Tech Talk #85 – How Not to Build a Racing Engine


By David Reher, Reher-Morrison Racing Engines

“Here’s a Reher analogy you’ve never read before: A racing engine is like a bowl of Jello.”

My daily fitness regimen is a brisk morning walk at Texas Raceway near my home in Kennedale, Texas. It’s just me, my dog, and a drag strip.

It’s quiet there on a weekday morning, and a man can think. Among things I’ve been thinking about recently are the common mistakes that racers make. One of the biggest errors is collecting parts for an engine that will be built “someday.”

Some racers build engines on the installment plan by stockpiling parts over a period of time. I know that a limited racing budget often dictates when parts can be purchased. However, the problem with this plan is that better parts are constantly being developed. Many components that were state-of-the-art a few years ago are now far behind the development curve. I think it’s a better strategy to collect the money to build an engine with current technology than to collect obsolete parts.

The pace of change in engine technology is accelerating. I see the evidence whenever I walk through our shop. Piston rings for sportsman engines rival the rings used in Pro Stock just a few years, at a fraction of the cost. Precision-machined dry-sump oil pumps that were once reserved for high-end engines can now be bought for a third of the price. Some sportsman cylinder heads can now outperform the extensively welded and modified castings that the pros used just a few years ago. And when I look at the valves and springs available now, I am amazed by the progress that’s been made in metallurgy.

It doesn’t take long for components to become obsolete. If you’d asked me two years ago about cylinder heads, rocker arms, and cam profiles, my recommendations today would almost certainly be different. My advice back then wasn’t wrong – it was simply based on what was available at the time. If you intend to race in the fast eliminators that have become popular with sportsman racers, then you can’t expect to be competitive with “antique” parts, even if they are only a few years old.

Another potential pitfall of buying parts piecemeal is ending up with an unworkable combination. Consider the staggering variety of big-block Chevrolet cylinder heads on the market – conventional ports, raised ports, spread ports, symmetrical ports, 18-degree heads, 14-degree heads, and many other variations. Each distinctive design requires specific complementary components, from intake manifolds and pistons to rocker arms, gaskets, and valve covers. Consequently that trick manifold introduced in 2010 may not work with the cylinder heads you buy next month.

It’s painful when a customer brings a pile of mismatched parts into our shop and asks me to build an engine out of them. I hate to turn business away, but I have a responsibility to be honest when someone shows up with a collection of incompatible parts. When a customer asks me to put a set of CNC-machined large-port cylinder heads on a marine engine that’s going to cruise at 3500 rpm, I really have to point out that he’d be much happier with smaller ports.

Here’s a Reher analogy you’ve never read before: A racing engine is like a bowl of Jello. Why? Because you can’t wiggle a bowl of Jello in just one place. Touch one part and the whole thing moves. To continue the metaphor, you can’t change just one part of a racing engine without affecting the entire combination. Change the intake manifold, and you may need to change the cam, the carburetor, and perhaps even the rearend gear ratio to get maximum performance with a new setup.

It’s easier than ever to purchase parts through online auction sites, virtual speed shops, and manufacturer websites. But are those parts in your shopping cart really the best for your application? Are they compatible? Are they high-quality components or cheap knock-offs? Before you hit the “Buy” button, it’s worthwhile talking with people who have real-world experience with building, testing, developing, and maintaining race engines.

Let’s say you buy pistons that are advertised as having a 14:1 compression ratio. And when you assemble the engine, you discover the actual ratio is 11:1 because the valve pockets and domes are designed to clear any conceivable cam/valve/cylinder head combination. A universal piston isn’t going to be effective and efficient in a serious racing engine. You don’t need or want .300-inch piston-to-valve clearance, but that may be what you get unless you talk with an expert.

There is no shortage of Internet experts who claim to have all the answers on engine building. Unfortunately, some keyboard gurus have little practical experience. I see endless discussions about rod length-to-stroke ratios on forums and bulletin boards, yet on the list of important factors in engine performance, rod ratio ranks about fiftieth. And please don’t get me started on cylinder head flow numbers. CFM is one of the least important characteristics of a competition cylinder head. Two head designs can have identical flow numbers, yet one will rev up and run on a racing engine, while the other is as flat as West Texas. In level of importance, the average air speed, port shape, and the efficiency characteristics of the port rank much higher than simple CFM numbers. Yet these vital characteristics are often overlooked, and most cylinder heads are sold on the basis of CFM figures.

Drag racing is a technology-driven sport that is continuously evolving. The winners constantly look for the next step forward, not yesterday’s hot setup.