9.9 Vs 15 Hp Honda

[JIM MCCARTHY]

I Have A 15 Hp Honda Outboard That Runs Fine. The Day Will Come However When It Doesn't Run Fine And With That In Mind, I Am Looking At A Parts Engine For Future Use. I Have Located A 9.9 Hp Motor That I Am Told Is The Same Engine With The Exception Of The Throttle Body Which Is Different. If That Is True, It Would Make A Good Parts Engine, But If Not, It Would Be Money Down The Drain. Both Engines Are About 1995-1997 Vintage, The Boxy Hood Style.

Can Anyone Tell Me If These Engines Are In Fact Identical Engines?

 

[gss036]

Re: 9.9 Vs 15 Hp Honda

I can not say for sure, but Honda basically had the 9.9 and 15 hp on the same block and just changed the carb a little. I have a 1997 15 and it looks identical to the 9.9.
The new models are 8 & 9.9 HP on the same block and the 15 & 20 HP on the same block. The newer ones have a few changes for EPA requirments.
I have been having an electrical problem on ine which I am at a loss on what is really wrong and the local Honda dealer charges almost $100.00 @ hr for labor. I am about to the point of just starting to replace parts, seems like every component is close to $100 each.

 

[catfishjon]

Re: 9.9 Vs 15 Hp Honda

I have a 2000 4 stroke 9.9 Honda motor. I was told the 15 horse engin was identical and the throttle had a screw that could be adjusted out to allow 15 hp performance. The salesman said the 9.9 rating was for lakes that did not allow over 10 hp motors on them. Honda did not want to build two different motors, so they put a throttle stop adjustable screw that would allow only 9.9 hp output. I have only had one problem with mine. Every year I have to change the water pump impeller out. I don't use the motor but 4 or 5 hours a summer, but it won't pump water unless I put a new impeller in. The old one looks fine. Any suggestions would be appreciated. Catfishjon

 

[JB]

Re: 9.9 Vs 15 Hp Honda

There are several approaches to building engines of different HP on the same block.

For example, an engine could be designed as a 15HP and "detuned" to 9.9 by simply reducing the airflow (smaller throat in the carb or even a restrictor plate between carb and intake).

However, that approach is simply a choked down 15 and has reduced efficiency. A 15HP "chain" with a 9.9HP link to weaken it.

A more sophisticated approach designs one version as a 15HP and a different version as a 9.9HP. Different air/fuel management (carb or EFI throttle body), different ignition advance curves, different port or valve size and timing, different compression ratio and different exhaust scavenging geometry. In some cases only the block displacement is shared.

Bottom line? Having the same block displacement and even the same part numbers for many moving parts does not mean that the engines are identical in anything more than a trivial way.

This confuses a lot of people and leads to some horribly bad attempts to "upgrade" the HP of a well designed, efficient engine.

Seems obvious, but if you don't understand exactly what you are doing. . .don't do it.

In your case, Jim, anticipating the need to go below 10HP I suggest that you plan on replacing the 15 with a 9.9 when the time comes or even before.

 

[JIM MCCARTHY]

Re: 9.9 Vs 15 Hp Honda

Thanks For The Responses. I Was Trying To Decide If I Should Buy A 9.9 Hp Engine As A Parts Engine For My 15 Hp. Nobody Could Tell Me (including The Honda Dealer) if They Were The Same Engine. I Finally Decided To Go Ahead And Buy It And Sell Whatever I Couldn't Use On Ebay. It Would Still Be Quite Helpful To Know If They Are The Same Basic Engine Though If Anyone Knows.