When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
A while ago I bought a crate LS3 from GM that I put in my hotrod. It runs fine and makes a boatload of power.
I just added an Autometer vacuum gauge to the dash and it’s only reading 14” of vacuum at idle. Not sure if living at 5400’ elevation has anything to do with it.
It’s connected to the 3/8” port on the back of the intake manifold.
I have a few other vac gauges and they all read within 1/2” of the new Autometer gauge.
If I goose the gas the vac drops as expected, letting off the gas the vac quicky jumps up to 18” and then back down to 14”.
Anybody know what vac should read?
My bad, it’s an LS3 6.2 liter Gen 4 with rectangular port 821 heads.
The crankcase ventilation is set up just like GM specified, hose from the passenger side valve cover to a port between the MAF and the throttle body.
And the hose from the rear port of the driver side valve cover goes? My thinking is that if you did not have the intake manifold pulling vacuum on the crankcase then you would have an increase in vacuum which is needed when you start running larger camshaft profiles and using power brakes. Do away with the vacuum booster for the power brakes and you'll gain even more vacuum. 14" is below threshold when stock LS is anywhere from 15-22" but I wouldn't say that it's super low and you stated this is a stock LS3 so I don't see there being an issue. Are you having any issues with stopping the car?
The rear drivers side port came from GM with a cap on it.
The valley pan port connects to the front side of the intake manifold after the throttle body.
I don’t have a brake booster so that port is blocked off, came that way from GM.
I’ve got 4 wheel disc brakes with better pads, it stops fine. Just need more foot pressure than a car that has a booster.
Stock LS3 430HP one, the only change was I added BBK shorty headers and a catch can.
The fresh air port from the pass side valve cover is connected between the MAF and throttle body.
The valley pan port is connected to a catch can and then to intake just past the throttle body.
The left rear drivers side valve cover port is blocked off with a plug.
This is how I have it connected.
Last edited by JimLev; Aug 17, 2025 at 07:37 AM.
Reason: fix typos added pic
6 Common C5 Corvette Failures and What's Involved In Repairing Them
Slideshow: From wobbling harmonic balancers to failed EBCMs, these are the issues that define long-term C5 ownership and what repairs typically involve.
Retro Modern Bandit Pontiac Trans AM Comes With Burt Reynolds' Autograph
Slideshow: A modern Camaro transformed into a retro icon, this limited-run "Bandit" build blends nostalgia with brute force in a way few revivals manage.
Top 10 Greatest Cadillac V Series Performance Models Ever, Ranked
Slideshow: Cadillac didn't just crash the high-performance luxury vehicle party, it showed up loud, supercharged, and occasionally a little unhinged...
Coachbuilt N2A Anteros Is an LS2-Powered C6 Corvette In Italian Clothes
Slideshow: A one-off sports car that looks like a vintage Italian exotic-but hides a C6 Corvette underneath-just sold for the price of a new mid-engine Corvette.
Gen 3 LS3 intake vacuum? 
