Want evil 90 hp drag motor for 13' allison have score to settle!

I want the fastest 3 cylinder avalible. have heard stories of a merc sst 90. Who has one? I have a 90 hp yamaha 20" in full trim weighs 265lbs. Anyone ever built one of these motors? Based on my personal experience with the 90 hp nissan, high rpms(7500) are not good for the center main bearing.

Going to try to get the yamaha to turn about 7200
Im going to start by shaving the head and get the compression up to about 180lbs, raise the exhaust port timing about .045 clean up the burs, radius all the ports install larger carbs(2mm), remove inner liner, vent midsection, gonna need some help with the exhaust tuner, last ditch effort will involve nitrous oxide.
Also going to need a 27-29 pitch wheel to fit this rig
I need help?

[Rusrog]

WHile it seems that you have a good handle on what you are doing I wouldn't go too wild with the ports. Also remember that you can always go back in and re-cut if you don't achieve the desired result on the first try. Another thing, if you don't have adequate intake port area & timing, all of that exhaust relief is of no use. Look for ways to stuff the crank case, double check your port dimensions, look into having the recip assy lightened as that will help with the RPM issue as well as longevity. It sounds like fun but be careful.......
You might also look to Hydro Tec of Nixa Missouri for some help on this project. They have some good 3 cylinder stuff that might be of some help to you.

[Bruce Washburn]

Steve Coler (?) out of SC raced a 90 yamaha on a 13-14 critchfield. It was always very competitive. I believe that he had it on an old 60 (cly) mariner short shaft housing. The motor should run well. I know that Dale Eaves in Soddy Daisy also built a couple of 3 cyl. Yamahas several years ago that ran very well.

The SSt 60 may also be a motor that you would want to consider. Add a little compression and porting and it should fly. It is already on a 15 inch center section.

[LJL]

Old early 80s 2 cylinder Mariner 48, Mariner 60 and Yamaha 55 midsections will all fit. They are the old Merc/Yam motors, and have the same mid. You can bolt the powerhead to the short midsection without modifications. And the cowlings should fit the short mid.. So, its pretty easy way to get a short mid. The only problem is that the old mids don't have power trim. So, you'll have to make one. I believe the old Merc powertrim cylinders will fit the bracket pretty easy.

Once you're on it, you can change to the smaller gearcase (40 to 70hp). You'll have a couple of problems. First, the drive shaft splines won't fit from the small engine to 90 hp powerhead. The drive shaft is in the right position, so you'll have to cut the splines from the big (yam 90 orig.) gearcase, and weld them to the small g/c driveshaft. The other problem is that the smaller g/c is ofcourse weaker. Try to find a g/c from 70C or Pro 50 (large gearcase in the new models). I believe they have the beefiest gears available. The third problem will be running an enough dia propeller in that g/c. I don't know what props you can run in that g/c, but all 40-70 props seem to be 12 + inch dia props. The problem will propably be the distance between the prop shaft and cavitation plate. You can chop off the old cavitation plate, but you'll loose the opportunity to use the large aftermarket hydrofoils. Well, you propably can weld the cavitation plate a couple of inches higher to be able to use the hydrofoil. Also, finding a nose cone can become a problem. You'll propably have to make one yourself.

LJL

[David]

How did you disable the Nissan rev limiter?

disconnect the blk/yellow tracer loop on the cdi box. disables the rev limiter

what kind of rig are you running the 90 on?

I tore the motor down this weekend. one cylinder needs a new piston and boring. I think ill stick with the standard 20" for now the gearcase is a good strong one. for $250 bob will put one of his nosecones on it. How much compression can i run pump gas with? at 180 psi I was running 93 octane in the nissan without any trouble the carbs had been rejetted. looking at the yamaha heads I suppose i can can get pretty close to 190-200lbs without custom heads.
how much can i port the reed blocks? do i need a least 1/16 overlap? will a set of boysen reeds help?
and the question I shouldnt ask does any on suggest using wiseco pistons in a high rev motor.

[Rusrog]

I am putting myself in the line of fire but I firmly believe that if you have the Wiseco pistons lightened and cryogenically treated you will have a set that will last. Get your tolerances set and you should be in good shape. If you really want to protect yourself...consider having the pistons coated with the ceramic domes and the moly coating for the skirt.

[Jeff_G]

cryogenically treated? Could you fill me in on what that is supposed to do? Thanks

[Rusrog]

Cryo treating is a process where the metal is deep frozen and then the temperature is cycled up and down several times. What this does is normalize the material and condense the material at the molecular level. I had a website that gave much better and more detailed info on the subject. Basically it normalizes the material. I had my entire recip assy done including the crank. JAY SMITH can explain more about it and also provide the service.

your boat looks similar to the formula v rigs. only thing missing is the rollcage.

[Jeff_G]

Actually I am familiar with cyrogentically treating metal but have never heard of it being applied to aluminium pistons. On cranks, rods I prefer to stress relieve them instead. I also use a rockwell hardness tester on the pistons that we reuse.

The picture is a Formula V but instead of a cage I went the whole way and put in a capsule. The boat is a 1983 15' Allison with a modified Velden capsule and no canopy. Hell I'm clastrophic enough !
We put the capsule in before the rules mandated it.

[Jason Johnson]

Sixabove, I am doing the same thing right now. I have a short (13") midsection. The shaft is being cut right now. I have a light flywheel and a shaved head (.200) and rechamberd. It is hard getting info on porting. I am cutting stock cages for a set of TDR reeds right now. I wish somone was willing to help mod and intake for some dual carbs for me. I can get any yamaha parts I need (USA) that is. Good luck stay in touch mabey we can help each other.

[pyro]

Dude, check the dates. This thread is over six years old!

[usamarshal]

Snap!!!