So same belt brand/type used, same belt length ? Tensioner feels good and strong and when installed tensioner is under max load so it has plenty of travel for when the belt stretches under load ?

It should be difficult to get a new belt on

 

 

So you havent fitted a new belt ? Why not ?

That would be a first thing to try.

 

 

This is a real long shot, but you have checked almost all possibilities. Please bear with me. I know it's a stretch.

The lower bottom pulley is very small. As a result it has a smaller diameter because the pulley is smaller. The smaller a pulley the faster it spins which causes more heat and shortens bearing life. The bearings in this pulley are 6904’s. The 6904 is also not as wide compared to the 6203 below which is 12mm. These two small metal shielded bearing spin very close to their max rpm threshold. They are metal shielded too and don't keep out moisture as well as nylon shielded.

The common size bearing in most GM idler pulleys as well as the idler pulley on the D1 SC J bracket/Aster are 6203 –style with a 17mm standard ID. The 6203 series is a common off the shelf bearing with plastic shields (if spec'd so). Measured width at the ID is 12mm - wider than the 6904 bearings. The narrower 6904's means less surface area - more heat.




So what you say?

If any of the bearings especially in the the lowest pulley is galled and not spinning freely, the belt might be slipping over it, heating up the belt and immediately causing belt stretch after a run or two. If the green on the belt is shiny or starting to wear off quickly that would be a tell tale sign as well.

I replaced this pulley with a 50 mm CTSV Metco pulley (the one on the left) that has the larger 6203 bearings because I didn't want to have to keep checking those little bearings, or what might happen if/when they fail at 6K.

The Gates RPM product line (aramid layers) doesn't stretch if any compared to the HD green series. However, it is hard to find in stock and very expensive. It's something you might switch over too, when you get your 'losing boost' issue resolved. Its part number would be Gates K080530RPM to match your Gates K080530HD.

 

Belt dust on the blower and around the blower, is an indication the belt is slipping.

I had a customer down in Mexico City, Mexico who had a similar situation, I had him checking everything out and it turned out his LS6 intake manifold had internal cracks in it. Once he replaced his intake manifold he had the boost he was looking for. Bob

 

I agree DuWayne, A stock LS1 engine with headers and exhaust will normally make around 8 psi with a 7.65 crank/4.50 blower pulley, unless the elevation is really high. Bob

 

 

Or pressure test the entire intake system as I described.

 

After everything you've done, that's the next place I would look. Bob

 

 

What was the result of the pressure test ?

 

 

So the intake did seal fine during the test ?

It must be belt slip then

 

I don't see where you provided info with respect to fwhp and rwhp when you were making 15 lbs of boost. Also what size fuel pump and injectors.

Here is an example (not my writing):

"theres SO many variables... but i understand what u mean.. with my 416 ls3 i plan on runnin 16lbs or so to around double my hp.. but every turbo is very different as well. but typically that boost will double your hp give or take id say up to 100hp.. also on my bone stock ls1 made 290 at the wheels theu an auto.. then made 565wheel with everything same except for a t7575 on 12psi. so i imagine around 15 lbs i should make right around 600 which is double the hp."

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My car:

My 98 A4 stock Z28 with Borla Catback made 301 rwhp on a (DynoJet).

With an LS6 manifold, 224/228 cam, cheap brand headers, and typical not so good ported heads back in the day (2002), same car made just south of 495 rwhp at EPP with D1SC at about 6-7 lbs of boost on Mustang dyno. Pulley was a 4.5. Brent was the tuner.

With all same mods and upgrade to American Racing headers, FMIC, and a 4.38 pulley same car made 565 rwhp on Mustang dyno on a very safe tune. Tuner refused to try to get more since still stock short block.

Now with all of the mods I have made specifically to handle 15 lbs of boost, even with a 3.9 pulley I don't think I will make 15 lbs of boost. Will post later after breakin when we dyno again.

So am curious how much hp you were making at 15 lbs of boost? Maybe that number, although still many variables, will tell just how much actual boost you were making. No need to chase after boost that's probably not there with your current setup. I am not trying to be offensive, but there are over a hundred owners in the stickies with only a handful at 15 lbs of boost.

 

 

 

My aster never slipped with either the d1sc or the f1a, spinning both to max rpm. For the F1a I went ahead and pre-emptively got the CBR 8" pulley to keep a larger pulley on the blower while the engine was exposed, always an option if you think slip is the culprit. Also make sure and try a few runs with the filter/elbow off, those can choke your boost down too.