PRP Velocity Lessons

Earlie A said

Look at this chart. This is not my information, this is used by guys like Darin Morgan and Larry Meaux in their seminars and calculations. For the street 260-300 seems to be a sweet spot.

The Speedmaster head that I am working with can get to 285 cfm on a 186cc port with a 4.937in centerline length. That is pretty much max velocity/flow when the port starts shutting down due to separation/turbulence.

So, (186cc)x(1ci/16.387cc)=11.35 cubic inches port volume

(11.35ci)/4.937in=2.3 square inch average cross sectional area

(285cfm)x(1m/60s)x(144si/1sf)x(1/2.3si)=297 fps average velocity

This port is having problems before the 297fps point, so 260-285 fps would probably be better for this port and my porting skill level. Good racing heads would certainly have higher average velocities at the target rpm. Really good street heads could as well, but it would probably be better for a street engine to shoot for the 285 fps velocity at a lower rpm, which makes the CSA smaller for the street than for race.

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Thank you very much, great info, good to know these formula's aren't just based around applications way beyond the average dudes build.

I think Hysteric thinks I'm anti velocity, it just like his eg.. That's Olds combo is only something a dude with a flowbench and a dyno is gonna come up with and a lot of time, it's making 1.4 lbs-ft per cid beyond what most are capable of, most would build a 350 to get those numbers.

Plus most are dealing with of the stock or off the shelf heads and a few options at that, that engine is basic very similar to a 318 and the 3 400 hp combos gonna vary similar cam and cr wise and making peak hp around the same rpm but 35 hp less with EQ, Stock Magnum, ported 302. And probably at a more realistic around 1.18 lbs-ft per cid the one that ran EQ's they did a ported version of EQ's and got 425 hp (kind of a disappointment with the head flow #), I'm sure the velocities still probably fall in acceptable fps range. But these are all what I consider average street strip build level engines, basically 10:1 with 230 to 250 cam. A more street level cam under 230 generally needs airflow to makes it's hp eg. Modern truck engines like hemi and ls.


I'm just saying these engine are a lot less efficient than the Olds 1.17 to 1.19 vs 1.40 lbs-ft, but for the average dude this route probably makes more sense, especially since most would just go more displacement to hit these numbers, not that velocity shouldn't be thought of.


https://www.motortrend.com/how-to/0810phr-chrysler-318-engine/

"For our street performance small-block, we selected the most conservative of this line of camshafts, the XE275HL. Specs for this stick come to 231/237 degrees duration at .050, and .525-inch lift with a 1.5:1 rocker ratio, all on a 110-degree lobe separation angle."

On The BenchEngine Quest Ch318b Cylinder HeadsSuperflow 600 Flow Bench 28-Inch Water Depression Tested At Dr. J's Performance

On The Dyno318 Mopar Street EngineSuperflow 901 Engine Dyno Stp Correction Factor Tested At Westech Performance Group


Here's one to confirm port velocity a set of ported 302 heads 215 cfm (size ?) made the most torque and made 400 hp with a smaller cam xe268h, besides the leaking problem is most can't port even if they trying don't mean their gonna get the same results. This does have the best efficiency out of the 400 hp builds 1.26lbs-ft per cid.

https://www.motortrend.com/how-to/318-small-block-build/


Might as well, here the magnum one, the easiest to build just add cam cr 4bbl headers to a 5.2l

https://www.motortrend.com/how-to/mopp-0409-318-engine-build/


Might as well add one more here's a 477 hp 323 built for engine masters challenge more inline with the Olds, making similar tq but more hp, ported R/T Magnum's and 247 cam.

https://www.motortrend.com/how-to/0901phr-mopar-318-magnum-engine/
On The Dyno DTS Dyno Data Performance Crankshaft 323CI Small-Block Mopar