Carb CFM?

[a1bummer]

Does anybody know how many CFM that these BDS 34x4 Mikuni carbs on our bikes put out. How about a simple formula to figure it out? pi*(bore/2)^2*stroke*number of cycinders*max rpm's/3456 = CFM needed or, pi x (2.99/2)^2 x 2.60 x 4 x 8500/3456 = 180CFM needed.

[Gearhead]

Dunno about these carbs specifically, but your formula looks good. I bet these carbs are capable of twice that.

 

Jeremy

[pegscraper]

The formula looks correct, right off. Make sure to use the bore and stroke dimensions in inches, which you did. With one carb per cylinder, you won't multiply by 4 though. You'd only do that if you're using one carb for all four cylinders.

 

I'm curious about the CFM capability of these carbs too, as well as the 35mm V Max carbs and the 32mm carbs from the RSV.

[a1bummer]

You right, I wasn't thinking about the one carb per cylinder setup. 45CFM per carb then. Thanx.

[pegscraper]

You do realize too, don't you, that the formula only figures what CFM the engine will draw, and says nothing about what these carbs will actually flow? It also assumes 100% breathing efficiency, which is virtually never the case. Even 85% breathing efficiency is really high. You could cut that CFM rating down at least to 85%, if not less.

 

I'll do some digging around and see if I can come up with any CFM figures for these carbs anywhere.

[zagger]

Ditto on the volumetric efficiency: gotta multiply your result by 0.8 or 0.85, no precise way to know the correct value. Out of curiosity, why do you care? Thinking of switching to some other carb?

zag

[a1bummer]

Just curious. I have no intention of changing carbs. though that total engine management with the fuel injection would be nice. I've heard of people putting custom intakes and Holley's on these engines before. I think it may have popped up on here a couple time too. I really wouldn't see the point if the engine couldn't make use of the extra flow.

[Gearhead]

Another thing about carb flow rating is you need to know the vacuum drop it is rated at. I read that when flow ratings were first established way back before "modern" 4-barrel carbs were around, they started rating carbs at 3" pressure drop. In other words, if you sucked air through the carb with 3" vaccum, that's the flow you would get. When 4-barrels came out, they flowed too much for the existing test machinery, so they established a new standard of 1.5" pressure drop. A given carb flows less at 1.5" drop than at 3" drop (because at 1.5" the machine is not sucking as hard). So I'm told that even now if you buy a 2-barrel automotive carb it will be flow-rated at 3", while 4-barrels are rated at 1.5". I'm also told that if an engine is pulling 3" manifold vacuum at WOT, it is inhibiting performance. If you can get that closer to 1.5" (with a bigger carb), that's much better and at that vacuum you're getting about all the airflow (and thus performance) you're gonna get and a bigger carb won't help.

 

Jeremy

[ctraylor]

The formula I use in my class is RPM X Displacement/3456 X Volumnmetric efficiency (.85% if a street car and 1.10% if racing) = CFM

 

I have often wondered what the bike would run like if I built an intake that would take a 2 barrell carb. It would be much easier to keep in tune.