Author Topic: Carb sizing.....torque vs topend  (Read 2315 times)

ludedude

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Carb sizing.....torque vs topend
« on: June 24, 2004, 12:49:16 AM »
Can't sleep  :roll:

And was wondering why. Why is a smaller carb size better for torque and a larger bore carb better for top end, but looses on bottom end?

Larger bore = more air flow and more fuel....right? Why does that hurt bottom end?

ludedude

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Carb sizing.....torque vs topend
« Reply #1 on: June 24, 2004, 06:17:18 AM »
It came to me......(not telling how  :P )


All relates to carb signal and flow.

Smaller carb will have higher charge velocity at low rpm than a bigger carb. Garden hose and thumb thing - stick your thumb over the end of the hose and the flow rate stays the same, but the velocity of the water leaving the hose goes up. Smaller hole, same flow, higher velocity, better cylinder stuffing.

The carb eventually becomes the limiting factor in high rev situations - the engine can take more than the carb's able to pass, so a larger carb will flow more at a higher velocity and you end up with more top end.

But the inverse is true with a bigger carb - it doesn't flow as "fast" as a smaller diam carb at lower signals, hence it doesn't give as good of a mixture.

There is a company that makes diverters that fit inside the intake bell of the carb http://www.scaryfastracing.net/ that trick the carb into thinking it's a smaller diam unit at lower throttle positions. From all the stuff I've read, the piece works extremely well - especially in four bangers. - even the MX mags say it's a very noticable, well worth the price, item.

ludedude

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Carb sizing.....torque vs topend
« Reply #2 on: June 24, 2004, 06:20:05 AM »
OK, this may look like a conversation with myself...maybe it is, maybe it's not :P

If the above is true...why is it still a debate as to wether or not oval boring makes a noticeable/worthwhile effect?

Oval boring a carb being the upper end (WOT) of the intake is a larger bore than the lower end (low end).

redrider

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not schizo
« Reply #3 on: June 24, 2004, 09:36:33 AM »
I'm only adding a reply post to say, "Hmmm, interesting stuff Lude.  Thanks for sharing."
But mostly I'm adding it to keep you from appearing so schizo.
 
Both of us here hope you're doing well   :)

PilotSniper

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Re: not schizo
« Reply #4 on: June 24, 2004, 03:51:48 PM »
Quote from: "redrider"
I'm only adding a reply post to say, "Hmmm, interesting stuff Lude.  Thanks for sharing."
But mostly I'm adding it to keep you from appearing so schizo.
 
Both of us here hope you're doing well   :)


LOL!!!  :lol:

Lude, I'm reading buddy! I find it interesting, but I don't have answers, so I'm just absorbing like a sponge.

So.....do you have an explanation on the oval carb???
I started out with nothing and I still have most of it left!  :shock:

ludedude

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Carb sizing.....torque vs topend
« Reply #5 on: June 24, 2004, 05:21:16 PM »
Not yet....but my answer man may come through yet :) lol

Odyknuck

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Carb sizing.....torque vs topend
« Reply #6 on: June 25, 2004, 09:51:45 AM »
The Oval Carb is a variable venturi to provide a smaller venturi at low throttle and a larger venturi at higher throttle positions. When the machining is done the cutter start position is right above the bottom of the venturi. Hence as the throttle valve rises in the venturi the opening gets larger. You get the besyt of both worlds in one carb. The limiting factor is you can only bore so much out of the venturi before you cut thru the casting

ludedude

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Carb sizing.....torque vs topend
« Reply #7 on: June 25, 2004, 11:09:18 AM »
I know "how" it's done. My question is, in theroy it sounds great, but I've heard it doesn't work, or not that well, doesn't make much difference.

Is that beacuse as you siad Odyknuck you can only bore out so much before punching through?

There's got to be more to it...or some one would cast a triangular bore carb!

Odyknuck

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Carb sizing.....torque vs topend
« Reply #8 on: June 25, 2004, 12:25:46 PM »
Below is a responce from MacDizzy to that very question. I have not used one my self so I can not offer an empircal answer. I would think that it is a viable alternative to be able to have an all around setup for trail riding and racing in one carb.
==============================================

The response - It is an attempt to make a large carburetor behave as though it were a smaller one (at lower throttle speeds) and a smaller one behave like a larger one (at higher throttle speeds).
 
By making the bore oval (taller vertically) you are able to give the carb increased air flow at the fully open position and less flow at
partially open positions. Since you didn't cut a round hole in there, the stock slide will still cover the throttle bore. Most carburetors can handle an increase in bore in the neighborhood of 1 to 2 mm. If you can add another mm vertically - you have gained something. Not much - but you have gained some increased air flow.
 
As far as whether this feeds the reeds or crankcase any better... I would guess that perhaps it does at low flow (1/4 to 1/2 throttle). But the margin must be very close. If the actual calculated oval area is the same as the actual calculated round area (of 2 different carbs), there would be no detectable difference at moderate throttle positions - therefore they would both be making the same power. There may be an advantage below and above this area to oval boring - if the engine is jetted correctly.

ludedude

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Carb sizing.....torque vs topend
« Reply #9 on: June 25, 2004, 05:20:47 PM »
Yeah, that's where I read it!

Quote from: "Odyknuck"

But the margin must be very close.


What does that refer too???