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Author Topic:   OB Engine: Re-tuning for more HP
Ferdinando posted 11-26-2001 07:10 PM ET (US)   Profile for Ferdinando   Send Email to Ferdinando  
I have a 1998 90hp Evinrude. I have been told that the 100 & 115 are the same basic engine, and that I can upgrade my engine
to a 100 or 115 with a simple needle change in the carb. Is this correct, will it hurt my motor in any way and is it a simple procedure. My engine has 150 hrs on it.

Thanks, Fred. San Juan, Puerto Rico

Bigshot posted 11-27-2001 09:45 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Little bit more than carbs. Has to do with port sizes, etc. You can try but the best I would say is that 115 carbs would make you a 100hp and for the price, it aint worth it for 10hp.
Jerry Townsend posted 11-27-2001 11:39 AM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Fred - I have the 1996 115 V-4 Ocean Runner. The specs show that the two engines are basically identical - same bore/stroke, weight, fuel/air inlet configuration and carburetion. There has to be, by definition, a difference - but that difference is not apparent in the specs, so any difference has to be quite subtle. Talk with a good service individual and see what he says. ---------- Jerry
dgp posted 11-27-2001 11:58 AM ET (US)     Profile for dgp  Send Email to dgp     
Dunno about OMC motors but Mercury uses intake restrictor plates on the lower HP units of the same displacement. You might check with the dealer parts guy to compare intake components of 90 vs 115 HP versions.
Bigshot posted 11-27-2001 01:59 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Oh it is definately in the reeds, ports, carbs, etc. Cheaper to buy a 115 and put 90 decals on it than pump one up. Make sure main bearings and other bottom end components can handle the increased hp.
Clark Roberts posted 11-27-2001 05:46 PM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
The exhaust tuning is quite likely to be completely different! All the way from exhaust porting (as mentioned above) through baffles and down to the megaphone(s) in mid-section. Too much/many differences to make conversion practical as Bigshot has suggested... happy Whalin'... Clark... Spruce Creek Navy
Ferdinando posted 11-30-2001 07:20 PM ET (US)     Profile for Ferdinando  Send Email to Ferdinando     
Thanks guys for all your input. Guess I'll leave well enough alone for the time being.

Thks again,


zpeed7 posted 12-03-2001 12:23 AM ET (US)     Profile for zpeed7  Send Email to zpeed7     
Ferdinando, go to Outboard Motor Center in the marginal street near Plaza Norte. The owner is an expert in OMC engines with years of experience. He'll be able to tell you in a second if you can do it...

hope it helps...

zpeed7, Isla Verde, PR

Ferdinando posted 12-04-2001 08:33 PM ET (US)     Profile for Ferdinando  Send Email to Ferdinando     

Know the place. Will go over and pop the question.

Thks for the input.


p.s. What type of Whaler do you own...

zpeed7 posted 12-05-2001 12:30 AM ET (US)     Profile for zpeed7  Send Email to zpeed7     
No problem.... I own a 1986 18ft outrage, Im currently rebuilding it... and u?
Ferdinando posted 12-06-2001 07:59 AM ET (US)     Profile for Ferdinando  Send Email to Ferdinando     

Own a 98 Montauk with a 90 Johnson which as you may suspect I purchased at Miramar Marine. I use it mostly in Salinas and once in a while in La Parguera......


jimh posted 12-15-2001 10:55 AM ET (US)     Profile for jimh  Send Email to jimh     
[Changed TOPIC; was "Engine Performance".]

It does seem tantalizing to be able to increase horsepower with just a minor change of parts, but often the higher horsepower is the result of several subtle changes in dimensions and tuning that may be hard to implement.

This brings to mind a story told to me while I was researching material for the article on the Bearcat-55 outboard engine ( ). A engineer with access to a dynometer said he had tested several modified versions of the Crosley-44 engine. In the case of one race-tuned engine, as each of the "go-fast" parts was removed and replaced with stock items, the measured horsepower increased! The fancy intake manifolds, carburetors, and exhaust manifolds looked great, but they were not adding any horsepower.

In the case of some modern engines, the mechanicals of two motors can be identical. The difference in horsepower is all obtained with software imbedded in their electronic control modules.

triblet posted 12-15-2001 01:00 PM ET (US)     Profile for triblet  Send Email to triblet     
And in at least some cases, the different
engine control modules may make sense.
The vehicles may have different weights,
and require different tuning to pass smog
requirements. They may have different uses
(sports car vs. pickup vs. family sedan, ie,
HP vs. torque vs. MPG).

In other cases, it's all marketing.


Clark Roberts posted 12-16-2001 09:25 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
The term "Break Mean Effective Pressure (BMEP) refers to internal pressure created by the compression and combustion process. Basicly, the higher the hp per cubic inch the higher the BMEP and the more stressed the engine is when operated under load at mid to peak rpms... a lot of engines put out about one hp per cubic inch and one I recall (the 3 cyl OMC 75hp "Stinger") had only 49 cubic inches therefore about 1 1/2 hp per cubic inch... very high BMEP. Now, the lower the BMEP the higher the longivity! The higher the BMEP the lower the longivity! Basic message here is that a hopped-up engine may suffer from a short life-span. Don't think I can get any more "wordy" with this so will shut up.... but not for long! Happy Whalin' and pass the "waterless hand cleaner"! Clark... Spruce Creek Navy
dgp posted 12-16-2001 11:04 AM ET (US)     Profile for dgp  Send Email to dgp     
Engine life is a function of fuel consumed!
JBCornwell posted 12-16-2001 10:27 PM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
It's not that simple, gents. Many other variables affect engine life.

BMEP is an important parameter. Essentially it is a measure of mechanical stress on the moving parts.

Metallurgy, manufacturing tolerances, maintenance, lubrication, balance, equality of dimensions and weights of parts in multiple cylinder engines and maintenance of optimum operating temperatures of each part will determine what BMEP does to engine life. They will, of course, also affect how much fuel is required to produce a given BMEP. There are other less obvious variables as well.

A 150HP Force and a 150HP Yamaha, producing the same BMEP or using the same volume of fuel will have very different life spans. The above suggests why.

Red sky at night. . .
JB :)

where2 posted 12-26-2001 07:44 PM ET (US)     Profile for where2  Send Email to where2     
I'm a little confused about this BMEP thing because the 75Hp Johnson Stinger is quite similar to the rugged 3cyl "Looper" engines praised for their longevity on this same list. The 75hp used a different gear set in the lower unit, different carbs, and rated the HP at a different (higher) RPM. Is a 70Hp 3cyl looper going to have a far greater life span than a 75HP Stinger?
JBCornwell posted 12-26-2001 10:17 PM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
Howdy, Where2. The Stinger's larger venturi carb allowed it easier breathing than the 70 loopers, and higher BMEP at higher rpm.

The gearset was selected to optimize prop selection for the higher rpm.

If everything else were equal, which it never is, the Stinger wouldn't last as long as the 70, which wouldn't last as long as the lower rated versions of the same engine.

With sound care and maintenance you wouldn't notice any difference in engine life.

Red sky at night. . .
JB :)

tlynch posted 12-29-2001 03:27 PM ET (US)     Profile for tlynch  Send Email to tlynch     

I think that I might have met you once. My girlfriend and I were in La Parguera about 2 years ago and we rented a boat and went out behind a patch of mangroves and anchored. A very nice man (possibly you!) showed up in a 17' Montauk with a 90hp on it. We talked for an hour or so just standing in the water. When we left you offered to come in and pick us up and bring us back out. I am 6'6" tall and I am from NYC - if you are the man I met then you have a brother in Brooklyn or somewhere around the city. Was it you?

In any case, we are returning to La Parguera the first week in March. Maybe we will see you out there (again?)!

Todd Lynch

WSTEFFENS posted 12-31-2001 01:46 PM ET (US)     Profile for WSTEFFENS  Send Email to WSTEFFENS     
I am not a OB mechanic, however I did investigate doing what you propose on a pair of V4 110 engines in the mid ,80's.

Generally the differences in hp in an engine series from OMC (90,110,130) etc was achieved by carb's & reed plates and exhaust baffel tuning along with diffrent pistons (for compression ratio as well as bore).

Switching parts is very expensive as you will find. Also the higher the hp in an engine series the higher the stresses. Some of the engines in the late 80's in the high end of the hp range had failures of the upper ring land and that would "lunch" the power head when it failed.

Since the advent of the ECM & anti knock sensors there is little to be gained by fooling with the mechanical ignition timing advance.

The only thing I can think of that you might do for relitively low bucks, would be to square out the intake and exhaust ports for extra cross-sectional area and more flow. You will have to be very careful not to not to change the port timing when doing this. This is the go-kart version of bench flowing a set of cyl heads for a car.

As stated above it is probibly cheeper and better to buy a remanufatured power head and just switch it out.



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