L-71 vacuum reading

[Robert D.]

Re: L-71 vacum reading

Duke i run the motor to get it warmed up then set the exhaust while the intakes are opening and intakes while the exhaust are closing

Bob

[Duke W.]

Re: L-71 vacuum reading

There's no need to warm up the engine. I've never found a difference in lash between "hot" (idling) and cold on a cast iron head/block engine because the iron and steel (pushrod) thermal expansion rates are about the same, and the exhaust valve stem in not very hot at idle, so there is little stem expansion at idle. I prefer setting the intakes at 90 deg. ATC and the exhausts at 90 deg. BTC. This absolutely positively guarantees that you are on the base circle.

The L-78 lash specs are .020/.024". They were loosened to .024/.028" for '67-up (same cam other than the rear journal groove), but I have no idea why. Based on the lobe clearance ramp height of .012" times rocker ratio of 1.7 (It may be less, but I don't have reliable measurements of ratio variation on big blocks as I do on small blocks.), clearance should be .020", but I recommend .020/.022" to give the exhaust valve stem a bit more expansion allowance since it's larger diameter than small block exhaust valves and likely runs hotter.

This cam is shorter duration and has less overlap than the SB 30-30 cam (The LT-1 cam uses the same lobe on the inlet side on a slightly smaller base circle) , so it is significantly milder especially when combined with the BB's longer stroke, and the lower overlap shows up as much higher idle vacuum at 900 - 14-15" versus about 10" on a 30-30 cam engine idling at 900.

The engine could be "deturned" to trade top end power for low end torque with a shorter duration lower overlap cam (like the 390 HP cam) but then it won't have SHP idle behavior.

See the attached pdf.

Duke

[Mark E.]

Re: L-71 vacuum reading

Duke's attached file says "...the Service Manual for all cars so equipped [30-30 mechanical cam]... says specifically to set them "hot and running".

I admit I've never owned or serviced a mechanical cam engine, but how on earth could one be adjusted while running? It seems that would just beat the feeler gauges all to hell.

[Duke W.]

Re: L-71 vacuum reading

Believe me that's how I did it back in the sixties - didn't know any better until I mounted dial indicators to the rocker arm pushrod socket and valve retainer in the early seventies, which yielded lobe and valve lift data and allowed me to compute the height of the clearance ramps and rocker ratio behavior. Decades later when I got hold of the lobe lift data from the engineering drawings I was able to compute clearance ramp height much more accurately and make equally more accurate clearance recommendations .

It wasn't really that tough on feeler gauges (They're hardened steel.), but it's an atrociously messy procedure and damned near impossible not to burn your knuckles on the exhaust manifolds. I really hated doing it.

The Hinckey-Williams procedure is sooooooo much more relaxed and easy, and likely more accurate. I've been timed doing the procedure in a mere few minutes, but that doesn't include R&I of the valve covers and other parts that are necessary to access the valves nor the time required to clean up the valve cover gasket surfaces and cement new gaskets in place.

I volunteer to adjust the valves on anyone's mechanical lifter engine - but just measuring the initial clearance and resetting them to proper spec. The owner gets to do all other grunt work including recording the initial measurements.

[Dan D.]

Re: L-71 vacuum reading

I used to do this all the while back in the 60s (small blocks). I set a piece of sheet metal in the lower lip of the heads to prevent the oil from getting on the exhaust manifolds, then go down the bank, first doing all the intakes, then switching feeler gauges and doing the exhausts. Did not take any time at all, and I always felt it was more accurate - doing it dynamically so to speak, instead of statically with the engine not running. I guess I had just honed in a procedure that seemed to work well.

-Dan-

[Robert D.]

Re: L-71 vacuum reading

Thanx for all your help guys

Bob

[Richard M.]

Re: L-71 vacuum reading

Bob, Did you get another idle baseline vacuum reading at the choke pull-off port or manifold fitting yet?

Rich

[Mark E.]

Re: L-71 vacuum reading

I used to do this all the while back in the 60s (small blocks). I set a piece of sheet metal in the lower lip of the heads to prevent the oil from getting on the exhaust manifolds, then go down the bank, first doing all the intakes, then switching feeler gauges and doing the exhausts. Did not take any time at all, and I always felt it was more accurate - doing it dynamically so to speak, instead of statically with the engine not running. I guess I had just honed in a procedure that seemed to work well. -Dan-

I'm missing something. For mechanical lifters, how can lash be checked while the valves opening and closing with the engine running?

The spec for valve lash represents valve stem to rocker clearance when the valve is closed. And obviously, valve lash is zero when the valve is actuated. This mean lash needs to be checked with the engine OFF and rotated so the valve being adjusted is closed.

[Dan D.]

Re: L-71 vacuum reading

Actually Mark, it works quite well. You keep constant pressure on the feeler gauge. As you say, it wont move when the valve is lifting or closing, but it will release when the lifter is on the clearance ramp. So with constant pressure you can quite easily tighten it until the gauge wont move, then back it off until you can move the gauge. The amount of much resistance you encounter allows you to make a loose lash, or a tight last. It is easy to make all the valves to same.

-Dan-

[Mark E.]

Re: L-71 vacuum reading

Actually Mark, it works quite well. You keep constant pressure on the feeler gauge. As you say, it wont move when the valve is lifting or closing, but it will release when the lifter is on the clearance ramp. So with constant pressure you can quite easily tighten it until the gauge wont move, then back it off until you can move the gauge. The amount of much resistance you encounter allows you to make a loose lash, or a tight last. It is easy to make all the valves to same. -Dan-

Thanks Dan. That makes sense. Is one method considered more precise than the other? Adjusting with the engine off would seem easier and less messy.

[Dan D.]

Re: L-71 vacuum reading

I think with the engine running it would be more precise, but others will probably argue that. The preferred method seems to be with the engine off, and I am sure it is less messy that way. I think others will probably comment on this. This subject has been discussed several times in the past.

-Dan-

[Robert D.]

Re: L-71 vacuum reading

Richard
i just got a chance to get the reading with a new gauge at the choke pull off port and im happy to say with some tweeking of the idle mixture i got it between 14 and 15 inches at 900 rpm

thanx so much for all your help