Chevy & GMC Vans

So I'm hanging out at the local engine rebuilding shop when a customer comes in with some parts he's just received from Jeg's. He's got a Chevy 350 in for rebuilding and he is delivering the cam gear drive. One guy on the floor is not impressed; he thinks that cam gear drives are the worst thing for an engine possible. I ask him why, he states that gear drives "transmit crankshaft harmonics" to the cam and cause valve train damage. I have also heard this reason time and again on various web forums, but I never get what I consider to be an adequate explanation of just what crankshaft harmonics are and why they could be bad for the valve train.
Anybody know what's up with all this? In particular, I'd like to really find out what's meant by "crankshaft harmonics."

Rob,,,first,,good to see you around my man.

A little on harmonics. The cause of crankshaft harmonics is the twist and rebound of the crankshaft after fuel ignition and then the rebound of piston returning to bottom of stroke. Both directions(twist & rebound) of each piston produce crankshaft harmonics. This harmonics or vibration is in all engines even if balanced and proper balancer is installed,,,you can't run from it. What the shop guy was saying is,,it's there, no matter what and when running a gear drive there is no where for the harmonics to play out or be reduced before arriving at the camshaft due to direct contact of parts versus a chain which, due to slack reduces transfer to cam.. This is the story but don't know if anyone has really done any kind of comparison as far as wear and time to reach a certain point with a gear drive versus a chain.
Did my southern thought process get this where it's understood. You do understand,,,we aren't wrapped up real tight doen here.

Larry, I can understand that the crankshaft isn't actually rotating smoothly, it has to be pulsing and even twisting according to the cylinders firing. But I have a problem with a chain stopping these pulsations from reaching the camshaft. My point is this, and I couldn't get the tech at the shop to understand, is that the chain had better be transmitting every twitch of the crankshaft or the cam wasn't really being kept in step and, I feel, valve and spark timing would only be an approximation. He was more concerned with what the distributor was doing with the spark timing if crankshaft harmonics were affecting it. I brought up crankshaft spark triggers and that most consider them a better way to accurately control spark timing than by using a distributor. I've never heard of crankshaft harmonics being brought up when crank triggers are discussed.

There's also the OEM use of gears, not chains, on some inline engines and I've never heard a discussion of crank harmonics affecting them, either.
But I have noticed that a lot of people just flat-out don't like the idea of using timing gears on a V8. The majority of them seem to think that a person who uses timing gears is trying to make others believe that there's a blower under the hood.

But, back on topic, I'd love to read something that really explains crankshaft harmonics and exactly what a chain, belt, and gear does with it.

Understand your thoughts big Rob. I can see what the guy was trying to say about the gear drives. Don't think I mentioned in my reply, that I don't think, for the life of a street engine, it would make a hill of beans difference either way but over the long haul and high RPM use I can see that a gear drive really doesn't allow any shock absorber for the harmonics, sound or movement. It's true, there is a direct link no matter if it's a gear or chain but think about the chain and the play, though very slight when new, the play is still there and by the design of the chain you can see how it would reduce the harmonics versus a gear but again,,,don't think it would make that much difference for street use but my guess is,,the wear difference could be measured over time.
I know you're a very technical guy and for me,,this is the best I can explain this deal...in other words big guy,,,this is all I got.lol

There is a good site on this subject. After a problem I had years ago with a 283 bored to 301 I ran acorss the site. I'll see if I can track it down. Got in to alot of detail,,,,stuff I didn't really want to know.haha

Thanks for the replies, Larry. This whole matter of crankshaft harmonics and timing gears is my current automotive thing to dig into. I really want to know what the truth is and when the nonsense (no matter how often repeated and highly held) begins.

Here's a photo of a Ford 7.3 diesel V8 that is being rebuilt for truck pull competition at the local engine shop. I'm not familiar with the Ford 7.3, but it appears to me that cam timing gears are normal for this engine. This gear setup doesn't have an idler gear, so the cam is ground for reverse rotation compared to the crankshaft. But if it is the norm for a 7.3, Ford doesn't have a problem with crank harmonics and gear driven V8 camshafts.

The Cummins 5.9 diesel also uses gears. The diesels are a much lower speed engine though and are designed for 300,000+ mile service life.



I can't see where the chain or belt is any different than using gears except when you start talking about noise and how quiet it is. The only issue I could think of would be if at a certain RPM the gears themselves give off a harmonic that causes issues.