# Hinda 8hp/9.9hp/15HP - what's real the difference?



## mikehoyt (Nov 27, 2000)

*Honda 8hp/9.9hp/15HP - what's real the difference?*

They all weigh the same and seem basically to be the same motor. So what is the real difference between them?

Mike


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## TrueBlue (Oct 11, 2004)

When comparing yamaha 9.9 and 15 hp outboards, I had a similar question, since the 15 was more expensive but of similar displacement. Apparently the difference is in the carburetors, which increase HP and fuel consumption.


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## DrB (Mar 29, 2007)

*I don't understand the question. Besides the obvious..*

of HP differences, what information are you looking for?

DrB


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## christyleigh (Dec 17, 2001)

Things may have changed in different places but 15 years or so ago when I bought my first 9.9 I was told that in some states or even lake specific the "Under 10hp" rules, limits, or benifits made the 9.9 come into existance basically as a de-tuned 15hp.


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## TrueBlue (Oct 11, 2004)

DrB,
Although it was badly stated, I understood the OP's question to refer to what differences exist in the engine's design to create different horsepower ratings, if the units appear to share the same engine?


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## DrB (Mar 29, 2007)

*TrueBlue, I understand a little better*

now of what information the OP may be seeking based on your and the other responses.

Thanks.

DrB


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## TrueBlue (Oct 11, 2004)

Not familiar with Honda, but most likely the 15 HP has a larger diameter cylinder bore as well - affecting engine displacement.


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## christyleigh (Dec 17, 2001)

TrueBlue said:


> Not familiar with Honda, but most likely the 15 HP has a larger diameter cylinder bore as well - affecting engine displacement.


I think your first post was more on target with carbs or jetting being the only difference - at least the way it was laid out for me a while back. Mine was a 4 stroke 9.9 Yamaha at the time but all the majors seemed to be doing the same thing to get under the 10hp thing. My dealer (small boat dealer not Yamaha-so nothing in it for him either way) told me he could sell me a 9.9 for 1000 or a 15 for 1500 (something like that) being the same displacement but carbed or jetted differently, and that with the high tourque prop the 9.9 would actually do better. Cubic Inches = tourque (basically - other factors of course) so to push my trailor sailor I would be better off all around ($$ especially) with the 9.9 Lots of vague statements I know..... but it was a while ago and some things may have changed..... but many of the same games are still being played.


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## mikehoyt (Nov 27, 2000)

OK - so we are starting to get into the meat of this.

An 8, a 9.9 and a 15 all weigh exactly the same so there is no sacrifice in terms of heavier to get more HP - just in money and perhaps in fuel consumption?

From what I have been told these are basically the same engine and therefore I would guess pretty much the same manufacturing costs so in theory could be the same price.

So if they are pretty much the same engine is there really more HP as advertised and to the same degree? What about torque and all the other issues?

I have a 4000 lb disp boat 8.33 beam, 26.67 LOA 23 LWL that currently runs with a Honda BF75 (a 28 year old Honda 4 stk 7.5hp very very similar to the BF100 10HP of the time).

So if the engines are so much the same I would think the 8HP would be fine and then would guess that the 15 would simply have more power for adverse conditions where the hull might not be up to hull speed?

OK - not really one question here but rather than 4 different posts - are these engines really different or just marketted differently? If just the carb what would stop an owner from putting a 15hp carb on the 8 hp engine? Is there any real benefit in my application of the 15 over the 8?

I am actually coming at this post from a backward position. I knew the 9.9 was 115lbs and figured I could make do jest as well with the 8HP but was dismayed that it weight the same.

Food for thought on a friday ...

Mike


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## TSOJOURNER (Dec 16, 1999)

*Honda 8,9.9,15*

Here is the spec from Honda Marine. Just do a google search. Too easy. It looks like the only differance in the 8 and 9.9 outboards is carb and rpm rating, the 15 is of bigger displacement.
8 HP
Type 4-Stroke OHC Twin Cylinders 
Displacement 222cc (13.5 cubic ins.) 
Bore & Stroke 58 x 42 mm (2.30 x 1.65 inches) 
Full Throttle RPM Range 4500-5500 RPM 
HP Rating @ Propshaft 8HP @ 5000 RPM 
nduction Scavenging SOHC 
Valves per Cylinder 2 
Fuel Delivery Single Venturi

9.9 HP
Type 4-Stroke OHC Twin Cylinders 
Displacement 222 cc (13.5 cubic inches) 
Bore & Stroke 58 x 42 mm (2.30 x 1.65 inches) 
Full Throttle RPM Range 5000-6000 RPM 
HP Rating @ Propshaft 9.9HP @ 5500 RPM 
Induction Scavenging SOHC 
Valves per Cylinder 2 
Fuel Delivery Single Venturi

15 HP
Type 4-Stroke OHC Twin Cylinders 
Displacement 350 cc (21.4 cubic inches) 
Bore & Stroke 59 x 64 mm (2.30 x 2.51 inches) 
Full Throttle RPM Range 4500-5500 RPM 
HP Rating @ Propshaft 15HP @ 5000 RPM 
Induction Scavenging SOHC 
Valves per Cylinder 2 
Fuel Delivery Single Venturi


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## TSOJOURNER (Dec 16, 1999)

*Oh yeh, weight*

8HP
Recommended Transom Height 
(S) 381 mm/15 inches
(L) 508 mm/20 inches
(X) 635 mm/25 inches 
Dry Weight Tiller (Man) (S/L/X) 42/44/48 Kg - 92/98/107 Lbs. 
Dry Weight Tiller (Elec) (S/L/X) 46/49/53 Kg - 102/108/117 Lbs. 
Dry Weight RC (S/L/X) 47/49.5/53.5 Kg - 103/109/118 Lbs

9.9HP
Recommended Transom Height 
(S) 381 mm/15 inches
(L) 508 mm/20 inches
(X) 635 mm/25 inches 
Dry Weight Tiller (Man) (S/L/X) 42/44/48 Kg - 92/98/107 Lbs. 
Dry Weight Tiller (Elec) (S/L/X) 46/49/53 Kg - 102/108/117 Lbs. 
Dry Weight RC (S/L/X) 47/49.5/53.5 Kg - 103/109/118 Lbs.

15HP
Recommended Transom Height 
(S) 381 mm/15 inches 
(L) 508 mm/20 inches
(X) 535 mm/25 inches 
Dry Weight Tiller (Man) (S/L) 46/49 kg - 101/108 lbs. 
Dry Weight Tiller (Elec) (S/L) 49.5/52.5 kg - 109/116 lbs. 
Dry Weight (Power Tilt) (L/X) 59.5/62 kg - 131/136 lbs. 
Dry Weight Tiller RC (S/L) 58/59.5 kg - 128/131 lbs


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## TSOJOURNER (Dec 16, 1999)

*15HP and 20HP*

It also looks like the 15 and 20 HP are the same weight and displacement.
15HP 
4500-5500 rpm range
[email protected]

20HP
5000-6000 rpm range
[email protected]

Carb looks like the culprit here also.


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## StoneAge (Sep 28, 2007)

It is my understanding that the camshafts are different as well.


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## TSOJOURNER (Dec 16, 1999)

StoneAge said:


> It is my understanding that the camshafts are different as well.


They probably are different. The RPM ranges are different so the cam you think would be. This is not on the spec sheet at Honda Marine though.


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## therapy23 (Jul 28, 2007)

Catalina274me said:


> This is not on the spec sheet at Honda Marine though.


They don't want you to know all that stuff.

They want to sell HP.

If the motors are essentially the same then get the smaller one.
It will work less and last longer.


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## TSOJOURNER (Dec 16, 1999)

therapy23 said:


> They don't want you to know all that stuff.
> 
> They want to sell HP.
> 
> ...


Actually the reverse is true, the more horsepower the motor has the less it will have to do to move the same boat the same speed. The more HP motor will be using less of its reserve capacity.


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## therapy23 (Jul 28, 2007)

Catalina274me said:


> Actually the reverse is true, the more horsepower the motor has the less it will have to do to move the same boat the same speed. The more HP motor will be using less of its reserve capacity.


I disagree.

All the bearing surfaces and working parts will be under less strain all the time.


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## SanderO (Jul 12, 2007)

I have the 8HP. It's heavy, quiet, a one pull start. Aside from the ethanol issue... and that you MUSTY add fuel stabilizer.. it's a good clean motor. 

The weight is a pain so I got a demountable Garhauer crane.

jef
sv shiva


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## T37Chef (Oct 9, 2006)

SanderO said:


> I have the 8HP. It's heavy, quiet, a one pull start. Aside from the ethanol issue... and that you MUSTY add fuel stabilizer.. it's a good clean motor.
> 
> The weight is a pain so I got a demountable Garhauer crane.
> 
> ...


How do you like that crane? I have a Garhauer davit and find the construction to be very good.


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## TSOJOURNER (Dec 16, 1999)

therapy23 said:


> I disagree.
> 
> All the bearing surfaces and working parts will be under less strain all the time.


No, because the motor will be working harder to do the same thing, like more rpm to get the same speed. Anyway who cares? This was supposed to be what are the differances and I showed the differances.


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## TSOJOURNER (Dec 16, 1999)

If 2 cars are identical except the motors, one has a 300HP V8 and the other has a 150 HP 4 cylinder, both are going up a hill, the V8 powered car will work less hard going up hill than the little 4 banger. The 4 banger will have to work it's tail off. 
The same with the boat. If I had the choisse of a 5 HP or 9.9 Hp for a boat that calls for a 6, I think the 9.9 HP would last longer cause it would not have to be working to it's full capacity (full throtle) all the time.


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## sailingdog (Mar 19, 2006)

The Honda 20 and 15 are basically the same engine... the eight and 9.9 are basically the same engine, but different from the 15/20.


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## timebandit (Sep 18, 2002)

Find a frendly parts guy and ask him to check the part numbers for the following
carburators
camshafts
pistons
intake valves
exhaust valves
cylinder heads
somewhere in there you will find the difference.


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