CR 33s on a 1045 gord bush MOTOR

It seems that your friend has put a great deal of thought ,effort, and money into this issue. its great to hear that he did indeed find a solution in the end.
I wonder if the pods inhibit sudden flow as u go to wot ?? could it be that the air box has an unrestricted supply of air in reserve?? Does this air box have velocity stacks inside?
i'm not sure that a race bike would see this problem since they never see low rpm's. I never have any trouble at the track myself.
Did he have this problem with vm29’s ?? I had it, but way back then (I had vm29’s in the 70’s) I never played with the carbs to try and resolve it. I didn’t have much knowledge of carbs at the time.
this is 1 area of disappointment in my build. a little more bottom end would be nice for the street.
Until now , I have just accepted it as the norm. i will have to follow this theory and see if I can find an economical solution.
Thanks for the information and inspiration .

He only has his time into it and the money to get it welded because he does not have a TIG welder.

To answer your questions :

With the pods there is an abrupt air flow transition, the carb tries to pull air in from all sides at once. There is also a chance for more turbulent air from cross winds... the only thing they are good for is fast jetting changes - once your motor is set up this is not required. The pods also shorten the intake tract to nothing reducing avaliable low end torque.

The volume of air inside the box is critical and the distance it creates from the carb mouth gives the air charge a chance to "straighten out", creating a smoother air flow. It is also coming from one direction without any interuptions from different air sources (side winds) which we believe gives more low end torque... which in turn creates better low end response.

(The comments above are our theorys on what is occuring on the intake side of the head)

The box was built with internal velocity stacks... if you look at a set of stock 900 rubber air box mounts they are molded as a velocity stack (I believe all rubbers are like this - for a reason?) which is hidden inside the box.

He tried the following carbs - VM26, VM28, 29 smooth bores, and 34 flat slides from GSXR750R...
none of these resolved the issue - and the first two were horrible compaired to the RS34s.

You might be loosing low end torque for two reasons - aftermarket cam timing overlap and the lack of an air box

The only way your making up for that is a larger bore.

I guess I'll just have to make something up and play with it a bit to see if ican improve the botom end.
Thanks

Has anyone modified velocity stacks to put pods on the ends? This seems like it would help with the low end torque, and possibly some of the turbulence from cross wind? Just a thought...

I'm like u T. i thought about it but never tried.
it may be just as easy to try a box. if i only had some stacks, i'll put the box around them.

mark1122 wrote:

larrycavan wrote:

If you can get the capacity the motor requires, an airbox is the better choice over open stacks or pod filters...

Larry, i understand that modern bike take advantage of wave signals in the air box, but can u explain how an air how may help for wot from low rpm bog?(as i experienced with the vm33's)

Fuel moves up the needle jet ONLY because the air pressure at the top of the needle jet is less than the air pressure above the fuel in the float bowl.

The engine doesn't suck fuel in. Atmospheric pressure pushes the fuel up through the needle jet.

The greater the pressure differences between the top of the needle jet and the atmospheric pressure in the float bowl, the stronger the push on the fuel.

The atmospheric pressure air in the bowl is trying desperately to fight it's way to the low pressure region. The fuel just happens to be in the way...

Keep in mind that air is elastic. It can be compressed but fuel cannot.

If you can't have a pressurized fuel delivery mechanism [fuel injection] then the next best thing is to design an air delivery mechanism that can best create an environment that yields laminar flow rather than turbulent flow of the air across the low pressure side of the fuel delivery device. The airbox is a solution.

Carburetors rely completely on air pressure differentials to operate. Fuel injection doesn't have that weakness.

If you look at the way Lectron carburetors operate, you'll see they take the low / high pressure concept to another level in order to setup an even stronger signal to move the fuel up the tube. They're nice for throttle response and making HP but they are not refined enough for street use with silky smooth throttling around town and they don't idle well at low RPM.

One of the things Keihin did with their CVK carbs was fill in the bottom of the slides. That helped create less turbulence at the needle jet and speed up the air in the exact location it needs to be fast.

PODS look cool but airboxes RULE

DoubleDub wrote:

Has anyone modified velocity stacks to put pods on the ends? This seems like it would help with the low end torque, and possibly some of the turbulence from cross wind? Just a thought...

I think if that was something that would work for motorcycles it would already be on the market... I have seen it for cars with unlimited space (sticking thru the hood) but there are space restrictions on stock motorcycle chassis.

Usually the outside ones will hit the chassis down tubes.

Plus I still think you run into the same side wind issues and not enough length for the air to make the turn into the stack to smooth out in time before it hits the combustion chamber.

When were talking about missing length it might be on the order of 4-5"

larrycavan wrote:

mark1122 wrote:

larrycavan wrote:

If you can get the capacity the motor requires, an airbox is the better choice over open stacks or pod filters...

Larry, i understand that modern bike take advantage of wave signals in the air box, but can u explain how an air how may help for wot from low rpm bog?(as i experienced with the vm33's)

Fuel moves up the needle jet ONLY because the air pressure at the top of the needle jet is less than the air pressure above the fuel in the float bowl.

The engine doesn't suck fuel in. Atmospheric pressure pushes the fuel up through the needle jet.

The greater the pressure differences between the top of the needle jet and the atmospheric pressure in the float bowl, the stronger the push on the fuel.

The atmospheric pressure air in the bowl is trying desperately to fight it's way to the low pressure region. The fuel just happens to be in the way...

Keep in mind that air is elastic. It can be compressed but fuel cannot.

If you can't have a pressurized fuel delivery mechanism [fuel injection] then the next best thing is to design an air delivery mechanism that can best create an environment that yields laminar flow rather than turbulent flow of the air across the low pressure side of the fuel delivery device. The airbox is a solution.

Carburetors rely completely on air pressure differentials to operate. Fuel injection doesn't have that weakness.

If you look at the way Lectron carburetors operate, you'll see they take the low / high pressure concept to another level in order to setup an even stronger signal to move the fuel up the tube. They're nice for throttle response and making HP but they are not refined enough for street use with silky smooth throttling around town and they don't idle well at low RPM.

One of the things Keihin did with their CVK carbs was fill in the bottom of the slides. That helped create less turbulence at the needle jet and speed up the air in the exact location it needs to be fast.

PODS look cool but airboxes RULE

so why doesnt the fuel pump on the rs34's solve the bog?

I'm going to be putting a rack of RS36's on my 1200 pretty soon......I'll keep you posted on the results.

Pretty similar motors except you have more cam.

What am I missing in the air box vs. pod debate...doesn't the dyno show more horsepower with pods and a re-jet every time?

79MKII wrote:

What am I missing in the air box vs. pod debate...doesn't the dyno show more horsepower with pods and a re-jet every time?

no cross winds on a dyno,id be willing to bet the dyno will show more power with 14:1 squeeze .580 cams and a set of lectrons also,problem being it is all between 8500 and 11000 rpms for about 5 seconds at a time! :woohoo:

That makes sense but I've never seen a roadracing bike with an airbox either - why not? I would think they use a lot more of the rpm range but maybe they want the best top end only too?

I guess this debate is similar to a decision I made about my air cleaner in my V-8 truck. I wanted to get a K&N intake kit for it until I saw the dyno chart. The only place it made more hp was wide open throttle. The other rpms actually lost power. That didn't make any sense to me so I kept it stock since I don't run that wide open very often. That argument would make a lot of sense for the motorcycles too but it seems that many of the jet kits show more power throughout the power band.