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.
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.
