Engine Vacuum - Team Camaro Tech
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post #1 of 3 (permalink) Old Apr 22nd, 02, 08:03 AM Thread Starter
Join Date: Aug 2001
Location: Lawrenceville, PA, USA
Posts: 13
What should the ported vacuum typical be at idle?

I have a gauge I put in the dash to replace my clock. The line is "T-ed" into the distributor vacuum can line that connects to the front of my Holley 650, dual feed with Vac. Secondaries.

The gauge reads about 7" at idle @1000rpms.
As I open the throttle the vacuum climbs to about 15".

Driving I have seen it as high as 18". Does this sound about right?

I am getting ready to recurve my HEI and wondering if I can use the vacuum can. I have read that with a radical cam you shouldn't use the vacuum advance because to idle the initial timing will already be set to about 18*. (I think I am at 14* now and it doesn't like to idle below 900rpms). Plus, if the vacuum rises off idle with a radical cam you wouldn't want it pulling more advance into the mechanical curve.

What signifies a radical cam?
If I unhook the vacuum can will I suffer gas milage or other performance problems?

Would switching to manifold vacuum be better? (I don't want to start a debate on ported vs manifold, just looking for suggestions.)

The car is mainly a street car, I will probably race it a few times once I get it broken in but more of a toy to cruise in and have fun. So I am looking for a curve and setup that will give performance but keep the motor safe from detonation.


'79 Z28, 4spd, 3.73 10 bolt, 350 4-bolt .030 over with stud kit, 10:1 Compression, Keith Black Flat Tops, Comp Cams Magnum 294 Solid Cam .525 lift, 63cc Heads, Stainless valves, hardened exhaust seats, guideplates, Summit Needle Bearing Roller Rockers 1.5 ratio, Offy Single Plane Intake, Holley 650 Vac. Secondaries, Stock HEI (for now), Headers 1 7/8" primaries, 3" Collectors into 2 1/2" exhaust with "H" pipe and Turbo Mufflers.
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post #2 of 3 (permalink) Old Apr 22nd, 02, 09:19 AM
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Location: Moraga,Ca.,USA
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I would recommend using a 'ported' vacuum set up for your street car. They tend to drive better and allow the timing to adjust to engine demand not throttle postion better.
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post #3 of 3 (permalink) Old Apr 22nd, 02, 09:52 AM
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Ported vacuum is above the throttle plates, usually on a Holley, its on the pass side of front metering block about midway on the side. Attaching a vacuum gage at this point will be 0 inches of vacuum at idle until throttle plates are opened, then there will be reading, closer to full manifold vacuum when cruising, under very light load, which is normal.

Your post reads you have connected it to the front of the carb, meaning you have placed it at a full vacuum point. Bigger than stock cam will show lower vacuum readings at idle. Then as you open up throttle plates, vacuum will rise to a value you have mentioned.

By the looks of the camshaft you are using, I probably wouldn't worry about vacuum can, just use the springs and weights to allow for a smooth gradual timing up to 2500-2700 rpm, then this will be maxed out, about 20 deg.

You also want to look at total timing, base (initial) plus vacuum plus mechanical. Add up all these numbers and you have total timing @3000 rpm. You must use either, a timing tape, degreed harmonic balancer, or a variable timing light. The very first you should do is to make 0 degrees on the H/balancer is 0 degrees on the pointer. If not, then remark the pointer scale of H/balancer.

Good luck, I'm sure others here will have some good suggestions, as given previously by others.

Having no vacuum can will not hurt anything, you're set-up to crunch some 5.0's I see...

Everett 68/350/PG/11.90/115mph
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