Project Muscrate

Part 2: Head games

by Jay Roeder

Hi everyone, it's time to get to work. This installment of Project Muscrate will cover the cylinder heads that I am using, and what can be legally modified, as per IHRA rules, to increase performance, meaning flow.

The 302/320 hp Ford crate engine comes from Ford with a pair of GT-40 Turbo Swirl aluminum cylinder heads. Now before anyone gets too excited, the only thing "turbo" about these heads is the name. In fact, they are moderate street/strip style heads with rather uniquely shaped combustion chambers designed to promote "swirl" within the chambers and provide a more efficient burn during combustion.

That being said, what do they flow, you ask? The answer is, not as much as they are supposed to. The problem with flow numbers from all manufacturers is they tend to be way too optimistic about the published flow numbers of a particular head. The problem for me, as an engine builder, is that I have a SuperFlo computerized flowbench and I know the truth about what a head flows at a given lift at a given pressure drop. I am continuously aggravated.

For instance, I have just completed a 406 SBC customer engine that I used aftermarket iron heads on. When I initially planned this guy's project, I looked at the advertised flow numbers for these heads -- 307 cfm at 28 inches of pressure drop. Knowing how things usually work, I figured that in the real world they would probably flow about 25 cfm less than advertised. WRONG! How does 262 cfm at 28 inches strike ya? That's 45 cfm less than advertised! You have to watch these things.

My heads for the Muscrate engine, on the other hand, were supposed to have an intake flow of 240 cfm at .550" lift at 28 in. drop and the exhaust numbers were advertised at 170 cfm at .500" lift. In the real world however, (on my SuperFlo bench) they peak out at 221 cfm intake @ .600" lift and the exhaust peaks at 141.30
cfm @ .600" lift. Both numbers were, of course, measured at 28-in. pressure drop. Why all manufacturers list their respective flow numbers higher than you or I will ever see, out of the box anyway, is a story all by itself and may be a topic for another column in the future.

The main piece of advice I would give to the consumer when trying to decide which pair of aftermarket cylinder heads to choose, is to seek the advice of a reputable engine builder and ask if they have ever flowed the particular heads you are interested in. Also, keep in mind that PEAK flow numbers are almost meaningless. Peak flow is a number that some people like to throw out during the deep discussions we all have had about our favorite thing, horsepower. The person that has the best mid-lift flow and average flow is always going to be the winner in the real world.

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