Topic: benefits of Keihin CR vs Keihin factory round tops on a cb750

[gschuld]

Just for curiosity, what specifically are the benefits to going with CR carbs over early round tops.  Obviously, CR 29 carbs flow more air than round tops do.  But carb for carb, say stock cb750 round tops vs CR 26 carbs of the same throat size, what makes the CR series better carbs on a performance engine?

George

[dusterdude]

Yea,inquiring minds want to know

[TurboD]

 Have a good day.

[gschuld]

Hopefully you will have a bored round top carb soon to compare with the others.

George

[Retro Rocket]

Fitting CR's will automatically give a rise in HP, they are race carbs for a reason guys...   The stock Keihin's won't flow as much as the CR's in the same advertised throat size..

[TurboD]

Have a good day.

[scottly]

Jerry Branch did some interesting flow testing back in the stone age.

[Retro Rocket]

And we have the back to back dyno results or flow bench data on this?

This type of answer annoys me, we all have flow benches and dyno's.. . You don't need it, go down to any track and watch the bikes and their set ups, Every fast guy on a 750/4 runs CR's, besides, if you've seen both carbs side by side you'd know what i'm saying, the Keihins are not a true 28mm carb, George would know this..  Do you think all the guys using CR's have been sucked into buying something they don't actually need...?  Didn't you ever own a stock old chugging V8..?  Stick a holley on and get instant HP gains..?  Its not really a secret that good carbs give HP gains, I've known this since i was first playing with cars and bikes nearly 40 years ago...

Just as a matter of interest, the Keihin  advertise a bolt on 10% horse power gain with CR's...

[FunJimmy]

CR carbs are considerably lighter and have a lot of jetting adjustments & options readily available.

[gschuld]

Let's settle down, nothing to get feathers ruffed over. 

Clearly CRs are the choice for cb750 road racers.  Factory early 28mm round top carbs measure out to the equivalent to 27.6mm throat size compared to 29s, 31s, 33s, 35s CR carbs.  Perhaps some may have guessed what I am really pondering:

That is ...what might the potential be for a set of my 29.4 throat size bored, blended, and polished throat early round top carbs compared to a set of equivalent sized CR29s.  Let's say for example in a road racing application, on a theoretical 12.5:1 915cc based cb750 motor, fully ported head, big valves, road race cam, Yoshi copy exhaust, etc. 

I would be SHOCKED if a set of CR29s would flow any more raw air volume in CFM than my 29.4 bored, blended, and polished carbs at WOT.  What I have no idea about is how well the two would compare regarding feeding the engine with a well atomized and optimized air/fuel ratio throughout the rpm range suitable for road racing.  I would seriously doubt my bored carbs would be on par or better than a set of CRs, but I'd be curious as too how far off the mark they would be.  So I am wondering if there are any features of the fuel delivery system or inherent overall design of the CR carbs that are specifically superior to early round top carbs, beyond the obvious larger throat size/cfm capacity.



For early "28mm" round top carbs:
 
Factory specs measure out to:  (same numbers for both K0 carbs and rest of rack throttle round top carbs, measured both)
 23.95mm wide
 30.57 mm tall
  11.975mm corner radius (15/32" corner radius) or 15/16" end mill
total area 608.95 square mm
equivalent to 27.6mm throat(less than the "28mm")
 
My bored carbs are:
24.6mm wide
30.8mm tall
 7.93mm corner radius,  (5/16" corner radius) or 5/8" end mill, not totally flat bottomed
total area 670.22 square mm
equivalent to 29.4mm throat

These are the dimensions of the throat on Frank's(754s) bored carbs:
 25.3mm wide
 30.8 mm tall
  7.93 mm corner radius (5/16" corner radius) or 5/8" end mill
 total area 725.31 square mm
equivalent to 30.4mm throat

By chance does anyone have a set of CR carbs(29 and or 31) that would be willing to measure a few things for me?
 
George

[scottly]

So I am wondering if there are any features of the fuel delivery system or inherent overall design of the CR carbs that are specifically superior to early round top carbs, beyond the obvious larger throat size/cfm capacity.

That is a valid question, George, that can only be answered on the dyno...

[gschuld]

A back to back dyno comparison on a strong motor would be very telling, but even that wouldn't really tell the whole story.  The up and down throttle movement and different loading on the motor during road racing would logically "challenge" a carbs true performance more so than a linear pull and resistance load on a dyno in a controlled atmosphere.  But yes, perhaps a dyno comparison in the future could happen and shed some valuable light on the question.


"Just as a matter of interest, the Keihin advertise a bolt on 10% horse power gain with CR's... "  That's a bold across the board statement.  It is of course totally dependent on what the airflow requirements on the particular motor in question is.  I seriously doubt CRs will add 10% HP on a dead stock 736 motor, yet I would fully believe that 10% gain would be expected on a 1080cc, fully ported, high compression, giant cammed, 11,000rpm+ race motor.  Factory 28mm round tops are just not capable of supplying the engine with as much airflow as it ideally "wants" in that situation, where larger CRs could.

George

[Retro Rocket]

I think another thing to take into consideration with the CR's compared to the stock carbs are the differences in the fuel circuits.
Gordon {Illbikes} wrote this about his experience with the CR's over stock carbs.

For anyone looking at the stock 28mm carbs in comparison to the CR 29's - you see something immediately obvious - you can almost stick your hands into the 29's venturi - it looks twice the diameter of the stockers. Why they call the Honda carbs 28mm I  don't know that either. It narrows down to about 22 in the center and the slide is tiny - clearly the CR's are kick-a$$ and will make power.

Most of the power increase will be in the top end of the rev range....

[Old Scrambler]

I like the conversation..................but why stop at 29mm?.............Jerry Branch concluded that a ported K head would flow 32mm carbs for a 736 to 762cc motor. I would think you would want even larger carbs on a serious big-bore

[bwaller]

Not 750's but Dr. Tom Marquardt strongly suggested I use a set of bored 28's instead of the CR29's. He preferred them over the CR's on his 592. He finally switched to CR31's when it grew to 675cc. I chose the Mic smoothbores. I know they're older but they're sorted and work great. I'd be curious to see if there was any extra power advantage on mine too. 

[TurboD]

Have a good day.

[FunJimmy]

I am very sorry that once again, I tried to offer input that was based on proven idea, facts and abstract thought to this forum. It won't happen again... Have a good day.

I'm sorry. I don't get it.
You've edited three posts in this thread with the same comment.
Is there something I'm missing?  Why would you do that?
Rewriting history has never worked.

[TurboD]

Not "rewriting" history, any input from me is not needed, all the answers are above..^^^

[scottly]

And we have the back to back dyno results or flow bench data on this?

Do you have flow bench data?

[Haybus]

George, since you're likely pondering this setup for your race bike, you might want to consider how easy jet and needle changes are with the CR's. I don't have any first hand experience with them, just what I've been told. Since they're race carbs, I believe they make those changes a cinch. John has them on his bike, maybe he'll chime in. However I have changed needle positions on Mikunis and these Keihin stockers and it's one of my least favorite things to do. I mean, could they make it any harder to get to the needles? I don't know if that's worth the clams they want for CR's to you, but it's something to consider.

Alan

[Retro Rocket]

I am very sorry that once again, I tried to offer input that was based on proven idea, facts and abstract thought to this forum. It won't happen again... Have a good day.

Hmmm, I must have missed that...?

[TurboD]

And we have the back to back dyno results or flow bench data on this?

Do you have flow bench data?

I was the one asking the question based on the two carbs in question in the OP's post.

Yes I have plenty of flow data on both the round tops vs PD carbs along with K heads vs F2-3 heads.

[scottly]

Were your tests conducted with the carbs connected to the heads, like the Branch tests? I would be very interested in seeing your data. Thanks in advance.

[TurboD]

I have done tests both ways. Possibly in the future with time permitting I could start a thread.

On the Hot Bike article. I have read it a number of times, as It has been posted here many times. In short I will say that the article, for me after 30 years of porting and testing cylinder heads, it leaves me with a few questions on a number of things.  This could be result of the testing itself or just possibly the way it was written. We'll never know at this point.

[POPS 911]

TURBO-D :  These folks are talking about road racing , right   We [ drag racers ] open up all the way open on a two step and let go of a button or a lever and the only time we up and down the slides is not to hit the wall or lift on a sand bag pass not to break out, right   DYNO and FLOWING give you data with numbers but seat time and a .501 light gets you the win ticket = you know the 1320' game which is a quick shot at the same FLOW on that dial in....... How many times have you lost by a thousand of a second ? And had them covered.......

POPS BK911