Airflow Fallacies: Avoiding the Pitfalls of the Flow Bench
By David Reher


"What's it flow?"

Whenever a conversation about cylinder heads begins with that question, I cringe. I know where this discussion is going, and it's not good. When a racer wants to distill the performance of a highly developed cylinder head down to a single number, I know I'm dealing with someone who is fixated on the flow bench.

I can speak from hard-earned experience, because there was a time when the flow bench was the center of my universe. When my partners Buddy Morrison and Lee Shepherd constructed our first flow bench in the '70s, it was a revelation – or so we believed. We were addicted to airflow, and like three flow bench junkies, we convinced ourselves that big flow numbers translated to quicker elapsed times. But that was more than 30 years ago, and since then I've learned to avoid the pitfalls of flow bench testing.

Unfortunately many racers coming into the sport haven't been taught the lessons that Buddy, Lee and I learned the hard way. Cylinder head manufacturers, porting shops, and engine builders constantly advertise flow numbers – and I confess that I'm sometimes guilty as well. In this environment, it's understandable that some racers think it's all about maximum airflow. They shop for the biggest cfm number at the lowest price, like finding a screaming bargain on a 52-inch TV at WalMart.

The strategy to win the "Biggest CFM Contest" is simple: Grind the largest port that will physically fit in the head, use the biggest valves that will fit the combustion chambers, and test it on the biggest fixture you can find. That head might win the prize for airflow, but it won't win on the dyno or on the race track.

The factors that determine the performance of a cylinder head are complex. A head that is ported without considering air speed, the size of the engine, the rpm range, the location of the valves, and a dozen other parameters isn't going to be the best head, regardless of its peak airflow. And yet I see racers who are seduced by big cfm numbers bolt a pair of 10,000 rpm cylinder heads on a 7,000 rpm short block and then wonder why the engine won't run.

The most critical area in a competition cylinder head is the valve seat, and the order of importance works its way out from there. There are many questions that are much more important than airflow: How far are the valve heads off the cylinder wall? What's the ratio of valve size to bore diameter? What's the ratio of the airflow to the size of the valve? What's the size of the port, what's its taper, how high is the short-side radius? The answers to these aren't as simple as comparing a flow number, but they are what really make a difference in an engine.

Airflow is simply one measurement among many that influence engine performance. With the availability of affordable flow benches and computer simulation programs, it's easy to fall into the airflow trap. A builder works on a cylinder head, sees some bigger cfm numbers, and keeps working for more flow. But if he doesn't stop and test the engine on a dyno and on the drag strip, it's very likely he's gone down a blind alley. What the manometer on a flow bench sees at a steady 28 inches of depression is not at all what the engine sees in the real world. The pursuit of a big cfm rating has ruined countless cylinder heads in terms of what will actually run on an engine.

I put more faith in dyno pulls and time slips than I do in flow benches. I'll cite an example from back in the day when Buddy, Lee and I were winning Pro Stock championships. Lee came up with an idea for a tuliped exhaust valve. He filled in the back of the valve with Bondo, and tested the new design on our flow bench. It was killer. We instantly saw a tremendous improvement in airflow with a small exhaust port, a nice tight radius below the seat, and much more stable flow. So we had some titanium tulip exhaust valves made and tested them on the dyno – and the engine didn't run well at all. We had great airflow on the bench, but the engine didn't care.

We were working late one night, and Buddy decided to yank the heads off the block and have Lee open up the exhaust throats. Well, Lee kept grinding and Buddy kept taking the heads on and off, and eventually we picked up 30 horsepower that night. We were porting from the dyno and not from the flow bench. When Lee finally flow tested the heads the next day, they were down 30 or 40 cfm, but that's not what that engine saw.y an experienced engine builder.

The final test of a cylinder head is on the track. Frank Iaconio was our chief Pro Stock rival, and he was a smart racer. Frankie used to change valves at the track -- he'd make a run, come back to the pits and switch from valves with a 30-degree back angle to a 20-degree back angle. We did similar tests on the dyno, but he did it at the track. I was impressed.

I'm not dismissing flow benches. In fact, we use them daily at Reher-Morrison Racing Engines. But a flow bench is a tool, and it's really not much different than a micrometer. A micrometer can measure the diameter of a piston, but you have to run the engine to learn the correct piston clearance. Knowing the sizes of the piston and cylinder bore doesn't tell you if the piston is going to gall or collapse a skirt until you run it. And knowing the airflow of a cylinder head doesn't tell you whether it will make good power on a given engine until you race it.

Experience is the most important tool in cylinder head development. A person with extensive dyno and track experience has been through it all before, and knows how to avoid the flow bench fallacies.

cuisinartvette wrote:
THe biggest cfm, biggest port possible as mentioned above usually nets a disappointment, wev'e seen that for yrs; proven over and over with the newer head tech that has been out, old hat. True.
You see overported heads all the time, people fall for it over and over. Question is what size/shape is the port for the given application and was it a good choice.



I had to chuckle when I read this. You can get all the fuel in the world through a head but if you cant get the air through to burn it what good is it. While there certainly is somethign to fuel flow you need air, too or its no good.
http://www.dartheads.com/dartboard/showthread.php?t=323

Btw your flow numbers are on there and verified if youre curious.

To be honest we almost never use flow numbers these days. Our newer heads are all developed on a wet-flow bench, which has completely different flow figures and reference points than a typical flow bench.

Yes, we do still make airflow numbers available, I'm not sure if we used conservative ones, we do like any figures we release to be accurate and not over-inflated, but it's also entirely possible that those flow numbers are spot on. In wet flow testing we have found that heads with approximately the same (Or sometimes, even lower) airflow numbers can have significantly better performance based on fuel behavior in the fuel/air mixture. Because of that we have started focusing less on airflow and more on the more realistic testing conditions that wet-flow provides.

But in any event, if you do want some idea exactly how conservative our stated airflow numbers are let me know and I'll talk to one of the techs about it.

cuisinartvette wrote:yup [/QUOTE]

why?

[quote]
Can be...I use one of two peoples benches I know. Both are Superflow 600s and readings are consistently off by 8-10 cfm even when flowed on the same bore. Come to the conclusion the inlet fixture used is the culprit. One persons fits flawlessly and takes extra time to clay it in perfect to mimic a perfect portmatch, the other isnt quite as perfect and that can throw #s off some.

Performace improved...It would be nice to see some examples.
You want my honest opnion, try marketing hype.
Sure, maybe their fuel flow did pick up. "Says someone".
An example would be nice. Again, if you cant pick up air flow also, what good does it do.

cuisinartvette wrote:
[QUOTE] more air does not burn more fuel.[/QUOTE]
Yes it does. Guys lean their cars out to gain power...
Illnever discount Vizard, the guy is a master. Think what he is saying is you can make that much power with that little flow but I guarantee you wouldnt want to drive it.
Lots of cam, compression and huge rpm to get it there.
These days you can get 400hp with a teeny cam , good flowing head and youll actually enjoy driving it..

cuisinartvette wrote:
Any head company that doesnt publish some type of number or say they care about it, thats a crock. What other reason is there to produce one? To make it better of course and that means (at least a measure of improvement) is...numbers.
Theres a reason.

Personally it makes no doifference to me, I just find it amusing at some of their responses.."We dont care", "It doesnt matter"...Ok

dan0617 wrote:
Would also like to add, before anyone thinks I'm a guy that is all about flow numbers, to me, a flow rating is a tool to help me get my setup where I want it, and a flow bench is a tool used to get that reading from the heads. The flow number is not the end all to performance, but it helps me choose what is going to work best for me. It is a number that is going to have some variables just like any other tool of measurement. You measure the crank to find out what size bearings you need. You measure the flow of the heads to find out what engine combo they will work best on.

dan0617 wrote:


OK, Either I am misunderstanding, or you are. Not sure which. It is my belief that fuel to air mix is controlled through carb jetting or computer tuning the injectors. In which case the more air you get in there, you regulate the fuel to get the right amount in there to burn it.

Where does the heads have that much to do with it? How would the design of the heads create a condition which results in lack of burned fuel due to lack of air? No matter what the wet flow or dry flow numbers are, that would be the tuner's problem if there is too much fuel in the mix.

The more air (oxygen) you get in there, for the most part, the more power you can make. That is because you add fuel as you need to. It is easy to add fuel. The hard and expensive part is getting more air in there.

Less than optimum heads for a specific motor means you need more cam to get more air in there, and street manners drop off quickly, which is what I think Ron meant when he said " these days you can get 400hp with a teeny cam good flowing heads and you will actually enjoy driving it". Better flowing heads with optimum port size for proper velocity at part throttle (since very little air is moving through there at light throttle) with a smallish cam will make roughly the same power as heads that flow less but have a bigger cam to let more air in. But the former will be much more fun to drive as it won't be learching and bucking all over the place.

I believe it is all about getting as much air in the engine as you can and still maintain manners that make you happy. A very rough general rule of thumb for me, Go with as many cubic inches as you can, then get heads with as much flow as you can with a port size optimum for your engine, then give it as much cam as you can without losing too many street manners for the general use of your vehicle.

You now have a certain amount of air in there, then use the carb jetting or tune to get the right amount of fuel in there to optimally burn said amount of air. Don't forget to choose an intake that matches the power range of the heads/cam setup. Go with as high of compression as you can get away with with your fuel and your combustion chamber design, and then set the timing accordingly. Last, set the converter and gears up to match the powerband that the combo has made as that is every bit as important as the rest of the package. Then, if you want more power you have a few choices. Give up some street manners, nitrous, or forced induction. I chose nitrous as it fits my budget.

Do I have it about right, or is it possible that the designer of the heads designed a head that "results in lack of burned fuel due to lack of air?"

PS: went to far already, not going to break it down further to quinch, spark plugs, hot spots, etc.

Define "optimum".

dan0617 wrote:
I can't afford a builder, so I have to figure it out for myself.

dan0617 wrote:

I don't need to define optimum for you, you just described what I mean. Optimum ports for an L98 are ones that are pretty small (not going to get into numbers) that flow as much as possible. Optimum ports for prOzac are much more than that. He chose the optimum cam for his goals, which is my point, which I'll better define below. The optimum ports for the guy who doesn't have a cam that lifts above .600 would be the ones that max out their flow much lower than .600. But, IMO, the best heads are the heads that have the most flow with the smallest ports, "Optimized" for the setup. Optimized by choosing the right ports for the cubic inches, the cam, and the intake. "Optimized" by choosing the ones by having the most flow at the right lift for the cam.

Going along with the above, you are right about the cam ramp rates, I just wasn't going to go that far into it. There is alot more to discuss there but I don't think that is necessary here.

There you go again with optimum. The OPTIMUM port is the one that meets your clearly defined purpose built spec. If the spec is torquey low end, then perphaps a smaller port is optimum. If the stated goal is 1/4 mile time with an operational range of 6000RPMs, then a bigger port, that may even flow less, may be optimum.

Exactly. Optimum for the clearly defined purpose built spec. The best thing one can do is have a clearly defined purpose. Then look for all the parts to achieve that. Yes, for a torquey low end a smaller port with the highest flow possible (wet or dry or however you want to measure the volume of the air fuel mixture that is needed to pass through there) would be optimal.

For the 6000 rpm operational range you would need the optimal port size that flows the most of any you can find for that port size. I can't go into what port size might be the best, depends on too many things. If you want some street manners in that car than you might want to go a little smaller on the port size and you may give up a little horsepower.

If it is a track only car you would go with the biggest, highest flowing ports you could find that wouldn't put you out of your 6000 rpm operating range. If you want to be in the 8000 rpm operational range then you would likely need even bigger ports. But, IMO, once you decide on a port size (or a narrow range of port sizes) that your cubes, cam, intake, operational range, converter, gears, etc. can use you then would look for the heads that flow the most and have that port size. To find the heads that flow the most at that port size you are usually looking at manufacturer advertised numbers.

You also need to look now at the cam lift and ramp rates to determine which will flow the most on your engine. IE, which will allow the most air in there since the advertised flow numbers do not take into account what cam you have stuffed in your engine. Now that your port size is chosen, if your cam has fast ramp rates then perhaps you need the heads that flow the best at .550 lift if that is where your cam is living the most. IF you have lazy ramp rates then you need to look a little more at the lower lift flow numbers, which will perhaps knock you down to a smaller port size. To go a step further, if you know your cam gets to .400 lift very quickly then stays between there and .600 and back down to .400 for a very long time, then closes from .400 very quickly you might be best to get out the calculator and average the .400, .500, and .600 flow numbers to determine which head would work best for you.

There is no single port size, flow rating, flow average, brand, or anything else that is best for all. I hope this thread helps the newcomers when choosing a head/cam/intake/converter/gear combo.

In the end, though, if you are a poor sap like me trying to "optimize" your dollar as you don't have many of them to spend, you need to "optimize" your components for your clearly defined purpose build. And, like most of us, all I have to go by is advertised intake powerbands, advertised cam specs, and advertised head specs. Part of the advertised head specs is the flow rating and as long as you are comparing apples to apples (wet to wet or dry to dry) it is an important part of helping you choose heads, or perhaps you already have the heads and then it becomes an important part of choosing the cam and intake.

cuisinartvette wrote:
Youll have to get inside GM to view one of those in action unless you know someone, no head mfr will let you in on all their research and basis for design. They will only say so much..pretty competitive field.

dan0617 wrote:
Wet flow might very well be a better measurement than dry flow, but until EVERY head manufacturer posts wet flow numbers, I can only compare dry flow as I can't afford to buy heads and have them wet flown for comparison purposes.

Once you choose an intake runner size, the heads that flow the most in correlation to where your cam keeps your valves the most would be the best for you. You must choose the proper intake runner size or you will be making power below or above your combo's powerband, and perhaps, if too large, you will have a learching and bucking ride. Not saying the smallest ports are best. If you think I am, reread this.

cuisinartvette wrote:
Go see Wes thread that will tell you the story on a modern smaller head that flows well something bigger Jsup..

cuisinartvette wrote:
http://www.corvette-guru.com/modules/ ... 39&forum=30&post_id=60665

I didn't put my own together because it would not have cost me any more, and it would have taken me a lot of time. I prefer to spend time with the kids on weekends. I'm paying for that. It probably cost me $2K more to NOT have to do the work and be at my kids soccer games and pizza parties, money well spent in my book. Great value proposition.

dan0617 wrote:
Quote:

I didn't put my own together because it would not have cost me any more, and it would have taken me a lot of time. I prefer to spend time with the kids on weekends. I'm paying for that. It probably cost me $2K more to NOT have to do the work and be at my kids soccer games and pizza parties, money well spent in my book. Great value proposition.

I respect that. I bought my Corvette with plans to mod for something for my son and I to do together. He is almost always either handing me wrenches, or picking them up when I drop them into a place where little arms fit and big ones don't. It is the perfect passtime for both of us to spend time together.

jsup wrote:
Maybe I'll take a ride to Bever Springs in the spring...

BeachBum wrote:
I'm too tired to read this whole thread....

But, I am pro-flow

dan0617 wrote:
Quote:

BeachBum wrote:
I'm too tired to read this whole thread....

But, I am pro-flow

LMFAO! When you do go back and read it will be a hard read. I went back and edited out alot of what I originally said because the thread took a wrong turn in my opinion and I didn't want it to get worse. Best to lose a bunch of typing and hopefully keep a thread that has some good tech info in it.

BeachBum wrote:
Quote:

dan0617 wrote:
Quote:

BeachBum wrote:
I'm too tired to read this whole thread....

But, I am pro-flow

LMFAO! When you do go back and read it will be a hard read. I went back and edited out alot of what I originally said because the thread took a wrong turn in my opinion and I didn't want it to get worse. Best to lose a bunch of typing and hopefully keep a thread that has some good tech info in it.

Its too long of a thread these days for my small brain.... I can't keep my focus for that long.

I have a good idea what it says anyway....its all good.

BeachBum wrote:
# 26 is talking about wetflow, which I know is what Dart has hung their marketing hat on.....

My only problem with wetflow is that its an unproven science at this point for street/strip cylinder heads.(actually unproven on any cylinder head.... they just don't know enough about it yet.... its not easy to see the results and then when you have figured out how to look at the results, what does what were looking at even mean and how do we improve it?)

Many years ago, some of the pro-stock teams utilized a make-shift wetflow machine and did some testing, I've heard with mixed results, but all gains were fairly modest. Thats not to say that isn't good.... it is, with those pro-stockers, every single hp they can find might be the difference between qualifying or going home on Saturday..... but it really hasn't been in-corporated in your typical off the shelf cylinder head yet, except, as advertised by Dart.....

I know they had an advertorial going around the publications that showed I think some dart pro 1s and then some platinum 1's that increased the hp by I think around 20 hp, and had around 5 cfm more flow on the intake and 10 more cfm on the exhaust. This test really wasn't all that impressive considering the test engine results..... thats not to say they didn't find some power with it, but the end result wasn't really anything to write home about. I can give a comparison of the exact same engine & cam with 1/2 point less compression and a different cylinder head that didn't utilize wetflow making 20+ more HP .... but it doesn't matter, and I don't want to go there anyway.

On cylinder head flow.... so many I don't think understand it. Its treated as an element of a cylinder head by many, but its really not, its a culmination of all the good stuff you did.... in otherwords, its the result of a well designed port, not another element..... cylinder head porters raise or lower the roof, tighten up the throat, clean up the pinch..... all of these things typically increase the flow of the cylinder head, and thats a good thing !

Wetflow and Airflow go hand in hand.... not against each other.... if you introduce a liquid in the same density and weight of the gasoline in the stream, you're hoping to see good things with that well designed, good flowing airflow.... in fact, you want to tie them together, you want lots of airflow and you want the fuel to stay suspended and enjoy the ride that the airflow gave you. As most know, fuel is heavier than air, thus it'll spatter on the walls if the air flow is too slow or in-sufficient to carry it... which is what this is all about.

Of course there are other things that make a good cylinder head.... for the serious racers, having a good thick casting that can easily be heavily modified by the serious porting guys. (Dart is one of the best with this).... also having the proper spark plug location, seat, etc..... its all important, but to be honest, all of the aftermarket cylinder head manufacterers figured out the basics a long time ago.... I don't worry about it.

Actually, at the end of the day, none of this matters..... there are around 5-10 aftermarket cylinder head manufacturers that all make good products that "will" give you a nice performance increase, and Dart is certainly one of those companies. My advise, pick your poison and go have fun with it !!

Disclaimer: All of the above is just my opinion, if I'm right, I want a trophy, a trophy girl and good cigar, if I'm wrong, my name is Pedro, and I don't speak english.

BeachBum wrote:
Good post Jsup !

I don't have anything to argue about on the subject.....I'll buy into it.

Lets find a new subject for everybody to debate.... let me think.

Okay, in my opinion, a Whale fin on the back of a C4 will increase aerodynamics 6 fold, thus resulting in incredibly impressive et gains in the 3-4 second range..... I have seen stock tpi corvettes run in the 9's at almost a 140 mph with just this one modification !!

btw, my website is www.whalefins.com if you want to discuss further.

dan0617 wrote:
I wish I could run a 240/248 cam in a 383. I'd have all the power I want but when heading out for dinner with my wife on Friday night I don't think she would appreciate the learching and bucking when in OD. For me, it's all about finding the happy medium between animal performance at the track or when I want it, and going out for an evening in a nice stock looking and driving vert. Main reason I went with nitrous to make the big power.

dan0617 wrote:
I wish I could run a 242/248 cam in a 383. I'd have all the power I want but when heading out for dinner with my wife on Friday night I don't think she would appreciate the learching and bucking when in OD. For me, it's all about finding the happy medium between animal performance at the track or when I want it, and going out for an evening in a nice stock looking and driving vert. Main reason I went with nitrous to make the big power.