why is my boost/response so weak?

[hypntyz]

I have a n2k kit for the 96-00 GM 5.7 on my denali. The blower oil seals were bad so I rebuilt it and everything is fine again. But the response with this thing is horrible...worse than any LARGE turbo I've ever dealt with. Here's some info about my setup.

8" crank pulley
3.75" blower pulley
10 rib (it is not slipping that I can tell)
no boost leaks (I pressurized the intake using shop air and a silicone coupler to be sure)
MSD 6a, msd coil, msd 8.5mm wires, msd BTM
2 additional injectors and HKS AIC to control them
plx wideband o2/display

At 3/4 throttle to WOT,

I see 1.5-2 psi at 2000rpm
I see 2.5-3 psi at 3000rpm
I see 5-6 psi at 4000rpm
I see 6-8psi at 4500-5000rpm which is the shift point

Even if I drop down to a 2.75 pulley, my impellor speed at 3500rpm will be about the same as it is now at 4500rpm, which means my boost probably wont be any more than 7-8psi then. I'd like to be seeing at least 5 by 2000rpm and 10 or so from 3000 up.

Granted, I dont have my fuel and AFR worked out fully yet (every time I dump in fuel with the AIC, the stock computer pulls fuel out of it's main injectors and leans it back out), but to my understanding a blower's ability to produce boost shouldnt have anything to do with that.

If the damn thing won't do any more than this, I'm not keeping it...it's not drastically faster with the blower than it was bone stock, maybe a couple of tenths in the 1/4.

[Paxtu22]

You need to pulley it up. The blower does not even start coming on until like 30,000 impeller speed. Try a 3.0 or an 8" crank and see what that gets you.
However remember that you are moving alot of truck with that blower. What is your rear gear ratio?

Eric

[hypntyz]

The only place I can find a pulley is superchargersonline.com and they list the novi 2000 pullies for mustang applications only. I know the head unit is the same, but I wonder if the offset/spacing is different for different applications? I have posted in their forum (as they asked) and emailed them without response. I'm sort of afraid to give them any money, with that sort of customer service.

I already have an 8" crank pulley. And a 3.75 blower pulley.

IF you're saying the blower is efficient between 30k and 50k impeller speed (the 50k number came from another thread on here), then here are the numbers. I currently see 30k impeller speed at around 4000rpm, the same time I see around 5psi boost. Even if I change the pulley and get that response sooner at say 2800rpm, I am still only seeing 5-6psi at that impeller speed. True, I might see 8-10psi near redline, but the bottom end response is still nowhere near what I would have expected out of a blower setup (being used to turbos and lag).

I have a t-78 (known to be slow to spool) on my rotary (known to have weak low end) and I still see more boost by 4000rpm with that setup than I do with this blower on the v8. OF course with the rotary I have about another 4500rpm to play with after boost comes on...with the v8 I have maybe 1000rpm.

[Hustler]

a smaller pulley to get you to like 10psi and a good tune would work wonders in your situation. I mean thats like running your turbo half assed and not having the car tuned and running like *** and then saying the turbo sucks. Not trying to be an ***, just pointing out the obvious.

[hypntyz]

With some more playing around I was able to stabilize and tune my AFR's. I had to unplug the front 2 o2 sensors to prevent them from correcting my additional fuel injection. After a few days of street tuning, I had solid 11.8-12.3 AFR's under boost. The boost itself did not change, though the truck did make a bit more power.

I called and talked to aaron at paxton about the situation with low boost. My existing pulley was a 3.75" with 8" crank. I told him my boost pattern and he agreed that it was pretty much what that system was designed for, and that not many centrifugal blowers would perform a lot better in the midrange.

We discussed impeller speed and pulley diameter and even with a 2.75" pulley we agreed that I would only get a couple, maybe 3 more psi boost in the midrange on a good day. Since this is a stock truck engine, and redline is relatively low (5000) it appeared that this blower would never make more than 9-10psi boost, and all of that would come after 4000rpm.

Also their pullies are $106 and he could not guarantee that it would actually fit my head unit, since the pulley they sell is ONLY made for the foxbody version of n2k head unit. We wondered if the offset of the pulley might be a little different. HE also told me that for sure I'd need a new, shorter belt...and in a 10 rib, that wouldnt be cheap or easy to find.

All things considered, I had 2 grand (used SC kit, ignition setup, AIC setup) and I was making maybe 50 more hp than stock, all the while putting more stress on the engine and suffering a rough idle. I decided it was not worth fooling with, removed the kit and sold it. I still need to sell the AIC and ignition as well.

I can easily get a wait4me hot tune and pick up 35hp for 200 bucks with no sacrifice in reliability or idle, and that'll give me enough extra grunt when pulling a trailer to make me happy.

Anyway, thanks for your help.