<VV> low vacuum

Dan Kidder iscdirector at yahoo.com
Sun Sep 3 19:31:31 EDT 2006


Bob, thanks for your input, but in response to your comments I refer you to the section in Chev shop manual, automatic trans diagnosis. It says that the trans pressure checks must be preformed with engine at idle and with 16 HG vacuum. This even counts for 110 autos with hi-perf cam. This is what led me on this quest. When I purchased this car (66 110 auto) I noticed that the modulator did not work I replaced it only to find the new one did not work either. I screwed the bad one back in the trans and hooked up the new one to the vacuum source on the engine and connected a vacuum gage so I could watch it and see what it was doing. At 14 Hg it did not pull in and it was actually buzzing in my hand instead. I raised the engine speed to obtain more vacuum and at 15 Hg it pulled in. Anything less than 15 HG and it will not work. At this point I consulted the shop manual and went to work trying to repair the engine to get the vacuum up where it needed to be. As long as the vacuum
 is low when you put the car in gear the tranny slams into gear because the fluid pressure in the trans is 50lbs higher than GM designed. Also the tranny slams when it down shifts as the car is coming to a stop. I believe this constant slamming when the car shifts into gear is why we are now seeing so many loose pinion gears in auto differentials as well as broken e-clips and broken band actuator cushion springs. I know that the 110 140 cam is not high enough performance and overlap to cause a no vacuum situation and I also don't believe that GM built these engines to run with that small amount of vacuum because nothing works the way it is supposed to when it is below 16Hg. I think many people are having this problem and have just decided to ignore it because basically the car still runs half way decent. I am now running my 66 as an experiment with valves at zero lash and the difference is amazing. Vacuum Gage sits steady and it reads 15 to 16 (should be higher because it
 goes to low when trans is put in drive) and now the trans works almost exactly like it is supposed to. So the problem is, what is it that needs to be done to be able to run rocker arm adjustments were shop manual says and still have the valves close all the way like they do at zero lash.

      Please see below for some comments.
  Regards, Bob Helt
   
  In a message dated 9/2/2006 6:32:52 PM US Mountain Standard Time, iscdirector at yahoo.com writes:
  All the corvairs I own as well as all the ones I have checked belonging to other people have readings about 12 to 14 Hg. in neutral gets worse in drive 8 to 10 Hg. Also the readings are very erratic the needle on my handheld gage vibrates and jumps.   That should be telling you something....hi perf Corvairs have this kind of vacuum at idle speeds. Probably the same for the base engines too.
   
   
  All the engines I have checked have aftermarket lifters, some with many miles, some fresh. This indicates valves not closing.    
  No. Not true.What's happening is that the high degree of valve overlap built into the cams makes the idle vacuum erratic as you have found out. The vacuum builds rapidly at speeds over idle and then drops under engine loading.
   
  I have tried replacing valve springs and it no effect. The only thing that seems to raise the vacuum any is to run rockers at zero lash then it gets much better. This problem is bad with powerglide as the modulator quits working when vacuum drops below 15 Hg.   Never measured it, but that just doesn't sound right. The modulator responds to vacuums around 3-6 in-hg as I remember.
   
   
  Any one else addressed this problem it seems to a common one as others in our club say they are having the same trouble.    
  Not sure just what the problem is from what you stated.
   I personally don't see a problem.
  Regards,
  Bbob Helt
  
   
   


 		
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