Bigger carbs vs. bottom end performance

[SLORYDER]

You say that going from 26mm carbs to 33mm carbs will cause you to lose bottom end performance.

 

Please explain why, so I will know

 

Thank you.

[SLORYDER]

I cant explain why, but I can tell you that bigger carbs actually do kill the bottom end. My banshee has 35mm carbs and I have NO bottom end at all. Im switching back to stock carbs.

 

You sure it isn't the jetting?

[pr0blumz]

My banshee has 35mm carbs and I have NO bottom end at all. Im switching back to stock carbs.

 

What's the rest of your setup?

[SLORYDER]

This is how I view it. I may be way off.

 

A banshee engine needs a certain volume of a/f to run at a certain rpm.

 

Say, for instance that rpm is 1,000; it may take say 100 cc's of air per minute. Now to me that can be achieved by having a 26mm carb running at 10% OR a 35mm carb running at 4%.

Either way, the same amount of air is being fed into the engine, and if the engine is drawing the same amount of air from each carb, and the orifice passing the mixture is the same size (which in my mind it would have to be), then the velocity would be the same, which leads me to have doubts about the velocity theory.

 

Anothe reason I don't but the velocity theory is that the final velocity of the stream is determined by the size of the transfer ports and the pressure pushing the stream.

IN MY OPINION

 

Thoughts?

[Hilarious]

You say that going from 26mm carbs to 33mm carbs will cause you to lose bottom end performance.

 

Please explain why, so I will know

 

Thank you.

 

a larger carb has a lower velocity (speed of air comming into the engine) where as a smaller carb will have a higher velocity at

lower rpms.

[SLORYDER]

a larger carb has a lower velocity (speed of air comming into the engine) where as a smaller carb will have a higher velocity at

lower rpms.

 

Say, for instance that rpm is 1,000; it may take say 100 cc's of air per minute. Now to me that can be achieved by having a 26mm carb running at 10% OR a 35mm carb running at 4%.

Either way, the same amount of air is being fed into the engine, and if the engine is drawing the same amount of air from each carb, and the orifice passing the mixture is the same size (which in my mind it would have to be), then the velocity would be the same, which leads me to have doubts about the velocity theory.

 

Anothe reason I don't but the velocity theory is that the final velocity of the stream is determined by the size of the transfer ports and the pressure pushing the stream.

IN MY OPINION

[SLORYDER]

The rest of my setup is T5 pipes, v force reeds and no airbox. I am going to pull the carbs off tonight and see what the jetting is. I have not had them apart yet. I have a pair of stock carbs that I want to try so I can see what the difference is between them and the 35mm carbs.

Make sure your jetting is right before you jump to any conclusions..

[JKJK]

Try to look at it this way.Engine A will use x amount of fuel regardless of what carbs are applied.The carbs job is to atomize the fuel mixture+supply the engine a consistent ratio of air to fuel,through out the whole rpm range.Basically when you over carb,The weaker signal,velocity or draw through the larger carb will not produce a consistently sufficient signal for the carb to work optimally through out it's three circuts.

[GNR101]

Say, for instance that rpm is 1,000; it may take say 100 cc's of air per minute. Now to me that can be achieved by having a 26mm carb running at 10% OR a 35mm carb running at 4%.

Either way, the same amount of air is being fed into the engine, and if the engine is drawing the same amount of air from each carb, and the orifice passing the mixture is the same size (which in my mind it would have to be), then the velocity would be the same, which leads me to have doubts about the velocity theory.

 

Anothe reason I don't but the velocity theory is that the final velocity of the stream is determined by the size of the transfer ports and the pressure pushing the stream.

IN MY OPINION

 

 

going bigger in carbs will loose u bottom end..

it happens ..

deal with it..

 

its not ur opinion that makes it happen

[SLORYDER]

Try to look at it this way.Engine A will use x amount of fuel regardless of what carbs are applied.The carbs job is to atomize the fuel mixture+supply the engine a consistent ratio of air to fuel,through out the whole rpm range.Basically when you over carb,The weaker signal,velocity or draw through the larger carb will not produce a consistently sufficient signal for the carb to work optimally through out it's three circuts.

 

Ps sorry for the caps lock

 

ok i SEE WHAT YOU ARE SAYING.

yOU ARE SAYING THAT TOO SMALL AN ENGINE DOESN'T PUT ENOUGH SUCTION ON A BIG CARB TO ALLOW IT TO PERFORM AS IT IS INTENDED.

bUT EVEN IN THIS INSTANCE, IT SEEMS LIKE YOU COULD JET DOWN ON THE SLOW JET/ DROP THE NEEDLE TO COMPENSATE FOR THIS...

 

tHE REASON i BRING THIS UP IS THAT i HAVE SPOKEN TO MORE THAN ONE BUILDER (SPONSER)THAT SAID THE BOTTOM END LOSS IS BOGUS.

[JKJK]

You will loose throttle response or snap.Your topend might be stronger,but getting there will be slower with the bigger carbs.

[AKheathen]

ok, you are basically look at this like a fuel injected motor, where the total air is set by the throttle body(s) not counting itake flow, etc. there, atomization is dependant on the fuel injectos them self, and one body can have as much bottom end as another 1/2 the size. it is possible, to tune a large carb to atomize low throttle/low rpm well, but, the adjusting increments are muah larger, and the tuned venturi is further away in cfm than a smaller carb. for most, the bottom end will simply just be "impossible to tune right". atomization controll via the airjet, needle jet, and needle, aside from expensive, will be more favaorable of one area, rather than another. if you want, go buy the rest to play with, tune it better, and praactice roll-on throttle and you may et some lowend back, but conventional tuning isn't gonna do it.

[Larry's Shee]

Slo , how about you get two sets of carbs and do a butt dyno comparison and report back. Remember that sceintifically you only change one thing at a time, ie, you cant put v-3 on stock and stock on 35's. A good example of all of this is the old spread bore carbs, small primaries HUGE secondaries.

[madjimmax]

I've kinda been wondering the same thing. I just got my shee back together a month ago with the stockers and it runs really hard, But Jeff @ FAST told me that I should switch to 33pwks and I'll have a much broader power band with no loss on the bottom at all. I'm going to make the switch, But I was still wondering about this.

[SLORYDER]

I've kinda been wondering the same thing. I just got my shee back together a month ago with the stockers and it runs really hard, But Jeff @ FAST told me that I should switch to 33pwks and I'll have a much broader power band with no loss on the bottom at all. I'm going to make the switch, But I was still wondering about this.

 

Yeah Jeff and i think it was Kevin Herr that told me the same thing.

 

It just doesnt make sense that 2 venturis of the same size (33mm at 10% and 28mm at 18%) could give you different results if they were passing the same amount of air.

 

People talk about velocity ect which is pertinent in a four stroke application because the flow goes straight from carb to cylinder but in this instance the transfer phase is a separate ordeal from the intake. A smaller carb just creates more pumping losses at higher rpm's

Edited June 8, 2010 by SLORYDER