Help with motor selection

[nate22]

Hi everyone,

First I would like to thank all the contributors of this form, I have learned a lot from reading the many post on here. Thank you

I'm planning to race in my local offshore circuit next year and need help selecting an engine.

The class I will be racing in gives you two options, stock 1.3liter 90hp (transom mounted water pickup allowed) or modified 850liter 70hp engines (no nitrous and exhaust stacks allowed).

The hull that I will be using is an Ocke Mannerfelt 19ft stepped V bottom hull that weights around 450lb. http://www.mdteam.se/boats/B19.asp?s=3&type=race

Boat transom is 15in

I'm thinking about using a sst60 motor, moded Stinger 75 engine, or moded Yamaha 70hp motor.

How do you think these engines would stand up to a stock 90hp engine? I'm manly worried about the torque being sufficient on the 70hp engine vs the 90hp for the weight of the hull.

what would you use if you were racing in this class ?

[Mark75H]

That's a real toss up. I think you would have a good chance with either.

Let's hear what others say

[JohnsonM50]

Its an old rivalry.. big 'stock' cubic inches vs smaller more hi performance motors. Tuff call but Id think the modified idea can work if the weight & drag of the hull are compatible. The stock 90 with a pick up on the other hand can be run at a hi performance set up. [In other words I have no idea] but Good Luck.

[nate22]

Lets compare to engines of the same manufacture.

90hp Yamaha
weight 261 lb
20in shaft motor
rpm range 4500-5500rpm

70hp Yamaha
weight 228 lb / 33lb less then Yam 90hp
I can modify it to a 15in shaft motor less weight maybe 10-15lb
rpm range after mods 6,500 to 7,000rpm
I can modify the lower unit for less hydrodynamic drag and use a low water pickup

So if rpm = speed the 70hp should be the whey to go? also I would have -40lb off my transom for better handling.

I'm gassing I can mod the 70 to create 90hp and still be reliable and still have less weight on the hull.

The only thing i'm concerned about is the toque of the 90 vs the 70. Is it better to have more rpms or toque all things being equal to create speed?

The hull is not a deep V. I have been told it uses the same concept as catamarans to trap air to create lift and use less of a running surface.

[JohnsonM50]

Lets compare to engines of the same manufacture.

90hp Yamaha
weight 261 lb
20in shaft motor
rpm range 4500-5500rpm

70hp Yamaha
weight 228 lb / 33lb less then Yam 90hp
I can modify it to a 15in shaft motor less weight maybe 10-15lb
rpm range after mods 6,500 to 7,000rpm
I can modify the lower unit for less hydrodynamic drag and use a low water pickup

So if rpm = speed the 70hp should be the whey to go? also I would have -40lb off my transom for better handling.

I'm gassing I can mod the 70 to create 90hp and still be reliable and still have less weight on the hull.

The only thing i'm concerned about is the toque of the 90 vs the 70. Is it better to have more rpms or toque all things being equal to create speed?

The hull is not a deep V. I have been told it uses the same concept as catamarans to trap air to create lift and use less of a running surface.

It may come down to what props you can spin [or not]. The more hi perf, the less range of what you can use. The 90 will have more options but doesn't mean better ones The minus 40 lbs is attractive especially if you need to be nimble on course. No getting round that torque tho, still a tuff call.

[nate22]

Some very good points made thus far.

I have talked to a couple of performance tuner shops, and they all think the modified 70hp can work and should perform better, because of the more broader rpm range once modified. One particular shop suggested that I use a 56 c.i power head vs the 49 c.i one. Said the added displacement was much better then the 49 c.i head, so I guess that eliminates the 70hp Yamaha and 49 c.i OMC out of the equation.

I guess my only question now is reliability. How reliable is a high compression 70hp outboard engine turning around 7000 to 7,500 rpm jacket up to about 100+hp? How many hours are you racers getting before doing a rebuild on the engines you run.

I have about 15 race days per season to complete.

keep you opinions coming