According to "Wikipedia":

Why? Manifold vacuum should be at zero at WOT. I dyno'd my car yesterday and I was at 0.2inHG @ 4000 RPM and 0.6 inHG @ 6000 RPM. So, my assumption was that the shorty headers could be causing the restriction at higher RPMS.

Why would they state

 

Because they're talking about carbed stuff and the signal gets funky when that happens?

 

Right. You need a depression for the carb to work. With fuel injection atmospheric pressure would be the best at wot for a n/a car.

 

And its normal for the vacuum to increase with the square of the air flow... if you increase air flow by 50% between 4000 -> 6000rpm (1.5X), your pressure loss (vacuum) would increase by a factor of (1.5)^2 = 2.25X. That would account for a large portion of the increase from 0.20"Hg to 0.60"Hg.

How did you measure vacuum? A U-tube manometer or the PCM's BAR minus MAP readings would probably provide the most accurate results. A mechanical vacuum gauge would be the least accurate.

 

FWIW, I don't use Wikipedia as the source authoritly for anything.

 

Thanks for the great responses. We unplugged the vacuum supply to the HVAC and used that port. The Vacuum tool was part of the Dyna Tune machine (it was a rectangular black box) which then converted it to a digital signal. We then data logged the signal and overlayed the results along with AFR, HP, & lbs/ft.

The digitized reading was very detailed, not a lazy line.

So the increase from 0.2 inHG to 0.6 inHG is normal and not indicative of a restrictive muffler (exhaust).

This was on a 1995 Z28.

 

an exhaust restriction would if anything reduce an engine's air demands not cause it to build vacuum at WOT.

The vacuum you are seeing is well within normal. I see more like 2" at WOT on mine.

 

I'm pretty sure that if the vacuum builds in the intake manifold during WOT that there is an exhaust restriction. For example, if a Catalytic Converter is clogged (ignition misfire, etc) that the vacuum will read close to zero at WOT and will build dramatically under load during WOT. Since the exhaust can't poo the fumes, the intake cannot take as much out of the manifold which would leave more vacuum in the manifold.

Nonetheless, if you are seeing 2 inHG at WOT then I feel in the safe zone.

 

I dunno... I'm a BioChem major and find that even some of the most cutting edge bio-chemical topics are represented very well in Wikipedia. Just make sure you only take referenced items seriously if you have any doubts.

Same goes with my Neuroscience classes... very accurate from what I've read.

Maybe it's just the "more advanced" topics that get that much scrutiny (based on who would even know to contribute such info). More general topics considered "common knowledge" are less reliable IMO... then its about on par with google

 

vacuum is caused by a restriction in the intake side
example
when at idle your throttle blades are closed closing off the intake from the atmosphere whilst the engine is still attempting to suck the air in there by creating a vacuum because of the throttle blades being closed which is your restriction.
At WOT the blades are fully open which will ideally feed the engine all the air it needs plus some if there is still vacuum there would still be some sort of restriction in the intake track.
anything that reduces the engines need for air will also reduce the chance of having a vacuum at WOT
i could be stupid though but im not usually

 

Dude, lay off the Wiki!

 

That vacumm is in the "normal" range. What is your intake setup and what are your mods?

 

I have the following mods:

95 Z28/1LE - T56
Induction - SLP CAI, K&N dual air filters, 58mm Edelbrock TB, TB Bypass, 1.6 RRs Pro Magnum, Manley Pushrods
Exhaust - Edelbrock Ceramic Coated TES Headers, Off Road Pipe, Ex. Insert Cone, Edelbrock Cat Back
Ignition - Jacob's Ultra Pack Ignition, Coil, Trigger, 8.5mm Plug Wires, NGK TR6 Iridium Plugs
Drivetrain - Hurst Short Throw Shifter, SLP Alum Driveshaft, LG Torque Arm, GM 4.10s with Eaton Posi, SLP Lightened Flywheel and Centerforce Clutch Package
Cosmetic - RK Sport Back, ported heat extractors
Suspension - Hotchkis Front Springs, Welded Sub-Frame Connectors, Hotchkis Lower Control Arms, Hotchkis Strut Tower Brace, RK Sport LCA relocation brackets, Rear weight jacks by Ground-Control with Eibach 175# springs, and Spohn Adjustable Panhard Rod, Bump Steer Tie Rod Ends by Baer, 285/50R17 Goodyear D1-GS D3s, custom powder coated 17" wheels
290 RWHP/ 324 RWTQ - (July 2002) Before most of the mods
365 RWHP/ 361 RWTQ - (March 2008) Superior Automotive Ported Heads & Comp Cams 446 XFI added

 

That setup looks fine. Pull your filter and compare if there is a difference.

The headers you have aren't great for power, but probably aren't causing any vacumm issues.

 

Unfortunately, I'm off the dyno so I can't run another test.