I have a 91Z with a paxton sn2000 mod impeller. I was getting 8.5psi and pretty good power. I purchased the VR4 impeller kit and was told I would be getting up towards 12 and possibly more boost with this new impeller. Well, I'm only gettin ten psi due to belt slippage, I can see the boost climb up to 5-6 fast, then, it struggles the higher the boost becomes closer to ten. The new impeller pushes 1400 or more cfm in cotrast with the old impeller @ 850. I can tell some difference in the power, but my car just looses power above 4800rpms. With the old impeller, my car easily climbs to 6K. At first, my car would fall flat on it's face @ 5K with the new impeller. I pulled my plugs and gaped them from .035 to .028. My car then seemed to run fine for a coupple runs. But now, the car starts to struggle @ 4800rpms, it does not fall flat on it's face, but it is really fighting to get power above 4800 rpms with the new VR4 impeller. It's lilke it wants to go beyond the 4800rpm's but it's surging and struggling to get past that rpm? With the old impeller, it would easily climb to 6K.

mods include; ported intake base and plentum with slp runners, AFR 190 heads, Crane hi6s ignition, tes headers, 3in flowmaster cat back. Also, Ford SVO 24lb injectors, vortech FMU, and LPE 213 cam.

My thoughts are to change my fmu ratio from an 8/1 to a 4/1. I"m not getting any, or very very few kock counts(i've set my timingin the chip to 20 @ WOT from 3800 and above).

Any other thoughts greatly appriciated

 

Just a bump. I'm interested in any ideas someone comes up with too!

 

do you have any way of checking duty cycle on injectors or access to a wide band o2 to see top end air/fuel ratio?

I'm thinking 24# inj not enough. I would have thought at least 24, if not higher, for your set up naturally aspirated- much less on 10psi +.

 

First off, thank you guys for the replies

After some thinking and experimenting, here's what I found.

I took her out for a spin and here's whut's up. I ran the 4-1fmu and no change in how the car ran. I put the 12-1 fmu disk in, added more fuel to the chip in the upper rpm range. The car ran better. It still popped out the exhaust, but ran better than when the car had the 4-1 fmu disk in. I have ford svo 24lb injectors and it's time to upgrade to the 30svo's or even 36lbs. I have added as much fuel as the tables will allow me to do with the 24's and I'm beyond their range i'm sure of that.

I just purchased a set of 30lb ford svo injectors and I'll keep you posted on my results.