After reading a few posts here last night on timing chain guides I got to wondering the following - perhaps someone can tell the me the answer:
Assume an early Q never had the guides changed - the guides crack and one chain skips a tooth (assume the RH side to eliminate the CAS) So the passenger side head is running retarded.
Before the chain skips a second tooth, the guides are done. Of course without pulling all the timing covers and rocker covers the tech won't see the timing is off.
How will the car run? Would it be like a car with a very stretched chain? What would the performance characteristics be? Would the car have problems with emissions?
timing chain question for the experts
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Q451990
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The idle would be rough, sluggish performance, and I'd assume the emissions would be off as well. Also that would be one lucky owner to have a guide failure that only slip one tooth! From what I understand, more from a reassembly standpoint, one tooth off and it runs rough, two and the valves and pistons hit.
Heath
Heath
If I recall right, Dennis has stated 3 teeth and the valves might barely hit. 38 teeth on the cam sprockets, so 28.42 degrees. Not sure if that assumes advanced or retarded cams, though. Remember that the valves begin opening before TDC. A 1-tooth retard, then, would actually put the valve and piston farther apart...almost exactly at TDC. Hypotheticals aside, it would almost certainly happen on the driver's side, since that's the guide that fails.
Hmmm. Driver's side eh? I was sort of curious more as a mental problem than anything that I've encountered. I just assumed that in case of timing failure it's the crank that skips ahead, retarding the head.
I was more curious about the math of the situation. How much less air will enter, and how much more horsepower would be lost due to increased back pressure in pushing out exhaust. What would a PBT show in this case.
My dad showed me some reports from the 50's where every year the same displacement Chevy engine was reported to have increased horsepower from the year before. He said it was because the measurement speed was changed, but after learning about variable valve timing I wonder if the GM guys were learning to cut better cams ... and that got me thinking about the effects of bad timing on a Q.
When I changed my Q's timing guides, the LH (driver) guide WAS down in the pan. When I tore the engine apart in Feb I carefully checked and the timing was still right ... so apparently the absence of a guide doesn't automatically cause the chain to skip. Meaning then that it's possible for a sproket to skip once and not skip again for a long time. I think.
dan
I was more curious about the math of the situation. How much less air will enter, and how much more horsepower would be lost due to increased back pressure in pushing out exhaust. What would a PBT show in this case.
My dad showed me some reports from the 50's where every year the same displacement Chevy engine was reported to have increased horsepower from the year before. He said it was because the measurement speed was changed, but after learning about variable valve timing I wonder if the GM guys were learning to cut better cams ... and that got me thinking about the effects of bad timing on a Q.
When I changed my Q's timing guides, the LH (driver) guide WAS down in the pan. When I tore the engine apart in Feb I carefully checked and the timing was still right ... so apparently the absence of a guide doesn't automatically cause the chain to skip. Meaning then that it's possible for a sproket to skip once and not skip again for a long time. I think.
dan
http://www.nissaninfiniticlub....12551Much has been written in the past regarding engine failures and how immediate failures from broken guides are, in fact, rare compared to eventual failures of engines whose guides are in the oil pan. Unfortunately it seems a lot of that info has been lost in server moves.
Without knowing the actual geometry of a cam lobe (beyond lift), I can't really guess how much less air is going in and out of the cylinder. I would guess that emissions are impacted, probably increased HCs from running too rich. Valve overlap would be the same, as would spark relative to piston. Retarding the valves would appear to have the same impact as dropping the CR while increasing fuel. Incorrect timing should show up in a compression test, especially if one head's compression was a lot different from the other head's. Allowable range is from 142-185psi I think, with small variations allowed across cylinders.
Without knowing the actual geometry of a cam lobe (beyond lift), I can't really guess how much less air is going in and out of the cylinder. I would guess that emissions are impacted, probably increased HCs from running too rich. Valve overlap would be the same, as would spark relative to piston. Retarding the valves would appear to have the same impact as dropping the CR while increasing fuel. Incorrect timing should show up in a compression test, especially if one head's compression was a lot different from the other head's. Allowable range is from 142-185psi I think, with small variations allowed across cylinders.