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Flow Bench Fallacies

by David Reher

Intro by Bruce Fulper


Since I began studying airflow at Chaffey college in 1990 I was taught early on how to relate

to port flow - air flow benches - and most importantly - how to make my own relationships to

the power achieved to the porting changes I made. Consequently from my beginnings I have never boasted about

dry airflow numbers taken from flow benches as some kind of ultimate relationship

to making an engine perform. We've actually had one guy (Mr. Sparks - a comptuer specialist) try

to sue us because we didn't furnish him with "flow numbers." When in fact we never promoted

or offered flow numbers.


People are impressed with our amazing results.

Non-performers like the has-been Pontiac writer Pete McCarthy and his legion

of non-performers try to belittle our accomplishments. He's heavily responsible for wasting ten years

of the Pontiac racers time. After I ran this story in my "The 421 Club" magazine, Pete had to give in.

(after claiming big air flow numbers for years, I have a set of his D-port heads he claimed

flowed 275 CFM. They actaully flowed 218. ) After ten years of B.S.

he nearly plagerized the Flow Bench Fallacies article in his Pontiac Enthusiast article

when he finally stated "don't believe flowbench numbers." Mutt.


Before I could write an all encompassing article on the situation, David Reher

of the famous Reher and Morrison did. And here it is in its entirety.


Open your mind. Learn from us that actually have a better understanding of engine dynamics.


If you're buying heads because someone claims their heads flow more

than ours, you're in deep trouble. Compare apples to apples. We'll help you understand the difference.

When I finish my book I will reveal the port volume relationships that no "Pontiac expert" writer has yet to figure out.

Remember, we're the ones that made 540 hp at 9.5 Cr. on 87 octane with a Pontiac 350 and

600 lbs. ft. at 3000 rpm with a 87 octane 455 and my .240's duration at .050 lift custom profiled hyd roller cam. Stock strokes. Stock rod lengths.

Those are friendly street reliable engines. Our race engines performance? Fagidaboudit. If you don't know - read the entire web site & ask me questions.




Flow Bench Fallacies

Our era is often refered to as the Information Age, but not all of the information

is necessarily useful. I am beginning to think flow benches should come with a government warning:

"Caution! Excessive reliance on flow numbers may be harmful to your engine!"

I'm kidding of course. Used wisely, a flow bench can be a useful tool in engine development, just like

a timing light or a dynomometer. Unfortunately, some racers believe that a flow bench is

the ultimate answer machine. When the subject is cylinder heads, the four words I dread

to hear are, "What do they flow?" Novice racers and magazine writers share a fixation about airflow.

Their mistaken belief that "more is better" is often the false assumption that produces an

under-performing engine. A flow bench measures air movement in a very rudimentary

way - steady-state flow at a constant depression (vacuum).

Obviously, the conditions that exist in a running engine are quite different.

The flow bench can't simulate the effects of pistons going up and down, the reversion pulses

as the valves open and close, the sonic waves that resonate inside the runners, the inertia of fuel droplets,

and all of the other phenomena that influence engine performance in the real world.


When you flow test a cylinder head, you are simply measuring how far you can move the

liquid in a manometer. The bigger you make the port, the more it flows. That's hardly shocking news.

Bolt a sewer pipe on to a flow bench it will generate terriffic flow numbers.

So should we use ports as big as sewer pipes on our race cars?

The flow bench says we should - the time slip says something completely different.


If airflow were everything, we would always use the longest duration cams we could find -

after all, more duration means more airflow. In fact, we know that there is a

finite limit to how long the valves can be open before performance suffers.

That is because the valve events have to be in harmony with the rest of the engine.

That same principle applies to cylinder heads. Simple airflow capacity should

never be the first consideration in evaluating cylinder heads. Characteristics that

are far more important include air speed, port cross section, port volume and shape,

and the relationship between the size of the throat and the and the valve seat.


If these attributes are wrong, you can work forever on the flow bench and not overcome

the fundamental flaws.


Here's a do-it-yourself example: Turn on a garden hose and the water dribbles out

a couple of feet. Now put a nozzle on the hose and the water will spray across

your backyard. The water pressure and volume haven't changed, but the

velocity has increased dramatically. Now think about air and fuel going into your engine's

cylinders. Which would you prefer: slow and lazy or fast and responsive?


An engineer will tell you that an engine requires a prescibed amount of air and fuel

to produce"x" amount of horsepower. In a perfect world, that may be true - but

we race with imperfect engines. The shape and cross sectional area of the runners

is absolutely critical to performance. For example, I have two sets of Pro Stock cylinder

heads that produce nearly identical flow numbers, yet one pair produces nearly

150 more horsepower at 9200 rpm than the other.


The flow bench can't tell the difference between them, but the engine certainly can.


There are software programs that claim to be able to predict an engine's performance based on airflow numbers.

Unfortunately, a critical shortcoming of many of these programs is that they are based on

inaccurate information or false assumptions. A computer is an excellent calculator, but it

is not an experienced engine builder. The software doesn't know if the short-turn radius

is shaped properly, whether the flow is turbulent at critical valve lifts, or whether

the flame speed is fast enough. Racers have a tendency to believe that computers are

infallible, so they accept the softwares solutions as gospel when in fact they

be badly flawed. Textbooks would have you believe that an exhaust to intake

flow ratio of 80 percent is ideal - yet a typical Pro Stock head has exhaust

posts that flow less than 60 percent of the intake runners, (Bruce here, YES!)


You can improve the exhaust flow tremendously with about 40 minutes of

work with a hand grinder, but the supposed improvements will just about kill

the engine's on-track performance. I know because I've been there.


We also have learned that the low-lift flow, (meaning anything below

.400 valve lift in a Pro Stock engine with a .900 lift camshaft) is relitively unimportant.


Think about the valve events in a racing engine: From the point when the valve is

first moves off its seat until it reaches mid-lift, the piston is either going the

wrong way, (that is, it is rising in the cylinder) or it is parked near Top Dead Center.

The piston doesn't begin to move away fom the combustion chamber with enough velocity

to lower the pressure in the cylinder until the valve is nearly half-way open.

Consequently, it is high-lift flow that really matters in a drag race engine.


The shape of the combustion chamber also has a significant impact on performance.

A conventional chamber with deep reliefs around the valve seats and a relitively

flat valve seat angle can produce terriffic flow at .200 - .300 vlave lift.

Today, a state-of-the-art chamber typically has 55 degree valve seats

and steep walls that guide the air/fuel mixture into the cylinder to enhance

high lift flow. This doesn't mean that every racer needs state-of-the-art

Pro-Stock cylinder heads - along with the high maintenance they require.


The heads have to match the application.


Conventional combustion chambers and 45 degree valve seats are just fine for a

dependable, low-maintenance racing engine that will run a full season between overhauls.

The classic Hemi combustion chamber is capable of producing impressive flow numbers,

but it's not going to make impressive power. Engine technology in all forms of motorsports

is converging around smaller, high-efficiency combustion chamber designs. You can see the

result in lower brake specific fuel consumption (BSFC) numbers, which indicate

improved engine efficiency. Twenty years ago, a racing engine with a .48 BSFC was considered

very good; today's competition engines produce BSFC numbers in the neighborhood

of .35. This means that a given quantity of fuel is being atomized and burned

more effectively to produce more power. A cylinder head's combustion efficiency

can't be measured on a flow bench, yet it has a huge impact on performance.


I am not against flow benches; in fact, we use computerized flow benches daily

at Reher-Morrison Racing Engines. What I am against is the over reliance on flow

numbers as the primary measurement of a cylinder heads's performance.

A flow bench is a valuable tool that can help a racer fine tune a

combination - but it is not the ultimate authority.


Thank you David Reher and NHRA's National Dragster that first ran this story.

Re-read the story and send your questions to