RPMs are 300 to high. What are my options?

[Otts]

I have a Suzuki 90 four stroke that should run at a max of 5500 RPMs. At WOT it is at 5800 RPMs. Will this extra 300 RPMs hurt the motor? Am I all right as long as I don't exceed 5500 RPMs? The boat is finally set up to run like it should. THey raised the motor and went to a 21 pitch prop, aluminum. Can they lower the RPMs by adjusting the throttle linkage?

Thanks for your help,
Otts

[T-Mac]

Personally, I would not be overly concerned about 300rpms when talking that 5500-5800 rpm range.
Once you load another passenger and some gear, you will lose a few of them. Plus, it would not be uncommon for a Tachometer to be off at least half of that.

[dave t.]

put the 20" pitch Stainless Suzuki prop on that boat and you'll be in the money.

[Hmmmmm]

If he is turning more rpms than he wants with a 21 pitch prop. I don't think he wants to prop down. His rpms will go up doing that.

[dave t.]

in theory that sounds right, but I have a Suzuki 90 and had the same problem. The Suzuki SS 20" prop will develop fewer r's than a 21" alum. prop. It is unlikely that this motor can turn the 21" SS like the 115 can.

[TheBone]

Send the Prop in to a performance shop and they can add more "CUP" to it. This will lower the RPM'S and also give you more speed. Even though your not worried about speed, this is one option.


BONE

[Hmmmm]

makes sense if he is running an aluminum now. thanks

[TravisC]

The Suzuki 90 & 115 are the EXACT same engines. The 90 is rated at 5500rpm while the 115 is rated at 6000. Therefore, I wouldn't be too concerned about revving a 90 to 5800rpm. That being said, I wouldn't expect too much sympathy from Suzi if something were to go wrong due to prolonged over-revving of the engine.

I've asked a lot of people what accounts for the horsepower differences in four stroke fuel-injected outboards of the same company having the same displacement/gear ratio, etc. So far no one has given me a good answer. The only thing I notice is that the "higher horsepower" engine is rated about 500 rpm higher than the "little sister" model. This leads me to believe the the additional horsepower is gained only from higher rpms. If anyone has a better answer I'd be very interested in hearing it.

Hathead

[Tom P]

Most HP increases from the same size cc or size motor is increased by a larger carburetor and/or port timing. The design of the motor has a lot to do with how much over the factory RPM you can safety go. The Merc in line 4 cylinder 100,115, and 125 hp had crankshaft harmonic problems that were not found out till they started having problems in the field. They then lowered the max RPMs from 5,500 to 5,250. Yet, most V6 outboards can safely go 500 RPM over factory RPM with only a slight increase in wear from the added stress. In most motors, you will lose a mile an hour or two going past the max RPM because you are way past the torque curve and just going past the max HP peak. Depending on the motor, it's induction system, and it's port timing you may not drop off the hp curve too fast and actually get back that 1-2 miles per hour and maybe another 1-2 on the plus side. BUT, this is very risky now a days with your motors computer saving what RPM you are at and when. You can guarantee they will use that against you if there are problems. I think you could get away with 100+ over max RPM on a V6, but after that forget it. They will just say you went too far over factory max so we are not paying for your new powerhead IF anything happened. That would be a great question for T-Mac or any marine mechanic that follows this board. Please chime in on how the main factories handle over max RPMing a motor these days. I myself have a 99 150 V-Max that is propped to 5,800+. A Yamaha 2.6 lower end is good till 6,300 as is, IF you had the compression, port size, port timing, and tuner mods to get her spinning happily there. I like a good holeshot and am willing to give up a MPH or two to get the hull out of the water faster. The carbed V-Max is dependable motor, so I followed as close as I could the factory recommended RPM the first 3/4 of the boating season to see that she held together and then upped her to my 5,800 RPM. Roll the dice with your warranty.

[dave t.]

hathead,

I think you are on to it, the higher RPM's do give the 115 a little more horsepower. This is because the crankshaft on these two motors is different which allows the 115 to crank out a few more R's. If it was a simple fix, I'd have my motor (90 Suzuki) converted over to a 115 to give me a little boost - that being said it isn't cost effective to do so.

These motors are awesome for those that have never driven one!

[REW]

Simply back the throttle off a bit, if you are by yourself, and running the 300 rpm over.
When you add gear and or another person, you are likely to load the motor enough to lose the 300 rpm as well.

The other option, as one of the other posts suggested, is to replace your existing prop with the Suzuki 20 pitch stainless prop for that engine.

Normally, going down in pitch will Increase RPM.
However, in this case I doubt that that will happen, due to the prop diameters.
If you have a "standard" after market 21 pitch prop, it is likely that the diameter is 13 or 13 1/4 inches.

However, I believe that the Suzuki 20 pitch prop is 14 inches in diameter. This additional diameter does a better job of matching the torque characteristics of the engine. i.e. the larger diameter gives a tremendous hole shot, as well as keeping the engine from over revving.

Remember, due to the torque characteristics of this engine, the lower unit has a different gear ratio than the typical two stroke engine. The suzuki is geared lower and thus has the ability to swing a larger diameter prop. Thus, if you have installed the same prop that might be used on a two stroke engine - i.e. in the 12 7/8 to 13 1/3 range, it is likely that the engine will overrev.

Check the diameter specs on your prop, and compare it to the diameter specs on the Suzuki prop. My guess is that you are going to find a pretty big difference.

Take care
REW