6psi out of an S-trim, why so low?

Alright, I just finished my set-up and had it chipped and dyno tuned. I am using the stock pulley set-up. On the intake side I have a 90mm LMAF and UPR boostmaster pipe, so there really should be no restriction there. My boost reference is tapped straight from the upper intake plenum. The belt is the tighetest it can go(maxed out the tensioner).

What could be the problem? This is a used kit, could the belt be stretched too much and possibly be slipping? Is the stock TB and spacer causing too much of a bottle neck? are the rpms just too low(5200rpm) to boost that high? something else?

I ran the part # on the blower and it is the 8-10psi S-trim, not the 5psi, so Im scratching my head here.

Any suggestions appreciated.

If you had restrictions you would see more boost. You have no motor information listed but less boost in psi does not mean less power. List information about you engine and your Dyno numbers. You do need a smaller belt but not sure it is slipping.

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Originally Posted by speedytang

If you had restrictions you would see more boost. You have no motor information listed but less boost in psi does not mean less power. List information about you engine and your Dyno numbers. You do need a smaller belt but not sure it is slipping.

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The motor is Stock, oil pan to TB. The heads have been milled so the comp. is slightly higher. The dyno #s are as follows:

284RWHP @ 5psi (367flhp)
365RWTQ @ 5psi (454fwtq)

All AFR's were between 11.5-12.2

Tune is Via an autologic chip.

Try out a shorter belt. You should have to spin the motor over to get the belt on.

Two things jump out at me:

1. Depending on how old that belt is it could very well be slipping but I don't think that's your problem. Also be careful just how much you tighten that belt....don't make it so tight that there is no play in it because it's murder on the front main bearing. If the tensioner is maxed and there's a good amount of slack in the belt (over 1.25") get a new belt. Check the belt tension for slack on the belt span between the blower belt tensioner pulley and crankshaft pulley.

2. The primary reason your boost numbers are so low is the engine RPM....5200 rpm isn't high enough....it takes about 6000 rpm to see 8-10 psi from an S-Trim on a stock motor. Also keep in mind that ambient air temp will affect final boost numbers.....if it's hot outside, the numbers can be lower by a pound or so.

with the stock motor (heads/cam/intake) and a powerpipe you should see at least 8psi from 5000rpm up using the 3.33" and 6.87" pulleys. A few things you want to check....

1. Is your motor high mileage and might possible have a lot of blowby due to worn rings??

2. Check your PCV setup. The stock PCV valve doesn't close off completely under boost and allows it to enter into the crankcase, thus loosing boost in the intake. Get a PCV valve for an '88 T-bird turbo.

Thanks for the opinions guys. I replaced the blet today and saw 7psi @ about 5400rpm. Were getting there!