CFM of intake round tubing per sq in

[Gschuld]

I’m working on a 4cyl vintage(70s) 1000cc road road racing motorcycle engine build.

We run individual carbs per intake. Our class legal carb types(round bore/round slide “smoothbore” carbs) offer on average 134cfm at 28” WC or 80cfm at 10” WC flow per square inch. It would be very helpful if I can find out what a smooth round pipe section flows per square inch(around 1.25” ID tube in my case). It clearly flows a bit better than the throats of round slide carbs with their turbulence inducing edges.

I’m guessing the bare smooth pipes will flow in the neighborhood of 150cfm or so at 28 and 86cfm or so at 10. I’ve tried to locate this information but have come up short. I’ve seen mention of 146 by Darin Morgan but was unsure about shape, with perfectly round cross section being ideal. Should I assume 146 is it?

My intake tract, FWIW, consists of a 4-4.5” velocity stack, round bore/round slide carburetor, followed by a round cross secion intake port until the intake valve pocket(2V engine). Tip of velocity stack to valve pocket is a dead straight line.

Any help here would be greatly appreciated.

Thank you.

George

[mag2555]

I can’t recall the exact number when at 28” but I think your very close because I want to say the number is 146 cfm.

[panic]

CFM flow per square inch area will not be linear and directly proportionate due to Poiseville flow, also varying with fluid viscosity.

[Gschuld]

Ok. Thanks so far.

Here is what I’m trying to deal with. My class requires 31mm restrictors if the carbs(4 carbs for 4 cylinders) are larger than 31mm, my intake valve will be 38mm, and a 31.4mm round intake tract would be 82% of the 38mm valve. I plan on running 33mm carbs which at 80cfm per square inch gives me 177cfm at 28” or 106cfm at 10”. This roughly equals what I can expect out of a 38mm (1.49”) intake valve at my .445” maximum lift.

I’ll likely keep the intake port from carb spigot to near valve pocket at 31.5mm round. Feeding the carb would be a stack enlarging from 33mm at the carb throat to about 57mm at the entry 4.5”-5” out from the carb throttle slide and with a generous radius at the entry.

From entry of stack to the valve pocket is dead straight. I’m looking to achieve 130RWHP at 10,000rpm and 75 ft lbs of torque with a .445” lift cam (maximum lift the head will stand without major surgery)

Bore is 71mm stroke 63mm. 4/2/1 exhaust with stepped primaries matching a compact exhaust port and small exhaust valve. Hopefully something Calvin Elston would approve of.

I am under the assumption that a 2” length of 31.5mm diameter round intake tract to the valve pocket will flow equal or greater than the 33mm carb throats at 177cfm at 28”. Seems logical to me but I wanted to check. 31.5mm equally 1.22 sq in. 1.22 x 146cfm is 176cfm at 28”.

The wild card is the 4.5”-5” long ram stack entry should be accelerating the air quite a bit between 7000-10,500 rpm. Not sure how this extra velocity effects the flow balance between the 33mm carbs and 31.5mm intake tract beyond it.

George

[cab0154]

146. and at that number they are close to sonic. 134 is still pretty fast. i have seen a lot of good running small blocks anywhere between 115-125. My profilers from Chad are around 127 if using the average cross section, and 131 if i use min cross section. But YMMV.

[pcnsd]

146 cfm per square inch assumes a discharge coefficient of 1. That is unlikely to be the case.

[cab0154]

Thats a really good point. Depending on the surface finishes differences, I could see the CD difference being rather significant in some cases.

[Gschuld]

Thanks for the replies.

Being and intake port, I’d be putting a surface on appropriate to good fuel air mixture retention so around 80 grit sanded.

George

[blackford]

I'm curious if anyone thinks this air flow calculator could be used to get some idea of air flow through an ideal pipe and how it could be used for getting a sense of air flow through an ideal port for changes in min CSA. I ran across this a few months ago when I was looking into the impact of minimum CSA of a port and how an increase in CSA increases air flow.

What i'm not so sure about is if my input values are valid. I entered the min CSA (you can enter the ID of the pipe directly and override Sch40 pipe info) and I used 1 psi for pressure to simulate 28" H2O. For air velocity, I entered 350 ft/sec as this seems like the max air speed you would want in an intake port (again, not sure). Temperature only affects SCFM and not actual air flow.

I'm just looking for some validation on using the following tool as it may help myself and others to get a sense of the impact of minimum CSA and to estimate air flow through an ideal port. Thanks

https://www.tlv.com/global/US/calculato ... dvanced=on

[digger]

Thats a really good point. Depending on the surface finishes differences, I could see the CD difference being rather significant in some cases.

its more about the shape of the entry (minimise vena contracta) and exit (maximise pressure recovery). The length obviously comes into it

There was also a post that explain why you can exceed 146 cfm/in2

[cab0154]

146 would put the mean airspeed at around 350 fps/ so depending on how close the pulses are to .55 mach, I am sure that you could exceed that average airspeed, and thus exceed that cfm. But just based on what I have seen, in the real world with an inline if the head is anywhere between 115-130 per sq inch, they can run good of the cross section, shape and velocity gradient are right. I have never seen anyone in the real world good running port that was anywhere close to that high of a cfm/per sq inch. but idk

with what i was referring to above with surface finish I was alluding to differences in boundary layer caused by finish differences.

[panic]

I have in my notes a comment from the net that the surface flow is nearly zero, and at a depth of 1mm from the wall it's nearly 100% (using air @ STP), and in between the flow was nearly proportionate. The idea was to deduct 1mm from the ID to get a rough guess.
Anyone read this?