200hp carbs on 1989 150

[BQUICK]

How much of the 50hp difference do you think will be gained by putting 200hp carbs on? The 150 is slightly lower compression 5.6 vs 5.9 and possible exhaust tuner difference but going from 24mm carbs to 32mm would have to make a top end hp gain, I would think....

Reeds and intake manifold are same part numbers.

Exhaust manifold is main difference....any way to derestrict it without removing powerhead?

Maybe this is wrong forum....

[zenoahphobic]

How much of the 50hp difference do you think will be gained by putting 200hp carbs on? The 150 is slightly lower compression 5.6 vs 5.9 and possible exhaust tuner difference but going from 24mm carbs to 32mm would have to make a top end hp gain, I would think....

Reeds and intake manifold are same part numbers.

Exhaust manifold is main difference....any way to derestrict it without removing powerhead?

Maybe this is wrong forum....

Just putting larger carburettors doesn't always result in a more "powerful" motor. Remember it is not 200 with one and 150 with the other. These are approximate hp at best and It depends on the actually RPM reached what the actual hp reached on either motor setup
At 6000rpm's I don't think the motors have reached their peak potential. That is they can rev more (and produce more hp) before the internal engine friction becomes too detrimental taking away significant horsepower then reducing hp.
Outboards are conservatively run at this maximum revolution, as they are often run at peak for considerable periods but still remain reliable.
So the different carburettors, as you noted, are not the only differences. You may find the 150 looses lower down torque to achieve an increase in top end horsepower if you swapped carbys. What could happen is that motor takes longer to get up to power, drives like a dog if you like; and you will need precise prop pitch to ensure you reach 6000rpm. It may fall off peak rpm to an unacceptable extent.

You can do this mod, you have speed as an indicator of power when you know the rpm' s, rpm' s that can be somewhat adjusted by prop pitch selection. Remember you will probably need to play with jetting - don't blow the motor by running it too lean at high rpm.

[zenoahphobic]

And I would think you may get a greater hp gain changing the exhaust alone than you would by changing the carbs alone.

[ausnoelm]

A 30% HP increase by just replacing carbs? like that's going to happen!

[BQUICK]

Never implied that......

Same cubic inches and surprisingly big carb size difference (24 to 32) between the two. Makes me think they had to really had choke it down to get it to 150hp. Usually only small differences in carb sizes. like 34mm vs 32mm with the 225 and 200.

[99yam40]

more air in needs more exhaust back out, I would think they both would need to be changed.
might also look in some racing forums for what all they do.
porting could be different, timing, and who knows what all they change to get there

[zenoahphobic]

Never implied that......

Same cubic inches and surprisingly big carb size difference (24 to 32) between the two. Makes me think they had to really had choke it down to get it to 150hp. Usually only small differences in carb sizes. like 34mm vs 32mm with the 225 and 200.

I agree, I think the 150 is choked, and am inclined to think the 175 was the happier median for the 2.6 litre engine.
I base this on the long running 115 four cylinder stalwart; the v6 is exactly 2 more cylinders. Do the maths add 50% equals nearly 175.
I have had both, and I believe in reality the 200 is not much more powerful than the 175. I suspect as explained before we don't really know their precise output, difficult to dyno, they are just badged with those numbers. I suspect at 6000rpm the 175 is much more hp, and at 5500 the 200 is much less than 200. From observation I suspect they may nearly have the same output if propped this way.

Nevertheless I betcha many people have done this before, just changing the carbys. This forum doesn't seem to encourage any modifications, it is sponsored by a parts supplier. Invariably you will get the reply, if you want to change the hp, you look up the parts lists and change all those components that are different.

[ausnoelm]

I don't think it's that the forum doesn't encourage modification, it's just that lots of people think that changing jets or the like will give a dramatic gain in HP, some engines are easy to to increase HP, some are not.

[99yam40]

To create more HP you need to burn more fuel, to get the proper amount of fuel to air you will need to increase the air along with the fuel at the proper proportion.
if you increase the air and fuel in, you need to let all of that back out without creating a bunch of back pressure.
some time you will need to port the cylinders different and change the timing.

If the engineers already designed the motor to run properly ,why try to redesign it on your own.
find the parts that are different and change them all just like the engineers designed it.

maybe cut open the exhaust and then put a nitrus kit on so it can breath and spit HP

[zenoahphobic]

I don't think it's that the forum doesn't encourage modification, it's just that lots of people think that changing jets or the like will give a dramatic gain in HP, some engines are easy to to increase HP, some are not.

I think that point is understood by the OP, he wasn't seeking 30% more power but wondered what the change in carby size would make in hp increase.

On that note I had a Bultaco trails 360cc motorcycle that put out only 12 hp - it was meant to rev slowly with grunt with the reputation of being extremely capable. I can't remember how small the carby was (something like 16mm), but when I merely swapped it with a Frontera carby (32mm I think), there was a significant, significant power gain! I changed nothing else. Now admittedly they were simple motors with the ignition fixed and that significant power gain came from the engine then revving significantly faster and more freely. That bike also lost its ability to pull smoothly really slow.

[zenoahphobic]

To create more HP you need to burn more fuel, to get the proper amount of fuel to air you will need to increase the air along with the fuel at the proper proportion.
if you increase the air and fuel in, you need to let all of that back out without creating a bunch of back pressure.
some time you will need to port the cylinders different and change the timing.

If the engineers already designed the motor to run properly ,why try to redesign it on your own.
find the parts that are different and change them all just like the engineers designed it.

maybe cut open the exhaust and then put a nitrus kit on so it can breath and spit HP

I think we are well aware of what your saying, but it is not the exhaust back pressure that necessarily holds the engine back. Powerful two strokes can have surprisingly tiny tiny exhaust outlets (look at Grand Prix motorcycles of old). It's the wave or pulse of the charge and exhaust that determines how this "pump" works.
The engineers designed an engine to meet particular horsepower that the market wants. Sometimes the market changes so they fiddle with an existing engine to change it's parameters. Eg. The 115 became also a 130 and 140, but the last two quickly disappeared. Imagine starting from scratch, what determines how many cubic inches you need for a particular maximum horsepower output?

[zenoahphobic]

Further on the point on what engineers design, in the case of motorcycles the tricks of the trade are well known. Where horsepower restriction is required; we in Australia have large capacity bikes (600 - 700cc) that are restricted for beginner use; they use restrictions behind the carby, restrictions at the exhaust port, restrictions in the muffler opening, leaner mixtures, and particularly in fuel injection restrict how far the throttle can travel at the handle or at the throttle body. It was easy just to remove these to gain horses. Even just enriching fuel creates significant increases.
Admittedly some mods are placebo, in the eye of the rider, but there is good evidence doing a number of alterations that the cumulative affect becomes obvious.

Ofcourse there is pressure from the government to make these restrictions harder to undo, like making ECU remapping impossible.

[99yam40]

well is seems even you now admit that restricting the exhaust is a wa to tune a motor down

[zenoahphobic]

well is seems even you now admit that restricting the exhaust is a wa to tune a motor down

I failed to mention the motorbikes in #12 are 4 strokes.
Ofcourse in principal restriction means just that, and it to some extent applies to two stroke exhaust but sometimes it surprisingly makes little difference although you would think the extra velocity the exhaust gases would now exit would create frictional restriction.
You need to see examples. I used to dabble in small Remote control engines. These you would design and fit an expansion chamber exhaust, a feature of which is the stinger end and as I recall it was 1/5 the area of the exit port or engine header pipe. To control noise that stinger could be pushed into the reverse cone having no affect on the tuned length or performance. To reduce noise further we used to make that stinger tube even smaller.
Off the top of my head, a five horsepower motor the formula may have called for a 12mm diameter pipe. You could go down to 8mm or more and not see much difference in peak rpm power. It is all about the strength of the pulse, the peak pressure and the depth of the negative pressure in the wave. The principal being the negative part of the wave pulls the charge through the combustion chamber into the exhaust, then rams it back in before the port closes under the high pressure pulse in the pipe. The burnt or expanded gasses just seem to ooze out under whatever pressure, like it is a separate process!
Now I suspect what outboard manufacturers would do is just muck up the pulse by whatever is simple and cheap, little science at all.

[99yam40]

if the exhaust tuner is a different part number it is different and the engineers made that change in design on purpose