Engine Tips - Biggest Engine-Building Mistakes

Valve Sizing

John DaLuz of JMC Motorsports in San Diego, California, shares this one. As with carburetors, cylinder-head port sizing, and header tube size, valve sizing is another one of those situations where bigger isn't always better. Rule of thumb is simple-no larger than 1.94/1.60 inches (intake/exhaust) for 221, 260, 289, and 302 engines for street use. Larger 2.02/1.60-inch valve sizes work for 289/302ci engines with closer examination of valve-to-piston clearances, and valve-to-cylinder wall clearances. Larger 2.02-inch intake valves make sense when the chamber and cylinder wall don't shroud the valve. To make the right decision, look at all cylinder-head designs and options available.

If you are considering larger valves, ask yourself if they will yield any benefit. Is the larger valve size shrouded? If so, how can you expect improved airflow? Flow-bench testing shows time and again that larger valve sizing doesn't always improve air flow because a shrouded valve can suffer from airflow shortcomings throughout its travel. Bigger valves don't always net bigger power.

Mismatched Components

When a customer comes to MCE Engines with an engine project, Marvin McAfee asks what he wants the engine to do most of the time. McAfee subscribes to a strict regiment of proper parts selection and compatibility in every engine he does. He says customers come to him with collections of parts seeking an estimate, but he won't do an engine unless he is certain all of the parts will work well together. As a rule, he prefers to select and acquire the parts, clearing all phases with the customer as he goes.

Expect disappointment when you top an intake manifold with a carburetor ill-suited to the displacement, manifold, heads, and driving conditions expected. Anticipate trouble when you fit an aggressive cam to a block topped with incompatible heads with soft valvesprings. Figure on lackluster performance when you opt for a mechanical distributor for the daily commute that doesn't have a vacuum advance. Plan your engine build like you would your career or a dream home. It all has to work well together or you're wasting both time and money.

Before buying anything or even choosing an engine, know exactly how you're going to use it most of the time-then plan each phase of your engine build with liberal doses of common sense and patience. It's easier to wait on buying parts than it is to be sorry later when you've made a bad decision.

Measure Twice-Cut Once

MCE Engines takes a methodical approach to every engine it builds. This means inspecting and measuring at least three times during mock-up and assembly phases. Measuring once is courting trouble, according to McAfee. It's too easy to misread a micrometer or dial-bore gauge. Go back and measure again later to ascertain precise measurements. Write them down on the first pass, then write them down again on the second and third passes. And remember, you should be able to turn a crankshaft using two fingers with all eight pistons and rods installed. The weight of a 11/42-inch breaker bar alone should turn the crank before pistons are installed. If it just hangs there, clearances are too tight.

Too Much, Too Little Endplay

Crankshaft endplay doesn't get checked enough during engine builds, in our estimation. We've seen enough of them where we had to remind the builder to check. Trust your parts, but verify dimensions always, even with new components. Too much crankshaft endplay can bite you in the butt just as badly as too little endplay. Much of it depends on how you intend to use the engine.

If you're going with a manual transmission, clutch engagement and disengagement causes the crankshaft to ride the thrust bearing-on the clutch, off the clutch. The result is wear every time you step on the clutch. The more endplay you have, the harder shifting is on the thrust. Step on the clutch and the crank rides the thrust bearing hard. Minimum crank endplay is 0.004 inch. Maximum is 0.008 inch regardless of whether you are running an automatic or stick.

Another important item that is rarely checked is camshaft endplay-again 0.004-0.008 inch maximum. Excessive camshaft endplay leads to timing-set failure, lifter damage, and other unthinkables.

Ignition Curving

Here's one we see all the time. Did you know improper ignition tuning not only adversely affects performance, but it also can do permanent engine damage? Here's why: Ignition timing isn't just about aiming a timing light and getting 6 or 12 degrees BTDC timing at idle. What really counts is total timing and how it synchs with engine rpm. Total timing, meaning total spark advance in degrees of crankshaft rotation/position, should be completely in by 3,000 rpm.

As a rule, total timing should never exceed 36 degrees BTDC at 3,000 rpm and higher. Some people push the envelope and crank a few extra degrees in there thinking it will net us more power. But, when you dial in too much timing, you run a high risk of serious engine damage from detonation/spark knock. This happens when fuel ignition occurs too early on compression/ignition stroke.

The objective is to light the mixture at exactly the right time as the piston rises in the cylinder to where combustion is fully underway when the piston reaches top dead center. This hinges on fuel octane rating, ambient temperature, humidity, cam profile, chamber size and shape, air/fuel mixture, and atmospheric pressure (or pressure altitude). Every situation is different, which means no two engines will respond the same way to ignition curve. What works on your buddy's 302 likely will not work on yours.

And remember something else: Total timing when the engine isn't under a load is different than total timing with a load. With a load, you often need less timing because detonation typically occurs heavy load at wide-open throttle at low rpm. This calls for retarding total timing one degree at a time.