Repower 150 merc 2 stroke to 140 Suzuki 4 stroke

[dizee25]

I have a 25' suntracker tri-toon, it has an older mercury 150 black max on it now with a 15" x 19 pitch 3 blade alum prop, WOT is 5300 rpm, I am repowering with a 2002 Suzuki df 140 four stroke, will the same size prop work or should I start with a 17 pitch to try and bump the rpm to around 55 5600 rpm? assuming the power is close to the same between to two motors I am thinking this should be a starting spot. Anybody done somthing close to this with any suggestions. I am kind of shooting in the dark.

[steelespike]

Can you tell us what an older Merc is?I found 3 different gear ratios 2 from the 90s one from the late 70s. I can tell you the Suzuki is 2.38 rpm ,the closest merc was 2.00.
The Suzuki might gain some rpm but your down 10hp. rpm may be a wash.
The Suzuki max rpm is at 6200. I think a 15 would paobably not be enough of a drop.
I'm thinking a 14 would be closer to what you need its better to be under propped than over.

[dizee25]

The motor i have now is a 1984 Mercury black max. So Steelepike, are you saying that one number drop in pitch should give me 200 rpm? it seems i recall hearing somthing like that before. I am a little concerned about over reving the four stroke verses lugging it where the opposite was true for me on the two stroke. that is only my opinion. with the gears being equal the 14 pitch should add 10000 rpm or put me at 6300 rpm. with the added .38 in gear ratio im alittle leary of dropping that many inchs in pitch as well as the diameter will decrease with the suzuki. I am looking for a cheaper prop so I think 14" is as big as I can get. I do not know what the effect of a smaller diameter is for sure, I am assuming it would be easier to spin moving less water. Keep in mind all of these assumptions are just that assumptions on my part. I am going only on my understanding of what I have been reading on forums.

[CharlieN]

I would start with the prop you have and see how it feels. The shorter gears on the Suzy would call for a bit more pitch to be equal therefore start with your 19 and get some data and then decide.

[steelespike]

Rule of thumb is 100 to 200 rpm per inch.Your 84 merc is rated at the powerhead the Suzuki
is rated at the prop so your actually working with about the same effective hp.
You do need to get up close to 6200 rpm.4 strokes are said to do better propped high.
After doing a little figuring I feel that a 19 will be too much prop but may be a good place to start.The suzuki may produce more rpm than the Merc with a 19 but doubtful to be anywhere near 6,000 or so needed.

[Silvertip]

Over-propping a 4-stroke can lead to the issue of "making oil". If you aren't familiar with the term, it is caused by "lugging" the engine so the fuel gets diluted with unburned fuel due to high throttle settings but low engine rpm due to improper prop selection. Some engines are more prone to this but it can happen to any 4-stroke. The oil level actually increases and as it becomes more diluted the fuel reduces the ability of the oil to lubricate properly and you eventually destroy the engine.