Author Topic: Is there a "rule of thumb" for HP vs intake flow for heads?  (Read 1378 times)

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FERoadster

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Is there a "rule of thumb" for HP vs intake flow for heads?
« on: August 03, 2020, 10:00:05 PM »
A friend much more knowledgeable then me and I were discussing head intake flow vs HP potential.
His statement seemed legit so I thought I'd throw it out here as a question/discussion point.

Hypothetical question
427 +017 block RPM intake and 850 Holley carb. 1-7/8 headers, cam: please someone name one that would be appropriate.
Compression 10.5

1st engine heads flow 275 CFM at 600 lift

2nd version exactly the same engine heads flow 300 at 600 lift.
Consider both versions a very detailed custom build

His thoughts are HP can be approximated or have the potential at 2 times the CFM intake flow.  plus or minus 10%
Your thoughts?

Richard >>> FERoadster



Joe-JDC

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Re: Is there a "rule of thumb" for HP vs intake flow for heads?
« Reply #1 on: August 03, 2020, 10:44:17 PM »
I have tried for years to find a correlation to airflow vs potential, and find that there is a lot more to the question than a simple answer such as you propose.  For years, the formula of  CFM x .257 x #cylinders was used, but every time I use that, it seems optimistic vs the actual dyno results.  I have tried average cfm airflow, and doubled that, which comes closer to actual potential, but that does not always work, either.  What I try to use is airflow through the intake manifold and head together, with the first formula, and it really comes close.  Not a lot of information available for that combination, but it works.  So, just for grins,  275 x .257 x 8 = 565.4 hp, and 300 x .257 x 8 = 616.8 hp.  This is theoretical, so take it with a grain of salt. 275 x 2 = 550 hp, and 300 x 2 = 600 hp.   That 10% fudge factor certainly comes into the parameters, and if you think about it, that is a lot of swing in potential.  Joe-JDC
Joe-JDC '70GT-500

e philpott

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Re: Is there a "rule of thumb" for HP vs intake flow for heads?
« Reply #2 on: August 03, 2020, 10:45:53 PM »
Most builders say double cfm for potential horse power, 300 has 600 horse capabilities but NHRA Stock racers kind of blow that theory out of the water . But besides 600 lift , flow at 200 lift up is important as 600 lift if flow only numbers of your cam has only a max lift of 600 meaning you can’t have lousy low lift numbers with the 600 lift cam and expect a miracle

WerbyFord

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Re: Is there a "rule of thumb" for HP vs intake flow for heads?
« Reply #3 on: August 03, 2020, 11:32:20 PM »
Most builders say double cfm for potential horse power, 300 has 600 horse capabilities but NHRA Stock racers kind of blow that theory out of the water . But besides 600 lift , flow at 200 lift up is important as 600 lift if flow only numbers of your cam has only a max lift of 600 meaning you can’t have lousy low lift numbers with the 600 lift cam and expect a miracle

The "double your intake CFM" = MAX horsepower potential on pump gas is about the closest to a rule of thumb I could think of.
And that's about the MAX - if everything else is right on.

Think about the top engines of the 1960s - the Hemi, the L72 rat, the Medium Riser. They had intake flows nearing 300, so 300 x 2 = 600 hp.
That doesn't mean you got 600hp from the factory - more like 460hp give or take a little.
But, in NHRA trim, they did indeed get to around 600hp, and that was with very little work to the heads as very little was allowed by NHRA.

jayb

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Re: Is there a "rule of thumb" for HP vs intake flow for heads?
« Reply #4 on: August 04, 2020, 08:22:28 AM »
I think the key word is "potential" HP per cfm of intake flow.  As Werby mentioned, you need a race motor calibration to really take advantage of that.

I usually use a factor of 2.1 HP per cfm if I'm building a real strong engine.  Here's a few examples from some of the engines I've done over the years:

511" Medium Riser used at Drag Week 2005, peak intake flow was 340 cfm at .700", HP was 706 at 6700, HP per cfm was 2.076

530" High Riser used at Drag Week 2013, peak intake flow was 401 at 0.800", HP was 850 at 7100, HP per cfm was 2.119

585" SOHC used at Drag Week 2016, raised and welded intake ports flowed 460 at .700", HP was 1004 at 7100, HP per cfm was 2.182

504" Medium Riser used as a dyno mule for testing the 351 intakes, if I recall correctly peak intake flow was 330 cfm from Joe Craine ported Edelbrock heads (Joe, maybe you can check this?), peak HP was 723 at 6300, HP per cfm was 2.191


None of these engines were really maxed out; for example, none had 15:1 compression, most were 13:1, and the first one on the list was only 11.5:1 if I recall correctly.  But I have seen legitimate dyno data of numbers of up to 2.4 HP per cfm, on a really high winding small block at another dyno, so it's possible to get pretty far past 2:1 for HP per cfm.  The intake tract is the critical piece...
Jay Brown
- 1969 Mach 1, Drag Week 2005 Winner NA/BB, 511" FE (10.60s @ 129); Drag Week 2007 Runner-Up PA/BB, 490" Supercharged FE (9.35 @ 151)
- 1964 Ford Galaxie, Drag Week 2009 Winner Modified NA (9.50s @ 143), 585" SOHC
- 1969 Shelby Clone, Drag Week 2015 Winner Modified NA (Average 8.98 @ 149), 585" SOHC

   

Joe-JDC

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Re: Is there a "rule of thumb" for HP vs intake flow for heads?
« Reply #5 on: August 04, 2020, 09:53:36 AM »
The problem I have with the cfm x 2 is the cubic inch of the engine is not taken into the equation.  A 302 with a 280 cfm head, and a 445 with a 280 cfm head will have very different horsepower and torque numbers.  I drove a SBF on the street for years with a 330 cfm TFS head, and thought it was a monster.  Put a 230 cfm head on the same shortblock, and it was just as potent on the street, actually felt stronger at times.  Like Jay, I think compression ratio has to be factored into the equation.  Compression ratio will come into the factor when you start figuring HP/CI and TQ/CI, regardless of the cfm of the cylinder heads.  This year, the Masters of Motors competition was supposed to increase the compression ratio from 10.5 to 11.5 for the Old Iron class.  It will be an interesting comparison to last year's dyno numbers since most everything else will remain the same except the headers must not have steps in the primary tubes.  Will have to wait until next year for that dyno comparison, though, because the MOM was put on hold due to the virus.  Joe-JDC
Joe-JDC '70GT-500

Barry_R

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Re: Is there a "rule of thumb" for HP vs intake flow for heads?
« Reply #6 on: August 04, 2020, 09:53:55 AM »
Airflow numbers can provide useful guidance as to power potential when comparing similar parts, but they are not anywhere near a fixed reference.  Most airflow numbers are given at 28"  - a value which has absolutely nothing to do with actual operating characteristics of an engine, but instead is tied to the capabilities of the test equipment in common use at the time the "standard" was adopted.

As examples, you can stick a 340 cfm wedge low riser on an engine, a similar flowing medium riser, a tunnel port, an SOHC, or a filled port high riser - - and you would get significantly different power numbers.  Port targeting relative to the valve, height off of the deck, port volume, runner length and shape are all characteristic not reflected in a simple flow metric.  Raise the flow demand on a given port with extra cubic inches, enhanced efficiency, and/or higher RPM and the flow characteristics of a given port will change as velocity goes up - some may get better, some worse.

My427stang

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Re: Is there a "rule of thumb" for HP vs intake flow for heads?
« Reply #7 on: August 04, 2020, 10:03:02 AM »
Joe, your rule seems plausible, but it also isn't far from 2:1 on a V-8 (2.056)

I like the 2:1 on an FE because it's easy and quick to explain differences, it can be conservative.  Not to bring a dirty subject in, the 460 guys usually use 2.2.  You can see from Jays numbers, and supported by Barry's comments, that engine design as a whole matters.  That being said, as Jay improved port design, even on the C intake, the numbers were a little better with the better port.

The key is of course match the entire build to itself, however, often we can explain why something happens too.  The guy with a 260 cfm Edelbrock and a HUGE cam can't make the numbers of a better head and less cam...easy to subtract the difference in flow and multiply by two to explain while bench racing.

Really simplified...fast guys make power with heads...certainly more to it, but assuming the car can hook and parts match properly, usually holds true...
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Ross

- 70 Fastback Mustang, 489 cid FE, Victor, SEFI, Erson SFT cam, TKO-600 5 speed, 4.11 9 inch.
- 71 F100 shortbed 4x4, 461 cid FE, headers, Victor Pro-flo EFI, Comp Custom HFT cam, 3.50 9 inch

blykins

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Re: Is there a "rule of thumb" for HP vs intake flow for heads?
« Reply #8 on: August 04, 2020, 01:26:41 PM »
Joe, your rule seems plausible, but it also isn't far from 2:1 on a V-8 (2.056)

I like the 2:1 on an FE because it's easy and quick to explain differences, it can be conservative.  Not to bring a dirty subject in, the 460 guys usually use 2.2.  You can see from Jays numbers, and supported by Barry's comments, that engine design as a whole matters.  That being said, as Jay improved port design, even on the C intake, the numbers were a little better with the better port.

The key is of course match the entire build to itself, however, often we can explain why something happens too.  The guy with a 260 cfm Edelbrock and a HUGE cam can't make the numbers of a better head and less cam...easy to subtract the difference in flow and multiply by two to explain while bench racing.

Really simplified...fast guys make power with heads...certainly more to it, but assuming the car can hook and parts match properly, usually holds true...

The 2.2 rule for 460 guys seems reasonable.   1206 hp with a 550 cfm head here recently.
Brent Lykins
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plovett

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Re: Is there a "rule of thumb" for HP vs intake flow for heads?
« Reply #9 on: August 04, 2020, 04:15:25 PM »
Discharge coefficient may help compare two heads with similar flow numbers?   Or average cross sectional area.  Or port volume.  In any case you will end up with ratios of flow to area or volume.   That might "flesh out" the simple flow numbers to some extent   

http://www.wallaceracing.com/discharge-coef.php

pl
« Last Edit: August 04, 2020, 07:23:49 PM by plovett »

FERoadster

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Re: Is there a "rule of thumb" for HP vs intake flow for heads?
« Reply #10 on: August 04, 2020, 09:32:45 PM »
Here is my cam card.  engine is a +030 427 with a 3.98 crank  Cam purchased new from Gene Kitchen about 8 years ago. I bought it primarily due to the larger exhaust potential for 100% street driving in a car right at 3000# with a 4.57 gear and wide ratio toploader. Intake will be the 6X2 Edelbrock with Edel. heads and 2.19 valves .  Heads flowed 306 at 600 lift.

Hope this helps with  the 2X HP rating

Richard
« Last Edit: August 04, 2020, 11:39:41 PM by FERoadster »

My427stang

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Re: Is there a "rule of thumb" for HP vs intake flow for heads?
« Reply #11 on: August 04, 2020, 10:15:51 PM »
I think the limiting factor will be your intake manifold but it should be a pretty stout combo

Of course the intake has a cool factor....I’d guess there’s probably 50 hp more with a modern intake

Super cool though!
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Ross

- 70 Fastback Mustang, 489 cid FE, Victor, SEFI, Erson SFT cam, TKO-600 5 speed, 4.11 9 inch.
- 71 F100 shortbed 4x4, 461 cid FE, headers, Victor Pro-flo EFI, Comp Custom HFT cam, 3.50 9 inch

FERoadster

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Re: Is there a "rule of thumb" for HP vs intake flow for heads?
« Reply #12 on: August 04, 2020, 11:50:41 PM »
Ross thanks for the reply. Yes the intake is totally for the wow factor.
The rest of the build: C3 Long CI headers. RC blowproof BH. Vertex Mag.  6 Stromberg WW carbs, Aluminum WP, Edelbrock Marine TC cover with the legs cut off.  Mustang II disc frontend with PS (way less weight than the 60 Vette huge cross member),  No conv. top, heater delete (not needed in SW Oregon) no wipers body stripped to the bare minimum.  Hoping for just under 3000#

BTW Jays testing showed that the Edelbrock 6X2 performed in the top 1/4  of his tests, Tuning will be the issue.

Richard
« Last Edit: August 04, 2020, 11:54:17 PM by FERoadster »

jayb

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Re: Is there a "rule of thumb" for HP vs intake flow for heads?
« Reply #13 on: August 05, 2020, 09:31:03 AM »
That 6X2 intake did a lot better than I thought it would, peak power with that intake was almost as high as the top intakes.  It lagged a little in the midrange, but still a really good intake.
Jay Brown
- 1969 Mach 1, Drag Week 2005 Winner NA/BB, 511" FE (10.60s @ 129); Drag Week 2007 Runner-Up PA/BB, 490" Supercharged FE (9.35 @ 151)
- 1964 Ford Galaxie, Drag Week 2009 Winner Modified NA (9.50s @ 143), 585" SOHC
- 1969 Shelby Clone, Drag Week 2015 Winner Modified NA (Average 8.98 @ 149), 585" SOHC

   

cjshaker

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Re: Is there a "rule of thumb" for HP vs intake flow for heads?
« Reply #14 on: August 05, 2020, 09:46:48 AM »
Am I getting this right? You're putting a 427 FE into a '60 Vette?
Doug Smith


'69 R-code Mach 1, '65 427 MR, 2x4, 4-spd, 4.30 Locker
2 1965 Galaxies with 390s
1970 F-350 390
1958 Ford Ranch Wagon 390