Dealing with a boost drop from on the higher rpms. I have a gen 3 5.3, with a mild cam and 1 7/8" long tubes in a silverado running a p1sc procharger 2.5" pulley with air to air intercooler. I'm seeing roughly 6-7psi at 4200 rpm depending on ambient temp/density, however drops to around 2psi at 4300 and stays consistent up to 5500 rpm. I have good tension on the belt, no signs of belt slip at this time. I am running a 6 rib setup, but I don't believe I need to go larger for this particular setup. There is a abrupt drop in pressure after 4300 rpm. I am seeing the the boost drop with a data log not using a boost gauge. I've double checked all couplers and clamps. I have to admit I am using a cheap china blow off valve, which had got stuck open on an occasion or two. The cheap bov was sold advertised as an 11psi spring, but who knows. Should I disconnect the reference source to the bov and gamble that nothing catastrophic will ensue? Any input is appreciated!

 

2.5"?! With a stock lower getting you 7lbs?

Something is funky. That's a tiny pulley and no boost. With a 3.5 in a 5.3 I could get 12lbs at 6500 p1sc

issues to look at are belt slip, impeller slip, bov venting and/or loose couplers

 

I'm a complete idiot on the 2.5" pulley. Its a 3.2". I feel stupid(er) now. That being said I'm going to build a testing rig to check couplers. I had the 3.7" pulley on and had the same issue on the boost falling off abruptly with a different (wasn't as tight belt wise with the 3.7") belt and tensioner combo. I've made sure the clamps on the couplers are tight multiple times. I know I need to upgrade my blow off valve, however I'd like to rule out any underlying issues so I'm not disappointed that a new part didn't fix my issue. At this point I'm looking for some sort of diagnostic flow chart.

 

There isn't one. You just go by process of elim and traditional diag methods.

Other things to check are charger bearing health (spin it by hand, feel for grittiness or notchiness in the bearings)

check the oil level

check idlers for a seized or worn out bearing

.5"-1.0" shorter belt

impeller bolt; check it and tighten as necessary. Impellers are press fit and your shaft may be spinning in the impeller under load. Gotta replace the impeller if that's the case. Look for fretting/galling under the cone washer after removing the center bolt

things like that are good places to check

 

My money is on the BOV. It's cracking at 6psi as the spring in it is probably not strong enough to hold. If it was belt slip or something mechanical to stay static at 2psi all the way up to RPM limit suggests it's just venting.

 

I have the same thought. Basically acting as a wastegate with too light of a spring at that point.

 

if it has a ref (it should) it will get pressure to keep itself closed as boost climbs

 

Is your crank pinned?

 

The bold statement caught my attention. Is there a noticeable power loss associated with the boost drop? Are you seeing an airmass change thru the MAF?

EDIT... I may have misread the statement. I read it as you are seeing the boost drop on teh log but not on the boost gauge however you may mean the log is the only reference you have?

 

Actually a solid point, damn cant believe I missed that