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Articles / Racing / Yamaha Stroker Engines

Yamaha Stroker Engines

04 November 2019
Article Yamaha Stroker Engines

PREFACE - During the later part of 1993 and early 94, numerous performance shops nationwide started selling Yamaha stroker cranks and components. By late spring, thousands of Yamahas with these stroker parts were run for the first time. In short order, many stroker owners found that their $2000 - $6000 stroker was no faster than their limited boat. The phones of Group K were besieged by unhappy stroker owners looking for someone who would "fix" their slow strokers. These owners told us they had been "blown off" by the folks that sold them their stroker parts. Our testing, in 1993, led us to believe that there is no "reasonably priced" way to make a stroker run significantly faster than our 55 mph 701 Hammer kits. It was not practical to describe the technical reasons to each caller over the phone. Instead of wasting our technician's time on these lengthy and fruitless phone calls, we produced the following pamphlet that could be mailed. The sellers of stroker engines did not like the pamphlet (a lot), however to date none of these shops has offered to resolve slow stroker motors from other shops.

ADA of Phoenix AZ, has linked their site to this document in an effort to discredit the information. Their respondent document "The Truth About Strokers" makes many weak and half true observations. It's context is often adolescent and vaguely worded. While most of the points in their document don't dignify an intellectual response...a few do. Our responses to those points are at the end of our article here. Happy Reading. Harry Klemm


In 1993 the IJSBA set the displacement limit for all modified classes at 785cc. The largest displacement Yamaha engine at the time was the 81mm bore x 68mm stroke 701cc Waveblaster motor. Almost immediately several performance shops started building versions of this 701 engine with an increased stroke. The most widely used has been the 73mm stroke (5mm over stock) that has resulted in the 82mm x 73mm (770.6cc). All these shops assumed that an increased stroke motor (also called a "stroker") would obviously be much more powerful and offer a significant competitive advantage.

The unfortunate reality, however, has been that the strokers are only sometimes better than a well prepared 701cc standard stroke engine. This is not because the builders of these 770cc stroker engines are incompetent or inexperienced. It's because the 701cc Yamaha engines lend themselves very badly to this modification.

At Group K we had many happy 701cc Hammer kit customers who requesting that we build them a 770 stroker. So we tested at length with our own 770 stroker prototype, in hopes of coming up with a competitive engine package to sell. After months of testing, we realized that the 770 stroker concept was wrought with many technical problems. We felt that it would be virtually impossible to build production units for sale to our customers that would be significantly better than their current 701cc Hammer kits. The following is an accounting of the problems we encountered. Some problems were matters of excessive expense, and others were a matter of defying the laws of Group K we avoid both of these.

THE THEORETICAL ADVANTAGES - From a theoretical standpoint, the displacement increase of the 770 stroker engine should have one fundamental advantage. That is, increased torque output through out the entire rpm range. This theoretically happens because of a larger volume of gases being involved with each power stroke, as well as a slightly longer power stroke. Understand that with regards to a two cycle engine, the "actual power stroke" refers only to the distance from top dead center to the opening of the exhaust port.

Along with these advantages, each engine builder understands that there is one premier disadvantage of a stroker...reduced peak rpm abilities. It is the hope of each engine builder that the trade off of increased torque vs. decreased rpm ability will eventually favor the stroker engine. Unfortunately for the 701 Yamaha engine, other pre existing design problems make the positive side of that trade off very difficult to realize. Understand that for any engine package to be considered truly "better", it must offer a significant power output advantage along with a maintenance cost that owners can afford to live with. Engines with short fuses do not qualify.

REALITY - This is much easier for the layman to grasp. In the real world, the only thing that matters is how many rpm can a strong accelerating engine turn a given prop pitch and nozzle diameter. In the August 1994 issue of Watercraft World Magazine, Group K prepared a 701cc Super Jet as a Limited, and then a "105 octane Hammer kit". Using the stock nozzle and a Skat Trak 10-18 variable pitch prop, the Limited ran 49.3mph @ 7000 rpm and the Hammer engine kit (at $1732) ran 54.5 mph @ 7300 rpm. Our tests confirmed that Wave Blasters with these same engine setups will net the same peak rpms. However, the Blasters equipped with these kits and the stock 15-18 Blaster prop, runs about a half mph slower (a result of the Blaster's additional hull drag). The next steeper prop for both machines is an 11-19 pitch. The Hammer kit and the Limited both lose an unacceptable amount of low end with the 11-19 pitch prop.

It would seem obvious that the 770 strokers would easily pull the 11-19, right? Not exactly. During all of our months of 770 stroker testing, our prototypes were never able to pull an 11-19 beyond 6900 rpm (51 - 52 mph). On testing done with numerous "over the counter" 770 strokers, we have found similar results. If 770 stroker engines could effectively pull an 11-19 into the 7300 rpm range, the makers of these engines would recommend that prop...THEY DO NOT! In fact page 34 of the October 1994 Splash magazine states that the Pro Tec 770 stoker engine (at $5750) will only turn a 10-18 prop to 7100 rpm. Being generous, that would drive a Super Jet hull to the 51 - 52 mph range.

At Group K we are not surprised. Our best 770 prototypes never exceeded 7200 rpms and never pulled an 11-19 with good overall acceleration. However we are surprised by the comparison of a $1732 / 7350 rpm engine package and a $5750 / 7100 rpm engine package.

It bears noting, that the owner of the Group K magazine boat used it to win the Busch Cup novice vet championship. He got nearly every hole shot, and reported never once being out powered by any Super Jet. Many of his regular competitors had fuel injected 770 strokers.

The April and May '95 issues of Personal Watercraft Illustrated featured the construction of a project 770 Blaster stroker. The finished machine, built from over the counter modification parts, cost over $9000 to build...and ran only 52 mph. As many Blaster riders know, a good limited can run 52 mph. This Blaster used a 17-23 prop, which would have put the peak rpm at about 6800. The editors and technicians involved with this project prepared the machine correctly. They were just forced to learn what many stroker owners already know...the results don't always match the money and the hype.

In all fairness, many 770 strokers (our prototypes included) have the ability to "leave harder" than a 701 Hammer. "Leaving hard" refers to the acceleration ability from an (IJSBA type) standing start. This type of start puts a tremendous load on the engine, since accelerating from low speed and "coming up on plane" are happening at the same time. On small Super Jet hulls this is not a big problem, however on the larger "sport" and "runabout" hulls, it can be. After our months of 770 testing, we simply couldn't justify an additional $4000 - $6000 expense to our customers just to make a race boat "leave a little harder".


AVERAGE PISTON SPEED - For each engine there is a theoretically "safe" maximum rpm. This theoretical peak is measured in the average feet per minute that the piston travels. However engineers and engine builders refer to this limit in "feet per minute of average piston speed", not rpms. They have found that exceeding an average piston speed of 4000 feet per minute, on high output racing engines, is very risky business. There is a formula used to calculate the rpm that equals 4000 fpm. The primary variable of this formula is the engine's stroke. Longer stroke engines have higher average piston speeds, and therefore, lower rpm thresholds. When an average piston speed of 4000 fpm is exceeded, the risk of component stress failures increases almost exponentially. Failures including, but not limited to, broken crankshafts or connecting rods, separating pistons, breaking rings, disintegrating lower end bearings etc. Operating an engine at 4000 fpm stresses all the moving pieces to their limit. Race engines that are run at 4000 fpm revs have component replacement intervals rated in operating minutes...not hours.

An added complication for PWC race engines is that they have direct drivelines with no transmission device. For this reason, pwc race motors run significantly higher compression ratios compared to most other high output racing engines. These higher ratios are needed to maintain strong low speed acceleration characteristics in the absence of a transmission. Unfortunately these higher ratios also reduce the "safe" average piston speed by 200-300 fpm. Another complication is the relatively high percentage of "not hooked up - non cooled operating time". When the pump of a race boat comes out of the water, it not only permits much higher rpm, but it also ceases delivering the badly needed coolant to the engine. The added stresses of unloaded rpm, and the higher temperatures of intermittent coolant supply further diminish the "safe" peak.

PWC engine builders understand that the 4000 fpm average piston speeds that work for tuners in other motor sports has been pared down to 3500 fpm neighborhood for their PWC race engines. They also understand that 3500 fpm is the high stress limit. If they want any kind of reliability, they must choose a lower limit. The following is a listing of the average piston speeds of the 68mm std stroke Yamaha 701cc engine vs. the 73mm stroke 770cc aftermarket engines.

------------------------------7600 rpm -------7400 rpm ----------7100 rpm -----------6800 rpm

68mm stroke --------------3389 fpm-------- 3300 fpm ---------3166 fpm -----------3032 fpm

73mm stroke --------------3640 fpm --------3544 fpm ---------3400 fpm----------- 3257 fpm

It's easy to see that the 68mm stoke engine has a much bigger margin of safety away from the 3500 fpm limit. This bigger margin would not be a big deal if the 770 strokers could pull an 11-19 or 12-20 prop. The ability to pull the steeper props might eliminate the need for turning allot of revs. However as previously noted, over the counter 770's cannot pull these props.

CONNECTING ROD LENGTH - The connecting rod is the primary piece responsible for converting the reciprocating motion of the piston into the rotational motion of the crankshaft. There will always be a certain amount of energy lost in this motion conversion, but additional rod length can help to minimize that loss. When a high output engine has the stroke increased, rod length is an important issue that shouldn't be ignored.

Stroke increases are accomplished by moving the crank pin location away from the center of the crankshaft. A side effect of this modification is that the connecting rod angle at 90° btdc and 90° atdc becomes steeper. This steeper connecting rod angle means that the rod will be pushing the piston harder against the cylinder bore on both the downward power stroke and the upward compression stroke. This extra piston pressure against the cylinder wall results in more rapid piston and bore wear, not to mention additional heat from the surface friction.

An easy way to reduce these problems is to simply use a longer connecting rod. The longer rod can significantly reduce the connecting rod angle, and the wear that comes with it. Unfortunately, none of the 770 strokers currently being built for sale are fitted with longer rods. Installing longer rods on the 770's would require the use of a very thick spacer plate between the cases and the cylinder. Most engine builders avoid using a spacer plate like this because it significantly increases the crankcase volume. Increasing the crankcase volume reduces the crank case compression ratio. High crank case compression ratios are needed to rapidly deliver the raw gases, in the lower end, through the transfer ports into the combustion chamber.

MAXIMUM PISTON SPEED - Another disadvantage of using the , relatively short, stock 701 rods on a 770 stroker is the terrific increase in the "actual maximum" piston speed. Calculating the average piston speed, as shown above, is an effective reference tool for engine builders. However it only indicates an "average speed" not the "actual speed". In reality, the piston is stopped at bottom dead center before it accelerates rapidly upward. The piston then accelerates to it's maximum speed, which takes place when the crank pin is 90° from bdc. As the piston passes the 90° position and approaches tdc, it begins decelerating rapidly until it comes to a full stop at tdc. While the average piston speed may be calculated at 4000 fpm, the actual maximum piston speed at 90° past tdc is about 10,000 fpm (or about 120 mph).

Coincidentally, this maximum piston speed and the maximum rod angle take place at exactly the same moment. This particular moment, where piston speed is at it's highest, is the worst possible moment for the additional friction of a steep rod angle. A longer connecting rod would not only reduce the rod angle, but at the same time, it would actually result in a slower "maximum piston speed" at the moment of 90° btdc and 90° atdc. This reduced "maximum piston speed" would not only result in less surface friction between the piston and cylinder bore, but it would also result in significantly longer piston ring life and ring sealing.

It bears noting that the 74mm stroke Kawasaki 750 engines are equipped with significantly longer rods than the stock Yamaha 701 rods. These longer rods in the Kawasaki are one of the design features that permits the 750 Kawasaki race engines to rev reliably into the 7500 rpm range (that's 3637 fpm average piston speed).

CYLINDER PORT LAYOUT - When the stroke of an existing engine is increased, the port timings automatically become more advanced. That is, the port timings are actually increased by sheer virtue of the changed connecting rod geometry. This automatic increase in port timing can be a benefit on cylinders with uncommonly mild port timings (like the 633cc Yamaha cylinders). It just means that you have to do that much less grinding to arrive at the proper port timing. However on cylinders that already have very aggressive port timings, this automatic increase in port timing can easily put the stock port heights beyond the desired range. This is the unfortunate case with the 701 Blaster and Wave Raider cylinders. A 770 stroker with a stock 701 Yamaha cylinder has a more radical exhaust port timing than the 7300 rpm Group K 105 octane Hammer kit. That is to say, it has allot more port timing than it needs. If the exhaust port on the 701 cylinder could somehow be lowered a few millimeters, it would end up with the same port timing as the Hammer kit cylinder. However the real benefit of this lowered exhaust port, would be a much desired increase in the length of the power stroke. (remember that the longer power stroke is one of the theoretical objectives of building a stroker in the first place) Since lowering the exhaust port of the 701 (or Raider) cylinder is not do-able, the stroker engine builder must choose between having a shorter than optimum power stroke, or starting out with the much milder ported 633 cylinder. While the bored out 633 cylinder can be cut to ideal port timings, engine builders who choose this cylinder will unfortunately get their fair share of problems as well. The 633 cylinder casting has very little spare material around the exhaust port. So little, in fact, that the exhaust port cannot even be made as wide as a stock 701 exhaust port. This is much less than the optimum width that a 770cc race motor would demand.

DECK HEIGHT - If dealing with the port layout problems wasn't enough, there is the added problem of the piston that now pokes out of the top of the cylinder at tdc. To say the least, this is not a desirable arrangement. The only option is to put a spacer or thicker base gasket between the crankcases and the cylinder. Unfortunately this will again increase the port timings and shorten the power stroke. A spacer that is thick enough to get the piston crown back down into the cylinder, is unfortunately also thick enough to put the port timings of the mild 633 cylinder at the raged edge of acceptable. This same spacer (between 1.5 - 2.5 mm) will put the port timings of a stock 701 cylinder into the twilight zone. Under other high output circumstances, port timings this radical might be acceptable. However we must remember that the theoretical advantage of a stroker is increased overall torque. It's difficult to accomplish that with road racing style port timings.

Perhaps the most effective way to deal with all of these problems would be to make a new 770 cylinder from scratch. Such a cylinder could have ports located at the right height, as well as the additional deck height to accommodate the added piston stroke and some longer connecting rods. Unfortunately a special purpose cylinder like this would further increase the price of an already too expensive engine.

It was at this point in our 770 stroker development project that we realized the 770 design had allot of problems...big problems. Furthermore, all the available solutions had equally big problems of their own. From this point we realized that any "over the counter" 770 stroker engine, made by Group K or any one else, was going to contain a collection of very poor technical choices. It appeared unlikely to us that such a race engine could be reliable...much less competitive. We have not yet seen any over the counter 770's that can run 55 mph in a Super Jet or Blaster hull. All the over the counter 770's we have seen are slower, less reliable, and much more expensive than our existing 701 Hammer kits. As a result we do not (and will not) sell any 770 engines or components.

ABOUT THE NEW 1996 753 cc RAIDER MOTORS - These new 84 x 68mm top ends offer an interesting option for the 785 class racers. It's very likely that this new top end can be adapted to most existing 701cc lower ends. However, more importantly, with a stroke increase of only 2mm they will yield a 775cc displacement. This 775, bored to the 84.5mm 1st oversize piston will yield a limit pushing 784.7cc. A Yamaha engine with only 2mm of stroke increase would not experience many of the technical problems outlined in this pamphlet. This shorter stroke will make the 84 x 70mm layout would suffer from fewer design problems and be potentially more reliable than any of the current 82 x 73mm or 82 x 74mm strokers. While this is speculation now, only testing during the winter of '95 will tell for sure.

FINAL THOUGHTS AND THE FINAL NUMBERS - The obvious question here is " How do 82 x 73mm Yamaha 770 stroker motors dominate the pro stand up and sport classes, if they only turn a 10-18 prop at 7100 rpm as claimed (theoretically 52 mph?) The answer is...the winning pro boats either turn allot more than 7100 rpm, and/or they are using a very different prop. Having an uncommonly talented and lightweight rider certainly wouldn't hurt. However, rider talents aside, there is no doubt that the winning pro 770 strokers on the IJSBA tour are probably capable of 55 - 57 mph. To do so, they would need to turn an 11-19 prop at about 7300 rpm or a 10-18 at about 7500 rpm. In either case, any 770 stroker capable of this output would likely require frequent replacement of all internal moving parts (crankshafts, pistons, rings, etc.) While this kind of maintenance might be affordable for a fully sponsored pro racer, it would obviously be out of the question for a reliability minded local level racer or weekend warrior.

At Group K we intent to continue development of our existing 701cc Hammer kits. We expect that we will be able slightly increase the rpms into the 7400 range. The resulting speeds (in Super Jet and Blaster hulls) should be in the 55-56 mph range. The Wave Raider hulls will likely run in the 62 mph range. These performance numbers may not be quite as good as the fully sponsored IJSBA national championship 770's. However, the 701 Hammer kit performance is still much better than the output numbers of any over the counter 770's being sold to the public. Even if new cylinders and longer rods are produced for the new generation of 770's, they will be unable to hold a candle to the price value of a 701 Hammer. That is...unless the laws of physics change in the near future.

"Some Additional Truth That ADA of Phoenix Should Consider"

Linking onto someone else's site, for the purpose of slamming it (as ADA has done), is really "asking for it" this case we are only too happy to oblige.

Item 4 of the ADA document discusses the limits of average piston speed. The ADA technicians should understand that engineers use average piston speed as a barometer for "all" the stress loads taking place with "all" the moving parts...not just the piston itself. While metallurgical advances in the last 20 years have certainly yielded better pistons, there has been little change in the physics of "skidding" rod bearings and excessive connecting rod angle. The 4000 fpm limit, cited in the 1973 Gordon Jennings book, IS NOT the "safe " maximum limit that ADA claims. 4000 fpm is the maximum piston speed at which the imminent failure of moving parts can still be "predicted". That is, you know that the crank will fail after so many operating minutes, and that the pistons will break if not replaced after every "x" number of operating minutes. All the road racing engines Gordon worked with were maintained in this fashion. Frequently exceeding 4000 fpm means a crank could last 2 hours or 2 minutes. ADA should also understand that while Gordon Jennings is very intelligent and experienced, the road racing engines he worked with never EVER "flashed-unloaded" to rpms well in excess of the engine's peak horsepower output (as all pwc race engines do constantly). Had this been the case, Gordon would have also recommended a buffer zone for anyone wishing to have "predictable" reliability of moving engine parts.

With a perspective toward the bigger picture, we would like to remind the technicians of ADA what the owners of high performance PWC (on planet Earth) are actually interested in. They want to go as fast as possible, for the least possible cost, and have the maximum possible reliability. Furthermore they want an accurate assessment of "exactly" how much speed they will get for "exactly" how much money. All of the Group K literature, and development work, is based on meeting these needs. We urge the web site builders at ADA to wake up to this reality. They would better spend their time posting specific price and performance information about "their" engines on their website. We think it's pretty silly for them to launch a website by taking pot shots at a company that sells reliable engines at 1/4 the price of their own.

For folks that don't already know, a complete ADA stroker motor sells for about $8000. We look forward to the ADA site showing comparisons between their $8000 stroker engines and the racing strokers sold by other shops (Riva, Pro-Tec, Power Factory, etc.)

As a footnote, we too encourage all Group K Hammer owners to go to ADA and take the Hammer challenge outlined in their website. However if you want your Hammer "upgraded" to an ADA stroker, you should bring about $8000 with you and prepare to have your boat run 2 - 3 mph faster...what a bargain!

By Harry Klemm -


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