I was recently talking with someone about the difference between static and dynamic compression. He assumed the published compression ratio worked out in math against atmospheric pressure would result in the published compression pressure. It doesn't though. That bugs some people. But career techs like myself think it odd that anyone would even think of this. You can't have been around bikes for very long and be mystified by it. A number of dynamics come into compression. First, the temperature change in the compressed air. Whatever its entry temperature, there's no way air is staying the same temp after being squeezed into a space a tenth or less of its original volume. Then there are fluid dynamics at work, such as the speed and density of the air entering the engine. The fact is, two engines having the same compression ratio can produce different compression pressures merely by having different intake air speeds and or volumes; even by just having differently-sized intake ports.

Many online information sources confuse a cylinder's compression with its compression ratio. They are not the same thing. Compression is the number on your tester's gauge. Compression ratio is just that, the ratio between the cylinder's fill volume and its compressed volume. Compression and compression ratio are linked, but not the same thing. For example, there are many ways to increase cylinder compression without changing the engine's compression ratio. Almost every external engine adjustment will change compression: valve clearance and ignition timing, for example. Also more involved changes to the engine: increased valve sizes and port sizes, for example, as well as cam timing changes. Compression ratio is unaffected by all of these things, yet they increase compression.

We've just described dynamic compression. That is, compression that is subject to more than just compression ratio. Some on forums have supposed that fuel entering the cylinder during a compression test raises the tested value. No. Virtually no fuel reaches the engine during a compression test. With the throttle held wide open and the engine turning at 200-rpm starter motor revs, there isn't enough pressure drop at the carburetor to cause fuel discharge. Even if a few particles of fuel did make it, mixed with all that air they would be miniscule in proportion. Moreover, they wouldn't be wet particles at all. They would be atomized into a fog between the carburetor and the intake port, then vaporized into a gaseous state between intake port and cylinder. They would enter the cylinder as a barely-perceptible ether. This is known intuitively by even a beginner career mechanic. He knows if he does a compression test with the carbs on and throttle held wide open, or with the carburetors removed, the results will be exactly the same. He doesn't need the science. Being an eyewitness to the fact thousands of times is enough.

One forum's revered "expert" is misleading people to believe that compression testing values published in Honda's official manual are part of a deception perpetrated on the public by Honda. This leaves me absolutely speechless. With this kind of misinformation characterizing the world's largest vintage Honda user forum, it is little wonder folks are desperately seeking information they can trust. And I do mean desperately. As I write this, most of the eight bikes in my shop were sent by truck from all over the country: Ohio, Iowa, even Florida. I even had a CBX sent me from Dubai.

Compression testers having screw-on adapters are a joke. The adapter inevitably separates when unscrewing the tester from the engine. Do you have any idea how hard it is to get that adapter out of a modern engine, one whose spark plug well is seemingly a foot below the valve cover? You almost can't see the thing, and extracting it is many times more challenging than seeing it. Moreover, how accurate are those funky tools? Not very. One of the silliest discussions to be viewed on forums concerns these Harbor Freight compression testers. The arguments center around the effect the screw-on adapter has on the tool's accuracy, with the supposition that it adds volume and thus lowers the reading. First, just stop. Get a real tool. Second, even if it were possible, how much difference could it make? Are there adapters longer than 1/2"- 3/4", and are their internal volumes enough that they add appreciably to the combustion chamber volume? Is it even 1cc? What would be the difference, 2-3 psi at most? Certainly not the 30 to 50 psi you can expect a fifty year old engine to be lacking, and that so many are grasping at straws in response to. Maybe in a moped's 50cc cylinder you might get a 10 psi difference. Maybe. Still a far cry from 30-50. Perhaps I'm missing something because my experience rebels against this dialogue ironically fueled by seemingly very smart people. I recognize however that my experience has been specialized, seeing as I am not drawn toward toy tools. Don't make excuses. Focus on the engine. My compression tester has almost two feet of hose between the engine hose and the gauge. Add the engine hose's length and you're looking at very nearly three feet. This tool consistently measures the 170 psi vintage SOHC four and DOHC four and six Hondas were built with according to factory documentation, when the engine has very low miles or is properly rebuilt. Right on the money. Consistently. Forum readers, wise up.

Another mystery is why folks still put oil in a cylinder during a compression test. Yes, I know it is a way to separate valves from rings in eliminating the cause of compression loss. But the tool called a leakdown tester replaced this oil technique in the mid-1940s. It's far more conclusive and definitive. More to the point, and much more bizarre, the folks that are doing the oil thing aren't in many cases even thinking of oil's outdated purpose but rather are trying to fudge their compression test results by 20 percent. Kind of like the person who puts one foot on the floor when weighing themselves. Playing games.

And compression is a problem on fifty year old Asian bikes. I don't understand why people on forums either ignore or push back on this. Industry people are well aware that all Hondas and Kawasakis made in the 70s and early 80s have soft valves. They so rapidly recede into the cylinder head that by 15,000 miles in most cases they have significantly stolen compression from their cylinders.

A few years ago one of my customers, a mechanical engineer who thought himself reasonably proficient mechanically, called me and was perplexed enough to quiz me on how I knew that loosening up his valve clearances would increase his engine's compression. He had followed my advice and was astonished at the result. I didn't know how to answer him. Every career powersports mechanic I have ever known is intimately, even subconsciously aware of this. How has it escaped the general public? A definite mystery. A career mechanic doesn't need to have an education in physics or engineering to understand these things. He knows from working with the tools and the parts what they will do and what it means. When he has been a keen observer of the industry as well as formally trained in mechanics, all the better. He's even more equipped to understand. He knows things because his work informs him of them.

Big bore piston kits. They're popular. And for good reason. They offer something that is not well understood, though universally welcomed. Making an engine 5 percent larger in displacement does not make it run better due to that increase. A big bore kit brings to the vintage engine something it needed, namely, new machining that compensated for wear. That alone is the boon that excites people, because it restores lost cylinder compression. Add to that the fact that when you enlarge the cylinder bore the result is even more increased compression, because the combustion chamber volume hasn't changed and thus the chamber has become comparitively smaller. Added compression makes up for many inefficiencies. Carburetion gets a break, a huge one. The engine likes it. Add to this one or two tuning tricks, and you'll have an engine making 20 percent more horsepower than your buddy's. No cams. No trick carbs. No placebo exhaust. Forget exotic. Go for meticulous. And good compression is meticulous.

Speaking of "good", every once in a while a customer fails to see the importance of cylinder compression. I ask them to tell me their engine's compression as I start on their carburetors, and they sometimes react, almost are offended. Or they simply tell me it's "good". Makes me cringe. Look. I have been around a while. But I have never seen the word "good" on the face of a compression tester. Is that what your doctor's lab results look like; after each of the dozen tests do you find the word, "good"? You know the answer to that. The reason everyone wants to make witchcraft out of this subject is they refuse to accept that their engine is worn out, or in the case of a just-rebuilt engine, that it wasn't rebuilt correctly. Very common. Never do you read in these discussions the term "valve recession". It's conspiculously absent. Instead, you read about "hose length" and "engine break-in". The former is nonsense and the latter is asinine excuse-making.

Compression Chronicles part 7