Scott H.

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Starfury

Well, the question says the garage replaced the camshaft. This meansit no longer has the original cam. For the purposes of this forum,thatsuggests a performance upgrade:P There is NO reason at all to use a factory grind camshaft for a classic Mustang. Add that to the fact that the technicians say that adjusting the cam timing "can" increase compression and vacuum, not "will," and you come up with C

I really hate ASE questions like these. They're worded so poorly that you'll commonly have 2 or 3 right answers.

jc92073

I have been ASE certified in A1-A8 plus L1 for a long time now, and still hate how their questions are worded. It is easy to over think and over theorize the questions. I am sure they would be just talking about a manual compression reading, which would be low if the cam timing was off a tooth or two, with low compression and incorrect valve timing you would have low vacuum, and possibly back firing, lack of power, etc....
I vote D.

67mustang302

D FTW!!!

And as far as factory tuning for max VE, I'd think they'd shoot for ahigh VE in their design, but at a lower rpm range. Tuned to get better VE at lower rpm where the engine sees most of it's time, gets better mileage, but makes less power. The opposite extreme being a race car of some sort, high VE at high rpm for making maximum power, but hardly the economic choice for the road. But it does seem quite often that some of the factory cars are dumbed down. It seems that some engines with a few basic mods can really be opened up, and with little/no impact on mileage. Maybe they keep factory power down for insurance purposes and let the customer bump it up if they like.

jlg2002

As an certified NIASE (ASE precursor) tech, you can't make more mechanical compression than the assemblied components will make. I vote D as well.

67mustang302

Dynamic compression can change, but static won't. If you had a cam where the intake valve closed at TDC of the compression stroke right when power begins, the dynamic compression would be 0:1. Dynamic compression is defined as the compression ratio that begins when the intake valve closes on the compression cycle, and is pretty much always slightly less than the static compression. My static for instance in my 302 is 9.44:1 but the cam timing and rod ratio(which has a minor affect on dynamic as well)makes my dynamic8.31:1.

Scott H.

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67mustang302

Valve timing can only be optimized to trap the maximum amount of air in the cylinder within a narrow rpm range(without using a variable cam timing setup). Without defining what rpm range the cam in it is optimized for and how the testing is done, you can't say for certain whether compression will go up or down by changing cam timing. If I advanced my cam, my cranking compression and idle vacuum would both increase because the cam is optimized for breathing at higher rpm, and cruise vacuum would likely increase as well. Like I said before, that's also why the new S197 Mustangs, and other cars such as Porsches, Ferraris etc, use variable cam timing/design of some sort. In the case of the S197 all that's happening is that the cam timing is either being advanced or retarded to change breathing efficiency at varrying rpms. Retarded timing favors efficiency in higher rpm, and advanced timing favors efficiency in lower rpm. The S197's start at idle with advanced timing, and as rpm increases the cam timing is retarded.

And if you don't believe me, look up things like the Miller Cycle engine, designed by an American engineer named Miller back in the 50's. It was a design that specifically used a cam with a late intake valve closing to reduce dynamic compression and allow for increased boost levels with a supercharger, but used positive discplacement supercharging to make the power and keep the efficiency up in lower rpm ranges where the later intake valve closing would normally cause inefficiency. Some of the new Acuras use this design as well, but acutally use the computer to control the cam timing to close the intake valve sooner or later based on boost, rpm, load etc to optimize power production. At lower rpm and part throttle the intake valve closing is advanced to boost dynamic compression and thermal efficiency, allowing for better power, mileage, and decreased emissions. As the rpm increases and the throttle is opened more, the intake valve clsoing is retarded, to reduce dynamic compression so the boost doesn't cause catostrophic preignition/detonation.

That's also why some race teams will change their cam timing when they go to different tracks, to optimize power in the rpm range that they'll spend the most time in based on the type of course they're on.

This is all valvetrain 101, so I don't understand how the book could say D. A piston engine by it's very design is only running optimally within a very narrow rpm range, based on cam design, but also induction, exhaust, heads, valves, bore, stroke etc etc. Even by changing cam timing on the fly to broaden the amount of efficiency across the rpm range, the other static parts such as heads and intake, still make the engine only run at it's peak efficiency within a narrow rpm range(which is also why some newer cars use computer controlled variable length runner intake manifoldsnow as well). Changing cam timing in either directiondoes NOTalways reduce breathing efficiency, it's not possible because of physics. At higher rpm, air moves at higher velocity, therefore the engine can breathe more efficiently, which can be increased by giving it a cam profile that works better with higher velocity air, but not at lower. Even high mileage street engines have cams that are optimized for some rpm a fair bit above idle, but well below peak horsepower rpm, typically the common rpm range the engine will operate in under cruise. Now, changing cam timing in either direction will reduce breathing efficiency at that one particular point, but will increase or decrease efficiency at other rpms. Changing things like cam timing move the peak efficiency point around on the rpm range, decreasing in some places and increasing in others.

That's why compression and vacuum can go either up OR down, depending on the cam profile, the rest of the parts, and which way the cam was moved. Why do you think race cars have such horrible idle and low rpm vacuum? They never spend any time there so they're not optimized for it. Any engine is a compromise of optimization in one narrow rpm range for mediocre efficiency in others, or sometimes maximum possible optimization in a narrow rpm range, for a total lack of efficiency in others. That's also why cam manufacturers reccomend static compression to run with a certain cam, to make sure the dynamic compression remains correct. Too high and you detonate/preignite, too low and you have a loss of power.

Anyway, just discussion for those that bothered to read to the end of my long windedness. [8D]