A question of power

[Amazon]

Hi

Nice at last to have a company showing a real interest in the 50s engine with an active forum on the website.

The powerhead fitted to the MB2/50S unit has the designation number BF75 and is 7.5 hp, an identical powerhead BF100 (which appears to be interchangable) is rated at 10 hp ?

If the specification data sheets of the 2 units are compared they appear identical stroke bore etc.
My question is how is the extra 2.5 hp achieved ? I have heard that is is due to a carb mod and the allowance for the B100 to rev a little higher, but find it difficult to accept this would give this type of % power increase. Could there be another reason ? eg higher compression, larger or different head valves not indicated on the data sheets?

It would be great to hear from anybody who knows the real answer.

My engine was fitted to a gaff cutter about 8 years back in place of the outboard, when working its just perfect, as Amazon has a long keel and a wide deadwood area at the stern the saildrive leg fits in very tight behind it causing very little drag when under sail. it is fair to say that the period since installation has not been trouble free,

A picture of Amazon is attached

[ajp]

hi i also have looked at this and have bought a 10hp for parts but because of the fittings for the fuel are differant the banjo fitting is tapped in to the housing and as im not sure if it will make any differace i have not tryed but im going to look at it when boat is liffted out end of the year

[Howard]

The extra 2.5 HP would be of interst to many users and I for one would like to find out. I do have a contact at Honda and I will try to get at the answer that way. I do believe thought that alowing any engine to rev higher and improving the breathing can have significant impacts well above 25%. This is in fact the basis of much performance engine tuning work I have been involved with over many years, regards Howard

Hi

Nice at last to have a company showing a real interest in the 50s engine with an active forum on the website.

The powerhead fitted to the MB2/50S unit has the designation number BF75 and is 7.5 hp, an identical powerhead BF100 (which appears to be interchangable) is rated at 10 hp ?

If the specification data sheets of the 2 units are compared they appear identical stroke bore etc.
My question is how is the extra 2.5 hp achieved ? I have heard that is is due to a carb mod and the allowance for the B100 to rev a little higher, but find it difficult to accept this would give this type of % power increase. Could there be another reason ? eg higher compression, larger or different head valves not indicated on the data sheets?

It would be great to hear from anybody who knows the real answer.

My engine was fitted to a gaff cutter about 8 years back in place of the outboard, when working its just perfect, as Amazon has a long keel and a wide deadwood area at the stern the saildrive leg fits in very tight behind it causing very little drag when under sail. it is fair to say that the period since installation has not been trouble free,

A picture of Amazon is attached
[/quote]

[Howard]

I found this post on the web and I am now planning to change my cam shaft & carb when my boat comes out on the water this winter, I will post the results the nexct spring, Howard

Date: 10/31/2004 04:24PM


Mr Moore;

Thanks to you and Affordable Yachting for the asssitance, it is greatly appreiciated. Although it did not specifically address my problem it helped in several other areas of interest. i was lucky to find another member with same motor, so i sneeked a peak at his setting and will make minor adjustments when I do my spring basin trials.

Here is some other info for consideration

The B75's (7.5 hp) were first, starting in 1973. They're completely white. The BF75 (7.5 hp) was an improved version of the B75 and started in 1978. The BF100 was introduced at the same time with bigger carb jets and different cam for 9.9 hp. In 1987 they started rating hp at the prop instead of the flywheel.

The BF75 was dropped (it puts out about 6 hp at the prop) and the BF100 was given the new designation of BF8 to reflect the 8 hp it put out at the prop. It is still available and the oldest continuous model in Honda's line up: 1978 to present.

With the BF100 (the 9.9 of the time) becoming an 8, Honda needed a new 9.9. That came in 1987. They engineered it as a 15 hp engine, the BF15, and detuned it by changing the cam and carb jets. When they did this they only got the engine down to 13.2 hp.

To keep the folks in lakes that require LESS THAN 10 hp legal and to legally go on boats rated at 10 hp they put a stop screw in the throttle bellcrank to limit rpm. That screw is there to protect the consumer, not Honda. Remember it's really a 15 hp-engineered engine. So if you remove the screw (which does NOT affect your warranty) you get the full rpm (still a little less than the 15) and get 13.2 hp.

So the differences between the BF100 & BF75 are carb jets and camshaft. The differences between the BF9.9 and the BF15 are also carb jets and camshaft. The peak torque of the 9.9 comes lower on the power curve than that of the 15 for more pushing power at lower rpm's. The marketing idea for the 9.9 is as main motors on sailboats and kickers on powerboats. The 15 is marketed toward main engines on small fiberglass and aluminum skiffs and inflatables

Now my new question, When the time comes to replace my good old BF 100 will the new Honda 8 HP High Thrust provide as much or more power than my old BF 100 ? WMT David

[Amazon]

Hi

Many thanks for the clarification on this one, I will check the difference of the two camshafts with interest.

[Howard]

I have a new completly rebuilt engine with the 10HP camshaft ready to be installed. The boat should be in the water and under test by the end on Febuary all going well. The carb jets proberbly will need adjusting, I will update this post with the progress, Howard

[mojarlets]

I feel happy to read that you have made a new 10 HP capable engine because I am facing similar problem with my boat.I am eagerly waiting for your next post.

[ajp]

hi justn to add i have run a modifed engine at 10 hp for a year no probs at all and have a thread listing this and this year added a dynastart