The Stovebolt Site Forums The Shop Area Engine Shop timing set with a vacuum gauge

I have a vacuum gauge in the dash of the 58. The 235 idles at 20 but when you get it a shot of gas it drops to about 5 and comes right back up.
Can you tell anything by that?

Lindsay

Vacuum is always higher with the throttle closed. When you open the throttle reducing the restriction it drops, until rpms pull more air through the carb, then the opened throttle becomes restrictive again (to the bigger chunk of air being pulled in) and vacuum rises again.

this may help some...
http://www.classictruckshop.com/clubs/earlyburbs/members/roster.htm
http://www.classictruckshop.com/clubs/earlyburbs/projects/how_to.htm
& finally
http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htm

You can use a vacuum gauge to set ignition timing, but it is best used along with a tachometer. I have found the best results by advancing the timing until either the RPM or vacuum reaches a peak, then retard the timing to give about 1"Hg drop in vacuum. I have found that method gives a close approximation to manufacturer's specifications. Using a vacuum gauge alone, going for peak vacuum will usually leave you with timing that is advanced more than is desirable for your engine and operating conditions- engine load, altitude, ambient air temperature, etc. Others have their preferred method of setting timing using a vacuum gauge, but they also advise to listen for detonation/spark knock/pinging, and to retard timing until that no longer occurs. The best approach is to use a timing light.

The information on vacuum gauge readings contained in the link in the previous post is weak or incorrect on several points.
If you search the forum using "How much vacuum?", there are responses there to give you more insight on the topic. Make sure you use the quotation marks, otherwise the search produces more responses than you will care to read.

Harvester

Harvester,

I'm having trouble understanding why a vacuum gauge is best used along with a tachometer. Is the tach just used to set the initial RPM before using the vacuum gauge.

You say to advance the timing until EITHER the RPM or vacuum reaches a peak, then retard about 1"Hg drop in vacuum.

If it is EITHER and the vacuum gauge reaches the peak then why a tach too?

Also, I would prefer to use a timing light but I'm not sure if the flywheel/BB and or bell housing are correct for my engine.

Tom,

I have found that with different engines, vacuum and RPM don't reach their peaks with the same amount of advance. Sometimes one value will peak ahead of the other, then drop off if you continue to advance the timing as the other continues to increase. I haven't found a predictability to that occurance, but I found that advancing timing until reaching peak vacuum alone resulted in timing that predictably was well advanced from the mfr's specification, regardless of the manufacturer. I found the best compromise is to use the tach and vacuum gauge together, stopping further advance when one of the readings reached its peak, then retarding for the 1"Hg loss of vacuum. The "best compromise" setting was not necessarily right on spec, but typically it was pretty close- within a reasonable tolerance, maybe slightly toward a retarded setting toward an than an advanced setting. But with that, I would still use a timing light as the first choice, and use the tach/vacuum gauge when I couldn't use a timing light, as when the outer ring of a harmonic balancer has turned on the hub. All this was arrived at experimentally, checking quite a few engines with various methods and comparing with mfr's specs.

Hope this helps explain my method.

Harvester