Cylinder Head Porting and Power Production

What I'm about to post is the honest to goodness truth concerning my knowledge about cylinder heads. I'm going to throw out all kinds of thoughts but that is all they are--thoughts. I don't have all the answers, I wish I did but I don't. I'm going to include some of what I consider to be real nuggets of information.

Let's make the hypothetical model engine a 10.0 comp .530" flat tappet , dual plane manifold, 750 carb, street headers, 408/416 SBM since they're so popular.

Let's throw out a flow rate of 250 cfm @ .500" @ 28" H2o for our discussion:

How much HP/TQ should an LA style head flowing 250 cfm be capable of producing? I use a rough benchmark of 2hp/cfm so 500hp is easy enough to calculate. Is a J head flowing 250cfm going to make 500hp on our model? Probably not. I'd say it would more likely produce 430-450 hp 470-480 tq. Now we switch heads to some EQ Magnums flowing the same 250cfm and we magically keep the comp ratio the same and all other specs. It would make at least 20 hp and 20 tq more in my experience and estimation. Why? Chamber? Yes the chamber has a bunch to do with it but that isn't all of it.

Some ports just "work" and some ports don't "work as well". I don't really have a hard and fast reason as to why. And I'm just talking dyno numbers here, when you throw in the acceleration in car performance and part throttle / tip in factors putting your finger on "IT" becomes even more difficult. Describing how good a cylinder head performs and why is difficult for me. I just know what head has a good/poor reputation for performance. I do find most people focus on CFM and I understand why.

Here's what I consider to be a nugget: When I see a cylinder head use ALL of it's flow and then some I know it's going to be something special. I strive for and use 2.06 hp / cfm as my benchmark. So in my opinion if our example made 515hp I would be satisfied that the engine is making the most of those cylinder heads. Would I expect our example to make 515hp? Not a chance, I already stated what I would expect. I'm trying to say that a head flowing 300cfm on an engine that makes far less than 2.06hp/cfm is going to be far less satisfying in performance. I'm also saying the example engine with EQ's flowing 228cfm and making 470hp would be a far better performing engine than if it were wearing a set of LA heads flowing 250cfm. I've seen it across all brands and platforms. I know I'm talking HP here but really what happens when you use all of the available CFM is velocity is optimized and torque production BELOW peak torque increases dramatically.

A W2 head at 250cfm on our example will make more than 500hp, and while an EQ Magnum head at 250cfm might only make 470hp I'm more than confident the Magnum headed example would be a harder accelerating engine and a better performing engine and a joy to drive. Even a dyno doesn't always tell you that.

Of course there are always exceptions to any rule but what I have stated is no rule or law, they are just my thoughts based on my observations. Yes people will throw in the GM LS3 examples or G3 examples but they have all kinds of other attributes to help them achieve what they achieve. I'm just trying to boil it down to the basic SBM cylinder head we all know and love here at FABO.

To recap I believe a 280-300cfm cylinder head on our example engine making even 500hp is leaving a lot on the table. I also believe tooling around town from stop light to stop light and manners will be inferior to a lower flowing cylinder head making the same 500hp. Maybe I'm wrong and maybe I'm expressing this wrong. Some heads flat out work and some don't. It becomes even more perplexing when you get to put a "magically" performing cylinder head on the flowbench and it doesn't flow what you think it should. Even industry gurus like David Vizard can't really quantify it and really gave it a good shot with his "Port Energy" moniker. I think he may be on to something there but doubt he can wrap it up into a easily digestible package.

I can't wait to see where this discussion goes because I bet I get more than I give, if nothing else it should be a bit of mental fun. J.Rob

p.s. As I sit here I'm listening to this recap Webinar from this past Wednesday.

There’s a couple pieces of info that I’d add as “ must knows”: the intake valve size, and the lift where that max flow is measured.

I also use “2hp/cfm”, but I add the caveat of “within the lift range of your cam choice.” Try as you might, a max effort 300cfm @ .700 lift port that flows 230cfm @ .400 is not going to make 600 hp. Yeah I need the rest of the info.

Chamber shape is important, but I’d also suggest in the example engine you provide, chamber is only a means to a compression ratio target. Nothing more. A higher hp/inch effort certainly. Restricted fuel rules certainly. But a pump gas 408 making 500hp, it’s just not.

moper said:

There’s a couple pieces of info that I’d add as “ must knows”: the intake valve size, and the lift where that max flow is measured.

I also use “2hp/cfm”, but I add the caveat of “within the lift range of your cam choice.” Try as you might, a max effort 300cfm @ .700 lift port that flows 230cfm @ .400 is not going to make 600 hp. Yeah I need the rest of the info.

Chamber shape is important, but I’d also suggest in the example engine you provide, chamber is only a means to a compression ratio target. Nothing more. A higher hp/inch effort certainly. Restricted fuel rules certainly. But a pump gas 408 making 500hp, it’s just not.

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Yes I agree with your points but I am speaking generalities in the interest of speed and effort. I didn't want to spend the time or the effort to nail down all the variables. I was trying to distill it down to the port design and shape while trying to single out flow rates. I don't really care about valve size and I did stipulate the 250 cfm @ .500" and a valve lift of .530". Again I know the chamber shape is important and that's why I said the compression ratio is "magically " maintained. J.Rob

I know that a head flowing 240 cfm @ .400" is probably going to work very very well for most purposes. I also know that a head flowing 230 cfm @ .400" is also going to work very very well also. What I'm trying to say is the flow rate really doesn't tell even half the story as far as I can tell. This is why the jury is still out on certain heads until they gain a reputation for just "working" . I have the EQ Magnum and Chevy Vortec in mind here--these heads --never mind the dyno results--just flat out work. So defining what "works" means is problematic as well. This is one reason why these types of discussions get convoluted so quickly and easily. J.Rob

Sorry if this is not a relevant question but beyond the obvious answer of "more air=more power", how does exhaust port flow affect overall power?

rmchrgr said:

Sorry if this is not a relevant question but beyond the obvious answer of "more air=more power", how does exhaust port flow affect overall power?

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That's exactly what I'm saying. More air doesn't always equal more power, unless you have a way out of whack combination like a 200cfm head on a 500 inch stroker. To answer your question I think the exhaust has a lot to do with it but I really don't know the answer other than I think a strong exhaust port helps an engine stay in "tune" longer so that it hangs on in power after peak. I also think cam events become more critical and header design becomes a bigger player with a good exhaust port. I've had W9's flow 260cfm through a 1.6" valve that fell far short of the 2.06 cfm/hp on the intake side. I've also had engines that flowed terrible on the exhaust perform stunningly well. I really don't know the answer to your question but I would revert to the reputation of whatever cylinder head you have in mind for the answer. J.Rob

I don’t like to look at the flow numbers “in a vacuum”.
Saying you should make 2.06hp/cfm is just too arbitrary for me.
Take a Ford 351C for example.
Not too hard to take the stock big valve 4bbl heads and get them well into the 300+ cfm range.
Installed on a 10:1 basic street combo with a small cam, stock intake and small tube headers....... does not equate to a 2.06hp/cfm combo.

RAMM said:

I know that a head flowing 240 cfm @ .400" is probably going to work very very well for most purposes. I also know that a head flowing 230 cfm @ .400" is also going to work very very well also. What I'm trying to say is the flow rate really doesn't tell even half the story as far as I can tell. This is why the jury is still out on certain heads until they gain a reputation for just "working" . I have the EQ Magnum and Chevy Vortec in mind here--these heads --never mind the dyno results--just flat out work. So defining what "works" means is problematic as well. This is one reason why these types of discussions get convoluted so quickly and easily. J.Rob

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interesting post, and very nice to hear.
My new motor is a 416( 418 actually) a local guy had spent a little time “ improving” my stock eddie heads. Had a 10 to 1 360 they were on
Now i have this motor built by a pro, first thing he tells me is the short turns are messed up they go turbulent because the air doesnt want to follow around nicely.
He “fixed” them as best he could( he is pretty handy) thought putting a 2.05 intake would make a few more horsepower, and got them to go low 270’s at 550 lift. My cam is 565 gross lift.
I was kinda dissapointed in the “ low 270’s” peak number.
He said the heads are really going to surprise me, and the 300 and 400 numbers were quite stout.
Said they will work very nicely. Don't know if he is just trying to make ne feel better that the heads didnt flow more, or is really happy that they will make good power... pretty sure its the latter.
It is 12.5 compression, so i am hoping the car will make nice gains. The heads and cam were on my 10 to 1 360. Heads of course the way they were before “ fixing” them
Still went almost 119@3300+ pounds before

I love posts like this with cylinder heads, camshafts, rod stroke ratio, etc. Discussing theory and not so much the hard and fast rules. It always makes me realize there is so much more to learn and so many people that have learned so much more than I have.

PRH said:

I don’t like to look at the flow numbers “in a vacuum”.
Saying you should make 2.06hp/cfm is just too arbitrary for me.
Take a Ford 351C for example.
Not too hard to take the stock big valve 4bbl heads and get them well into the 300+ cfm range.
Installed on a 10:1 basic street combo with a small cam, stock intake and small tube headers....... does not equate to a 2.06hp/cfm combo.

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Arbitrary or not all I was trying to say was when I see 2.06hp/cfm from my flowbench to my dyno the engine seems to over achieve in real world use. The corollary is also true from what I have seen. Also any 351C I have done for the street turned out less than great. J.Rob

I would say that most of the “real street” motors I’ve been involved with, that the owners are pretty happy with, would fall short of the 2.06hp/cfm.

Maybe my expectations are just low.

A 10:1 440 with ootb RPM heads and a small hyd roller cam....... how much “should” a motor like that make?

The ones I have tested have fallen short of that 2.06hp/cfm criteria....... yet seem to run quite well in the car.

At the lift of the cam, a set of TF240’s would be about 30-35cfm better.
Would you be expecting to see a 60-70hp improvement if you did nothing but swapped the heads(assuming the CR stayed the same)?
On my bench they’re 311cfm@.550 lift...... so 640hp if you’re getting 2.06hp/ci.

I’m pretty confident a 10:1 446 with TF240’s and a low-230’s @.050 HR cam isn’t going to make 640hp on the dyno here.
But I think it would still be a very satisfying street combo.

RAMM said:

Arbitrary or not all I was trying to say was when I see 2.06hp/cfm from my flowbench to my dyno the engine seems to over achieve in real world use. The corollary is also true from what I have seen. Also any 351C I have done for the street turned out less than great. J.Rob

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I believe they REDUCE the port volume on the Cleveland heads for street use.

I missed the @.500 I guess. Sorry. I also may have misjudged the goal of the post which I saw as discussing the use of airflow numbers in arriving at a post dyno conclusion about efficiency of the entire package. I don’t look at overall flow numbers as more than an indicator of performance. Because the bench is only the closest thing we have to a running engine- not a running engine with which to gage improvements vs changes. The reason certain heads just work is because they work within designed in parameters. A vortex heads works well, until your out of its comfort zone. Same with a magnum port design like the EQ heads. And the guys making bigger numbers with them are aligning the other parts to exploit their main feature- which is air speed.

My opinion, if you really want to gage a head to see if it works- look at the horsepower number and bsfc and dyno it as it will be in the car. Otherwise put it in a car and run it. Otherwise we’re all just spitballing.
Or is spitballing the intent here?

I was coming at my question from a "real world value" angle. I have an iron head 10.3:1 416" motor that made 500.2 hp on a dyno. (sheet below) According to the published head flow numbers (293 CFM), the combo should have easily made more power but hey, it didn't. Cam is a decent solid FT, around 260 @ .050 and almost .600" lift with 1.6 rocker arms, single plane intake, 1 5/8" step headers. BFSC was over 5 almost all the way through so as I learned that number was an indicator of less than optimal efficiency. The intake might be the cork, it's an old Strip Dom. HV oil pump might rob a few ponies too but it does have a crank scraper. Unfortunately I don't have any ETs because the car has been in storage for a while.

Anyway, I started looking at new heads and for what it's worth compared some published flow numbers. The comparison aluminum head flowed almost exactly the same on the intake as the current iron ones but the exhaust flow from .400" - up was significantly better, to the tune of almost 40 cfm at each interval. Have to think that an aluminum head combo would be more efficient but if everything else stayed the same - cam, intake, exhaust etc., how much of a realistic improvement could there be? I know there's no way to answer that question without actual data, just trying to figure if it's worth the money for a "perceived" improvement based on published flow numbers.

The other benefit of aluminum heads is the idea that they tolerate more compression so in theory more C.R. should help efficiency.

PRH said:

I would say that most of the “real street” motors I’ve been involved with, that the owners are pretty happy with, would fall short of the 2.06hp/cfm.

Maybe my expectations are just low.

A 10:1 440 with ootb RPM heads and a small hyd roller cam....... how much “should” a motor like that make?

The ones I have tested have fallen short of that 2.06hp/cfm criteria....... yet seem to run quite well in the car.

At the lift of the cam, a set of TF240’s would be about 30-35cfm better.
Would you be expecting to see a 60-70hp improvement if you did nothing but swapped the heads(assuming the CR stayed the same)?
On my bench they’re 311cfm@.550 lift...... so 640hp if you’re getting 2.06hp/ci.

I’m pretty confident a 10:1 446 with TF240’s and a low-230’s @.050 HR cam isn’t going to make 640hp on the dyno here.
But I think it would still be a very satisfying street combo.

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Dwayne you are making it sound like the engine won't run well if it doesn't make 2.06hp/cfm. I never said that. Of course your examples would make for very satisfying results. Again I'm just saying the engines that are around the mark I stated seem to run exceedingly well as in way above the average. You almost make it seem like an engine won't even run at all if it isn't around 2.06 hp/cfm--of course you and I both know that isn't the case. J.Rob

moper said:

I missed the @.500 I guess. Sorry. I also may have misjudged the goal of the post which I saw as discussing the use of airflow numbers in arriving at a post dyno conclusion about efficiency of the entire package. I don’t look at overall flow numbers as more than an indicator of performance. Because the bench is only the closest thing we have to a running engine- not a running engine with which to gage improvements vs changes. The reason certain heads just work is because they work within designed in parameters. A vortex heads works well, until your out of its comfort zone. Same with a magnum port design like the EQ heads. And the guys making bigger numbers with them are aligning the other parts to exploit their main feature- which is air speed.

My opinion, if you really want to gage a head to see if it works- look at the horsepower number and bsfc and dyno it as it will be in the car. Otherwise put it in a car and run it. Otherwise we’re all just spitballing.
Or is spitballing the intent here?

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Really I'm just spitballing. Totally agree with you about the bsfc and all that too. J.Rob

rmchrgr said:

Sorry if this is not a relevant question but beyond the obvious answer of "more air=more power", how does exhaust port flow affect overall power?

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In my opinion- not much. They pressure differential is so great the hole size and shape just had to be big enough to not impede the blown down, and then the header has to work to scavenge. The port itself doesn’t have a lot of impact, imo.

rmchrgr...... I would invest in a dyno session at a different facility before spending a pile of money on new heads.

Maybe I’m thinking of a different situation, but I seem to recall you had mentioned some unwillingness to do much tuning at the facility where the motor was tested last time.

I remember thinking I def wouldn’t be taking those results as “gospel”.

If the fuel flow numbers are accurate...... it’s using way too much fuel.
For comparison, I grabbed a dyno sheet for a street 446 making about 50hp more than your SB, and at 6200rpm it’s using 20lb/hr less...... and has a 12.8:1 a/f ratio with a bsfc of .479.

So, either the tune is way off...... or the combo is really that far off(or there’s a problem with the dyno room causing low power output).

Edit- found one more similar to yours:
416, RPM heads flowing 270-ish, 265@.050 solid cam, SV intake with old school 850.
561hp@6200, 246lb/hr fuel, bsfc .46

I think it boils down to having a good match: expectation to engine choice; engine size to heads; heads to camshaft. The engines that excel with the magnum and vortec based ports show how good they are. the question is if you had say, that same engine as you started with, but with a large volume race ported head, and then stuck a set of EQs on it. Would it get better or worse? Why?

I think it is the over all quality of the average airflow in concert with the port shape and volume is the way new guys should be enquirering (SP?) about the head which should be a match to the goal at hand for the size of the engine being used.

This is the big question to research IMO and the mystery at hand.

rmchrgr said:

I was coming at my question from a "real world value" angle. I have an iron head 10.3:1 416" motor that made 500.2 hp on a dyno. (sheet below) According to the published head flow numbers (293 CFM), the combo should have easily made more power but hey, it didn't. Cam is a decent solid FT, around 260 @ .050 and almost .600" lift with 1.6 rocker arms, single plane intake, 1 5/8" step headers. BFSC was over 5 almost all the way through so as I learned that number was an indicator of less than optimal efficiency. The intake might be the cork, it's an old Strip Dom. HV oil pump might rob a few ponies too but it does have a crank scraper. Unfortunately I don't have any ETs because the car has been in storage for a while.

Anyway, I started looking at new heads and for what it's worth compared some published flow numbers. The comparison aluminum head flowed almost exactly the same on the intake as the current iron ones but the exhaust flow from .400" - up was significantly better, to the tune of almost 40 cfm at each interval. Have to think that an aluminum head combo would be more efficient but if everything else stayed the same - cam, intake, exhaust etc., how much of a realistic improvement could there be? I know there's no way to answer that question without actual data, just trying to figure if it's worth the money for a "perceived" improvement based on published flow numbers.

The other benefit of aluminum heads is the idea that they tolerate more compression so in theory more C.R. should help efficiency.

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I hate to say it...but I will...this looks like a perfect case for a burr finish. All that fuel going through the engine isn’t being used, it’s just wall flow that blows out the exhaust or ends up in the chamber as stratified layers of fuel that burns so late it does nothing to make power. As much as I don’t care for EGT’s as a tuning tool, you would have probably seen elevated EGT’s because the late burning fuel is getting into the header. Elevated EGT’s can be late timing, too fat, both and what you may be seeing with wall flow issues.

Too bad you can’t test it. If you burr finish and drop 3-4 jet sizes, even if it makes the same power on the dyno it will be quicker in the car and at the track.

There are engine dyno’s out there that measure the rate of acceleration. I’m going on one here in a few months if everything goes well and it is an inertia dyno, not a water brake. It will tell you if the engine accelerates the load quicker, rather than measure torque against the water brake.

PRH said:

rmchrgr...... I would invest in a dyno session at a different facility before spending a pile of money on new heads.

Maybe I’m thinking of a different situation, but I seem to recall you had mentioned some unwillingness to do much tuning at the facility where the motor was tested last time.

I remember thinking I def wouldn’t be taking those results as “gospel”.

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Good memory Dwayne. Yeah, admittedly my dyno session was not the greatest. Part of it was my lack of experience and not knowing what to ask/do but from what I know now the operator was basically just mailing it in. I will be much better prepared next time.

So again, just because of some life stuff, (moving) the car has been in storage since 2017. Since it's been sitting so long, IMO the motor will need to be taken apart and checked, have not turned it over in over a year. While it's apart I'm going to re-check cam timing at a minimum. Might change the oil pump to a standard volume as well. Figured that if I have the thing apart new heads might wake it up. I do think there is some power to be had in bumping the compression up.

Other side of the coin is that I did want to run it the way it is and see what the actual results are, might surprise me!

RAMM said:

What I'm about to post is the honest to goodness truth concerning my knowledge about cylinder heads. I'm going to throw out all kinds of thoughts but that is all they are--thoughts. I don't have all the answers, I wish I did but I don't. I'm going to include some of what I consider to be real nuggets of information.

Let's make the hypothetical model engine a 10.0 comp .530" flat tappet , dual plane manifold, 750 carb, street headers, 408/416 SBM since they're so popular.

Let's throw out a flow rate of 250 cfm @ .500" @ 28" H2o for our discussion:

How much HP/TQ should an LA style head flowing 250 cfm be capable of producing? I use a rough benchmark of 2hp/cfm so 500hp is easy enough to calculate. Is a J head flowing 250cfm going to make 500hp on our model? Probably not. I'd say it would more likely produce 430-450 hp 470-480 tq. Now we switch heads to some EQ Magnums flowing the same 250cfm and we magically keep the comp ratio the same and all other specs. It would make at least 20 hp and 20 tq more in my experience and estimation. Why? Chamber? Yes the chamber has a bunch to do with it but that isn't all of it.

Some ports just "work" and some ports don't "work as well". I don't really have a hard and fast reason as to why. And I'm just talking dyno numbers here, when you throw in the acceleration in car performance and part throttle / tip in factors putting your finger on "IT" becomes even more difficult. Describing how good a cylinder head performs and why is difficult for me. I just know what head has a good/poor reputation for performance. I do find most people focus on CFM and I understand why.

Here's what I consider to be a nugget: When I see a cylinder head use ALL of it's flow and then some I know it's going to be something special. I strive for and use 2.06 hp / cfm as my benchmark. So in my opinion if our example made 515hp I would be satisfied that the engine is making the most of those cylinder heads. Would I expect our example to make 515hp? Not a chance, I already stated what I would expect. I'm trying to say that a head flowing 300cfm on an engine that makes far less than 2.06hp/cfm is going to be far less satisfying in performance. I'm also saying the example engine with EQ's flowing 228cfm and making 470hp would be a far better performing engine than if it were wearing a set of LA heads flowing 250cfm. I've seen it across all brands and platforms. I know I'm talking HP here but really what happens when you use all of the available CFM is velocity is optimized and torque production BELOW peak torque increases dramatically.

A W2 head at 250cfm on our example will make more than 500hp, and while an EQ Magnum head at 250cfm might only make 470hp I'm more than confident the Magnum headed example would be a harder accelerating engine and a better performing engine and a joy to drive. Even a dyno doesn't always tell you that.

Of course there are always exceptions to any rule but what I have stated is no rule or law, they are just my thoughts based on my observations. Yes people will throw in the GM LS3 examples or G3 examples but they have all kinds of other attributes to help them achieve what they achieve. I'm just trying to boil it down to the basic SBM cylinder head we all know and love here at FABO.

To recap I believe a 280-300cfm cylinder head on our example engine making even 500hp is leaving a lot on the table. I also believe tooling around town from stop light to stop light and manners will be inferior to a lower flowing cylinder head making the same 500hp. Maybe I'm wrong and maybe I'm expressing this wrong. Some heads flat out work and some don't. It becomes even more perplexing when you get to put a "magically" performing cylinder head on the flowbench and it doesn't flow what you think it should. Even industry gurus like David Vizard can't really quantify it and really gave it a good shot with his "Port Energy" moniker. I think he may be on to something there but doubt he can wrap it up into a easily digestible package.

I can't wait to see where this discussion goes because I bet I get more than I give, if nothing else it should be a bit of mental fun. J.Rob

p.s. As I sit here I'm listening to this recap Webinar from this past Wednesday.

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Exactly. How well an engine uses the air (BSAC) and fuel (BSFC) and what it makes HP/CID is what matters.

An engine making 2.1 HP/CID is much more impressive than an engine making 1.6 HP/CID making the same horsepower.

In the above exaggerated example, you could have a 380 inch engine making 798 HP and a 500 inch engine making 798 HP and for me, the 380 inch engine is much more impressive.

If you at least run it before the change, then you will have a bad to compare with. Run Greg! Run it!

My point was, I think it’s entirely possible those dyno numbers aren’t an accurate representation of what that combo is capable of.

275lbs/hr is enough fuel to make over 600hp.

Looking at a sheet for a 440 with a roller and good heads.
661hp@6000 using 278lbs/hr, bsfc of .438, a/f ratio 13.1