivery that is as linear as possible. Coupled with this engine package, needs to be a pump that can completely hook up all the power being made … especially during quick engine accelerations. While all of this sounds like the same kind of common thinking used to develop closed-course machines, there are a lot of tuning variables to accomplishing freestyle goals, that “are not” part of the average superstock closed course setup.

Besides the lower cost factor compared to the high-dollar “big block motors”, the Group K 845 offers some other important technical advantages that makes it as practical as it is effective.

Here are a few.

Crankshaft – The stock SuperJet crankshaft has a stroke that is 6mm shorter than an SXR, and 10mm shorter than the GP800 motor (being used for many big-block freestyle customs). This shorter SJ stroke means the SJ crank is considerably smaller in diameter, and quite a bit lighter than an SXR or GP800 crank. The lighter rotating mass of the Super Jet crank allows it to accelerate to peak rpms much quicker than the other longer stroke motors can. In addition, the SuperJet ignition rotor is also much lighter than the others, further benefiting quick acceleration.

An added benefit of a big-bore shorter-stroke engine (like our 845), is that it experiences significantly less vibration than the other “big-bore long-stroke” motors. In every other motoring genre, large displacement long stroke twins are equipped with counter-balancers to abate the otherwise violent vibration that they generate (as is the case on the Sea Doo 951s). The current crop of custom big-block free-style motors have no counter-balancers …. And consequently deliver violent vibration at a wide range of engine speeds. The dramatically milder vibration of the 845 improves the long term reliability of every part of the hull and drive train.

Perhaps the greatest advantage of this lesser vibration is that the carburetors are able to calibrate much more precisely. Among many other carburetion issues, excessive vibration can easily shake the float needles away from the seats. This causes an on going rich condition at low throttle openings that is virtually impossible to “tune away”. The reduced vibrations of the 845 allow consistent needle to seat contact that results in much easier low speed fine tuning. This easier low speed tuning allows for very precise low range carburetion. Precise low-speed carb calibration is a fundamentally important component of getting the instant throttle response that a pro freestyle setup must have.

Why not a Stroker Crank? – To be sure, the displacement of a longer stroke is always nice, but in the case of this particular engine setup, crank stroking brings some very negative technical luggage (besides added vibration). Because of the changed stroke geometry, when you increase the stroke of any engine, the port timings become inherently increased (longer in duration). Any stroke increase worth doing on the SJ crank (4-6mm) would result in port timings that are much more radical than what you would want for a pro free-style motor. In addition to that, most stroker motors need a spacer plate under the cylinder to keep the pistons from poking out of the top of the cylinder. Installing this spacer plate (as most strokers have) would increase port timing even more, and result in port timings that are close to the upper limit for a high rpm closed course setup … and far more port timing than what you would want or need for a good free-style powerband.

The last big handicap of the stroker crank is “rod angle”. When you increase the stroke of any crank, you also increase the rod angle when the crank is 90’ between top dead center and bottom dead center. This means that, on the power stroke, the piston is pushing down on a rod that is at a much steeper angle. This steeper rod angle results in increased piston-face/cylinder-wall wear, as well as much more stress on the rod bearing surfaces. Normally, these combined problems are resolved by installing a longer connecting rod. Sadly, by the time you add the cost of crank stroking, longer custom rods, and the thick cylinder spacer plate to accommodate that rod length, the resulting package cost increase would be about $2000+. The saddest part of such a setup is that, once built, it would always require a cylinder and head with “long-rod stroker” specs that cannot work on a stock stroke lower end. In contrast, the Group K 845 crankcase can easily accept any conventional 760 or 701 top-end in future applications.

About the 845 Cylinder Modification – For many years, Group K has be building 85-86mm re-sleeved big bore kits for the 61x cylinders that come stock on all ’94 and later SuperJets. Unfortunately, the 61X cylinder casting doesn’t have enough “beef” or excess material to safely accommodate the 845cc sleeves. Fortunately, the stock 62T 701 & 64X 760 cylinder casting both have quite a bit more “beef” than the old 61X casting, and they do allow for the fitment of the 845cc sleeves.

With respect to cylinder head castings, the earlier 61X head casting does not have enough wall thickness to allow for reliable sealing on the 89mm diameter, and 760 heads have a stock dome design that does not lend it well to the 845 modification. However the stock “64U” 1996+ SJ head casting has a dome design that is ideal for modification to 89mm, and it also has the casting ”beef” to maintain a lasting seal.

What this means is, anyone wanting the 845cc big bore must get a 62T or 64X cylinder, and a 1996+ 701 “62T” head for the project. For customers who do not want to hassle the process of finding these parts, Group K will have good condition used core cylinders & heads available.

Why Not Go To Piston Diameters larger than 89.0mm? – To be sure, a sleeve and piston set can easily be made for diameters over 89.0mm, but the installing of a large diameter piston, or a thicker wall sleeve will cause several serious weakness areas that have a very high reliability risk.

Sleeve Thickness – The base sleeve (that extends into the crankcases) of our 845 cylinder has a 2.2mm (.090″) wall thickness when the 89.0mm piston is fitted. While this wall thickness is a bit on the thin side for an 89.0mm bore, our sleeves are made from a very tough high-nickel content alloy that offers excellent long term reliability. That said, boring this sleeve to a larger diameter would result in sleeve wall thickness that would be ” risky” ….. no matter what material was used. To fit bigger pistons would require the use of thicker wall sleeves.

Cylinder Casting Strength – While the 62T cylinder casting is beefier than the 61X casting, it still has it’s limits. To fit our 845 sleeves into the 62T cylinder (for 89.0mm pistons) , we remove about 4mm from the stock casting diameter that holds the sleeves. Boring more material from the 62Tcasting (to fit a larger yet sleeve) will greatly increase the risk of the casting fracturing from the sheer interference fit needed to properly hold the sleeves in place. In addition, the casting wall will become so thin that merely torqueing the head down against the sleeve can crush down the very thin remaining cast material that is holding the sleeve. This results in a non-stop re-occurrence of head surface leaks as the thin casting material slowly crushes more and more with each head installation torqueing.

For engines that will be run for very short periods of time, and be torn down often for inspection, these technical problems can be “managed”. However, for owners that enjoy riding for long hours between teardown service intervals, setups running bore diameters larger than 89mm will be at high risk to be a very problematic setup.

Transfer Port Area – When the 62T casting is bored to fit the 845 sleeves, there is a measurable amount of transfer port volume that is lost. Losing this transfer port area might start to be a performance problem for an 89.0mm setup that was intended to make big power at very high rpms. However for the lower rpms of Free-style type riding, the available amount of transfer port area is still up to the job of properly feeding the 89.0mm bores. If larger diameter sleeves are installed, even more transfer port area is lost. This is a big problem because it asks transfer ports that are being increasingly reduced in volume, to feed cylinders that have greater and greater transfer volume needs. We stopped at 89.0mm because we felt that was the limit of being able to “properly” feed the cylinders with the transfer port area at hand. Bigger bores, with even smaller transfer passages, was a technically “very bad” idea.

Exhaust Manifold & Head-Pipe – The 62T/64X cylinders have the same bolt mounting pattern as the 61X cylinders, but the 62T/64X cylinders utilize 10mm bolts instead of 8mm bolts fitted in all 61X cylinders. Even though the 62T/64X cylinders have the same exhaust manifold bolt pattern, the “exhaust port location” within that bolt pattern is different between the 701 & 760 castings. This means that anyone who has the popular FPP “B” pipe on their 701, will have to either purchase a new FPP 760 exhaust manifold (and 10mm bolts) to match the different 760 exhaust port location, or have Group K modify their 701 exhaust manifold to match the port window sizes and locations of the new cylinder.

An added detail is the big variation in port diameters on the FPP manifolds and head-pipe. The early FPP SuperJet pipes had an exhaust manifold/head-pipe inside diameter of 43mm. While this diameter offered enough exhaust port area for most 701s, it seriously choked big-bore versions of the SJ engine. In later years, FPP increased this diameter to 46-47mm. For our 845 kit, we increase the diameter to 48mm. This means that for each 845 kit, we must also have your FPP head-pipe for diameter increase modification.

Porting Considerations – One difficulty involved with any big boring modification is the slight reduction in transfer port volume that takes place when fitting a larger sleeve. This is a particularly important issue for the 845 because, of course, the same transfer port passages are being asked to fill a larger displacement cylinder. While developing the 845 kit, a big concern was making every effort to maintain the maximum amount of transfer port area to insure that the new larger bores get an ample supply of fuel mixture. To help accomplish this we kept sleeve thickness for the 845 to a minimum, so that only a small amount of material would need to be bored from the stock cylinder casting to fit the new sleeves. In addition, transfer port shapes were altered slightly to further assist with good cylinder filling. Finally, a boost port hole was added to the rear of the piston to increase the amount of mixture being delivered by the rear transfer port. This boost port hole also has an important second purpose. With each inlet stroke, this boost port hole (located as close as possible to the crown of the piston) allows cool inlet gases to pass across the underneath of the crown of the piston, helping to significantly reduce the piston crown temperatures of the big 89mm piston.

Head Sealing – While “O” ring heads are a very popular technical fashion for many closed course racing platforms, sealing the large 89mm bore diameters of the 845 is a tall order for any head, much less an “O” ring head. We built the 845 kit to utilize an all metal head gasket in order to offer the widest possible sealing surface area around the bores. In addition, the sleeves of the 845 are manufactured with a very wide sealing rim on the top flange to further increase sealing area width. This arrangement allows the 845 to have a head sealing surface width that cannot be matched by any “O” ring type head setup.

Many owners understandably want a “girdle” type cylinder-head to help reduce the possibilities of fracturing the cylinder casting (as happens to numerous modified 61x cylinders). It bears noting that the 62T and 64X cylinder castings have additional casting supports, and are thicker in the areas where the 61X cylinders routinely fractured. Even with this extra casting mass, we felt that girdling support for the 845 was still in order. Since virtually all cylinder fracturing takes place on the inlet side of the cylinder, we opted to use a “Half-Girdle” that bolts on top of the head on the inlet side only. This half-girdle offers as much support as any “O” ring girdle head, and still allows the use of the stock 62T head casting. It bears noting that this girdle makes for very close clearance between the girdle plate and the flame arrestors of a dual 38 setup (dual 44s have plenty of clearance). To resolve this clearance issue, the dual 38s must be fitted with extended flame arrestor adaptors that raise the connection for the pods safely away from the girdle plate.

About Pump Gas Compatibility – Understandably, everyone wants to get the most power that can be had, while still running (readily available and more affordable) 91 octane pump gas. However the term ”pump gas safe” means something different to everyone. In the case of the 845, it has been developed as a “pump gas safe” free-style arrangement. This makes the assumption that it will regularly and repeatedly be accelerated very aggressively for relatively short bursts (as freestyle is). Under this kind of riding scenario, the 845 can operate happily on quality 91 octane fuel. However if an owner decides one day to go closed course racing, or just make a long peak rpm on glass water, 91 octane may not be enough to control temperatures. The 845 is mechanically sound enough to do sustained high rpm operation, however that kind of operation generates a heat level that cannot be supported by 91 octane fuel. For 845 owners that are planning an “exceptionally hard” day of high rpm operation, a 50/50 mix or pump gas and race gas will easily be able to manage with the higher operating temperatures.

Pump Setup – Most freestyle riders are very particular about every part of the setup of their machines. We will not claim to be proficient at setting up or evaluating “all” of those tiny details of a freestyle machine. However we do claim to be extremely proficient at setting (what we consider) the number one most important aspect of any freestyle boat …. Impeller pitch.

By far the biggest problem we have seen during our testing with freestyle machines (big bores and others) is that most freestyle boats are over-propped …. That is, they are running much to steep of an impeller pitch. We have repeatedly seen free-stylers that have selected a “too-steep” prop for their machine, and then proceeded to make other modifications in an effort to recover the instant acceleration that they are seeking. These mods include excessively high compression ratios, excessive spark advance, aluminum flywheel ignitions, etc. Doing these “two wrongs to make a right” mods on a setup that is “over-propped” results in a setup that is temperamental, unreliable, and often difficult to tune …. we see it a lot.

Having too steep of an impeller pitch results in other tuning problems that go beyond the easily understandable loss of low end acceleration. The worst of these is that the ideal high-speed carb setting is very difficult (if not impossible) to find on an over-propped setup. The most accurate means of finding the best high speed adjuster setting is peak-speed smooth-water runs using a digital tachometer … the best high speed adjuster setting always yields the highest sustained peak tack numbers. On an over-propped setup, the engine cannot get close enough to it’s ideal “target peak” to show clear rpm differences from one carb setting to the next. In this situation, it is common for high speed screw settings over one full turn range that will make no change in peak rpm at all.

For owners opting to use different pumps, props and carbs from what we specify in this document, the process for perfectly setting up the correct impeller pitch is covered in the tuning instructions with your kit. The most important item to accent here is that the 845 offers power and acceleration that is unmatched by any other pump-gas SuperJet setup ….. when it has the correct impeller pitch setup. Having this correct pitch is one of the most important components of getting the 845 to deliver it’s best performance. After all our prop testing on the 845, we got the best results (in the stock SuperJet Pump) with a re-pitched Solas Concord 13/17. The Concord hub and blade design offers excellent hook-up at all engine rpms, and still nets very respectable peak water-speeds. The 13/17 is the mildest pitch in the Concord series of Solas props, and (out of the box) it is too steep for “any” SuperJet application, including the 845. Group K will offer the Solas Concords already re-pitched to suit the 845 kit.

About Customized Props – There are plenty of folks customizing props …. “set-back”, “cut-back” , “trimmed” etc. Unfortunately, there is no standardized “language” for what all these different terms mean from one shop to the next. While some of these mods have some merits, they can affect engine loading and smooth-water peak tach numbers in a way that is not helpful.

There are two customizing mods that are most common. The first is machining the prop to sit closer to the vane body (different shops have different names for this). The intended result of this mod is to improve rough water hook-up … and for the most part it works well. The only down side to this mod is that it loads the engine harder at all speeds, and usually results in a drop of peak rpms (as measured in a smooth-water tach test). In our testing, we found that we had to slightly reduce the impeller pitch of these modified props to recover those lost rpms.

The other popular customizing mod is cutting the impeller blades to give them a slightly different leading/trailing edge profile. Regardless of how this blade cutting is done, it generally results in slightly less blade overlap, slightly less blade surface area, and slightly less engine loading (i.e. higher rpms). Our test riders had very mixed feelings about this mod, and the benefits are very hard to measure. From a sheer technical standpoint, we got our best overall results with full length impeller blades, and precise pitching that netted us the ideal target peak rpm. In the end, we found that, for the 845, correct pitching consistently netted better overall acceleration results any of the custom mods.

Carburetion – Since the 845 has such huge bores, it also has more “inlet signal” strength than any other SuperJet engine package ever built. In layman’s terms, inlet signal strength is the actual vacuum that the crankcase uses to draw fuel through the jetting circuits of the carbs. This vacuum is generated by the upward movement of the pistons. Having a lot of “signal” is a big advantage, because stronger signal results in less temperamental carburetion, and less fine tuning for changes in altitude and weather conditions. The strong signal of the 845 kit allows for very precise jetting of nearly any carb you install on it, and it will require very little fine tuning down the road when weather conditions change significantly.

The stock SuperJet comes with dual 38mm SBN Mikuni carbs. Group K routinely modifies the throats and jetting of these carbs to increase air and fuel delivery for modified engines. These modified throat 38s are a very popular alternative for 701/760 SJ owners wanting to get the best of both performance and fuel range, and they also work very well on the 845 kit.

During our testing, we also used the larger 44mm carbs (and manifold) that comes as original equipment on the 760 Yamaha engines. While we consider these carbs to be a bit too large for a 760cc application, they worked perfectly on our 845 kit. Since there is such a big focus on getting enough fuel and air to the 845, these oem 44 carbs are a very practical and effective choice for owners that want to get the most from their 845 kit. Unfortunately, the 44s do have considerably higher fuel consumption than the modified throat 38s. Given that, owners who have a high priority for fuel range should consider fitting their 845 with the modified 38s.

About Aftermarket Carbs – There are many aftermarket carbs (and manifolds) that fit the 1996 and later “62T cased” SuperJet engines. Among these are numerous custom modified 46mm and 48mm versions (Novi, Full Spectrum, RedTop, Buckshot, BlackJack, etc). The majority of these carbs gained popularity in high-rpm closed course applications where high rpm power is the top priority …. And precise partial throttle metering is not. Among all these carbs, the only ones we consider to have “acceptable” partial throttle metering abilities are the Novi, and Full Spectrum….we consider all the others “not acceptable for freestyle use. All that said, we also strongly question the benefits of “any” 46mm or 48mm carbs on the 845. The stock throat 44mm SBN Mikunis do a fine job of delivering all the mixture needed, and still offer excellent precision of fuel metering. If you opted to use larger carbs on your 845, the jetting calibration work would be up to you.

For owners wishing to “put together” their own dual carb kit, it’s worth noting that all 44mm SBN Mikunis are not the same. As best we can determine, there are 5 or 6 different versions that were imported to the USA. The big difference between these 5-6 types is the transition circuit layout (the “transition” orifices are in the throat of the carb just downstream from the opening edge of the butterfly). Each of these different transition arrangements meters a different amount of fuel throughout the full range of throttle openings. This means that each version would need to have considerably different overall internal jetting to net exactly the same total fuel delivery. The most common of the 44mm SBN transition arrangements is the 4 hole pattern that is common to the 1990-1993 650 SuperJet carbs (the 701s were very different). We have jetting data for this 4 hole “650” spec carb, and for the oem 760 carbs. If you opt to use carbs with a different transition arrangement, the jetting calibration will be up to you. It’s worth noting that about half the SuperJets we see equipped with aftermarket 44mm carbs, have a different transition layout between the two carbs …. This virtually guarantees that the engine will never carburate properly.

About Single Carb Setups – For reasons of desired simplicity and ease of tuning, some freestyle owners have converted their 62T cased engines to a single carb layout (running a single carb from 44mm – 48mm). Even though this is a setup that Yamaha has adapted to some oem models, we consider this to be a very poor choice for the 845 or any other any other freestyle setup for that matter….. here is why.

As mentioned above, maintaining the strongest possible inlet “signal” to the carbs is crucially important for any freestyle setup that is expected to have instant acceleration and precise fuel metering. Unfortunately, the “two into one” inlet manifold used for the 62T single carb conversion brings with it a huge increase in total inlet tract volume (much more volume than the individual runners on a dual setup). The increased volume of this manifold dramatically reduces the signal that is so badly needed for instant throttle response and precise fuel metering. Yamaha engineers were fully aware of this problem when they designed the very low volume 2-into-1 manifolding of the stock (61X) 701 crankcase/manifold setup. While this earlier 61X single carb case design is not “chic” technical fashion for hardcore free-stylers, the 61X cases are definitely the lowest-volume and highest-signal single carb manifold setup available. Even though the 61X reeds are slightly smaller than the 62T reeds, that difference does not out weigh the difference in inlet signal. The larger reeds of the 62T motor may be an advantage on an extremely high rpm closed-course racing package, but no freestyle setup needs to turn up into that extremely high rpm range.

All that said, we contend that the exceptionally strong inlet signal of the 845 makes dual 44s as easy to keep in tune as any single carb setup. Can a single 46/48 on 62T cases work okay on the 845??…. Likely so… would it equal the response or acceleration of well jetted duals?? …. No way.

For owners that are absolutely dead set on running a single carb setup anyway, we would strongly recommend they consider using 61X cases instead of the 62T cases. It bears noting that on our single carb 61X packages, we modify the stock 61X manifolds to further reduce inlet manifold volume and increase signal …. It makes a difference in throttle response that can be easily felt on any single carb engine setup.

Ignitions – The only ignition mod that we mandate for the 845 is the rev limiter removal. The stock cdi has a limiter at about 7050rpm, and the 845 peak rpm target is between 7200-7350rpm. The most reliable and affordable of the rev limiter modifications is the ProTec cdi box modification that we used during all our testing. Other aftermarket ignition boxes can be used with the 845, so long as they do not carry significantly greater ignition advance …. especially above 6000rpms.

The 845 creates so much sheer acceleration that lightweight flywheel ignitions are not really needed. Group K does offer an ignition flywheel lightening modification for the stock flywheel. We remove the greatest amount of weight possible without compromising the long term strength of the stock charging flywheel. While this mod makes a noticeable improvement in the quickness of throttle response, it is by no means as light as an aluminum (charging or total loss) flywheel …. But on the 845, an ultra-light ignition flywheel is not required to have instant and violent acceleration.

Cooling System – During all our 845 testing, we retained a single input cooling system. Included in the 845 kit is an additional water bypass on the cylinder head that helps to offer more even cooling of both cylinders. We also fit each exhaust manifold with a dual input “T” arrangement that also helps to more evenly cool both cylinders. Many owners have boats already set up for dual waterline inputs to the exhaust manifold. The dual inputs work fine on the 845, but are not required.

Pipe Cooling – Exhaust pipe cooling, and exhaust plumbing is a very important part of the 845 kit. The FPP “B” pipe head-pipe comes with three adjustable water input screws (we refer to them as top, middle and bottom). The primary water input point for all SuperJet setups should be the bottom screw (1/2 to ¾ turns open from bottomed). The middle screw should always remain completely closed. Opening the top screw has the ability to induce a large improvement of bottom end power, but opening this top screw also results in a significant loss in peak rpm ability. The 845 has such abundant bottom end power that opening the top screw is virtually unnecessary, and questionably beneficial. We conducted all of our 845 testing and development with water input at the bottom screw only, and we strongly recommend that setup to most 845 owners. We have observed that many freestyle riders run their machines with the bottom and middle screws closed, and admitting all the water at the top screw. We consider this to be a very bad setup for several reasons. Since water admitted at the top screw seriously harms peak rpms, it is virtually impossible to do meaningful and accurate impeller pitch testing, or high speed carb tuning while the top screw is open. In addition, the admission of water at the top screw causes much higher back-pressure in the exhaust that results in a slightly increased detonation risk (when compared to admitting the same amount of water at the bottom screw).

All in all, we recommend to open the top adjuster as little as possible (on any SuperJet setup) to get the bottom end acceleration you are after.

We conducted all our 845 testing with a Jet Worx pressure valve in the cooling line that feeds the pipe stinger. This valve helps to reduce the amount of water being admitted into the waterbox at lower engine rpms. The result is much improved acceleration and response during lower rpm maneuvers. We strongly recommend this valve for any 845 owner.

Lower End Mods – To accept the larger sleeves of the 845, the crankcases need to be bored slightly. The amount of material removed is small, so there is no significant compromise in the strength of the crankcases themselves. The only snag caused by the larger sleeves is that the base gasket does need to be trimmed slightly to accommodate the new larger diameter sleeves.

Since there is considerable mismatch between the transfer port openings on the cases and the cylinder, case porting can offer a clear improvement in performance at all engine speeds on the 845. Note that case ported engines may require some custom trimming of the base gasket to assure it matches the enlarged transfer base openings. We consider case porting a great option for the owner that wants to be sure that he is getting the very most power available from the 845 … however we do not consider it a mandatory modification for good performance. All our prototype testing was done on a machine without case porting.

Crank Welding – Crankshaft “truing and welding” has long been an upgrade favored by higher performance SuperJet owners. The scenario where it is most needed is in a high rpm engine that is run at peak rpm across rough water. The loads on the crank journals are huge when such an engine is at the rev limiter, when the pump makes a sudden “hook-up” connection to the water. The rotational loads on the crankshaft from that kind of rough water riding “can” twist the press fitted journals of a stock Yamaha crank. However very few free-style setups turn those very high rpm’s of a closed course setup, and very few freestyle riders are as abusive to their drive trains as the average closed-course racer. For most freestyle riders, a stock un-welded Yamaha crank will never experience any rotational “twisting. However exceptionally aggressive or abusive riders may still wish to error on the side of caution, and have their crank journals welded.