® The truth about cylinder compression

Cylinder compression. First, let's chop to pieces the first sacred cow, the seemingly widespread misconception that cylinder compression is to an engine what a "walker" or an oxygen bottle is for an individual; something you have to do only if it/he/she is old, nearly invalid. Just yesterday an inquirer protested, "my bike has only 10,000 miles on it!" So? Do it anyway. I can't understand why something that is obvious to a mechanic is so hard to understand for everyone else. A cylinder compression test is no different than adjusting your carburetors. It's akin to a medical checkup; something to be done regularly. It's not an angioplasty.

And that warming up the engine thing! Mechanics have traditionally begun each full service of the motorcycle with a compression test. Warming up the engine? No career mechanic will ever do that, because he would then have to wait an hour to three hours before he could adjust the valves. Not gonna happen. Besides, a warm engine makes virtually no difference in the outcome. The incoming air will be warmed by the engine as it enters and will expand before it reaches the cylinder, losing whatever potential pressure advantage. You've gained nothing. And in my experience the manual's compression numbers always match the results on a freshly-rebuilt but cold engine. Literally my entire adult life has been spent in repair shops of many kinds, working on motorcycles, and I have never witnessed another mechanic warming up the engine before doing a compression test. A pro just wouldn't think of it. You shouldn't either. I know, Honda manuals specify it. But some of them also suggest putting thread locker on fork damper bolts, another thing no pro Honda tech will ever do. Don't waste your time.

I was recently talking with someone about the difference between static and dynamic compression. The published compression ratio worked out in math against atmospheric pressure never results in the published compression pressure. 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 almost a tenth 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.

One forum's revered "expert" is misleading people to believe (and no one has called him on it) that compression testing values published in the 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. Most of the eight bikes in my shop were sent by truck from all over the country: Cleveland, Iowa, even as far as Florida.

Some on forums have even supposed that fuel entering the cylinder during a compression test is raising 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 every beginning 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.

If you're thinking of buying a compression tester, good for you! That's something everyone needs to do. But try to spend more than $49 on one. Compression testers having screw-on adapters are a joke; like carb rebuild kits and chicken McNuggets-- worthless. 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 Auto Zone 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 40 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.

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.

Then there are those who believe that having multicylinder test compression results whose values are close together is more important than is their median value. I have been a professional mechanic all my life, formally trained and certificated, with time as a trainer of other trainers, and I had never heard this one until I read it on a forum. It's nonsense. Sure, it's good to have compression numbers close. But it is far far better to have them adequate. I'll take pressures that average 150 psi but vary by 10 to 15 percent any day over ones that center closely around 130. Most definitely.

And compression is a problem on 40-50-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. Nature of the beast. There is even a factory Honda service bulletin that lets this particular cat out of its bag.

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.

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.

Why all this emphasis on cylinder compression? One simple fact: compression is marvelous. It not only moves the jelly in your spinal column, it also makes sweet music in your ears. That is, compression directly affects torque, i.e. acceleration. It also determines how your engine uses fuel, effectively leaning or richening the engine, thus resulting in satisfyingly musical throttle response. Low compression makes carburetion leaner, high compression makes it richer. Bike forums are virtually silent on this, but performance shortcomings that virtually everyone blames on carburetion can in most cases be traced to low compression.

Strive for the efficiency and pleasant benefits of good cylinder compression.

Recommended reading:
Forum backwardness
Lies from the ether
Real issues
Don't touch
Powersports orphans
Forums and me
They hate Honda
Where did the compression go?
Reasons for low compression
Valve recession

Last updated January 2022
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