Holeshot- Does Displacement matter?

[LarryMc]

Been looking at 90hp 2 Strokes to replace the 4S 60/40 on my boat. The boat is 16' long, 55" wide and heavy for it's size. Seems like it takes forever to get on plane with the 4S compared to the 2S I used to have. There is a lot of difference in the displacement of the motors I have been looking at, but they are all rated at 90hp and all use the same 7 3/8" impeller. Brian of Outboard Jets told me that they shoot for 5000 RPM at WOT for all their jet kits. Below are the Displacement figures for the motors I have been looking at;

Johnson 90-105.4 CI
Mercury 90 - 84.6 CI
Yamaha 90 - 69.9 CI

There is about a 30-60 lb weight difference between the motors. But I think that can be discounted. Obviously, the Johnson has the lead on the displacement. So does that translate into a better holeshot for the Johnson over the other motors assuming the same boat and same WOT RPM for all the motors? I'm thinking it will, but don't really know. Opinions?

 

[rotus623]

Yes! Displacement will make an absolute difference!! Most of the OMC 90's are actually de-tuned 115-130hp engines.

Engine manufacturers only make a certain number of block sizes and then have 3 or more hp rated Engines for that particular block size. Remember the old saying, no replacement for displacement? Hp is just a number.

If you take a Mercury 84.6 cu in 70 hp and put it besides a Yamaha 51.8 cu in 70hp, the merc will completely blow it away!! HP is just an equation of torque and rpms.

If you look at a dyno curve, the peak hp is made at the engines upper rpm limit. The torque actually starts to fall of before the peak hp is hit. Big blocks make their torque early. That's why we use big motors to get big loads moving.

So take that "all x hp engines are created equal" out of your head and get you the biggest displacement blockthat you can fit on that HP tag!!

I'd be right there with ya but I do a lot of idling and slow anchoring so I really enjoy the 4 stroke for that.

 

[LarryMc]

My boat is rated for 75hp. May need to hunt up a 65 decal to put on the cowl of the motor I get. :mrgreen:

 

[handyandy]

The omc will be the most powerful of them. If you go v4 omc look for a 115 it weighs the same just take the decals and badging off. The yamaha is the lightest and will be a big improvement over the 4s 60/40 but it won't compare to the omc v4 in power. The v4 115 yamaha would be more of the equivalent to the omc in terms of displacement/performance. If you can find a 3cyl omc they are solid performers and would weigh less than your 60/40 four stroke the later ones 1987 and after are 56ci it would perform like your like old 2s 60/40 maybe a little better. Any of the engines would be a big improvement over your 4s 60/40 your running currently.

 

[redrum]

Yes displacement matters a LOT.

Check out this video https://www.youtube.com/watch?v=gej5dBiVEVo then compare the displacement between the two.

If a V4 is an option then you should definitely consider 115/80.

 

[rotus623]

Larry,

I got a good buddy that has a late 80's OMC 75hp Stinger. These motors were SCREAMERS!! Compression is great and she runs like a banshee. It needs the trim system rebuilt, but I may do that before he sells it. Smack a jet lower on there and you will be PLEASED!!

 

[LarryMc]

I like the idea of changing over to an OMC 2S from my Mercury 4S, but there are also financial considerations which factor into my decision. I checked out the control parts needed to change and it will run around $700-$800 just for the parts. Using used parts would be somewhat less expensive, but not a lot. I still haven't made a firm decision but the odds are that I will stick with Mercury and try to find a 90hp 2S in good shape.

 

[eshaw]

My boat is rated for 75hp. May need to hunt up a 65 decal to put on the cowl of the motor I get. :mrgreen:

I don't think that would be necessary unless you just want to do it. Since the pump is making approximately 65h.p. you shouldn't have any problems with the authorities.

I had beem playing with the idea of using a 90 horse Johnson on my boat but by the time I bought a pump for it, approximately 2K and then built up my transom another 6.5 to 7 inches to accommodate the engine I was looking at a pretty fair piece of change.

I also agree that there is no substitute for more cubic inches.

 

[LarryMc]

Yep, no matter how you cut it, a considerable amount of change is going to change hands before it's over. Still that's better than trying to deal with an underpowered boat. Luckily, I recently sold an old Ranger bass boat I had which will help a lot. I also have the Merc 4S motor to sell which will help even more.

Welcome to the site.

 

[rotus623]

This particular engine has controls with it. It will not be the cost of controls, but the cost of the jet lower that will jack the price up on this one.

 

[LarryMc]

I decided to go the easy and less expensive route with a rebuilt 2005 Mercury 90hp 2S from Outboards Unlimited in Astor, Florida. The owner let me buy it without the prop lower which also saved me a little money. Still need to sell my 2016 4S Jet 40 and buy a Jet lower for the 90. Would like to find s used one, but I guess it would be too much to hope for.