I was tooling around the web and ran upon some really good tech articles. The guy who writes them is David Reher of Reher-morrison racing. Of course the guy knows more about engine building for race cars than your average Joe. I see around this site a lot of people talking mostly about flow #'s but I'm wondering if this directly translates to HP and probably more important Tq. From what I can gather it's the combination of parts, for heads, it's ultimately the speed and volume of the A/F that is important and actually having increased flow is sometimes detrimental. With this in mind how does a novice like me go about determining what is best suited for me when there are so many variables in place? Take a few moments to read this ARTICLE. It's worth the time.

So much knowledge to acquire so little money to aquire it with.

 

Flow number's are VERY IMPORTANT!!!...I see alot of people on this board saying that flow number's arent everything...well they are...if you have higher flow numbers (ALL THE WAY ACROSS THE BOARD!) Than another head...the higher flowing head will ALWAYS outpower the lesser flowing head...but to outflow another head takes alot of other things into account as well...if a head out flows another...one that has 239cc intake volume and peaking at say 305cfm@.600 and the other head has a 210cc intake volume and flows exactly the same..i would take the 210's in a heartbeat! baicly look for higher flow numbers with the smallest volume..that will give you velocity and tourqe..yes flow numbers mean ALOT..but not everything

 

It depends HEAVILY on the rest of the motor. Ill use big blocks as an example, because of the huge range of displacements and heads availible.

You can take a 396, and stick the newest, most expensive heads, with the highest flow numbers around on it, but if the heads have 320cc intake ports to get that kind of flow, and you dont A: have 500ci+ underneath, or are B: Spinning it to hell and gone (8-9k rpm) its gonna be a dog.

On the other hand, the same shortblock with a properly prepped set of oval port heads will probably outpower the 396 with the uber-heads.

Its about COMBINATION. Not flow numbers. The Mustang guys are the worst with that. They wont hesitate to stick a set of heads on a 281ci motor that simply DEMAND more cubes, or insane RPM. But they dont care. Theyve got heads that flow 350cfm at .700. Not that their cams lift that high anyway. And not that they have good flow at .200 or .300.

Combination Combination Combination.

 

Kobwo,
Yes, cfm's are certainatly important, they represent a % of oxygen content being injested, and how the intake tract manages the flow is equally important too.

Joe.

 

Whats that saying?

"You can never have too much headflow, just not enough motor underneath it"

 

The flowbench is a good tool. A motor is an airpump, and flow numbers are a way to quantify and measure flow which can be directly correlated to performance. As an example you can correlate HP for flow by multiplying your flow I.E. 300 cfm on your intake side will make over 600 HP ion a race motor. ON a race motor you multiply flow by 2.15. So each cfm of airflow is worth 2.15 HP. On a Pro Stock motor it is closer to 2.35. ON a street motor it is more like 1.8-1.9. But, a flow bench can be made to lie.

In a recent dyno test. A set of X heads went up against a set of Y heads. They flow well on the bench (as good as a Y), but don't seem to make the same power on the dyno,etc. X removes material in areas that will show a gain on the flow bench, but will not make anymore power on the dyno. They are big in the wrong spots and too small in others.

As an example a set of X stage 2s 5.3l heads on a car that had all the bolt ons and a 224 cam. It made 379/370 or so before adding the heads. After the heads, the car did 413/385 after tuning. A week before, a set of Y stage 2 5.3l heads on a car that already had the same 224 cam as well and the IDENTICAL mods as the other car (FLPs, LS6 intake, ASP, stock rear/4:10s,etc). Both dynoed the exact same before swapping heads on (the curves practically overlayed oneanother). This car pulled 443/418 with the Ys! On the flow bench, the Xs actually flowed better from .100-.550, while Y flowed slightly better past this. Same thing on the exhaust side. This just goes to show that a flow bench is an excellent tool, but not something that should be used as the determing factor in how a head will perform when put on a motor. The CC volumes were identical on the two heads, so compression difference wasn't an issue.

So, a vendor does what they think is good work to make the heads look like they flowed well, but in fact they didn't. And although they look good on a bench, in the real world they didn't live up to the hype.

But, a flow bench is an invaluable tool.

 

So what do most guys use to determine what head/cam to buy. I've seen to many times that the results aren't completely repeatable from one car to another. How do I know that, before I spend some cash, brand X heads will work optimaly for my configuration without just talking to the vendor that is actually marketing the head? They obviously have a reason to push their product. But, I dont want to put on brand x just because they flow better on a bench, it's track results that show the actual combination is good. So what should I do to determine the best setup for my car and who to buy from? If flow #'s don't show the whole picture.

 

Look over thread in this section. Don't look for the one fluke car that comes from a shop. Look at repeatability. If every set of heads puts out X on the dyno, and runs Y in the quarter then you may want to take a hard look at that shop. Alos proven packages from "tuners" are a good way to get a proven combo.