<VV> Lifter question
Mon, 30 Aug 2004 11:57:40 EDT
In a message dated 8/30/04 8:12:55 AM US Mountain Standard Time,
> It's possible that the gear on the crank slipped. Check that the #1
> pistons are exactly at TDC when the timing marks are opposite each other.
> [<Barry>] What timing marks? Before disassembly, the harmonic balancer
> showed 0 deg, with pistons at TDC using my thumb first, then I used a small
> driver just touching the piston head as a 2nd method.
> [<Barry>] But, what you're saying is, and I agree, I still haven't compared
> the location of the timing marks with the location of TDC. I think I see
> now. I should have checked the timing mark at the cam/crank gears, with the
> piston at TDC.
> Yes. There is a timing mark on the crank hub and another on the cam gear.
Locate both of these. then, scratch a mark on the face of the cam gear so that
it points fon the cam axis to th can timing mark. this is to allow you to see
when you have alignment. rotate the crank until both timing marks are opposite
eachother. then check to see that both #1 and #2 pistons are exactly at TDC.
> I didn't fill them with oil, but they easily collapse. I'll test this. I
> fear though, that this is just a "feature" of these lifters.
If ti's afeature, then it's a BAD BAD feature and lifters should be replaced
> we don't know how to read a table in the stock or aftermarket cam section,
> specs@50degrees There are things like "3A-74", "5B-7A" etc in the intake and
> exhaust columns. The one with ramps is not quite as confusing, but I can't
> find where these columns are explained anywhere in the book. Help?
Well, you are really interested in lobe lift which is measured from the base
circle to the highest point on the lobe. Measurements at 0.050 in aren't
necessary in your problem.
But to explain, the specs......The 0.050 in. points are selected to avoid
trying to make accurate measurements on the ramps.
In the idealized four stroke cycle, the intake starts at TDC, compression
starts at the following BDC, then the plug fires at the following TDC and we
the power stroke. Then, the exhaust valve opens at BDC, and the cycle is
repeated. You have to fully comprend this in your mind before going further.
OK, got it?
Now for the real world. Opening and closing the valves at the points of the
idealized cycle won't work in practise. This is because of the lag (inertia)
getting the valves open and getting the gasses moving as they should. So the
valves are actually opened (and some closed) before or after the time stated
in the idealized cycle. For instance now repeating the cycle above, the intake
valve opens BEFORE TDC and closes AFTER BDC. This gives a really long period
to fill the cylinder with gas/air mixture. Similarly on the power stroke the
exhaust valve opens BEFORE the piston reaches BDC. Why? Well engineers found
that getting the exhaust gasses out of the cylinder was best accomplished by
using some of the power stroke pressure to blow the exhaust out the exhaust
valve. (called blowdown)
OK so reading the cam specs, B stands for before and A stands for after and
the number is the crankshaft degrees the event occurs. So 5B-7A means that
valve opens 5 crank degrees before TDC and opens 7 degrees after BDC. But
this isn't an actual stated spec, so lets look at an actual one. Use the OTTO
OT-30 for example. Look at the 050 in. spec for the intake. It's 7B-35A.
Idealized, the intake would be 0 BTDC And 0 ATDC. But to make this cam work
efficiently the intake valve is opened 7 degrees BTDC and closes 35 degrees