TOPIC:

Nessism post=860245I am not an expert on this subject, but from what I've read, the RS34 performs much like a VM29.  Your questions are good ones, and I can't answer why, though.  

I've been poking around the web and I don't see any good answers.  But here are some random numbers I came across, the first in an old GS thread that it seems like Nessism posted in, where someone claims:

29 mikuni smoothbore carbs flow 75 C.F.M. per carb (so X4 for the whole bank)
33 mikuni smoothbore carbs flow 97 C.F.M.

www.thegsresources.com/_forum/showthread...&p=767856#post767856

Combine with some wisdom from Larry C:

Essentially you have 3 OEM cylinder heads to work with.
* KZ 900 / 1000 [68-70 CFM]
* J Model [72-74CFM]
* GPz11 Bathtub Chamber [82-84 CFM]

www.kz-1000.net/post/130285291257/faq-ab...kz900-kz1000-engines

If those numbers are roughly close, it would suggest that the VM29 would supply all that was needed for a KZ head that had not been worked by Larry.  Where a VM33 would probably be overkill on a KZ, and even a bit large on a bathtub GPz head.  I have no idea if any of that is accurate, but if true it may explain the popularity of the VM29.

I'm assuming the CRS and RS carbs are similar or better than the VM for the same bore diameter, as I don't see why they would be worse.  And I'm guessing the BS/CV carbs are less efficient for a given diameter, given the large jump in diameter when Kawasaki moved from the VM to the BS carbs.

One more random thing I found was a page on sizing Weber venturis on Porsce motors, which maybe is sort of similar?  It's based largely on valve size, sort of.

www.performanceoriented.com/throttle-bod...-main-venturi-sizing

I'm not exactly sure what to make of all of that, but I can tweak the numbers and get anywhere from 26mm to 33mm, so the answer might be in there somewhere?




 

I'm not exactly sure what to make of all of that, but I can tweak the numbers and get anywhere from 26mm to 33mm, so the answer might be in there somewhere?

Here is some math -
55ci/4=13.75ci
13.75 * 4500 = 61875/1768(ci to cfm) = 34.997cfm *.8(% efficiency) =27.997cfm
So a 73 engine sucks 28 cfm per cylinder at max rpm, that's why they used vm28 carbs.
 

So you are saying that a 28mm carburetor flows 28cfm at whatever level of vacuum that hypothetical engine makes at 4500rpm? I am confused.

wrote:

So you are saying that a 28mm carburetor flows 28cfm at whatever level of vacuum that hypothetical engine makes at 4500rpm? I am confused.

If the cylinder needs only 28 cfm, why would Kawasaki design a head that flows 70?

wrote:

Here is some math -
55ci/4=13.75ci
13.75 * 4500 = 61875/1768(ci to cfm) = 34.997cfm *.8(% efficiency) =27.997cfm
So a 73 engine sucks 28 cfm per cylinder at max rpm, that's why they used vm28 carbs.


 

To unpack that a bit, 55ci is 901cc. I think the 4500 is (Max RPM * 0.5), because 4-stroke, so max RPM=9000?  And you used 80% for volumetric efficiency.

Using this engine CFM calculator and those numbers I get nearly the same answer (28.6 cfm).

4secondsflat.com/Carb_CFM_Calculator.html

Which certainly brings up the very good question that TexasKZ asked.  The engine's required CFM and the port flow numbers that Larry posted don't seem to correlate.  I don't have what to make of that.

I will say that I'm skeptical that the carb diameter of 29mm directly translates into a flow rate of 29CFM.  That would be way too convenient.  

 

If the cylinder needs only 28 cfm, why would Kawasaki design a head that flows 70?

Which certainly brings up the very good question that TexasKZ asked.  The engine's required CFM and the port flow numbers that Larry posted don't seem to correlate.  I don't have what to make of that.

You want the manifold and carburetor the same size or a smidge larger then the intake valve so air can get into the engine for maximum performance.
A 73 engine sucks 28 cfm per cylinder at max rpm.
You can install bigger pistons, bigger valves, high lift cams to increase the cfm a small amount, you will never reach 70 cfm.


 

wrote:

If the cylinder needs only 28 cfm, why would Kawasaki design a head that flows 70?

Which certainly brings up the very good question that TexasKZ asked.  The engine's required CFM and the port flow numbers that Larry posted don't seem to correlate.  I don't have what to make of that.

A 73 engine sucks 28 cfm per cylinder at max rpm.






 

Sorry! but that is incorrect..
It will be flowing around that per cylinder on average just approaching 1/2 throttle.
There was a tuning article in one of the mags years ago that printed the results using an inline mass airflow meter on a stock running Z1 engine.
I also remember that the flow rate was not linear to rpm but rose fairly rapidly after around 1/3rd throttle.(which is largely down to the cam profile etc).
A stock head flows around 68cfm so if it only required 28cfm at max revs then port work and bigger cams to increase flow capacity and increase power or simply bigger carbs on an otherwise stock engine would make no difference which is obviously not the case.
The CFM calcs being used are either incorrect or the results are being misinterpreted..

 

The CFM calcs being used are either incorrect or the results are being misinterpreted..

The numbers I posted are correct.
Math and physics are the truth, not a magazine article from years ago.
You can check the math, you can consult with an expert, let me know if you get another answer.
 

Wasn't this article from Larry Cavanaugh?
Steve
Essentially you have 3 OEM cylinder heads to work with.
* KZ 900 / 1000 [68-70 CFM]
* J Model [72-74CFM]
* GPz11 Bathtub Chamber [82-84 CFM

Completely out of league but does this help at all

www.4secondsflat.com/Carb_CFM_Calculator.html

www.homebuiltairplanes.com/forums/thread...roke-carb-cfm.26481/