Idle Vacuum, How low is too low?

[tehzohan]

Freshly rebuilt 5.3, bored .020 over, BTR lifter and spring kit with a truck norris cam in a 2008 silverado. About 1500miles since rebuild. Live at about 5400 ft of elevation

I don't expect stock idle vacuum and quality, but it is only making about 10inhg of vacuum so the truck keeps throwing the "Service Brakes Soon" and has a stiff pedal. Idling at 750rpm and 22degrees of timing, I've played with everything between 10deg and 28deg without any noticeable change to vacuum. According to BTR and various places around the internet I should be making enough vacuum to run the stock brake booster. How much vacuum should I realistically be seeing with this cam? Any chance it's a tuning issue vs mechanical? If mechical, any ideas on where to look first?


The truck runs and drive pretty well, but hot idle in neutral, it also seems to have a slight miss or shake to it that comes and goes. It's like a chop, chop, chop, chop, cough, chop, chop. I don't believe it's in the tune since it's not super consistent but feel free to roast my tune if there is anything horribly wrong.

[Vbb1994]

On the motors with a truck Norris cam ive tuned/messed with, there was at least 14 in/hg , close to sea level where I am

I'd like to look at that tune later this evening

[Alvin]

The vacuum in this log looks perfectly normal.

[Vbb1994]

nevermind, i had the site on mobile mode

yes i see its idling at around 14 ?

[edcmat-l1]

Heck yeah that's a ton of vacuum at a mile high. You figure you're 20 Kpa off sea level.

[tehzohan]

I only have 10inhg of vacuum as measured by an OTC vacuum gauge off the intake. Do you know if the MAP sensor will read with an offset from the actual measured vacuum with an analog gauge? My Truck still throws the "service brakes soon" with a hard pedal after a few brake pumps. I've verified booster/check valve holds vacuum overnight and never had this error or hard brake pedal prior to motor rebuild/cam swap.

[gtstorey]

Bigger cam will lower the vacuum.

[edcmat-l1]

Try a different gauge. Your computer thinks it has more vacuum than that.