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Author Topic:   Pumping up a "de-tuned" motor
tbyrne posted 11-05-2003 10:27 AM ET (US)   Profile for tbyrne  
In late 2001, I bought a boat rated to maximum of 90 hp. I decided to try a four-stroke and ended up with a carbed Yamaha F80, which was essentially a de-tuned F100, which is similar to the injected F115. Since then, Yamaha has "adjusted" the ratings on both the F80 and F100, bringing them in line with the Mercury 75 and 90 hp motors. (I am not sure whether the Mercury 90 hp four stroke was available when I bought the Yamaha - I know I talked to a dealer who carried the Mercury two stroke and he didn't mention the four stroke). Had the Yamaha been rated 75/90 when I was buying, I would have opted for the 90, since it weighed the same as the 75.

The performance of my boat (a Rothbilt 18 - sort of a downeast skiff with a stepped hull) with the 80 is very good, with a cruising speed of about 25 kts. and a top speed of about 32 kts., but like most boaters, I like to have as much power on tap as possible.

My question is whether anyone knows what the difference is between the Mercury/Yamaha 75s and 90s and whether there is a quick and easy way to "re-tune" my 80/75 to a 90? If the 75 had a more constricted exhaust system, I would not bother messing with it, but if the 80/75 is limited by a chip or simple mechanical means(both the 80/75 and 100/90 have the same displacement, compression and redline), I'd consider changing it. Given the permissible 10% fudge factor outboards can have, maybe all four motors put out 82.5 hp and the only one that was not correctly rated was the Yamaha F100!

For what it's worth, my boat with the Yamaha 80 soundly outperforms the Suzuki 70 tested on the same boat in "Offshore" magazine in early 2001.

Can anyone offer any ideas?

Bigshot posted 11-05-2003 10:38 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Buy a new ECM for about $1300 and you will have a 100hp.
TampaTom posted 11-05-2003 11:18 AM ET (US)     Profile for TampaTom  Send Email to TampaTom     
I've flashed the bios on my computer. I wonder if there is a way for the "do it yourselfer" to rip the F100 program and flash it to the ECU in a F80?

The Rothbilt is a beautiful boat.

tbyrne posted 11-05-2003 05:14 PM ET (US)     Profile for tbyrne    
Thanks for the replies, gents -

I guess I'll leave it as is - Iam pretty underequipped as far as computer knowledge - I'd probably turn it into a 55hp Bearcat!

Tom - thanks for the compliment on the Rothbilt - I think its sharp looking, too, but the main reason I bought it was so well put together - solid bronze hardware (bowlight inspired by BW!), terrific joinery on the teak seating and decking (the solid teak console top is a full inch thick!), outstanding glasswork, fully accessible internal fuel tank. The layout (no Euro transom or useless rear jump seats) provides plenty of room for chasing fish around the boat. I am very pleased with her.

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