Scott H.

.....

mustangmatt63

Sounds like neither mechanic is correct but I am not an expert by any means.Wouldn't the timing just need to be set properly?[8D]

plcdude

Honestly, I just guessed.

67mustang302

Is that an actual ASE question, or did you take liberties with it? I've never seen one like THAT.

Both of them though, since compression is really dynamic, the compression reading is based on cam timing, particularly the intake valve closing point. That's why cam manufacturers reccomend higher static compression with larger cams, to keep the dynamic compression high enough as well. Higher compression readings would come from advanced cam timing, lower from retarded, and changing cam timing can change manifold vacuum readings as well, since advancing/retarding cam timing changes engine pumping efficiency, and thus, vacuum. Incidently, the new S197 Mustangs' Variocam timing is actually the cam starting off in the advanced position at low rpm, and as engine rpm increases the cam timing is retarded to shift the power higher in the upper rpm ranges.

falconfixer

See i thought you could have LOWER compression (overlap) not higher (already measured at 'highest' and not higher than stock). Vacuum would be affected in any case, but 'won't run right' is too vague though. Won't idle? Won't make the power it should? Backfire through the carb? Exhaust?

(The same follow-up questions we would ask a newB poster...) Or is my lack of ASE knowledge quite apparent now? [shrug]

JMD

SInce the question did not specify whether the timing was advanced or retarded, IMO either mechanic could be right, although it is unlikely that theraised timing or raised vacumm could be used as a diagnostic tool, so the shop manager is right again!

I am inclined to believe that a mildlyadvanced cam would tend to raise compression, and a slightly retarded cam might tend to increase vacumm.

Never really studied it before or given it much thought, but that is what I just came up with...

JMD

By slightly changing the "center of the overlap" either "forward" or "backward" as it related to TDC, either the vac and/or the compression can change.

I am trying to describe something I can see in my head,,,,, does anyone else understand the "mental picture" I am trying to project, or am I all alone..

Starfury

No, you'remostly right A is definitely correct, as an advanced cam can cause higher dynamic compression, just as you said. B is vague, but possibly correct, as a cam that normally has low vacuum straight upshould have higher vacuum when advanced (not retarded) due to the increase in dynamic compression. But B does not specify when the vacuum readings are taken. Maximum vacuum is not likely to change, but idle vacuum is.

67mustang302

The thing with the ASE test questions is you have to not think too much into them. Since vacuum is generally checked at idle, it's safe to assume it's talking mainly about idle vacuum, and since idle vacuum can be changed by cam timing it would be considered a correct answer. That's how a lot of the ASE questions are, you have to answer them more in generality sometimes than in specificity. If it's a specific thing they're dealing with, they give you the specifics, like specifying only idle, or only cruise vacuum etc. That's how they try to trip you up.

That's what I mentioned in the other thread, about some of the ASE questions they have now that have more than 1 possible right answer. The rule for those tests is that if they don't give you spcific data or ask for a specific answer, then they're speaking in generalities. Another rule is that if one of the answers is much longer than the others, and has a bunch of specific info in it, then it's most likely the correct one.

Starfury

Yeah, and C would be the 'correct' answer for an ASE test, but for education purposes, B needs to be more specific