Could somebody give me a rough estimate as to what the vacuum on a small block chev should be on idle with a?

Answer 1

compression and cam size plays a major role in vac. readings.

Most stock SBC v8's wil have between 17 and 23" HG vac.

as compression goes up the vac reading will go up, as the cam size goes up the vac readings will go down.

My 10.18:1 355 with a 218*/230* @.050 (272/284* adv.) cam has 16-18" HG vac at 700 RPM idle.

Now as the one person said.. Your timing and carb idle mixture adjustment will change the vac readings.

To tune the carb, you use a vac gauge and turn the idle mixture screws 1 at a time till you get the highest vac reading you can.

Timing, well I DO NOT belive in setting timing with a vac gauge. Even with a large cam or over camm'ed engine if it won't idle at norm high performance timing specs then you have other probs to deal with.

Setting the timing based on vac is a major risk in a real high performance engine.

My engine has 212-218 psi cyl pressure, its on the ragged edge of detonation with pump 93 octane fuel.

I'm limited to 34* total timing before I get detonation. At 35* total, I have a slight bit of ping, at 36* total I detonat pretty bad.

If I was to set my total timing by a vac gauge, it would be around 38* and I would have a pinning monster on my hands that wouldnt last too long.

Set your timing with a timing light.. I use a digital advance timing light with a built in digital tach.

Answer 2

My stock '70 300hp-350ci always showed 19.5 inches of vacuum when in park and idling 800rpm with no a.c. or any lights on. That Monte carlo would get 22 mpg on the highway in steady road conditions during the day with 5 people aboard.

Answer 3

With a standard camshaft with minimal lobe overlap and lift you should see 12- 14 inches of mercury below the throttle plate or throttle body at idle 550-650 RPM.

The more radical - (lobe separation, duration and rate of lift) the camshaft, the less inches of mercury in the intake manifold at idle. For instance: 112 lobe separation 222 degrees duration on intake and exhaust and .447 lift you'll see 8 - 9 inches of mercury. This camshaft was standard Chevrolet OEM in the L-79 350 HP. 327 cubic inch motor.

Answer 4

Uh, I use to have to setthe timing on my 73 Nova(w/ a big radical cammed 350ci/4spd) using a vaccum guage and the best I could get was like 17" of vac. advancin the timing til it peaked. Then, I'd back it down an inch and that's where I'd set it.

I got this procudure off of one of the dirt track racin in the area.

Good luck!

Answer 5

I believe Scott. that's a question with actually hundreds of solutions. human beings could even say get a crate for that variety of money. you will not detect a crate 383 with 500 HP for decrease than $8000. My hat is off to you considering that's accomplished.

Answer 6

15 to 17 at idle adjusting carb and timing can affect it alot.

Answer 7

i belive it to be 12 to 15