i have a vacuum question?

[13lazer]

i have read all i could about determining how well a motor runs based on measuring manifold vacuum. i am running a vacuum gauge in my dash and it never goes above 14psi and drops as low as 0psi during acceleration. however, it is a well tuned and maintained motor that nobody but me has touched since it was built. its a mild performance built stroker with 264/270 split duration cam and a set of kick ass world products .194/.150 heads. everything i've read says vacuum should read between 17psi and 20psi all the time. am i missing something or is this normal for a streetable performance motor? it seems like my vacuum advance would be affected but i haven't noticed anything, nor do i have any brake booster problems. i normally drive at sea level if that matters? according to what i've read about diagnosing the condition of my engine with a vacuum gauge; my valves are worn or i need a bigger carb. i really don't think i have either prob. my carb is a 670 street avenger and i'm not burning much oil if any? i know there are some experts on this subject around here, i'd appreciate some opinions.

[JMac11]

Not to worry.. you are probably OK.

First- Confirm your factory guage is 100% right on, if you want. Steady needle a little low by your 30 year old guage, nothing to worry about. It should climb higher as your engine revs, mirroring your tach, on gradual acceleration. I am thinking stuck needle, because if your engine never pulled over 14 inches, your vaccum idle, it would never rev upbeyobd an idle.

B- Dropping to zero then climbing back up is normal. That is why you have power valves and pump squirters on your carb. They "cover" the lack of vacuum during a quick wide open throttle event.

III.- Split duration cam and a 670 might be hurting the vacuum a tad. SOunds like you have plenty of vacuum, as your booster works properly.


Tuning by vacuum is more of what the needle does, than the initial reading. I ran one in a drag car, as my hearing is whacked in the " loose valve" range. A well placed and accurate VAC guage is better than a tach, and can tell you when you need to break out the tool box.

[makovai]

I have a 290 horse crate with lots of goodies for bout 330h.p. My vacuum is all over the place from idle to acceleration to deceleration.

Good thing i dont need to brake when i accelerate cause i have no vacuum at all.

[13lazer]

First- Confirm your factory guage is 100% right on, if you want. Steady needle a little low by your 30 year old guage, nothing to worry about. It should climb higher as your engine revs, mirroring your tach, on gradual acceleration. I am thinking stuck needle, because if your engine never pulled over 14 inches, your vaccum idle, it would never rev upbeyobd an idle.

jmac11
stuck needle might very well be my prob. on this dash my tach needle was stuck for a long time, i actually bought one of the $450 clusters at the swap meet. all of the gauges are oem, but im not sure nos or not. where should i pull vacuum for the dash gauge? right now im using the threaded port in the manifold behind the carb, i use this spot for my brake booster too. does it matter? i have a few other spots i can pull from.

[JMac11]

That spot is as good as any, but not sure where they were originally hooked up.

[jhow66]

You will not have the vacumn (17 to 20) with the cam you have . 14 at idle is good for this cam. Vac will always drop to 0 with wide open throttle. The more duration that you have on the cam the less vacumn it will pull so disregard the amount on the gauge at idle for tuning purposes. Just tune to pull the highest you can.