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Old 08-19-2003, 09:42 AM   #1
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All else being equal, how much HP per cfm?

I posted this on the LT1 forum, but didn't receive any responses.

What HP difference would be expected with a 5cfm difference in heads?

Jim
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Old 08-19-2003, 12:20 PM   #2
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i would say, depends on the cam, but maybe a rough estimate would be something like ... 7-10hp? just a guess
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Old 08-19-2003, 01:00 PM   #3
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Thats a good question.

Generally it would be true to say that with a well tuned combo and compression that will support pumpgas you can typically see 1.9 to 2 hp per cfm. With higher racing compressions I have seen as much as 2.2 per cfm on raised runner 23 degree heads. That seems to be a pretty good number for race engines.

It does depend on where you see that cfm increase. If you dont see it until the higher lift numbers it probably wont make as much difference as if you see an across the board 5 cfm increase. And you probably wont see any difference if the increase comes at a lift that is above what you cam has for lift.
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Old 08-20-2003, 07:16 AM   #4
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I agree with dano. Right around 2 hp/cfm.


Dave Vizard, in his 12 yr old book on building and modifying SBC heads referred to a method from Superflow's users manual. For a 350 SBC with 10:1 static CR:

Intake flow @ 25in. H2O for full lift point of intake valve
[convert 28 in. H2O figures: (25/28)^.5 or square root of pressure ratio or .945]

Multiply this flow figure by a factor detrmined by cam type:

1.95 for flat hydraulic
2.0 for flat solid
2.05 for hydraulic roller
2.1 for solid roller

For displacements different from 350, use these additional multipliers:

400 cubes 1.1
327 cubes 0.95
302 cubes 0.90

Additionally, CR's above 10:1 use an additional multiplier:

11:1 1.017
12:1 1.033

EXAMPLE: 11:1 350 with 300 cfm (@28 in H2O) at max lift point of hyd. roller cammed engine:

300 x .945 x 2.05 x 1.017 = 591 hp

This all assumes the output isn't limited by port area rule.

State-of-the-art of camshaft design is better now than in '91, so upper end is probably a bit higher.

Last edited by OldSStroker; 08-20-2003 at 08:10 AM.
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Old 08-20-2003, 05:58 PM   #5
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2 hp per cfm is a good pump gas mark but it's out of the reach of most people who are just picking very complimentary parts from a catalog and bolting the stuff together. it takes a bit of development work to get that from a streetable every day driver. most will be closer to 1.8 hp per cid and i believe that to be a much more realistic goal for the do it yourself guy with sound building skills and good machine work. and yes you can make 2.2 hp from raised runner 23 heads but for those considering such a build, unless rules restrict them, you probably should have went with a more race oriented head in the first place cause then you could make 2.5-2.6 hp per cid or make the same power with a less aggressive camshaft profile.

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Old 08-20-2003, 08:08 PM   #6
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Good points Jeff. I was going to post a link to that post were you stated some HP per CFM numbers before.

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Old 08-24-2003, 09:58 PM   #7
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Quote:
This all assumes the output isn't limited by port area rule.
Could you explain what is the port area rule?

Thanks,

Dave
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Old 08-25-2003, 07:35 AM   #8
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Quote:
Originally posted by Dave69Z
Could you explain what is the port area rule?

The short answer is that if velocities anywhere in the intake port exceed more than 0.55 to 0.60 the speed of sound (about 690 ft/sec or so), the air has a tough time accelerating past this velocity every cycle and you get an "inertial block" on the mass flow, as Vizard calls it, when airflow into the engine and power take a dump. The research on this was done by C. F. Taylor, a while ago. Taylor and Taylor wrote The Internal-Combustion Engine textbook.

The minimum port area, when the piston is at it's max velocity (75-80 degrees ATDC) and valve lift is past 25% of the valve diameter (or about .400 in.) isn't the valve curtain area, but in a typical SBC head, the area where the port passes the pushrod.

With some empirically developed constants based on type of camshaft (race roller, flat hydraulic, etc), Vizard published a few formulae for determining limiting port velocity (LPV), or min port area or max power rpm based on min. port area. Bore^2 and stroke (displacement) are part of each.

A larger displacement engine should therefore reach the limiting port velocity at a lower rpm than a smaller displacement area if there is a limiting port area in the heads used. It's a mostly linear relationship with displacement.

Read Mr. Horsepower's posts in this thread:
http://web.camaroz28.com/forums/showt...hreadid=102549

Not that short, I guess.

Last edited by OldSStroker; 08-25-2003 at 08:21 AM.
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Old 08-25-2003, 08:51 AM   #9
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This is why I love this board...

OldSStroker, you and a couple other advanced tech guys make CZ28.com the greatest board out there. I can look in every post in advanced tech and learn at least somthing. The amount of knowledge in here still amazes me.
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Old 08-25-2004, 07:43 PM   #10
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Re: All else being equal, how much HP per cfm?

I'm blown away.
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Old 08-26-2004, 09:38 PM   #11
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Re: All else being equal, how much HP per cfm?

Agree!!

I follow one list and only one list. The F-body guys are way ahead of everyone else by a bunch.

Good going guys! I appreciate your inputs.
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Old 08-27-2004, 02:46 PM   #12
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Re: All else being equal, how much HP per cfm?

You will gain 10.28Hp with a 5CFM increase,however you get it.
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Old 08-27-2004, 10:47 PM   #13
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Re: All else being equal, how much HP per cfm?

Quote:
Originally Posted by 1racerdude
You will gain 10.28Hp with a 5CFM increase,however you get it.

Maybe and maybe not. If you do something to disrupt the wet flow chacteristics of the port you'll hurt power... don't care if you gain 10-20 cfm. I can name some examples I've been involved with that did just that.

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Old 08-28-2004, 01:51 AM   #14
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Re: All else being equal, how much HP per cfm?

i refuse to think that my engine with my new heads should do around 500rwhp n/a even with 8.5:1 CR

add the supercharger and.. hmm...
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Old 08-28-2004, 01:55 AM   #15
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Re: All else being equal, how much HP per cfm?

MG,
I don't recall the original post saying anything about"wet"flow.
The question was how much HP for 10CFM!!!!!
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Old 08-28-2004, 01:55 AM
 
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