Stup Q of the Day - TDC???

[cab83_750]

At TDC, both in and ex valves are closed, right?

[ZAMIRZ]

yes. TDC is right before the engine's power stroke.

[norustscott]

yes , but TDC of which stroke is the question... compression stroke?

[cab83_750]

Ok, how about this....

When you adjust the valves, and u r adjusting cylinder #4 per the Technical Article, .......

[ischmitz]

Windsor,

You obviously want to have some intake (and exhaust) valve lift of the spedified value for your cam while you adjust the cam timing. So you do it on the exhaust stroke at TDC overlap of the respective cam.

Ingo

[john70t]

For every time the cam rotates, the piston reaches top dead center twice:
-once on gas interchange(valve overlap means that both valves are being pushed open at the same time briefly, and the rockers may both be tight).
-and once on compression/ignition. This is when the valves are adjusted.

The engine has to be dead cold because the clearance that your setting is the distance the valve stem will expand when it gets hot so that the rocker and the valve will dance close together.
Too far apart and they will wear, make noise, and not let gasses in or out(power). Too close together and one of the valves stays open-letting combustion backfire through the intake or exhaust and burning the sealing seat of the valve.

Top dead center of the piston/crank/marking pully may not be "TDC" of the camshaft however if the chain is stretched. Because the valve is adjusted on the circular area of the cam(not the lobe) its usually not a problem, but valve timing way out of wack can lead to crashed valves in interference engines and definite loss of power.

[ischmitz]

Good points John,

What I meant to say was that you adjust valve lash on or around TDC ignition and cam timing at TDC overlap.

Ususally, you can move the rocker ever so slightly on the rocker shaft when the follower is not on the lobe of the cam. That tells you that you are on the ignition TDC stroke. Another way is to look at the distributor rotor position. The shop manual shows in what direction the rotor points for TDC #1 ignition.

Cheers,
Ingo

[Wil Ferch]

Maybe all he wanted was a simple answer....

both valves are closed before the power stroke.... exhaust valve is open before the exhaust stroke

...both cases you're at TDC.

Wil Ferch

[thabaer]

No more beers tonight Will.....

1/2 right, other TDC is INTAKE valve open before intake stroke....

[HawgRyder]

Easy way to remember the order of engine.....
Suck...Squeeze...Bang....Blow!
And the plug fires to make the bang.
Bob

[Wil Ferch]

Hold on....

What did I say ...??

Hmm...maybe ..."exhaust valve is open at end of exhaust stroke"....wouldn't that put cylinder at TDC ??

Yeah...it's been a tough day ...

Wil Ferch

[Wil Ferch]

oopsy!...

I guess the better answer is that the "other" TDC I seem to be wrestling with... is the point of overlap..the exhaust is partially open ( on its way going closed soon) and the intake is partially open ( and going more fully open soon..initiating the next intake cycle).

Sorry...not so simple with words after all !

---Wil Ferch ( tired....)

[john70t]

Sorry, trying to simplify makes me more confused. If it's remembered correctly:

The gas exchange happens at funky times. The exhaust valve is not just opened when the piston is going up, and the intake valve is not just open when the piston is going down.

It's formulated to the specifics of a constant stream of air in and out of the engine according to the pulsation harmonics of that stream due to the start/stop of that stream.

The exhaust actually opens well before BDC because the work of the expanding air is usless to the piston and the push of exhaust gas out the engine helps suck in the fuel/air charge.

The intake usually opens well before TDC because the suction of exiting gasses helps clear the cylinder of "waste" gasses and pull in the fresh charge.

Except when its designed into the camshaft to keep a little in the cylinder and act as an EGR in economy cars, or (with a lot of camshaft overlap in the case of the dragsters) it loses a lot of the intake charge out the exhaust for the sake of being able to flow air quickly through the engine at high RPMs.

(Sorry if any above info is not correct, please post alternative. Too many bears tonight.)

[Wil Ferch]

This might help...

look at caliber60's graph here:

Valve adjustment question again


Wil Ferch

[cab83_750]

This is educational and surprising for someone reading up on Wayne's rebuild book.....though I am not currently 'actually' rebuilding.

I am monkeying around my SC, ready to drop and replace the tranny. While I was there, I started checking the compression. Then, as if I have a lot of time, I wanted to test my new leakdown tester. The manual mentioned about TDC.

So the basic assumption/requirement behind the leakdown tester is that both valves are closed and they have almost perfect air sealing to the heads. Then, air/pressure would escape via worn rings, pistons, cylinders, etc. Correct?

However, if timing is incorrect, air would escape via the intake and/or exhaust, thus preventing you to diagnose rings, pistons, cylinders, etc. Better yet, if the heads are not perfect, or you have head studs problems, this is yet another avenue for air/pressure loss. Right?


As I said, I am just religiously reading and learning about rebuilding, thus the title of "Stupid Question of .....".

Thanks a lot guys!

[cab83_750]

Wil,

Thanks for Caliber's graph. Very educational. I always wanted to know, visually, at what degrees both valves are open.

[Wil Ferch]

You're welcome..but like so many things..this is a good approximation for any engine...but *actual* opening and closing points ( the actual valve timing) can vary a bit around these generalized norms....I think for the purposes you intend, the graph should work.
---Wil Ferch