Carburetor CFM rating question

[XL-erate]

Hi all,

Wondering if anyone can tell me the CFM rating of a single GS550 carb, or all four?

If not for GS550, any Suzuki CFM ratings?

If not for Suzuki for another similar high performance MC engine, with displacement info?

Also like to know engine side throttle bore size in mm's of the GS550 carbs, or other Suszuki carbs.

Thanks for any info!

 

[Guest]

someone can correct me here, but it should be a simple math problem

550cc's * (1in^3/2.54cm^3) = 33.56 in^3

33.56 in^3 * 9,000 rpm = 302 067 in^3/min, however this should be divided by 2 because only 1/2 of the cylinders draw air on each rotation. - ~150,000 in^3/min

which is about 87 cfm total. Each carb is good for about 22 scfm.

Rating is likely some multiple of this. also, I have guessed as I don't know the exact displacement of a 550 or its redline.

 

[XL-erate]

Thanks, that's a start!

 

[Nessism]

Carburetor cfm ratings are an airflow rating and have nothing to do with the engines capacity. Common cfm ratings we see here on aftermarket automotive carbs in the US are based on an SAE (Society of Automotive Engineers) testing standard. I suspect the Japanese OE motorcycle manufactuers don't place a lot of value on rating their carbs using a US test standard, thus I doubt CFM ratings are available for Mikuni motorcycle carbs.

 

[XL-erate]

Thnaks for additional info, Nessism. I have used CFM ratings to match carburetors to most types of 4 strokes, as a basis for approximate size. It seemed to me that most motorcycle applications only mention bore/venturi size in mm's, so was hoping to find some useful info on carb fitment.

May sound pretty stange, but looking into the feasability of running one single carb, with a plenum box and individual runners to each cylinder. Threfore wanting bore AND cfm to see what the numbers look like. Knowing mm diameter can help a lot so I'll see what that looks like.

 

[Suzuki_Don]

Thnaks for additional info, Nessism. I have used CFM ratings to match carburetors to most types of 4 strokes, as a basis for approximate size. It seemed to me that most motorcycle applications only mention bore/venturi size in mm's, so was hoping to find some useful info on carb fitment.

May sound pretty stange, but looking into the feasability of running one single carb, with a plenum box and individual runners to each cylinder. Threfore wanting bore AND cfm to see what the numbers look like. Knowing mm diameter can help a lot so I'll see what that looks like.

The VM slide type carbs for the 550 are 22mm venturi size.

 

[trippivot]

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.

at the shop I work at we have a flow bench. However we haven't flowed little stuff. sorry
just a reference for you to make a educated guess at the ball park numbers

 

[drhach]

Obviously you know that one of your carbs flows 1/4th of what all of them flow. Using the engine's theoretical flow capacity only tells you that suzuki put carbs on the engine that were up to that challenge (maybe). Unless you have a flowbench, you won't find this number. As was previously stated. However, you do have options. There are many single cylinder engines out there. Use a carb that came stock with one of them. Remember that your 4 cylinder 550 probably revs higher than a single cylinder 500. So, factor that in to the equation. You probably could get away with a carb from a 650 thumper. I believe the XS650's came with single carbs. I know some of the Triumphs did, if you want to go with Amal carbs. Lots of bikes came with single carbs. Hell, you clould use a carb from a Harley if toss in a restrictor plate.

 

[Steve]

Thnaks for additional info, Nessism. I have used CFM ratings to match carburetors to most types of 4 strokes, as a basis for approximate size. It seemed to me that most motorcycle applications only mention bore/venturi size in mm's, so was hoping to find some useful info on carb fitment.

In reality, all you need is a number, any number to compare one item to another. If you know that 32mm throated-carbs work with a certain size engine, does it really matter how many cfm? (And yes, I saw why you are asking.)

May sound pretty stange, but looking into the feasability of running one single carb, with a plenum box and individual runners to each cylinder. Threfore wanting bore AND cfm to see what the numbers look like. Knowing mm diameter can help a lot so I'll see what that looks like.

I have thought of something like this, too, but have not pursued it at all.

A couple of things to keep in mind while working on this project...
1. Some carburetors, like the Weber carbs, depend on a pulse to get proper mixture. Other carbs rely on smooth flow. I don't know how our carbs fare in that respect.
2. Because the cylinder is only drawing fuel for just over 1/4 of its complete cycle, there are major times when nothing is going through the carb. IF the carbs work well with relatively constant flow, I would think that using a carb just a little bigger than stock would work. One example of this is my Gold Wing. It only has two carbs, each one feeds three cylinders. The three cylinders total 760cc, the carb is a 36mm downdraft unit. Admittedly, the Wing is in a lower state of tune compared to the GS, but it might give you a starting point. It will be a chore to find the correct jetting, but you will make up the time spent with a lack of time spent doing a carb sync. \\/

By the way, before you go trying to create an intake manifold, look into one from a turbo kit. Manifolds are not generally listed separately, but a phone call to the manufacturer will answer that question rather quickly. I am sure there are other kits, but here is Mr. Turbo.


.

 

[XL-erate]

Thanks, priceless information!! I certainly got more than I expected!

It was just those early bikes that got me to head scratching, why not? Certainly some small bit of preformance might be lost this way but I'm not headed to the GP circuit afterall.

Planning to play with tuned intake length and a plenum to modulate pulses, unfortunately all seat of the pants testing. Looking for bottom end to midrange anyway, so not as complicated as shooting for the moon with the tacho buried.

It took a while for it to dawn on me some time back that carburetor SIZE won't damage an engine, but the air/fuel mixture from that carb may. No problem at all under-carbureting as long as A/F is Stoch. Only performance will suffer, not engine life. Go too lean with any carb setup and damage is waiting to happen.

Thinking to tune to midrange with a single carb and plenum on a fairly long ram induction and to pull into upper rpm's with tuned exhaust. That is bring the carb's in, a little oversized, for say 4-5K and design exhaust for 6-7 peak, to suit my riding needs. Plenum will absorb intake pulses and keep flow nearly constant so I'd probably avoid Weber. Engine would still rev higher but with power dropping off or remaining weakly constant.

This should be fun and interesting and thanks much for the help. If I can do this I'll post results as it ambles along. Thanks!