# proper carb sizing

Discussion in 'Performance Mods' started by dave1490, Dec 11, 2007.

1. ### dave1490Guest

One general rule of thumb uses a formula to determine the CFM requirements of your engine. It goes like this: You need to know the CUBIC INCHES of the motor. You also need the maximum RPMs the motor will be spun to. Finally you also need the VOLUMETRIC EFFICIENCY PERCENTAGE (VE%) of the engine. The first two items (CUBIC INCHES and RPMs), are relatively easy to determine. The engine VE% is another matter. If an engine could use all of the air it ingested, it would have a VE% of 100%. Many performance engines reach this level. Certain race engines can actually exceed this and reach a VE% of over 100% at certain points in their RPM range. Most production engines and most street performance engines have VE levels below 100%. In fact, stock, production, low performance motors will fall around 75%-85% volumetric efficiency.

volumetric efficiency=raming effect{what i call it}

{cubic in x rpm}divided by 3456 mulitplyed by volumetric effeciency.

this will give you the cfm,s rating needed for your bike now just fine a carb or 2 rated for it.otherwise over carbing it will make it bog.

for more detail heres a link.

http://www.mortec.com/carbtip1.htm

2. ### jpilotGuest

Sorry to have to disagree but the article you cut and pasted the information from is talking about 4 stroke automobile engines. Most motored bikes use small, single cylinder, two stroke engines.

There are several reasons that the formula given in the article is not applicable to our two stroke engines:

1. A four stroke engine ingests air/fuel every other revolution of the crankshaft. Two strokes, of course, ingest air/fuel every revolution of the crankshaft.

2. The actual displacement of a two stroke engine is not simply a function of the piston swept area and stroke. The displacement of a piston ported two stroke like most of us run is less than the result of a simple bore/stroke calculation because the position and size of the intake and exhaust ports will greatly affect the effective displacement.

3. Who has data on the volumetric efficiency of these engines? You could make a guess at what you think it might be but guessing will only lead to incorrect results.

A two stroke is a completely different animal than the engine in your car so the theories and practices applicable to an automobile engine don't generally apply.

3. ### dave1490Guest

ya your right.i keep thinking you can combine the two.see that,s what happend,s when you had a couple of beer,s.

Last edited by a moderator: Dec 13, 2007
4. ### EgorGuest

There is probably a formula for the Thermal efficiency of the two stroke engine, and volumetric also. Because of the fact that the two stroke intakes every revolution, with all its inefficiency it might approach the flow volume of the 4 stroke, I know they like the same size carbs. My 70 Dax likes a 16mm carb. I know there is controversy as to how the engine size is determined but the swept volume is how it is done. The bore and stroke can be altered but it has to be 50cc in the 50 class, they will not let you subtract the ports. My old Yamaha 55 would always beat the Honda 55 there was never a chance it would win. the two stroke Kreidler 50 I think holds the speed record at around 120mph, they just need a bigger tank, (there goes the Thermal efficiency). Have fun Dave

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5. ### kevboGuest

The carb size matters only when air is moving through it, which for a single cylinder engine is only 1/4 the time with a four-stroke, and 1/2 the time with a two-stroke. (well, much less really with piston valving on our HTs)

A two stroke, and a four stroke with the same displacement draw the same size breaths at the same rate, just that the two stroke draws them twice as often.

And that is why the two-strokes and four-strokes of a given displacement run nice with the same size carbs.

Last edited by a moderator: Dec 21, 2007