Manifold vacuum question C1.

[Bruce B.]

How much manifold vacuum can be expected from a reasonably fresh stock 1962 340 HP engine?

How much will valve adjustment affect the vacuum signal?

Thanks,
Bruce B

[William C.]

Re: Manifold vacuum question C1.

At what RPM? Valve adjustment will directly affect the vacuum (tighter clearances cause lower vacuum) due to increased overlap.

[Bruce B.]

Re: Manifold vacuum question C1.

Bill,

Lets say at 800 rpm idle and at 1500 and 2000 rpm if thats available.

I usually adjust the valves at 0.010" and 0.018" hot.

Bruce B

[William C.]

Re: Manifold vacuum question C1.

Not sure what year, but chevy spec (1958-1962) was .012 intake and .018 exhaust, HOT. cold lash would likely be looser. I haven't actually checked my '62 for vacuum numbers, but the theory doesn't change regarding lash vs vacuum at lower rpm's especially. (My cam is not an original grind so I can't run your test and correlate)

[Bruce B.]

Re: Manifold vacuum question C1.

Bill,
Thanks for the information.
My Owners Guide calls for 0.008" intake and 0.018" exhaust clearences.
I'll take some vacuum measurements at the current clearences( 0.012" and 0.018") and then after adjusting to the 0.012" and 0.018" values.

[Jim L.]

Re: Manifold vacuum question C1.

My FI '60, with an original, GM-sourced 097 cam, which should be the same as in your '62 340, makes 14" vacuum at idle.

No idea about the valve lash.... it's been at least 10 years since I've checked it. I would hazard a guess that I set them cold at .010/.016. though.

Jim

[Duke W.]

Re: Manifold vacuum question C1.

Higher idle speed means higher vacuum, and the higher total idle timing with a vacuum advance distributor will increase idle vacuum relative to a non-VAC distributor at the same idle speed.

I recommend cold valve clearance of .010/.016", but .008" is an option on the inlet side. (See the Hinckley/Williams valve adjustment paper for the reasons. It's available as a download on the Web and has been referenced here many times.)

On a cast iron block/head engine there is no signficant difference in hot (idleing) and cold clearance because the thermal expansion coefficients of the cast iron block/head and steel pushrods are virtually the same, so setting clearance cold is much easier. more accurate, and less messy, but you must pay attention to proper crank indexing to be sure each lifter is on the base circle as you adjust that particular valve clearance. Read the referenced paper, and be sure you get the 9/08 revision. Earlier versions on the Web are obsolete.

Duke

[Stuart F.]

Re: Manifold vacuum question C1.

Bruce, FWIW; I recently reset my valves using the Hinckley/Williams method cold @ .oo8"/.016". I first applied a timing tape on my damper and used a flex head drive and socket to bar the engine over (mine is a 63 L-76, 340HP). I try to set my idle at 750/800 RPM with initial timing @ 12 degrees (using the tab correction factor for a 63 w/8" damper). Mine pulls a steady 14" Hg under those conditions with my 3721SB AFB carb (my 3461S is unsteady at 13" Hg, as it is still a work in progress).
I have always used those settings so I can't relate as to what it might be with the valves looser, but I do recall that I did see less Hg using the old 1 and 6 cyl. position valve setting method.

Stu Fox

[Duke W.]

Re: Manifold vacuum question C1.

You can ONLY use the TDC number one and six positions to adjust PRELOAD ON HYDRAULIC LIFTER CAMS.

This method will not give proper clearance on ANY MECHANICAL LIFTER CAM because several of the lifters will be on the clearance ramps, which results in running lash looser than what you measured with a feeler gage.

The Duntov cam duration including the long constant velocity clearance ramps is about 450 crank degrees. The 30-30 lobe is about 540.

At TDC of any cylinder both the inlet and exhausst lobes of the 30-30 cam are still on the clearance ramps and actual clearance with be two to three thou LOOSER than you measure if you set both clearances of that cylinder at TDC. For the LT-1 cam the exhaust is on the clearance ramp. The inlet is not, but just barely. This is why the method calls for setting all the inlets at 90 ATDC and exhausts at 90 BTDC with the above two cams (or ANY mechanical lifter cam) as these positions ensure that the lifter is on the base circle.

For the Duntov cam it is optional to set each cylinder's valves at TDC of that cylinder because the total duration - including the clearance ramps - is such that both lifters are on the base circle at TDC of the compression stroke.

Duke

[Stuart F.]

Re: Manifold vacuum question C1.

Yes Duke, this I'm sure is all clear to us now, but there were a number of manuals back in the day that didn't differentiate between the hydraulic lifter engines and those with solid lifters. Many were fooled into trying to dry set their solids this way. Perhaps I should have made that more clear as there may be others out there still doing their adjustments wrong. Your post should help clear that up.

Stu Fox

[Bruce B.]

Re: Manifold vacuum question C1.

Thanks guys.

Bruce B

[Duke W.]

Re: Manifold vacuum question C1.

The latest version of the paper is on the Web site:

www.lbfun.com

The half dozen other sites that show up in a Google search (hinckley williams valve) are obsolete versions. I even contacted a few of them with a copy of the latest version attached to my email asking them to replace their obsolete version. I was TOTALLY IGNORED, and the obsolete versions persist.

...and you wonder why so many internet myths persist... Don't believe everything you read

Duke