Dynoed my LT-4 - got 350 HP (flywheel) and a problem - mixture is leaning out as from 4500 rpm's and at 6k rpm it is dangerously lean. Fuel pressure showed 41PSI all the way up to redline so guess no problem there? Anyone can suggest how to find a problem? Car has stock injectors and pump - can they keep enough flow to ensure 350 hp?

 

Yes, the injectors should be fine. They can support up to about 350 rwhp. Have you tuned the PCM. Sounds like a computer problem. If you have an editor program, you should try to increase the numbers in the PEvsRPM table.

 

Yes, computer tuned by Fastchips. Is pressure 40-41PSI ok? Have heard it should be around 46-47PSI but can't find any reliable info source?

 

Stock fuel injector flow rating is at 43.5psi. Factory specs on fuel pressure (no vacuum compensation) is 41-47psi. Remember that your rail pressure is reduced by intake manifold vacuum, which even at WOT could account for a 1psi drop in rail presseure.

 

OK then, how I can find out what causes the leaning out?

 

Who did the tune for the cam?

 

I bought car as it is now - the previous owner did all below:

LT4 heads and intake
GMPP Hot Cam kit
BBK 58 mm TB
K&N Cold Air Intake
LT4 heavy duty timing set
Fastchips programming
Accel Extreme 9000 plug wires
Hypertech 160 F PowerStat
LT4 knock module
1LE air intake bellow
!EGR, !AIR, !TB

I scanned the computer for errors just now - nothing. Also I can feel that on 4.5-5k rpm (I went on track for for roadracing a bit) engine starts to stumble a bit - like it doesnt have fuel enough. But again - fuel pressure is steady all the way to readline.

Any suggestions how to find the fault?

 

What do the O2 sensor voltages look like at WOT? Have you ever had it on a dyno with a wide-band?

There are numerous ignition reltated problems that would give you the same problem.

 

I can't believe this thread has this many posts and not once is the actual A/F ratio stated. Are you gonna keep us in suspense forever???

Ken R.

 

We put wideband sensor into exhaust pipe, at WOT it is 950 mv all the way until 4500 rpm, 5500 it is 800 mv and 200(!) mv at 6000 rpm.

 

If its reading millivolts, its not a wideband.... output for widebands is 0-5V range. What was the A/F ratio reading on the wideband?

 

Yes, youre right its not wideband. Anyway, I found the fault: the connection of air intake elbow before MAF sensor was very loose (apparently somebody forgot to tighten it...) and air was sucked in at high rpm! Tightened it and immediately mixture was nicely rich until redline and power also jumped to 380 hp. Sometimes sophisticated problems have simple answers... Thanks for your input anyway!