FuzzKill

Background: I have a 2005 Colorado that I swapped a 5.3L from a Trailblazer into. Stock heads, stock cam, LS2 intake/tb. Soon after, I got the idea to make my own supercharger kit from another thread here on LS1tech. I used an M112 off of a Jaguar XKR. Anyway, I tuned it myself and had it running great for a stock 5.3L on about 7-8psi. Decided I wanted more power and got a 6.0L for basically free. This is where my problems start.

When I got the 6.0L I installed a 'small' blower cam as well. Mid 220's duration, .580 lift. Used the LS9 head gaskets, new comp 918 springs and hardware. Stock heads. New oil pump, timing set. With the blower installed, A/C off, the engine was pulling only about 12-13in hg. Would idle fine, but would stall on decel (automatic) or sometimes after a quick rev. I messed around with the tune (A LOT) and finally gave up realizing that the low vacuum was the cause of the symptoms. It drove ok under load, didn't seem to have as much power as I was expecting, but I also didn't have a chance to properly tune it either since I could never nail down the idle.

With the A/C on, it dropped to about 10in hg vacuum. With the 5.3L, it would pull about 18-19in hg A/C off and 16-17in hg A/C on. I know with the (slightly) larger cam it will pull less vacuum but something else is not right. I removed the blower, and left the belt off. With no blower, no accessories, and the booster capped the most vacuum I can pull is 15in hg.

I've checked the MAP sensor values vs. the boost/vac gauge, and used a second vacuum gauge. They all agree on the numbers.

I've removed the intake, checked all gaskets, removed TB, checked all hoses. I pressure tested all of my charge piping (since it is under vac as well) and then completely bypassed it all by running the TB right off of the intake. I pulled the timing cover to confirm cam/crank timing. It was correct.

Finally tonight I just did a compression test. All cylinders were 150psi+/- 5psi after 3 bumps. Plugs looked fine. I'm at a loss. Only thing I can think of is that the cam is ground off and needs to be degreed in? Either that or there is a massive crack in my intake. No hissing sound from a vac. leak though. PCV is a fixed orifice type, no moving parts to get stuck.

Anything I've overlooked? I'm almost tempted to swap cams since I still have my 5.3L sitting on the stand and don't have a degree wheel sitting around. I've never degreed in a cam before either. That would rule out the cam being the issue but I almost feel like I am grasping at straws here. Not sure what else to check.

Any ideas?

TL;DR
New engine/very mild cam can only pull 15in hg with no accessories, no booster, no blower. 12-13in hg with blower.

Old engine, stock cam would pull 18-19in hg with blower.

Slo_240sx

iTrader: (17)

Did you properly check pushrod length? Too long of a push rod will not allow the valve to close all the way and make vacuum very low. Usually when you wipe a lobe of the cam the vacuum gauge will flutter/flicker, indicating you lost a cylinder.

LLLosingit

iTrader: (4)

Where are you checking your vacuum source? Manifold?

LLLosingit

iTrader: (4)

I was looking at the Eatons build diagram and they show an actuator on the side to control boost at idle, it says that valve is open at idle to bypass boost then closed when under throttle to bring boost up. Is that working correctly? If it's not it may be trying to feed boost while at idle dropping vacuum in the manifold.

FuzzKill

Vacuum gauge is steady, all cylinders were equal on the compression check.

I didn't check pushrod length, but if the valves were hanging open wouldn't that show on the compression check too? Or should I do a leakdown test for that?

I have a bypass valve setup for the blower, but for all the troubleshooting none of the blower stuff is even on the motor currently (including the blower). Wanted as few variables as possible.

99z28383

iTrader: (1)

How much timing advance are you running at idle?

FuzzKill

I don't know off the top of my head, I started out with my custom tune from the 5.3L and adjusted for the injectors and displacement increase. Ended up wiping my tune and starting over from stock numbers from a stock GTO file. I'll check my tune and scans when I get back home.

99z28383

iTrader: (1)

Gotcha, depending on how much overlap that cam has - 15Hg may not be too far off from what you can get out of it. Bigger cams tend to like more timing at idle, try bumping it up.

FuzzKill

That 15in hg is with no w/p, no alt, no blower, no p/s, no booster, no a/c ... nothing.

If I could pull 15in hg with the blower back on I'd be happy judging from what I got with the old setup.

99z28383

iTrader: (1)

Yeah I understand, that's why I said it's not too far off from what you can get out of it - possibly bumping the timing may get you there.