Vacuum at idle with cams??

[can04cobra]

Just curious what everyone running "bigger" or custom cams is seeing for vacuum at idle?

I've been searching and haven't found much info on this. My car is seeing maybe 8in of vacuum. which seems super low, I expected it was a vacuum leak somewhere I've taken the blower and lower off at least 6 times now trying to figure this out. Everytime I used new gaskets and am 99% sure I dont have a leak. I also tried spraying all the vacuum lines with carb cleaner no change in idle. I tried using a vac tester and everything held vacuum solid except for my fuel pressure regulator which was wierd! tried plugging that vacuum line and no change in vacuum. Any help or insight would be appreciated.

Info on the car
2.9 whipple
return fuel system
80lb injectors
stock maf with mafia
mmr 4.75l
stage 3 ported heads
livernois custom cams

 

[can04cobra]

little up date on the top vac port of the supercharger I am seeing 8in vac at idle and on the lower port boost/vac I am seeing 3in vac at idle

 

[Bmeagher]

I'm missing the supercharger, but at idle i have 15-17 inHg

 

[brentsSVT]

I would think 8-12inches should be about right? Cams+heads will have a bigger effect on vacuum than cams alone.

 

[can04cobra]

I would think 8-12inches should be about right? Cams+heads will have a bigger effect on vacuum than cams alone.

I can see it being lower but doesnt it seem super low? I ran a smoke test and only place I had smoke was coming out the exhaust.

 

[Brutal Metal]

seems low to me I'm at 21-22 with stock cams

 

[can04cobra]

ok so i got tired of thinking about it and decided to take the 3.50 pulley off and put on the 3.10 to get ready for tune day on tuesday. Get it all on and figure i"ll start the car to see if it changed vacuum and it did I lost 3psi!!!! what could be going on here boost bypass issue maybe? any feedback would be greatly appreciated I'm about to pull out my hair!!
Thanks in advance

 

[IUP99snake]

Are you sure you don't have a vacuum leak with the PCV valves or something? That's typically what you see when you have a vacuum leak. And with aftermarket cams, it's even harder to notice you have a vacuum leak because a leak makes the car lope even with stock cams.

So, your car might actually be loping more than it should be even with the aftermarket cams.

There should be 20 in/hg of vacuum at idle with stock cams.

With aftermarket cams with the idle set to around 1000 RPM, it should still pull at least 10 inches of vacuum. Anything less than that, and I'd start looking for leaks somewhere.

I hate vacuum leaks by the way.

Homer

 

[can04cobra]

Are you sure you don't have a vacuum leak with the PCV valves or something? That's typically what you see when you have a vacuum leak. And with aftermarket cams, it's even harder to notice you have a vacuum leak because a leak makes the car lope even with stock cams.

So, your car might actually be loping more than it should be even with the aftermarket cams.

There should be 20 in/hg of vacuum at idle with stock cams.

With aftermarket cams with the idle set to around 1000 RPM, it should still pull at least 10 inches of vacuum. Anything less than that, and I'd start looking for leaks somewhere.

I hate vacuum leaks by the way.

Homer

I hate them too!! car is idling at 750rpm but the idle is not hunting like a vac leak if I unplug any vac line the idle starts hunting/surging so I'm 99% positive there is no vac leak

 

[can04cobra]

ok another update vac is 8-9in on pcv system, boost bypass, and top vac port the only place im getting like 2in vac is on lower boost port. I know most vehicles run the bypass off the same port but on the 2.9 there is seperate ones. I'm thinking about blocking the one port and t-ing into the one the bypass is hooked up to. Would this be a bad idea??

 

[fbody83]

What are you using to measure your vacuum? I have custom cams and was chasing a similar "problem" but it turned out that my aftermarket boost gauge was leaking and the line to the boost gauge was reading lower than the lines under the hood. I determined this by hooking a vacuum pump up to the lines under the hood with the car running and see 15 in/hg at idle. Right then I knew something was going on with the gauge since it was reading 10 in/hg at idle.

 

[IUP99snake]

What are you using to measure your vacuum? I have custom cams and was chasing a similar "problem" but it turned out that my aftermarket boost gauge was leaking and the line to the boost gauge was reading lower than the lines under the hood. I determined this by hooking a vacuum pump up to the lines under the hood with the car running and see 15 in/hg at idle. Right then I knew something was going on with the gauge since it was reading 10 in/hg at idle.

Great you pointed that out. A simple hand vacuum pump/gauge is a great diagnostic tool. Not only can you use it like you did as a secondary vacuum gauge to connect to different vacuum ports and lines on the engine to see how much vacuum you are getting throughout the system, but you can also hook it up the opposite way and pull a vacuum through the accessories to see if those accessories (or the lines going to them) will hold the vacuum. I used it to diagnose a blowoff valve that had bad O rings and was leaking vacuum, plus a whole bunch of other things.

I originally bought it to speed bleed my brakes. Turns out it was worthless for that, but has more than paid for itself to do vacuum diagnostics.

 

[can04cobra]

What are you using to measure your vacuum? I have custom cams and was chasing a similar "problem" but it turned out that my aftermarket boost gauge was leaking and the line to the boost gauge was reading lower than the lines under the hood. I determined this by hooking a vacuum pump up to the lines under the hood with the car running and see 15 in/hg at idle. Right then I knew something was going on with the gauge since it was reading 10 in/hg at idle.

I am using a vacuum pump to read vacuum and have used it to pull vac against the acc. to make sure they are holding vacuum and they all are.

What RPM does your car idle at??

 

[fbody83]

I am using a vacuum pump to read vacuum and have used it to pull vac against the acc. to make sure they are holding vacuum and they all are.

What RPM does your car idle at??

900 - 950

Hows the car drive? Any signs of a vacuum leak while cruising?

 

[RacerXHG]

If you see a difference in vacuum between the lower plenum and the upper intake part of the sc, I would think your bypass valve is stuck shut when it should be open at vacuum.