Freestyle Why so big carbs???

[Kawastyler]

Hallo , im wondering about carbdiameters we use for our engines. All big 2strocke crosscycles are using 38 up to CR 500, only CR 500 bigbore/strocker 680 single need 41/44mm carb and makes 90hp with it. Is there a great difference betwen SBN and MX carburator/engine???

Thanks for your help, Andy

 

[Vumad]

Hp numbers are not comparable. Remove the clutch and the transmission, and numbers will be much lower.

 

[Kawastyler]

Hp numbers are not comparable. Remove the clutch and the transmission, and numbers will be much lower.

You mean a KTM 380 with full desingnes ports and pvs has less hp then the half of a 760 yami with tranfers from the 70's???

I dont think so....

 

[tor*p*do]

actually transmission robs power, highest hp is at the crank.
Andy, you are comparing apples and oranges
Marine application is fundamentally different than a road application
and some engineers who know a lot more about engine design and carburation set them up that way!

 

[LimitedSlip7]

Hp numbers are not comparable. Remove the clutch and the transmission, and numbers will be much lower.

You're thinking of torque. HP cannot be changed with gearing, however torque can.

 

[Nuklear6]

The difference between the jet ski performance world and other power sports is that jet ski Engines can not benefit from gear reduction. We expect our engines to make great power from 1500-6500 + rpm. Because of this port timing must be kept lower among other compromises that are made in performance to keep such a broad power band. I build micro sprint engines, my best combination is 269 cc and makes 94HP, however it only makes good power from 7500- 10000 rpm. Under 7000 rpm it is a complete dog. Would be the worse jet ski engine ever, but because it has a 5 speed gear box it can be kept in its power range at all times.

Now to better answer the op's question. As said above other power sports have power band rpm numbers that are much higher. Therefore there will be alot more drawl or suction created by the engine which allows the engine to get the needed air/fuel volume through a small diameter carb. As we increase performance in our jet ski engines we can't increase the power band rpm by enough to increase the drawl the engine needs to get more air/fuel out of the stock size carbs so we have no choice but to go bigger and make it easier for the engine to get the air/fuel it needs to make the power we ask of it.

 

[augustaf]

The difference between the jet ski performance world and other power sports is that jet ski Engines can not benefit from gear reduction. We expect our engines to make great power from 1500-6500 + rpm. Because of this port timing must be kept lower among other compromises that are made in performance to keep such a broad power band. I build micro sprint engines, my best combination is 269 cc and makes 94HP, however it only makes good power from 7500- 10000 rpm. Under 7000 rpm it is a complete dog. Would be the worse jet ski engine ever, but because it has a 5 speed gear box it can be kept in its power range at all times.

Now to better answer the op's question. As said above other power sports have power band rpm numbers that are much higher. Therefore there will be alot more drawl or suction created by the engine which allows the engine to get the needed air/fuel volume through a small diameter carb. As we increase performance in our jet ski engines we can't increase the power band rpm by enough to increase the drawl the engine needs to get more air/fuel out of the stock size carbs so we have no choice but to go bigger and make it easier for the engine to get the air/fuel it needs to make the power we ask of it.

Above is the best answer yet. Weil Jetskis kein Getriebe haben muessen die Vergaser einfach groesser sein um genuegend Kraft bei allen Drehzahlen zu erzeugen!

 

[Balduccimoto]

The difference between the jet ski performance world and other power sports is that jet ski Engines can not benefit from gear reduction. We expect our engines to make great power from 1500-6500 + rpm. Because of this port timing must be kept lower among other compromises that are made in performance to keep such a broad power band. I build micro sprint engines, my best combination is 269 cc and makes 94HP, however it only makes good power from 7500- 10000 rpm. Under 7000 rpm it is a complete dog. Would be the worse jet ski engine ever, but because it has a 5 speed gear box it can be kept in its power range at all times.

Now to better answer the op's question. As said above other power sports have power band rpm numbers that are much higher. Therefore there will be alot more drawl or suction created by the engine which allows the engine to get the needed air/fuel volume through a small diameter carb. As we increase performance in our jet ski engines we can't increase the power band rpm by enough to increase the drawl the engine needs to get more air/fuel out of the stock size carbs so we have no choice but to go bigger and make it easier for the engine to get the air/fuel it needs to make the power we ask of it.

So pretty much a larger carb will let the engine breathe easier for those lower rpm. A smaller carb, inletwould result in higher intake velocity and atomization at higher RPM?

 

[Christian_83]

Nice write up Chuck, that makes good sense, i actully though about it, if the rpm could affect the draft through the carbs, and there for the carb size!
Ps check your PM

 

[Kawastyler]

Ok, but i owned a CR500 from99-02 and it had awersom torque from 2000rpm in every gear. Sold it because it was to strong for me. Dont know who need a bigbore/strocker from it???))

 

[Vumad]

You're thinking of torque. HP cannot be changed with gearing, however torque can.

HP is just torque x rpm. It's not even a real thing. It's just a theory.

A bike motor with no transmission or clutch would produce so little horsepower below it's peak that it would be a useless motor. The numbers are high on bikes because the transmission allows that narrow power band to be used over a large range of speeds via gear reductions. Jet skis must make enough power throughout their entire RPM range. They give up peak hp to improve low end torque, something a bike doesn't have to do. Remove the transmission and the bike wouldn't move. So I stick by my claim, they have high numbers because they have transmissions and clutches. Make it a direct drive, and that motor wouldn't make that bike move.

The difference between the jet ski performance world and other power sports is that jet ski Engines can not benefit from gear reduction. We expect our engines to make great power from 1500-6500 + rpm. Because of this port timing must be kept lower among other compromises that are made in performance to keep such a broad power band. I build micro sprint engines, my best combination is 269 cc and makes 94HP, however it only makes good power from 7500- 10000 rpm. Under 7000 rpm it is a complete dog. Would be the worse jet ski engine ever, but because it has a 5 speed gear box it can be kept in its power range at all times.

Now to better answer the op's question. As said above other power sports have power band rpm numbers that are much higher. Therefore there will be alot more drawl or suction created by the engine which allows the engine to get the needed air/fuel volume through a small diameter carb. As we increase performance in our jet ski engines we can't increase the power band rpm by enough to increase the drawl the engine needs to get more air/fuel out of the stock size carbs so we have no choice but to go bigger and make it easier for the engine to get the air/fuel it needs to make the power we ask of it.

He said it much better than I did. Hah.

 

[wanderer1]

HP is just torque x rpm. It's not even a real thing. It's just a theory.

A bike motor with no transmission or clutch would produce so little horsepower below it's peak that it would be a useless motor. The numbers are high on bikes because the transmission allows that narrow power band to be used over a large range of speeds via gear reductions. Jet skis must make enough power throughout their entire RPM range. They give up peak hp to improve low end torque, something a bike doesn't have to do. Remove the transmission and the bike wouldn't move. So I stick by my claim, they have high numbers because they have transmissions and clutches. Make it a direct drive, and that motor wouldn't make that bike move.

Off topic here I know but, don't we have crankshaft horsepower and RWHP which is after adding a transmission of any type. How'd they get the crankshaft horsepower (which is usually higher) without the tranny? Built a few motors over the years and been around a few dyno rooms (no transmission) and chassis dynos (with transmission) so your statement confuses me. Gonna need a little more.

 

[Vumad]

Off topic here I know but, don't we have crankshaft horsepower and RWHP which is after adding a transmission of any type. How'd they get the crankshaft horsepower (which is usually higher) without the tranny? Built a few motors over the years and been around a few dyno rooms (no transmission) and chassis dynos (with transmission) so your statement confuses me. Gonna need a little more.

It probably confuses you because I don't know what I'm talking about. Sometimes I participate in discussions I know a bit less about to have people tell me I'm wrong. Lots of people have their head up their asses and are stuck on "my way or the high way". Certainly a lot more experience around here than what I have and I'm open to feedback. We all have limited resources and a lot of my understanding about a topic like this is through research rather than hands on. I just haven't had those opportunities. That's why I try to keep my replies in polite debate (here's my side, what's yours?) format rather than as an argument (I'm right, come to my side) format. I'm very interested in what you guys have to say and very open to being told I'm wrong.

Edit: I assumed that everything with a tire was measured off the rear wheel. I was just trying to make the point that it's not a apples to apples comparison of power when one engine gets widen it's usable power with a transmission and another has lower numbers because it's a direct drive. A dirt bike never has to make any real power at 2000rpm. Not a fair measurement against a jet ski.

 

[wanderer1]

Wasnt trying to argue,I guess you took it that way. Keep it apples and apples,crankshaft (as I mentioned)to crankshaft. What I was asking about,I guess I shouldve been more clear, was the no power without transmission idea. I don't see HP as a theory or a product of torque coupled with rpm range,but I guess it could be viewed that way. Just asked for some clarification on where you were coming from. Hope ya have a good one.

 

[Vumad]

Wasnt trying to argue,I guess you took it that way. Keep it apples and apples,crankshaft (as I mentioned)to crankshaft. What I was asking about,I guess I shouldve been more clear, was the no power without transmission idea. I don't see HP as a theory or a product of torque coupled with rpm range,but I guess it could be viewed that way. Just asked for some clarification on where you were coming from. Hope ya have a good one.

I didn't take it as arguing. I was discrediting myself as a credible source of information and expressing my interest to learn.

 

[kevbo]

Hallo , im wondering about carbdiameters we use for our engines. All big 2strocke crosscycles are using 38 up to CR 500, only CR 500 bigbore/strocker 680 single need 41/44mm carb and makes 90hp with it. Is there a great difference betwen SBN and MX carburator/engine???

Thanks for your help, Andy

A standard SBN has a throttle shaft and venturi where a MX carb does not. An SBN 44 has only 39mm of airflow area where a bike carb is almost unobstructed. A bike can be lugged down at full throttle so their carbs cant be too large or else they will not draw fuel. Watercraft dont have this problem, this is why giant carbs can work on small engines. In the world of watercraft, an oversized carb makes the pipe hit harder than one that is just big enough to achieve good power. Plus this market will buy anything that is bigger without question.

 

[Joker]

Plus this market will buy anything that is bigger without question.

This is probably the most true statement I have ever heard in my life.

 

[KTM434]

Vumad, transmissions with gear reduction helps to keep an engine in the usable torque range but when measuring an engines output it's not done in first gear with all that gear reduction to multiply the engines power output, it's normally done in direct drive. A chassis dyno also measures wheel speed and engine rpm to factor in gear reduction. BHP or crankshaft power is the most accurate since different drivetrain setups will have a different amount of power loss. 20% drivetrain loss in a car/truck is average from what I've heard. A jetski is obviously direct drive but doesn't have a direct connection to the surface. A dirtbike has a direct connection to the ground with the wheel but a jetski has an impeller which spins faster than its actually hooking the water

 

[Vumad]

Vumad, transmissions with gear reduction helps to keep an engine in the usable torque range but when measuring an engines output it's not done in first gear with all that gear reduction to multiply the engines power output, it's normally done in direct drive. A chassis dyno also measures wheel speed and engine rpm to factor in gear reduction. BHP or crankshaft power is the most accurate since different drivetrain setups will have a different amount of power loss. 20% drivetrain loss in a car/truck is average from what I've heard. A jetski is obviously direct drive but doesn't have a direct connection to the surface. A dirtbike has a direct connection to the ground with the wheel but a jetski has an impeller which spins faster than its actually hooking the water

But do they advertise the torque curve? Peak horsepower at 7000rpm may be higher, but the torque curve at 3000 would be much much lower. The engine will have a transmission so they can push the torque towards the higher rpm and leave the lower rpm lacking in torque. That was what I was getting at. Crank shaft horsepower might be lower on the direct drive, but the torque cube is more level and continuous, while the engine that will be getting a transmission has very low starting torque and peaks very high in the later rpm. A 70 horsepower dirt bike engine won't work in a jet ski because it has a very high rpm torque curve and wouldn't produce power in the low rpm. What I have been trying to get at is the torque curve and how motors designed to be used with transmissions have very different curve styles. Cant just compare one number, have to look at the big picture.

Btw, that explanation about the carb venturies and the lack of slippage on a tire was great.

 

[wanderer1]

Torque and HP produced by a engine is not changed by a transmission, I cannot change gear ratios and gain or loose crankshaft HP or torque.The wheel (inertia) dyno knows the time it took to spin the drum, the drum speed (MPH), the drum mass, and the engine RPM. From that, it computes the power that would be required to spin the drum at the rate of acceleration. If the drive ratio was 1:1 (including the fulcrum arm of the wheel) and the engine produced 100 TQ and there was no loss in the drivetrain then 100 TQ would have been applied to the drum as well. Change the ratio to 10:1 reduction. The engine still produces 100 TQ, but 1000 TQ is applied to the drum. If the Dyno is doing what it should do, it should still report 100 TQ from the engine ... clear as mud? Current chassis dynos can give correct cranshft HP if given feilds are accurately entered,thus negating the addition of the transmission. A engine makes the power and torque it can make and cannot do anymore with a transmission,the transmision can help the engine apply that force in different ways.Maybe a bad explanation but maybe it helps.