RS34 Flow Test

KZ 900 head with J intake valves ported for Ben's 1075 street motor.

RS34 Carburetors - flow tested using matched J intake boots and Superflow calibration plate largest hole as the base mount.

113.4 CFM @ 10"
110.4 CFM with APE Filter mounted to carb.

Head with Cam Motion Z1 Grind cam at .385" lift / RS34's mounted to head:

91.7 CFM



For reference 10" of water = 209 FPS.

Velocity Test:

Velocity Probe runner entrance using J bend Pitot with above setup.

180.6 FPS @10"

For reference 10" of water = 209 FPS.

So...the head flows 91.7 cfm? I'm assuming this beings the carbs flow 113.4. Beings the filters slow down the flow by 3 cfm, how much horsepower would that work out to be? Sounds like the ports are working good if you are only losing under 30fps. How do you use the velocity numbers in your work...do you shoot for a certain number or use it more for reference of how the port works after porting as compared to stock?

racer54 wrote:

So...the head flows 91.7 cfm? I'm assuming this beings the carbs flow 113.4. Beings the filters slow down the flow by 3 cfm, how much horsepower would that work out to be? Sounds like the ports are working good if you are only losing under 30fps. How do you use the velocity numbers in your work...do you shoot for a certain number or use it more for reference of how the port works after porting as compared to stock?

Rule of thumb for port velocity...

Average Flow velocity should never exceed test pressure velocity. By the same notion it's desirable to keep velocity up, especially for a street motor.

I have target numbers I use depending on the use of the engine.

It's a basic math function for avg. velocity.

Then we have local velocity numbers where problems are discovered....too fast sections. The posted velocity number is a local velocity number measured reaching through the carb with the pitot to test speed right at the port entrance, where the boot meets the aluminum runner.

:laugh:

Hey Larry,

Out of that 30fps lost. Could any of that be recovered by running an intake cam, with a fatter duration then the exhaust?

kzz1p wrote:

:laugh:

Hey Larry,

Out of that 30fps lost. Could any of that be recovered by running an intake cam, with a fatter duration then the exhaust?

Excellent question No easy answer unfortunately.

For those few brief degrees of overlap, intake velocity will get that supercharge effect. But the catch is trapping the charge in the cylinder.

Look at what's happening during that period. Valve lifts are going to be relative to cam profile and timing. There's going to be waste out the exhaust to some extent.

It's another example of the upside / downside of poppet valve 4 cycle engines. Get some here...loose some there.

Some head porters will intentionally try to kill off low lift flow to fight charge loss. In some cases good low lift flow can have the affect of adding duration to the cam....

so what do u think of the rs34 #'s compared to others Larry? with the data from dif carbs, can u tell if they would be any more responsive down low (or anywhere)than another carb ?

larrycavan wrote:

kzz1p wrote:

:laugh:

Hey Larry,

Out of that 30fps lost. Could any of that be recovered by running an intake cam, with a fatter duration then the exhaust?

Excellent question No easy answer unfortunately.

For those few brief degrees of overlap, intake velocity will get that supercharge effect. But the catch is trapping the charge in the cylinder.

Look at what's happening during that period. Valve lifts are going to be relative to cam profile and timing. There's going to be waste out the exhaust to some extent.

It's another example of the upside / downside of poppet valve 4 cycle engines. Get some here...loose some there.

Some head porters will intentionally try to kill off low lift flow to fight charge loss. In some cases good low lift flow can have the affect of adding duration to the cam....

We know the hot exhaust gas, moves quicker. If you get some intake waste out of the exhaust? Does that mean the cylinder is full?

Larry, what are the specs on those cams?

kzz1p wrote:

larrycavan wrote:

kzz1p wrote:

:laugh:

Hey Larry,

Out of that 30fps lost. Could any of that be recovered by running an intake cam, with a fatter duration then the exhaust?

Excellent question No easy answer unfortunately.

For those few brief degrees of overlap, intake velocity will get that supercharge effect. But the catch is trapping the charge in the cylinder.

Look at what's happening during that period. Valve lifts are going to be relative to cam profile and timing. There's going to be waste out the exhaust to some extent.

It's another example of the upside / downside of poppet valve 4 cycle engines. Get some here...loose some there.

Some head porters will intentionally try to kill off low lift flow to fight charge loss. In some cases good low lift flow can have the affect of adding duration to the cam....

We know the hot exhaust gas, moves quicker. If you get some intake waste out of the exhaust? Does that mean the cylinder is full?

Larry, what are the specs on those cams?

How could the cylinder possibly be full at TDC overlap? There hasn't been any movement of the piston downward yet.

The incoming charge, once it's moving, will flow where the lowest pressure region is. That's still the exhaust port at TDC. Hopefully some will get caught in the chamber by piston dome diversion and at some RPM the exhaust system will come into tune with a positive pressure wave shoving some of the lost mixture back in before the valve says other wise....

On these particular cams with the lobe centers I chose there's .100" lift on the intake valve and .097" on the exhaust valve. There will be a few more degrees before the exhaust completely shuts but at this point there's very little curtain area for flow at the exhaust valve.

Specs are .400" 246@.050 A nice street cam IMO.

mark1122 wrote:

so what do u think of the rs34 #'s compared to others Larry? with the data from dif carbs, can u tell if they would be any more responsive down low (or anywhere)than another carb ?

They're fine. They'll feed a 1200cc street motor no problem.
They're never going to deliver the response of a Lectron but they're sure a lot nicer all around carb.

They out flow most J heads with 38.6 intakes.....

They carburete better than the old round slide 33's at partial throttle openings....much better.

With a short duration cam, they would be excellent on a 1200cc street Z for all around performance....

JMO

so what would your preferred cam choice be with my build now.
are the cams u have now your new fav's ? or would u still like to try a little less duration ?
can i assume that since u are not seeing the same flat spot as me, that my cams are to blame ?