Setting Valve Clearance using Vacuum Gauge

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dhowlan

dhowlan

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I was trying to educate myself on ignition curves and read of someone who swore by this. He sets valve clearance (engine running) to the highest vacuum and then measures what worked for future reference. Does anyone out there do this? Is there a reason to or not to?

D
 
iv been around race cars all my life and never herd of setting valves in a race motor like that.are you sure he wasnt talking about seting carb ajustment.
 
With aftermarket carbs and cams, using a vacuum gauge to set timing and fine tune the carb is the only way to get the engine in tune since so much has been changed...that's how I do it:read2:
 
On an LA style head it would be impossible as the adjusters are on the working end of the rocker. It could be done with a magnum (pedestal style) head but I've never heard of it, nor would I recommend it.

Highest vacuum at idle would be with the valves at their loosest possible as this would reduce duration.
 
Good way to wipe out a cam or damage valve gear. The specified lash has a lot to do with the ramp angle of the lobe. Too tight lash will cause the lifter to maintain pressure on the base circle too long and not allow oil to get between the lifter face and the cam. In severe cases the valves may not seat completely and they will overheat and burn. Remember the valves grow in length as they heat up, this may be several thousandths at full throttle. Too loose of lash and parts will beat themselves to death. And it sounds like crap. Cam cards have a specified lash, which should be followed.
 
Depending on the valve train design he could cause misalignment of the valve train geometry which will wipe stuff out. As Daredevil said, don't believe everything you read on the internet.
 
Thanks for all the input. So's you don't think I'm making this up, I read it in this ancient thread: http://www.moparchat.com/forums/archive/index.php?t-668.html "Ive even used a vac. gauge to adjust my valves while the engine is running ,I adj. to max vac. at idle if i want more torque," (Does anyone know who "PRO", the person who made that comment in 1999, might be? Location says Grand Junction , CO)

I also found a page that had a newsprint copy and transcribed version of an article describing much the same thing, although it was for a chevy 231 or something like that. I can't find the darn thing now. It did give specific minimums and maximums for the clearance, but said to maximize the vacuum within those parameters. Obviously not a good idea to have things so tight or so sloppy that metal bits are coming off.

Seeing is believing so I'll see what's possible (within reason) next time I adjust my valves.

D
 
I used to be there alot but have since mostly left due to irate people beilieveing there right and not letting up. Narrow veiwed people.
I know PRO a little bit. Not much.

Look, is your ride a race ride or street ride?
Street to mild strip, vacuum advance (If used) gets a ported nipple.

I'm with "GodFather" on this and I have never ever heard of this
Valves are set prior to running in the engine and then after warm up, the final adjustment should be made. No other way should be done for the adjustable rocker until you reach genuis level and know what your doing. And pray your as good as you think you are if you decide to skip normal practice or break something that'll cost alot of money.

I don't care if the "Chat" bash's me to high heaven for this comment, but stay away from there. To many non-sense talkers and wanna be wrench heads talking to much trash.

I aint so great, but tried and true practices work great for me.
 
All I want to know is how did they keep the allen wrench (or box end on the lock nut depending) on the fastener while it's bobbing up and down? Stud rockers are easy... the adjuster doesnt move while the rocker does. On a mopar.. no way. Maybe he's driving a magnum? I dont know. Never mind the mess... my hands would be trashed...
 
I have heard of people setting lash on solid lifter \6's with the engine running and I have even tried it but it beats the snot out of your feeler gauges and and as moper has stated it's tough on your hands. I found by doing it I wound up with to tight a setting.

The referenced article by PRO sound like he was doing it on a hydraulic cam. If you are in the operating stroke of the the lifter I can't see how you could influence the manifold vacuum at all. There may be a momentary change until the lifter either pumps up or bleeds down but once it stabalizes there should be no difference.

Typically you get higher manifold vacuum with less valve over lap. If you use that technique I would expect that you would wind up with very loose valave lash (more lash = less duration = less overlap).
 
I hadn't considered hydraulic. That may be what he meant. I have a slant and, yes, the FSM instructs you to adjust the valves hot and running. That may have been what caught my eye initially. If you alter the lash, the overlap will change, but the amount of vacuum will also depend on the timing. If the change in overlap moves the timing point to where the explosion is less efficient, the vacuum should drop to indicate this. Theoretically :-D.

Like I said, next time I adjust the valves, I'll see what happens a little this way or a little that way.

BTW, the slower the idle (I can get mine down to about 450), the easier it is to adjust the valves with everything moving and the less oil gets everywhere. I'm told, angled wrenches and feeler gauges make the job easier.
 
Maybe it is the Power Valve that the original author that the original poster quoted? That would only help in the selection of the PV. Then, yet again, it could be a secret Chevy engine trick to get 950 HP out of a 305 small block on a $84.99 rebuild.....
 
dhowlan said:
I hadn't considered hydraulic. That may be what he meant. I have a slant and, yes, the FSM instructs you to adjust the valves hot and running. That may have been what caught my eye initially. If you alter the lash, the overlap will change, but the amount of vacuum will also depend on the timing. If the change in overlap moves the timing point to where the explosion is less efficient, the vacuum should drop to indicate this. Theoretically :-D.

Like I said, next time I adjust the valves, I'll see what happens a little this way or a little that way.
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Timing doesn't change with lash. The timing is in relation to crank position. By increasing lash the valve opens later and closes earlier.
 
I think I phrased that poorly. I didn't mean to imply the timing of the spark was changing. I meant timing in the broader sense. The valves are opening and closing at a different time in relation to the spark.
 
dhowlan said:
I think I phrased that poorly. I didn't mean to imply the timing of the spark was changing. I meant timing in the broader sense. The valves are opening and closing at a different time in relation to the spark.
Click to expand...

the spark is in relationship to tdc of the piston not when the valves open....

changing lash will have a very small change on the opening and closing of the valves...
 
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