[Tom Vaught]

Very easy to get a 300 cfm head (at 28") using a typical 72cc Edelbrock head. I have a set that were barely touched in the intake side (for a blower project) that went 305.

If you have a lot of 400 cid Pontiac guys who bought 72 cc heads to go on their warmed over combinations and expected to run 6200-6400 rpm, I would think that they got screwed if they would have to buy expensive valvetrain parts to run 8000 rpm to be able to use the heads.

8000 rpm is no big deal for me with a 3.5" stroke forged moldex crank engine but some do not have the crank and other parts to go there.

Please explain.

Tom V.

[Motor Daddy]

I don't understand what you are asking to explain.

If you put 300 CFM heads on a stock 400 shortblock that has a 6,000 RPM redline, you wasted your money thinking you were going to make over 600 HP.

Please explain what you want explained.

[NHRASuperStock455SD]

Originally Posted by js machine

A simple CFM/HP formula would be:
HP=.27 x cfm x 1 = HP PER CYL
This is considered ALL components of engine assembly maxed or optimal for application.
Example:
HP=.27 x 300 cfm x 1 = 81 HP per cyl 81 x 8 =648 HP
In conclusion a head that flows 300 CFM at peak valve lift should net a MAX of 648 HP. Information (formula) from Superflow !

We use numbers like .27 or so for an average race engine. We use .285 for average Super Stock engines. NHRA comp engines with high compression, multiple carbs, and light rotating assemblies are as high as .300 to .310.

This number only works without cylinder head choke. If you are way short of HP, then you can reasonbly be sure that CHOKE is occuring. Did you notice that in the equation there is no calculation for Cubic inches? Why do you think that is?

I think you guys are ignoring the importance of CSA in the HP equation. This is one of the biggest problems in many Pontiac motors out there. It is the main reason behind the posting of this thread.

Lynn

[johnta1]

Tom, you could use this calculator.
Head Flow From RPM and CID

For 6000 RPM with a 127% VE, 292 CFM would get you there.

Then using this calc:
HP from CFM

You could get 600 HP.

Then you could use this calc:
Accelleration Calc

You could run a 10.47 @ 134 MPH in the 1/4 with a 3400 lb GTO .

This is assuming your heads CSA is big enough for your Bore/Stroke or otherwise it will be choked.

[johnta1]

Thanks to Lynn for some of my formulas and suggestions.

[Tom Vaught]

How about explaining the 127% Volumetric Efficiency number and where the author got it from.

Tom V.

[johnta1]

The 127.5% VE would be what the head would have if it had a .6 mach or 699/700 fps. It would then become port limiting velocity.
This would be the max cfm the head could flow without being port limited.
Any more cfm and it would not be making max peak HP at your rpm.

100% VE would be the minimum velocity the head would flow at your chosen peak HP rpm.

[NHRASuperStock455SD]

Originally Posted by johnta1

Tom, you could use this calculator.
Head Flow From RPM and CID

For 6000 RPM with a 127% VE, 292 CFM would get you there.

Then using this calc:
HP from CFM

You could get 600 HP.

Then you could use this calc:
Accelleration Calc

You could run a 10.47 @ 134 MPH in the 1/4 with a 3400 lb GTO .

John,

Thanks for the note. Please edit this to include the minimum CSA required to make 600HP with his bore, stroke without Choke because unless he as the cross Section, the flow wont get him there.

Lynn

[R 70 Judge]

johnta1 - Those calculators are awesome! Thank you for making them available to everyone.

I'll admit that when I first started reading this thread I felt the information was over my head. Now I feel I've learned something and have a greater understanding of the importance of selecting the right combination of parts for a given application.

My choke point with a CSA of 2.8, Bore of 4.35, and stroke of 4.25 will be at 6056RPM. Is this correct?

In turn, I assume choosing a cam profile that makes it's peak power below 6200RPM would benefit the overall performance of my engine. I'm now starting to wonder if a ported Torker II or modified Tomahawk intake would be a better choice over a Victor due to the "choke point" RPM limitation of 6056RPM.

Thoughts?

[Ron H]

Originally Posted by R 70 Judge

johnta1 - Those calculators are awesome! Thank you for making them available to everyone.

I'll admit that when I first started reading this thread I felt the information was over my head. Now I feel I've learned something and have a greater understanding of the importance of selecting the right combination of parts for a given application.

My choke point with a CSA of 2.8, Bore of 4.35, and stroke of 4.25 will be at 6056RPM. Is this correct?

In turn, I assume choosing a cam profile that makes it's peak power below 6200RPM would benefit the overall performance of my engine. I'm now starting to wonder if a ported Torker II or modified Tomahawk intake would be a better choice over a Victor due to the "choke point" RPM limitation of 6056RPM.

Thoughts?

Is the 2.8 CSA in the Victor or your heads? I would think the unported Victor will flow more than a unported T2 or Hurricane.

[johnta1]

Actually, I believe a higher lift cam will help the 'choke' situation if more power is wanted at a higher RPM.

[R 70 Judge]

Originally Posted by Ron H

Is the 2.8 CSA in the Victor or your heads? I would think the unported Victor will flow more than a unported T2 or Hurricane.

My heads - 2.38" x 1.18"

I was thinking of a cam in the .630" - .660" range

28” ½” big adapter 4.25 Bore
INT - 2.110 22* b/c 30 top/radius
Intake
.200 – 146 (69.5)
.300 – 212 (67.5)
.400 – 265 (84.6)
.500 – 298 (95)
.600 – 313.7 (100)
EX - 1.770 28” 30* b/c 35top/radius No Tube
Exhaust
.200 – 119 (54)
.300 – 157 (71.4)
.400 – 189.6 (86)
.500 – 216 (98)
.600 – 238 (71.6)

[slowbird]

John I could be wrong but if you are have a problem with to high of air speed in a port (choke) in a running engine, then adding lift to the cam will only increase that problem. As the lift is increased the air speed in the runner will increase. So taking away alittle lift and adding duration I believe is the best way to help with high air speeds. That is why you see most Pontiacs with less lift and more duration than the other makes.

[slowbird]

R 70 Judge, a couple of things your choke point will not be at the intake flange (gasket area), it will be at you pushrod area (on stock and e-heads that area will not be any wider than 1.100 unless you have sleeved it or moved the pushrods). So you need to figure out what that area measures. Also you have to take into account the corner radii. The corners can take up anywhere from .050 to .300 of that area depending on what the radius is.

[johnta1]

I thought/think that too, slowbird.

I did a read out on his combo in Pipe Max, and it showed his optimum cam lift around .840" lift.

I'm wondering if the additional lift slows the speed down to extend the RPM range up. That's the speed of the air not the CFM.

With the .660" lift cam, which BTW was what I used originally, the Pipe Max choke was at 5900 or so RPM's.
With the .840" lift cam inputted, it showed 7000 or so RPM choke.

Of course going to a higher lift, usually entails more duration, so it might be the duration helping the choke situation.

[slowbird]

What you are seeing there is at what point the valve window area no longer will cause a choke effect at that rpm (you have to get the valve out of the way), but it is not looking at the port itself. The port still has to have the right cross section. That is why Pipe Max also gives you the min. and max. port diminision. Pipe Max is loaded with info.

[johnta1]

Try my Mach calc.
Mach Calculator

Use different cam lifts, etc to see what the actual Mach index is for your combo.

I agee slowbird, Pipe Max is an excellent program.

[Ron H]

That calculator shows me at .57 mach. Should I be concerned? Doesn't the port volume, CSA or cam duration need to be factored in?

[R 70 Judge]

Is it better (higher volumetric efficiency) to be closer to .50 mach or .60 mach?

Is the idea to get the mixture velocity as close to sonic as possible at your target RPM limit to achieve maximum V/E and power?

Example engine:
4.35 Bore x 4.25 Stroke
2.20" valve
.660 Max lift
6500 rpm target limit

Using John's calculators I came up with a .59 Mach index. Would this combination yield the highest V/E and most power with the 6500rpm limit? Or would a change in parts with a resulting Mach index number closer to .50 yield more power?

This is a very interesting topic.

[johnta1]

Needs more cam lift.

Closer to .5 would be more efficient.
Closer to .6 you are stalls the efficiency.
It still can make power, just not what it could be capable of.