KLR

Would you bend a valve, or would the motor still turn over OK, but with timing retarded or advanced by about 10 degrees? Just curious.

944Ecology

Usually, off by one tooth is not all that critical, and you should not bend valves. You may have a bit more low rpm or a bit more high rpm responsiveness, depending on whether it's advanced one tooth or retarded one tooth. The ignition timing will probably not change, since the width of the contact on the rotor will allow for a few degrees of advance or retard anyway, and it's controlled by the DME, not by the position of the rotor.

That said, it's better if its timed correctly.

KLR

That's not what I expected. I would have assumed that the car would run terribly. If I had good mid range but thought I was a little weak above 4k rpm, could the timing being off by one tooth explain that? My A/F ratios are good and the car doesn't run hot. I'd have at least thought that the mixture would be off if the timing were off by one tooth.

944Ecology

Why would the mixture be off? It is not controlled by the valve timing... as a matter of fact, all of the injectors fire at the same time, and just wait for the fuel and air to be sucked in when the valve opens.

gb

KLR

I guess that I was assuming that the valves openning and closing at the wrong time (slightly) would affect how much of the fresh mixture gets in and, depending upon what's left in the cylinder if the exhaust valve doesn't allow all the spent gas out, potentially impact the mixture. Come to think of it, though, I guess that it really doesn't matter, does it?

"That said, it's better if its timed correctly."

What kind of damage, if any, would running the valve timing off by a tooth cause?

BeerBurner

I'm not sure that it would cause any damage. Many people use adjustable cam sprockets to advance or retard the timing a few degrees to alter the powerband, and as far as I'm know it doesn't cause any problems, beyond possibly needing higher octane fuel or something.

But I could be wrong...

BB.

theedge

My mechanic was telling me his 951 was off by a tooth, when he changed it he did notice a performance difference (as he put it, laying rubber all down his street in second g ear, couldnt before) and there were no bent valved or anything.

Matt H

Every time I have put a timing belt on any car off by one tooth the car ran like **** (a few Hondas and one or two Fords). As to the mechanics of it, I dont think the valves are closing all the way which would/should cause it to run like ***.
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BeerBurner

They'll still close all the way, just not at the time that they are supposed to.

BB.

Matt H

BB - yes, that makes far more sense.
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Danno

Being off by one-tooth is about 10-degrees. The difference in performance would depend upon which way it was off. Having the cam advanced would give you more low-end torque, in this case, it would probably be right around when max-boost hits, so you'd get a bigger torque-peak, but it would also drop much faster than stock. So a -30% to -40% drop in peak torque by redline may be the result.

If the cam was retarded, you'd have a lower torque-peak, but it would hold much flatter to redline. In fact, 10 degrees would be enough to optimize high-RPM HP around 6000rpm. So you may actually end up with a dyno-chart with higher HP-peak than torque.

"I guess that I was assuming that the valves openning and closing at the wrong time (slightly) would affect how much of the fresh mixture gets in and, depending upon what's left in the cylinder if the exhaust valve doesn't allow all the spent gas out, potentially impact the mixture."

Yeah, this makes a difference at low-loads and low-RPMs, like idle. Due to the batch-fire injectors, some cylinders would get a shot of fuel when the intake valves are open, others when it's closed. Not that big a deal really, because it would even out after a couple of revolutions. But it may affect emissions on those first couple of revolutions of running the engine. And it can give you a loping idle with large injectors. The solution here is sequential injection so that all the cylinders have their fuel injected at the same valve-timing.

Dilberto

Couldnt have said it better, Danno! Your logic is a reflection of your choice of ride....

Zach

josephsc

I recently did a t-belt (on a 968) and had my t-belt off by one tooth. One of the symptons was a terrible compression (like 130s instead of 170s).

dr914

so could the car run if the belt was four teeth off or would it interfere and crash the valves?
We just found a car in for service four teeth off and cannot believe it was running (although poorly)

Dan Martinic

Who the heck put that together? Wow. I know there's a popular mod for 4deg cam key but I'm curious about this mod lol