New PTS build - help with numbers

[Malvin]

Have you looked here ??
http://vacuumsonline.net/en/vacuum-parts/motors

https://www.blowermotorsplus.com/Ametek ... s_s/49.htm

[Heph]

Thanks for your help Tony and Heph.
50hp per pot happens to be about right for these motors after modding so reckon I'll go for 4 motors so there's some left in the bank.
Cheers
Adrian

I just realised I mixed up the numbers.

For a 4 cylinder engine, you will need roughly 1 CFM @ 28" per HP.
So if a 4 pot makes 200 BHP (50 bhp per cylinder) the head will flow around 200 CFM.

So you have to multiply the expected HP per cylinder by 4 to have a rough estimate of how much CFM @ 28" it will make.

[Tony]

Are you sure that is right ?
I have always understood 1.5 CFM per HP induction, and 2.2 CFM exhaust flow per HP on a running engine.

On that basis, 50 Hp = 75 CFM for one cylinder (depending of course on test pressure).

Adrian,
Here is a link to Pipemax, and Larry Mieux.
It contains vastly more than just tuned pipe lengths, very highly recommended and extremely good value at $75 U.S.
http://www.maxracesoftware.com/PIPE395.htm

[storm]

Tony's correct. 1.5 cfm is able to produce, when everything is spot on, 1 horsepower.
I've never come across, or I can't remember ever coming across, an exhaust figure but I'll trust Tony on that.

[Adrian]

Thanks for the info gents. Have ordered Pipemax - thanks Tony.

Been doing a bit of reading:

Regarding predicted HP from CFM at 28". If you use the Superflow/Harold Bettes formula it's more like 4CFM per HP.
That's the flow through the entire intake tract for one cylinder. Therefore 50HP per pot ~ 200CFM @ 28" H20

From what I can see, for 1.5CFM per HP, you'd have to be testing at less than 10" H20

Adrian

[Tony]

There may not be much correlation between the total air volume consumed in a real running engine, and steady continuous CFM measured on a flow bench through a fixed wide open valve.

Real airflow through a running engine is intimately associated with violently pulsing pressure waves and thermodynamic effects that need to be optimised along with combustion.

All of which has very little to do (directly) with smooth continuous airflow through a flow bench at room temperature.

Adding a correctly tuned intake runner will have a major effect on Ve at different engine speeds, but adding the same runner to a bare cylinder head on a flow bench may show as a drop in airflow.

So what does that say about steady state port CFM versus, actual measured dyno results ?

While there may be a relationship, I don't think its a solid fixed non varying relationship where you can predict with certainty that flow bench CFM will accurately predict dyno Hp.

[Adrian]

Yes, I remain skeptical about the degree to which steady flow is useful in predicting the behavior of the organised kaos in an IC motor. Guess I'm about to find out!
I get the distinct impression that a good deal of the learning curve will involve mapping the boundaries of its limitations.

[Tony]

The trick seems to be to aim for very specific flow velocities.

Too high a velocity and pressure drop will limit mass flow.
Velocities set too low decreases the inertial energy of pulse tuning, and that too limits what otherwise may have been possible with regard to peak Ve.
This is where Pipemax comes into the picture.
It gives excellent guidance on required flow areas and tuned lengths, as well as optimum valve timing.

Armed with that, you can then try for optimum flow profiles across the port, without making the ports and valves too large and losing velocity.
In other words, waking up low flow areas with some very careful port profiling without making the port too big.
Easier said than done !

I must admit that it is a very long time since I did any of this myself.
Modern factory four valve heads are pretty good to begin with, and usually only need light cleaning up.
Forced induction is the lazy way, which is my own preferred method these days for street.

[storm]

From what I can see, for 1.5CFM per HP, you'd have to be testing at less than 10" H20

In general, so there is no restriction at all, a 900cfm 4 barrel is good for 600 HP. 4 barrel carbs are flowed at 1.5"HG which is approximately 20"H2O. You can get 600 HP out of a smaller carb but the restriction builds up the smaller you go.