Absolute Max RPM's on 2 stroke

Texasmark · May 6, 2006

Been a lot of yakin about WOT max rpm's and I have been surely involved as I bought this boat in December, '05 and it had a 17" pitch prop. The engine is rated for 5500 WOT upper end. I had to go to a 24" pitch to get the rpm's down to 5600 and this is an '02 engine (stock 90 hp 3 cyl) and the previous owner bought it with the 17 and used it 3 years. Now it ran 6250 rpm's with the first replacement prop I bought which was a 21". You aren't going to tell me that the previous owner didn't max out the throttle while he was using the boat and with that said the rpm's could have gone to 8000, and did it for 3 years and apparently it hasn't hurt it. On my first trip out, after buying the boat, before proping up, I left the dock and hit the throttle and almost got whip lash at the hole shot. Took about half a second from idle to planed out and hauling buttox. Tach hit 7500 before I could grasp what I was seeing and shut her down. ------------------------ So what is there to limit your rpm's? Everything in the power delivery system (crank, rods, bearings, pistons) is rock solid with no slip (to speak of) except maybe the piston rings (the wedge shaped ones) jacking back and forth on the pistons. So it seems that it boils down to air flow. If you can get the fuel/air to the reeds and the reeds can get it to the combustion chamber, and the pistons can push it out the exhaust pipe, she will run right on up there. Since the reeds only move like .020" and the length of the reed is at least 1" you are not stressing the metal so they should keep on going physically. I know that there is a point where they are moving so fast that they flutter and I assume that's their upper limit. Don't know. Info appreciated as I am sure that I am not the only one curious about this. Thanks, Mark

rodbolt · May 6, 2006

Re: Absolute Max RPM's on 2 stroke

where are you getting your reed stop information? as long as its cooling and lubricating two strokes love RPM until the point that the wristpins cannot stop and then accelerate the piston. I would double check that the tach pole setting is correct. the yamaha 90 2 stroke should be set for 12 pole/6 pulse. if its set on anything else the reading will be off. they will live a lot longer underpropped than over. what kind of hull is this motor on ? the propeller will govern engine max RPM, the no load RPM is rather high, I had a gear case failure on a 90 when it was on my dyno at 5000 RPM. I am not sure how fast it was turning but the needle had pegged when I was trying to shut it down. but do double check your tachometers accuracy.

Texasmark · May 7, 2006

Re: Absolute Max RPM's on 2 stroke

Picked it up on this site. One of the old salts (DHadley?????) was talking about how far reeds move. I was surprised as I thought they moved farther than that. This is an'02 Merc 90. On the tach, it's right on. You can do a prop check (www) and my current slippage is 8% at 52 mph, 5600 rpm's, at 40 degrees F, 700 ft. The boat is a 750# (dry) 17' alum bass boat with a 3 step padded hull. The third step is only about 10" wide and only about 3' of it are in the water. Rivertrail built in Clinton (as I recall), Ark. Mark

rodbolt · May 7, 2006

Re: Absolute Max RPM's on 2 stroke

you will find theory actually baffles math. double check your tach. also look up reed stop info from your service manual and post it. I think the reccomened WOT on that motor is 5250 but I cant remember. much above and stock reed life becomes iffy. however I have seen production stock run at 8500.

gss036 · May 7, 2006

Re: Absolute Max RPM's on 2 stroke

If you plan on running it with that high RPM, if it is that high?, I would be running 100% synthetic oil until you get it figured out. I had a 125 Yamaha dirt bike suck a lower end crank seal and ran out to about 12,000 rpm before I could get it to shut down. I was running "KLOTZ" synth racing oil at the time and myself and the mechanic feel that is what kept it from holing the piston or worse.

Texasmark · May 8, 2006

Re: Absolute Max RPM's on 2 stroke

Well, I am topping at 5600 now and I am not bothered with that. I am using some 100% syn and some of Merc's best oil in the vro, with sometimes adding some more to the tank; 100% syn is in the gearbox and is fresh. WOT range is 5000 to 5500 for this engine. I was just fishing for info from someone who may have wrung out their stock mil and was wondering where it ran. 12,000, now that's more than I expected. I figured they'd be good for 10 grand. Thanks, Mark Mark

Dhadley · May 8, 2006

Re: Absolute Max RPM's on 2 stroke

If we're just talking rpm then there's lots of ways to get an outboard to spin up. At certain rpm levels certain things start to be the limiting factor. But if youre just talking theory and rpm (with no rev limiter) here's what we've found -- Some stock steel reeds start to flutter in the mid 6000 range. Others will flow good right up to 7000. Most everybody will run fiber (not fibergalss) reeds of some sort as they approach 7000. Most stock stators start to give up in the low 7000 range. Stock OMC crossflow porting is a limiting factor in the low 7000 range. Stock 90* looper porting is good past 8000. Stock flywheels don't like much over 7000. I wouldn't use cast pistons over about 6700 but I know lots of folks who spin them 8000 +. But they do crack the skirts. Modified / race outboards have no problem spinning well over 10,000. Well over. A 2 stroke outboard is nothing more than an air pump. You just have to get more air in and more air out. It'll go.

Chinewalker · May 8, 2006

Re: Absolute Max RPM's on 2 stroke

Some model airplane engines will spin out over 20,000 revs. Alky modified racing outboards can run upwards of 14,000. The simplicity of a 2-stroke - gotta love it... - Scott

Texasmark · May 10, 2006

Re: Absolute Max RPM's on 2 stroke

Thanks guys. Chine Do remember the days of the model airplane engines. Boy were they ever screamers. D Thanks, figured you'd know. Thanks

tommays · May 10, 2006

Re: Absolute Max RPM's on 2 stroke

My R/C car does 44,000 + but it is rotary induction no reeds

its allways funny when a .21 is BIG BLOCK

tommays

Texasmark · May 11, 2006

Re: Absolute Max RPM's on 2 stroke

Hey tom, You mean it's a Wankel design and runs 44,000? I knew they were hummers, but boy, that's really a-hummin. Mark

Dave Abrahamson · May 11, 2006

Re: Absolute Max RPM's on 2 stroke

Rotary induction Mark. Not rotary engine. I also fly r/c. Have a neat little four stroke. .60 cubic inch. Pushrods and overhead valves. That'll never see 44000 rpm!!

They do make rotary r/c engines. .30 cubic inch I believe. Expensive little buggers.

Dave Abrahamson · May 11, 2006

Re: Absolute Max RPM's on 2 stroke

O.S. .30 Rotary 49-PI Wankel

Neato huh?

dajohnson53 · May 11, 2006

Re: Absolute Max RPM's on 2 stroke

How big is that thing? It is neat.

walleyehed · May 11, 2006

Re: Absolute Max RPM's on 2 stroke

.30 of a cube.... swings a 9-10" low-pitch prop.

waterinthefuel · May 11, 2006

Re: Absolute Max RPM's on 2 stroke

I also fly RC. The Wankel design is awsome. It's tiny, it can easily fit in the palm of your hand. But unlike the piston engine, it's smooth.

Dave Abrahamson · May 12, 2006

Re: Absolute Max RPM's on 2 stroke

It's about 3.5 inches from front to back and about 1.5 inches in diameter. It's little.

Texasmark · May 13, 2006

Re: Absolute Max RPM's on 2 stroke

That is a work of art. Who built it? On the .60 4 stroke that must be awesome to see also.....man that's tiny. Mark

dajohnson53 · May 13, 2006

Re: Absolute Max RPM's on 2 stroke

deleted already answered!

Dave Abrahamson · May 15, 2006

Re: Absolute Max RPM's on 2 stroke

O.S engines builds them...and lots more of just about any configuration you can think of. Twins, four cylinder opposed engines like full scale ones, radials, etc...

Absolute Max RPM's on 2 stroke

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