I need a little edjumacation on Mercury engines

[Gomer50]

Just curious!!!
Is there any special reason why Mercury kept the rpms down (4500-5000) on a 78 inline 6 90hp.
Most inline 6's all run in the mid 5000 rpm? Seems kinda odd that this is the only model that I am aware of that runs in this rpm range.Hell even my 850 runs in the mid 5000 rpm's well (4800-5300) with the same pistons,carbs,timing.On another note what is the difference between a low dome and a high dome pistons?

By the way edjumacation misspelled for some humor.I think with whats going on in today's world we need a little more humor.

 

[ONERCBOATER]

Re: I need a little edjumacation on Mercury engines

i think we need less gov.... but that entirely dif discussion...

now not specifically relating to your engine.... I know that several models by several manufacturers (most in fact as i understand it) are essentially detuned versions of the largest HP engine in that size block... and that most of the time the differences were small... or smallish. usually they involve standard tuning/detuning methods carbs (throat size,jets) reed blocks (sometimes different blocks sometimes spacers) and the possibility of finger ports ect... as to how fast you can rev it... better make really sure all the parts are identical to a dif engine before intentionally exceeding the manufacturers recommendations.... there are some tuners on this site that may be able to give you far more info.

 

[Gomer50]

Re: I need a little edjumacation on Mercury engines

Thanks Buddy,
How are ya?? Long time no hear.

 

[Bamaman1]

Re: I need a little edjumacation on Mercury engines

Back then, different horsepower motors in a given family of motors was often just the size of the carbs. I wouldn't hesitate to run a 90 Merc stack 6 motor the same as a 115 hp--5500-6000 rpms.

Mercury had to get off their engineering rear ends when Yamaha came in arond 1984 and took much of their retail market share (in the same stores.) Then engineering changes quickly came to the boating world.

The different dome pistons put out different compression ratios--if the heads are the same. The higher compression engines would rev faster and higher rpm's, but the lower compression engines will run on regular gas and possibly be more durable.

 

[Gomer50]

Re: I need a little edjumacation on Mercury engines

I appreciate the input but I am a little on the concerned side as per manual RPM spec's 4500-5000.If memory serves me correct this was one of those designs where the exhaust ports werent like the 115hp and up motors therefore the reduced rpm range.Dont get me wrong it runs fine @ 5000 rpm's.Just didnt seem to make alot of sense that all the inlines were rated @ WOT around the mid 5000 rpm range except the 90hp.I was all thrilled at rebuilding this motor and kinda bummed out these only turn @wot 5000rpm,I would have enjoyed another 700 rpm

 

[emckelvy]

Re: I need a little edjumacation on Mercury engines

The only way you could do that with that 90hp is to enlarge the ports. You'd be better off changing the block and re-jetting carbs. Years ago I had a 90hp block ported out and it ran OK but was an expensive proposition.

It won't hurt to spin the motor higher than 5K but she's probably gonna run out of breath due to the smallish intake & exhaust ports and it's just a futile operation. Propping at the higher end of the recommended rpm range is probably a more reasonable thing.

If you're propping down in pitch to get the rpm up, you'll know when you've hit the rpm 'wall' with this motor, 'cause rpm might go up but speed either won't, or will actually drop, since she just don't have enough torque to spin the prop any harder.

Enjoy the economy of this motor, it's the most Fuel-Frugal of all the Inlines!

Cheers........ed

 

[Texasmark]

Re: I need a little edjumacation on Mercury engines

The 90 was a choked down 115 with a 2.3 gearbox rather than a 2.0. Forget the rating. Necessary to market the hp. I bought an '89 115 new and the way I had my Ranger setup I ran a solid 6000 rpm. That was '89. My son-in-law is still running the engine with no major engine work......................

Mark

 

[Gomer50]

Re: I need a little edjumacation on Mercury engines

Thanks guy's for this info as for fuel economy I am not so sure (she sure can suck it up quick).I am going to look around for one of these 115hp or bigger inlines.

 

[Faztbullet]

Re: I need a little edjumacation on Mercury engines

The 90 was a choked down 115 with a 2.3 gearbox rather than a 2.0.

I hate to disagree but all inline 6's are a 2:1 ratio except a couple of early models, they never made a 2.33 ratio for a inline 6 as thats for the inline 4's and the 89 115 was a 4 cylinder with 2.07 ratio.
The reason the 90 has lower rpm as it was designed as a torque engine thus the: lower port timing,reed stop clearance,non ported piston and block,different carbs(internal passages).

 

[Texasmark]

Re: I need a little edjumacation on Mercury engines

I hate to disagree but all inline 6's are a 2:1 ratio except a couple of early models, they never made a 2.33 ratio for a inline 6 as thats for the inline 4's and the 89 115 was a 4 cylinder with 2.07 ratio.
The reason the 90 has lower rpm as it was designed as a torque engine thus the: lower port timing,reed stop clearance,non ported piston and block,different carbs(internal passages).

You are the certified mechanic and you should know. It has been a long time but I sure thought I remembered the lower gear box ratio to assist in the "workhorse" 90. I still vaguely remember seeing the two engines in the sales brochure and some of the verbiage as to the intended applications. Sorry for the disinfo.

Mark