Tech Talk #2 – Reher’s Rule: RPM Determines Reliability

DavidTechArticlesBy David Reher, Reher-Morrison Racing Engines

“Dead valvesprings, like dead canaries, are a sure sign of trouble ahead.”

This is the time of the year when a racer’s thoughts turn to next year. The brief break between seasons provides a welcome opportunity to spend some time thinking rather than wrenching. In that spirit, I’d like to suggest that the decisions you make in December will have an enormous impact on your chances of winning races next summer.

The most important thing a racer can do right now is to develop a plan. Without question, the most common mistake in racing is to buy parts impulsively without having a clear vision of where you want to end up. The result is often a pile of mismatched pieces that will never work together properly.

I recognize that not every racer has the resources to buy a complete engine from a professional builder. Even though you may not be able to afford a professionally built motor, it ‘s still important to think like a pro: You have to select components that will work together as part of a total combination. Think about what you expect the end result to be: the engine’s projected horsepower output, its rpm range, and the chassis/powertrain combination. For example, do you plan to use a tight or a loose torque converter? The answer to that question will affect your choice of carburetor, cylinder port volume, and cam profile.

There is certainly no lack of information on engines, but not all of the available information is valid. Don’t be misled by inaccurate magazine articles and armchair engine builders touting this week’s hot tip. You really don’t need – and you really don’t want – a tall-deck block with 7-inch long connecting rods and Pro Stock cylinder heads for a 13-second bracket car. You need pieces that are compatible – pistons that fit the cylinder heads, valve springs that compliment the camshaft, and an induction system that is tuned for the engine’s operating range.

Perhaps you’re just thinking about upgrading your present engine for next year. Keep in mind that there is almost no part you can change in an engine that does not affect some other component. If you install a bigger cam, you’ve got to have valvesprings and pushrods that will work with the new profile. If you bolt on parts that allow the engine to turn higher rpm, then you may need to look at improving its lubrication system. Your old oil pan may work perfectly at 7,000 rpm, but if you spin the motor to 8,000, the pan may not be able to control the increased windage. The likely results: aerated oil and spun bearings. Almost invariably, when racers suddenly encounter reliability problems in a previously troublefree engine, it’s due to some change that produced an unintended consequence.

The one lesson I’ve learned in 30 years of building race engines is that rpm plays a major role in determining reliability. It is simply impossible to turn and engine faster and have it last longer. Higher engine speeds not only increase the loads on engine components, but also increase the number of cycles that each piece must endure. Although the actual time spent on the track may be reduced slightly if the car runs quicker, higher engine speed will subject the rod bolts, valves, pushrods, lifters, and every other part to more stress cycles on every quarter-mile run.

Coal miners took canaries into the mineshafts to warn them of poisonous gases. In a racing engine, the valvetrain plays a similar role as an early warning system. Dead valvesprings, like dead canaries, are a sure sign of trouble ahead.

It’s absolutely critical to maintain the valvetrain, especially if you are using an aggressive cam profile. The valvesprings are usually the first place that you see the detrimental effects of high rpm. Roller lifters also require regular inspection and maintenance. We seldom see bottom end problems in an engine, but lifter failures are a constant concern, especially if a racer lets the valvesprings get weak.

My advice to anyone who wants a powerful bracket engine – let’s say a big-block that produces in the range of two horsepower per cubic inch – is to use titanium valves. Yes, titanium valves are more expensive initially than steel valves, but they dramatically increase valvespring life and significantly reduce the chances of valvetrain failure. In my opinion, lightweight titanium valves are a good investment for any big-block that routinely runs above 7,000 rpm. I know that the cost of titanium valves is usually far less than the cost of repairing the damage caused by a broken lifter.

We performed a back-to-back comparison test between stainless steel and titanium valves in a 555ci bracket racing Rat motor with ported Dart 360 cylinder heads and a roller cam with .475-inch lobe lift. With steel valves, the engine made 920 horsepower at 7,000 rpm, but the power curve dropped at 7,200 rpm. We pulled the heads, swapped the valves, and had the engine running on the dyno again 90 minutes later. With titanium valves, the engine had the same power at 7,000 rpm. At 7,200 rpm it was up 20 horsepower, and it maintained that output to 7,800 rpm. Best of all, after two years of racing and hundreds of runs, this engine hasn’t broken a valvespring!

When you’re making plans for next season, be realistic about your own goals. Do you enjoy working on your engine or do you want to build a low-maintenance motor? A fast race car isn’t like a motor home; it requires constant attention. On the other hand, if you want an engine that’s as reliable as a refrigerator, you’ll have to change your expectations accordingly.

Consider the difference between two of my customers: one runs an auto supply and loves to work on his car; the other owns a successful landscaping and pool business and he doesn’t want to take off a valve cover between races. I can’t build them the same engine because their requirements are completely different.

A lot of bracket racers really like to go fast, and compete regularly in Quick 16 and Quick 8 shows. Other guys want to make 300 runs without doing anything to their engines except changing the oil and filter. You need to decide which category you fit in before you begin ordering parts. If you don’t have the time or interest to work on an engine, use parts that have a long life and keep the engine rpm low – and don’t be disappointed when the guy who is willing to work on his engine has a faster car.

If you’d rather race than wrench, why not build a bigger motor and keep it conservative? There’s no replacement for displacement; put a small cam in a big-inch engine and you may be able to get close to your performance goal without spending every evening out in the garage.

So here is the first of Reher’s Rules on Race Engines: Maintenance goes up with power. Period.