Hey guys,
Kind of been banging my head against the wall for the last month and half. In December, the truck ran 11.59sec @ 115.98mph (1.65sec 60ft) on 12-13psi, 15 degrees timing, and 11.1afr.

I increased the boost to about 14-14.5psi, kept afr at 11.1, and I'm currently at 13 degrees timing. The results aren't what I expected. After several trips to the track and about 15 passes, the best time I've been able to get was 11.87sec @ 115.14mph (1.78 60ft).

Observations:
-Seem to have more steam pushing out of my catch can breather, however it is colder out so IDK
-Trans is NOT slipping
-Converter isn't slipping anymore usual
-Plugs look good
-No DTC's
-No Knock
-Brakes aren't dragging
-IATs are increasing much faster during a run on 14.5psi than they ever did on 12-13psi
-I've done a compression check - all cylinders 150-170 (may have to redo this as I didn't prop open the throttle blade)
-An increase of timing from 11 to 13 degrees only seemed to net .2sec and 1-2mph in the quarter

I guess I am looking for suggestions on where to start for diagnosing my predicament. This is a 4.8l with 179k miles on it, and a used 4l80e with only a shift kit. Any help on what to look for/at next would be greatly appreciated! Thanks in advance. I just wanna go fast

 

Add timing back to motor, its very well possible your combo makes more power on timing vs boost...im referring to as turbo is getting out of its range so you may see a gain of 10 or 15 vs 20-25 earlier

So your timing could be worth 50 rwhp

Also timing makes torque

 

Hmmm, thanks George. That makes sense, although I am somewhat hesitant to add more timing past 13 degrees on 14.5psi...done a lot of research on how to read plugs, but never actually done it. Guess theres only 1 way to learn! Still on straight 93.

I would have to agree on the turbo getting out of its efficiency range due to the rapid increase in IATs now. Also seemed like IATs increased more rapidly when I increased timing from 11 to 13 degrees on the same boost level...not sure if that makes any sense.

 

I have a hard time believing that size turbo is getting out of its efficiency range on a 4.8 at less than 15PSI. What size is the exhaust side? Is the 3" downpipe becoming a restriction? That would be my first guess.

 

how long in run .before you hit full boost ,have you tried stock intake

 

Vortec, downpipe is 3", then immediately increases to 4" through a conical reducer below the passenger seat, to a 4" cutout. You may have a point about the downpipe...!

 

Hey Tim. I'm launching around 10psi and make full boost about a second into the run.

I haven't tried the stock intake - I know there would probably be some gains from it. I may swap it back on at some point. At the moment however, I'm just confused and frustrated in the fact I've taken a pretty big step backwards after 2 more psi.

I appreciate the help everybody!!

 

Vortec.....one thing youbhave to realize with turbos is turbine has the most effect...a 65mm turbine is small and will choke up......it should carry further but his IATs show its falling short



His truck would be much faster down low with a stock intake...his cam, turbo, intake, gears dont match all that well....swap intak and hell gain alot on short side of track

 

Next question....what valve springs

 

I'd be lookin squarely at that downpipe if it were me. It will be a cheap upgrade...

I know with blowers a 3" exhaust starts to create backpressure at or before the power you are making.

Yea that makes sense. Thats what I was asking for when i asked what size the exhaust side was.