Mercury outboard engines

[Jenko]

Still waiting on delivery of my new boat.........

I was checking out the spec for the engine it comes with a merc 75, interestingly enough I noticed the 90 is the same weight, size, capacity, compression ratio, stroke, etc, etc, the only difference is the max rpm (on the 75 its 4500-5000, on the 90 its 6000).

So, obviously Mercury are just rev limiting the 75.....Question is How??. Is it done via ECU, or a turn screw on the carbs?, also, would the torque curve be different on both engines at the lower end of the rev range.

Cheers.

Paul.

P.S. The 90hp was a £1000 optional extra!!!!

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[David2452]

The differences are far more than just rev limiter, port timing is different, exhaust stack is different and a few other small things. Bottom line is you can't upgrade for anything like the difference in origional cost.

David

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[Jenko]

But surely, port timing is controled via the ecu?????. I know some car manufactures do this (e.g. BMW on thier straight 6 engines).

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[David2452]

No, Ignition timing is controlled by the CDI unit, port timing is fixed on a 2 stroke and controlled by cam profile on 4 stroke (we are talking 2 stroke here ??)

David

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[Jenko]

Ahhhh no, we are talking 4 stroke...Sorry, should have mentioned that.
Also, I believe the 75 and 90 are the older 4 strokes with the older cdi ignition rather then the newer EFI......

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[Jenko]

Ahhhh no, we are talking 4 stroke...Sorry, should have mentioned that.
Also, I believe the 75 and 90 are the older 4 strokes with the older cdi ignition rather then the newer EFI......

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[David2452]

In that case it would be even more expensive, Exhaust tube, camshafts, re jet and probably larger valves. You are correct, the current 75 & 90FS are carbed with capacative discharge ignition.

David

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[jfm]

David, do you know that for sure? Not disputing what you say at all, I dont know this model. But SOME mercs with various hp (eg old strate 6s) were identical apart from carb jets, whereas others have different exhaust etc. So tis worth checking

We have both 115 and 125 2-strokes (not on same boat!) and the only diff is carb jets afaik. When I bort the 125 the dealer (Barnet) even offered me the same price for both engines, he said same price to him so I could choose which I wanted. List price was £500 different.

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[Jenko]

I also believe the 115 4 stroke is the same engine, although this time it does have a completly new ignition / injection system.

If the cam profile is changed for a higher top end, then would you not be loosing some low end power / torque. If I had just bought a 90hp and learnt the 75 had a better torque curve I would be a little pi$$ed off.........I did put this question on the Mercury technical forum, but so far have had no replies (wonder why!!!)......

It's an interesting discussion.....


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