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Author Topic:   Increasing Horsepower of 1976 Evinrude Outboard Motor
blux posted 01-29-2008 06:09 PM ET (US)   Profile for blux   Send Email to blux  
I've got a 1976 Evinrude 70. She runs great pushing a 17-foot aluminum fishing boat. I am thinking of moving to a larger (85-HP) motor, but can anyone suggest modifications to this before going that road? Are there any aftermarket powerhead bolt-on's, larger cylinders, pistons, reeds? Could [changes like that] seriously compromise the drive [train]?
jimh posted 01-29-2008 08:33 PM ET (US)     Profile for jimh  Send Email to jimh     
I don't know of any bolt-on components, but the 70-HP Evinrude three-cylinder motor can be tuned to over 100-HP by various techniques. At my local Evinrude dealer, one of the mechanics is also a boat racer. He has been rebuilding and tuning his three-cylinder Evinrude 70-HP motor, and he estimates it now produces about 100-HP. He also winds it up to about 8,000-RPM. It has all sorts of modifications. He has cut new ports into the block, ground and "ported" the exhaust passages, changed to different pistons, changed reeds, changed fuel pump, changed carburetors, and so on.

I don't think that this sort of modification is within the scope of the typical outboard owner. The mere fact that you are asking about it tends to make me think that you are probably not sufficiently familiar with the motor and with hot-rod techniques to perform the necessary modifications.

If you want more power in about the same size and weight, look at the Yamaha 90-HP three cylinder motor.

Clark Roberts posted 01-30-2008 06:41 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Your 70hp Evinrude displaces 49 cubic inches so is already producing over one hp per cubic inch and at about 1 1/2hp is about tapped out for reliable useage (in my opinion). You can move the reed stops back a little and increase size of high speed jets but idle may suffer... Happy Whalin'... Clark... Spruce Creek Navy
seahorse posted 01-30-2008 07:30 AM ET (US)     Profile for seahorse  Send Email to seahorse     

Hopping up a 32 year old motor will probably result in much more money spent than the motor is worth. Due to it's age, it will also need new rings, pistons, and bearings just to work reliably with the modifications.

You may be dollars ahead by investing in a larger motor.

The following link is for a company that specializes in 3 cylinder mods.

brisboats posted 02-01-2008 05:48 PM ET (US)     Profile for brisboats  Send Email to brisboats     
Your money would be better spent finding the right propellor and a jack plate. Dial the boat in first before looking for more horsepower. Your motor power to weight is a stout performer right out of the box. As the others have already said the cheapest way to more horses is a bigger motor but make sure your boat can handle the additional power and weight.


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