79 Evinrude 100hp...better performance?

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rennevat

Recruit
Joined
Sep 26, 2003
Messages
2
Can anyone tell me how to get more performance from my 1979 Evinrude 100HP outboard...It runs fine and is jetted properly...however I'm at 5-6000 ft elevation. Searching the database I get bits-n-pieces that the 79 100hp and 115hp basically are the same...only difference is carbs and exhaust...is this correct...what about intake/reeds/ports...Comments have said that this mod might not work and just cause excessive fuel consumption but the 115hp carbs have bigger diameter throats thus allowing more air/fuel (w/ proper jetting) right?...What exhaust modifications need to be performed...what year frames will these parts cross-over to the 100hp...Instead of carbs/exhaust how about shaving the heads for higher compression?...Can anybody give me specs on how much to remove and part #'s for thinner head gaskets...do I need colder plugs and their part #'s... What octane gas do I need for this modification...Most importantly if there is a flat out recommendation not to shave the heads please tell me why. Thanks, rennevat.
 
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ledgefinder

Ensign
Joined
May 2, 2002
Messages
916
Re: 79 Evinrude 100hp...better performance?

The 1979 85, 100, 115 and 140 hp OMCs are all 100 cubic inches. The difference in the engines is porting, a tuned exhaust on the 140, and carb size. Your 100 will have better low/mid range than the 140, but less top end. <br /><br />Before you look at engine mods, you should be sure your setup is optimized. The motor wants to be as high on the transom as possible, without losing water pressure, or cavitating too often. Once you get up to speed, generally trimming the motor back to the optimum will gain ~300 rpm. The optimum transom height & tilt are found by experimentation. Once you have them in the ballpark, prop so that you're at ~5800-6000rpm with a light load (just you in the boat).<br /><br />Make sure your carbs are clean & your WOT timing is set to spec.<br /><br />DHadley (and others) know a lot about performance with that era V4, and you can search the archives here for his posts. Not sure what size carbs the 100 has, or whether the 115/140s are bigger. <br /><br />In general, the forum has recommended against planing heads - with today's gasonlines & steady loads boat motors are under just seems to be a recipe for touble. Since you're above 5000 ft though, I'd go for it. There've been some discussions here & on the "Scream & Fly" technical forum on how much to cut them. The exhausts are pretty restricted on outboards, and it doesn't make sense to do too much to the rest of the motor, while you've got stock exhaust. Scream & Fly has had discussions on that as well (offshore exhaust, 'dry stacking', etc.). Keep in mind the exhaust mods are really a pain in the ears to others, so appropriate only a ways offshore, on big lakes, etc..
 
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