Originally Posted by KAAMA (Post 3564428)

I thought this was an interesting article I came across within the Total Airflow website...

In the “Cylinder Head Selection Factors” issue of Tech Talk, we mentioned the media’s fascination with “racing” flow benches. Here’s why that type of “competition” is a poor method of predicting performance or selecting components.

An excellent example of air flow versus horsepower was seen, way back in 1981, with the 1980 Firebird Pro Stock of The M&M Boys. Their engine was...340 cid Oldsmobile with aluminum heads, 2.08” intake valves, 1.60” exhaust valves, a fabricated intake manifold with two Dominator carburetors and a .720” lift roller cam. The combustion chambers were machined to fit the cylinder bores and the intake ports received straight forward porting to make good numbers throughout the flow curve. At .750” lift, the intakes flowed 393 cfm and the engine made 640 peak horsepower.

At the same time, they also had a 331 cid small block Chevy engine that made 690 peak horsepower on the same dyno. The main difference between the two was smaller intake ports on the Chevy. After reworking – reducing – the Oldsmobile intake ports to flow 301 cfm at .750” lift, the engine jumped to 705 peak horsepower. How? Port SIZE and SHAPE!

Every time this story is told, the first comment is “Oh, making the port smaller just increased the velocity.” Maybe, maybe not. Flow benches don’t measure velocity, they measure cubic feet per minute. According to the flow bench, the revised Oldsmobile ports flowed fewer cubic feet per minute than the previous ports and the engine showed its pleasure with the change by making 65 more horsepower. Remember, port size and SHAPE. The Oldsmobile’s intake ports weren’t just reduced in size, they were reshaped.

Then, attention was turned to the Oldsmobile exhaust ports. The .700” lift cfm numbers went down, from 237 to 225, but the horsepower went up to 710. All we did was mill ½” off the exhaust ports at the header flange. This illustrates how flow benches can “like” certain things that engines won’t. Another example would be adding a pipe to an exhaust port. The longer the pipe, the more air the port will flow. Not only that, the pipe will smooth out some of any turbulence or imperfection (which you can hear and see on the manometer) that may be in the port. That doesn’t mean the port is fixed, though. The extension is more like a band-aid.

Conversely, flow benches typically like short intake ports. All said, two different engines cannot be evaluated on air flow numbers alone. It’s not necessarily correct to say that a Hemi makes more power than a big block Chevy because the intake ports flow more. The Hemi intake port is shorter, from the intake flange to the valve seat, than the Chevy’s port.

Comparing (or “racing”) flow benches is just as difficult, as benches are different from brand to brand and even from model to model. Fixtures, over-the-counter or shop-built, also differ and will affect flow readings. The engine or bench bore length can change flow numbers throughout the curve. The thickness or shape of the ideal entrance on the intake side makes a difference. Operators and the operating systems can make a difference when warming up the bench or correcting for ambient air. Even the quality of the wet & dry bulb for checking relative humidity, the barometer and thermometer can affect readings.

To be sure our readings are consistent, we have a series of model heads for benchmarks. They have been precisely measured and we never change them. Periodically, throughout a test, we flow an appropriate model to check for changes in the ambient air and to verify changes to work piece. If we picked up 1 cfm on the work piece, and the model also picked up 1 cfm, then we know the air changed, not the work piece. In the end, a flow bench test is a comparative for measuring before and after work on a cylinder head.

To illustrate the difference between flow benches, let’s look at a customer who called about a set of small block Chevy heads. The manufacturer (NOT Total Flow Products) claimed they flowed 310 cfm, but didn’t provide any substantiating data on bore, depression, etc. The customer had another local shop flow the heads and they saw only 298 cfm. Now he was asking us which number to believe and what to do. In this case, the head was a good piece that was known to make horsepower in the customer’s application. We simply told him that both numbers were probably correct and bolting on the heads would make his car run just fine.

Originally Posted by articfriends (Post 3564588)

This re-iterates that port shape and design are as much of a factor as raw flow numbers, especially on a exhaust port that is just being dumped to the atmosphere/room vs thru a pipe that mocks a real world exhaust runner port, Smitty

Originally Posted by baja208 (Post 3563952)

Frank, didn't know you were considering heads for the Apache. Give me a call if you need any help.

Originally Posted by JimV (Post 3564606)

Did you not read paragraph 5?

The longer the pipe, the more air the port will flow. Not only that, the pipe will smooth out some of any turbulence or imperfection (which you can hear and see on the manometer) that may be in the port. That doesn’t mean the port is fixed, though. The extension is more like a band-aid.

Originally Posted by Rookie (Post 3563758)

I have no idea if the Michigan Total Flow CNC program out performs Darts CNC heads. If you are looking to get the most out of the heads you have and you are getting them ported I would send them to JimV for porting. You will not be disappointed.

Originally Posted by innerrage (Post 3564956)

does jim v have a website

Originally Posted by innerrage (Post 3564972)

THANK YOU BILL:drink:

Originally Posted by articfriends (Post 3564947)

I also read in paragraph 5 that "flow benches can like certain things that motors WON'T":drink: