Tech Talk #6 – Head Games

DavidTechArticlesBy David Reher, Reher-Morrison Racing Engines

“If we had Pro Stock cylinder heads fifteen years ago that were as good as the heads you can buy off the shelf today, we’d have killed ’em.”

I am truly astounded by the choices in competition cylinder heads that are available today. When I started my racing career 27 years ago, the only source for cylinder heads was a junkyard. Now racers can buy heads that are better than anything we raced in Pro Stock not too many years ago.

Unfortunately, too many choices can also lead to confusion. Back when the only alternatives were cast-iron and aluminum factory heads, the decision-making process was simple: you used what you could afford. Now, however, an engine builder has literally dozens of aftermarket cylinder heads to consider, with a staggering variety of port layouts, runner volumes, and combustion chamber designs.

I’ve seen the effects of this revolution in cylinder heads firsthand at our shop. We developed our line of “Super Series” big-block Chevrolet V8’s to take advantage of the choices in Rat motor heads. By selecting the right castings from manufacturers such as Dart and Brodix, we can match the heads to the engine displacement and rpm range almost perfectly without expensive porting and elaborate machining. As a result, the cost per horsepower has declined tremendously.

The most common mistake racers make when selecting cylinder heads is to choose ports that are too big for the engine displacement. Racers have a tendency to believe “bigger is better,” but that is usually not true when it comes to runner volumes. If you are debating between heads with different port sizes, my advice is to choose the smaller runners.

For years we have heard that oval-track engines want small ports because they need torque over a wide rpm range, while drag racing engines require big heads because high-rpm horsepower is important. Well, I’m not sure these rules of thumb still apply. On a long oval track like Daytona or Talladega, the engine speed in a NASCAR stock car varies only a few hundred rpm. A drag racing engine, on the other hand, has to operate over a much wider rpm band when the clutch engages (or the converter hits) at the starting line, and then on each successive gear change.

The best tool that we have available to simulate what happens during a quarter-mile run is a dynamometer acceleration test. Unfortunately, even an acceleration test on a conventional dyno can’t accurately recreate what happens when a load is suddenly applied to an engine. During a typical gear change, the engine speed drops 1500 to 2000 rpm in less than .05 seconds. This rapid change in engine speed can disturb the entire intact tract, so it is essential to keep the velocity of the intake charge as high as possible during these transitions. That’s why I advocate a conservative approach when choosing cylinder heads, intake manifolds, and carburetors. Smaller is often faster.

I also believe that you should buy heads that match the engine you are racing now, not the engine you plan to build in two years. For example, one of our customers who races a 468ci big-block recently purchased a pair of heads that would have been just right for a 555ci engine. Naturally, he was disappointed when the new heads did not produce the dramatic improvement in e.t. that he had anticipated. I pointed out to him that the big heads were a poor match for the relatively small short-block he was using now. Until he had the cylinder displacement that could take advantage of his new heads’ higher airflow capacity, he wasn’t going to realize their benefits. If you are serious about building a bigger motor, then it makes sense to buy big heads – but if you’re not really committed to a new short-block, you should choose heads that are right for your current combination.

Sharp throttle response is essential in bracket racing. Oversized ports can make an engine sluggish and unresponsive, and that’s not what you want when you’re trying to judge both your opponent and the finish line. If you’re a heads-up racer, you need an engine that’s sharp when it comes off the throttle stop. Having the right cylinder heads is as crucial for consistency as using the right converter and carburetor.

I think it is a bad investment to spend a lot of money on porting cylinder heads for a bracket racing or heads-up engine. That statement may sound strange coming from a professional engine builder, but it’s the truth. We can order heads for big-block Chevrolets with port volumes ranging from 260cc to nearly 400cc, with dozens of steps in between. If you need heads with 345cc runners, it is more cost-effective to buy heads with the correct as-cast runner volume than to port a pair of 320cc heads.

You should also have a clear idea about your objectives before you spend big money on cylinder heads. If you have any thoughts about racing in a fast eliminator, I urge you to consider the Pro Stock-style big-block heads that are available from Dart (“Big Chief”) and Brodix. (“Big Duke”). You can spend thousands of dollars modifying a pair of conventional siamesed-port big-block heads and still not achieve the airflow that is available out of the box with these spread-port heads. If we had Pro Stock cylinder heads fifteen years ago that were as good as the heads you can buy off the shelf today, we’d have killed ’em!

Many of our customers who race in quick eliminators at local tracks are learning that the competition to make the field is intense. It can take serious horsepower to qualify for a Quick 8 or Quick 16 show. If you make the decision to use Big Chief or similar Pro-style heads at the outset, you can save money by buying the right pistons, intake manifold, valve covers, and valvetrain components at the beginning. If you have to convert an engine with conventional siamesed-port heads to keep up with the competition, you’ll end up buying all of these parts twice.

My final advice is to use titanium valves with any of the Pro Stock-style racing heads. These heads use valves with longer stems and larger heads than the valves in conventional big-block heads, and that adds to their weight. These heads are also designed to operate at higher rpm, so you really need a lightweight valvetrain for reliability. As I mentioned in a previous column, titanium valves are one of the best investments a serious drag racer can make.

I certainly don’t miss the “good old days” of welding and grinding on cylinder heads. The speed equipment industry has done a remarkable job of producing better cylinder heads at lower prices than anything we could have imagined back when I was scouring junkyards for castings.