SBM head velocities and there application?

[273]

I'm trying to come to grips with this part of head performance, people talk about velocities a lot when discussing heads, carbs, headers etc... But in an vague abstract way no definitive numbers on what the head velocity is for a given head and what those head velocities will work with (cid x rpm). I hope we can figure out the velocities of the heads we generally have available and what displacements and rpms they work best with.

Now I'm not saying these formulas and these fps guidelines are 100% accurate and if people have better ones and or reasons why these guidelines and formula's are wrong or somewhat wrong, feel free to post. But I figured we'd start here with these.

Induction Formulas
Average_CSA = Port_Volume_CC / (Port_CenterLine_Length * 16.387)
MIN CSA = (Bore x Bore x Stroke x RPM x .00353) / 613.8 (.55 MACH)
Port_Volume_CC = Average_CSA * Port_CenterLine_Length * 16.387
Port_CenterLine_Length = Port_Volume_CC / ( Average_CSA *16.387 )
FPS = ( Flow_CFM * 2.4 ) / Average_CSA
Flow_CFM = Average_CSA * FPS * .4166667
Average_CSA = ( Flow_CFM * 2.4) / FPS
cfm demand = cid x rpm x .0009785 / # of cylinders

 

[273]

So according to these guidelines a velocity of 260 fps according to it's calculated average CSA seems to be the ideal target. And the formula to figure that is Cfm x 2.4 / 260 = average csa.

Now to figure a port volumes ave. csa = port cc / (port length x 16. 387), Now I've been going off Earlie A's 4.95" as center line port length, is that close to the average of most sbm heads or it's just the speed masters length?

But if it's close average length the formula is just ave. csa = port cc / 81.12

 

[273]

So eg. Say Trick Flows 300 cfm with a 190 cc port.

Is trick flows port length 4.95" ?
For now I'm gonna go with that.

Average csa = port cc / 81.12
190 / 81.12 = 2.34" ave csa.

Port fps = cfm x 2.4 / ave csa
308 fps = 300 x 2.4 / 2.34"

To get 260 fps the ave csa and port cc would need to be 2.77" and 225 cc a pretty large port especially for 300 cfm.

 

[273]

cfm demand = cid x rpm x .0009785 / # cylinders

Or rpm = (cfm x cylinders) / (cid x .0009785)

Rpm = 6,012 for a 408 cid

Min csa = b x b x s x rpm x .00353 / 613.8

2.25" min csa so that's supposed to be the smallest area you have anywhere and that's usually the push rod pinch.

I generally just use my made up one seems to always come close min csa = cid x rpm / 1,100,000, which is 2.22" min csa.

So according to this a 408 making peak around 6,012 rpms needs 300 cfms the pinch area of 2.25" and a port volume of 225 cc with an average csa of 2.77" to have recommended velocity of 260 fps. But with the Trick Flow ports is got an average of 2.34" csa and velocity of 308 fps, which states "high usable port velocity (good hp & tq ) with possible hp loss." So obviously ain't terrible but probably given up some hp if the guidelines are somewhat accurate.

According to this it seems hard to have too little velocity, even 250 fps is recommended minimum say good hp and have go 240 fps where there's torque loss. Which is 234 cc and 243 cc respectively, you'd have to go to 243 cc for 300 cfm where torque becomes an issue.

Now obviously these aren't carved in stone and I'm assuming TFs have a port length of 4.95".

If port length was 5.15" or 4.80" the velocity would be 320 fps or 298 fps respectively compared to the 308 fps I'm going by, makes a difference but it don't change by that much.

 

[273]

340/360 2.02/1.88

1st, not sure if the port length is 4.95" ? for these heads but using it for now, as long it's not to far off should be close.

From what I understand 340/360 heads are in the 150-160 cc range and flow 200-225 ? (1.88 vs 2.02)

Average csa = 1.85"-1.97"

Velocity 200 cfm = 244-260 fps, 225 cfm = 274-292 fps they seems like there around the so called ideal velocity range 250-285 fps.

Engines cfm demand

273, 200 cfm = 6000 rpms, 225 cfm = 6750 rpms
318, 200 cfm = 5150 rpms, 225 cfm = 5750 rpms
340, 200 cfm = 4800 rpms, 225 cfm = 5400 rpms.
360, 200 cfm = 4550 rpms, 225 cfm = 5100 rpms
408, 200 cfm = 4000 rpms, 225 cfm = 4500 rpms

Min csa 200 cfm = 1.5", 225 cfm = 1.69"

Close as I can with the info I have.

 

[Bewy]

Sounds like you have the Bettes book, a great book. I find that a lot of assumptions are made with some of the calculations, resulting in wrong final numbers.
D. Vizard also has an excellent porting book, SA 215. He claims the common 28" used is a compromise.

 

[273]

Sounds like you have the Bettes book, a great book.

Nope, but sounds like I should get it.

I find that a lot of assumptions are made with some of the calculations, resulting in wrong final numbers.

That's why started the thread hope to get to the bottom of it, not saying anything I calculated above is accurate just a jumping off point.

 

[273]

The one thing that has been surprising me with this is how seemly hard it is to have low velocities.

If the port length average for all these heads is around 4.95" that makes about a 195 cc as the crossover point where cfm and fps are equal and any ports that has less cc than 195 cc fps will always ways be higher than cfm.