WE MAKE SERIOUS PONTIAC HORSEPOWER
AND WE KNOW MORE ABOUT BUILDING STOCK PONTIAC ENGINES THAN ANYONE.
Flow Bench Fallacies
by David Reher
Intro by Bruce Fulper
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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.
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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.
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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.
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Open your mind. Learn from us that actually have a better understanding of engine dynamics.
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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.
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BMF
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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.
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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.
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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.
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If these attributes are wrong, you can work forever on the flow bench and not overcome
the fundamental flaws.
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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?
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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.
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The flow bench can't tell the difference between them, but the engine certainly can.
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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!)
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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.
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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.
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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.
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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.
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The heads have to match the application.
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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.
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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.
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Thank you David Reher and NHRA's National Dragster that first ran this story.
Re-read the story and send your questions to Bruce@PontiacEngines.com.