Applied Pressure
- WEEPING ENGINES
Before you can truly begin any serious tuning, you have to be sure that the engine you are going to spend hours working on is in good sound condition. It makes no difference whether the engine you have is a week old or if it has 30,000 km on the clock. There is only one way to accomplish this tune up correctly and that’s with a leak down test process. Compression tests might suffice for a ‘run about’ sedan, but remember, we’re talking about an outright street race, hi-performance engine. The main theme behind a “leaking down test” is to pump a given amount of air into a cylinder and then measure how much escapes via the piston rings and valves. You might be surprised when you hear the excessive compressed air blowing by a worn and leaking exhaust valve or perhaps an equally worn out intake valve. (Exhaust valves wear faster than intake valves).
Similarly, air can blow by the piston and it’s rings and enters into the sump. While performing the test, keep in mind that a good healthy engine can leak less than 5%, while a chronic leaker can exhibit numbers nearer the 50% mark. Some of the best-sealed engines in the drag racing circles are found under the bonnets of NHRA stock racing cars. Take the ‘leak down’ numbers into account and what should they be? Less than 3% on some of the better engines. Use your own judgment when it comes to a leak down test, the figures should be well noted from cylinder to cylinder. Be advised that an engine that leaks over 10% is asking to be stripped apart, closely examined and accurately measured up to see where it can be improved upon. Compression testers are available from a number of sources (single-gauge models). Aircraft supply shops are also a good source if you can’t easily locate a leak down tester at your corner speed shop. Whatever you do, make sure the one you purchase or use is of a good quality.
- THE PISTON SQUEEZ
Compression tests are routine tune-up steps, but on a high-performance application, they should not take the place of a leak down test. Essentially, the idea of a compression test is to obtain a reading that is close to that specified in a factory service manual. Now, if you have bumped up the static compression ratio (high compression pistons), changed the camshaft profile i.e. (changed the lift and duration figures from the standard factory figures), then the compression test numbers you get will be totally meaningless to anyone and that leaves you with an educated guess.
There are two areas however where a compression test can be of some assistance. Low readings usually indicate that the rings or valves require maintenance. This can be checked by squirting some lightweight oil into each spark plug hole. Crank the engine over with your bump switch for a few seconds to allow the oil to get in under the rings and into the places that will help seal off the rings as best as possible and then repeat the compression test.
If the compression readings are significantly higher, then you have a problem with the rings, piston or cylinder bore. If no changes are evident, then the problem lies with the intake or exhaust valve or both.
A drastic drop in pressure in one (or more) cylinders can mean numerous things. The worst could be a cracked piston or broken rings and a broken ring land combination. The most common problem is a blown cylinder head gasket. If a pair of side-by-side cylinders show approximately the same compression readings (while other cylinders are good), this is a good sign that the head gasket is blown between the cylinders. This is by far the most common problem, especially if the engine has been bored out to the max, bringing the cylinder bores closer together, thus narrowing the deck surface between the two cylinders.
Many engine builders opt for the largest oversize slugs available and most often end up fitting foreign pistons to the build and end up machining off the top of the piston, pockets or dome – a common practice amongst engine builders in time, somewhere along the line.
But if you can afford the right equipment then you will find the correct hardware at your local speed shop.
