*****EDIT, the below problem was a mix of bad plug wires and re-flashing pcm and tuning*************** Problem Resolved!



Car breaks up at 4200-5k rpm under boost...runs finr when charge pipe is disconnected. Also runs fine for at 3/4 throttle through the 4-5k range. Runs fine for a sec at WOT above 5k. Lost.... 40 pulls, A/F okay, boost holding, 60lbs psi fp, power simply falls off and the dyno, or car stuuders to almost a stall on the street at WOT in the 4200-5k range. Any ideas where to start.??? Or finish...we've been knocking our heads.


Makes 500ftlbs at 4200 with 7lbs boost, then drops.

 

What valvesprings are you running?

 

978's. 500 miles on them, I was thinking float too, but doesnt make sence...ran fine, changed turbo location and head gasket.
6lbs boost on this pull....boost stays soild foe entire pull.
.

 

What is the timing doing through the whole pull?

 

I'll pull the data, but nothing that looks bad. Zero knock... wanna say 19-22 degrees

 

My money is still on the fact that the springs don't have enough pressure to keep the valve closed.

 

400 lbs?

Possible, but it held 20lbs before turbo change, now cant hold 7?

 

Are you getting sufficient fuel? I noticed you do not have the AF readout down there. Sounds suspect to me.

 

Well, this is sorta ugly,but was one of the first pulls, gives ya an idea...This was just a pull result . Make more HP on motor, so at this point the tune was way off.//but here was A/F for the above graph.

 

The lambda rises to 0.8+ right where you have trouble,
that means something like 12:1 while elsewhere you
have more like 10.5:1? I'd look at the PE table first,
may need to roll it on harder at lower RPM. You might
want to do a little Excel exercise, scatter plot your PE
vs RPM (or 14.7/PE(RPM) vs RPM) and the pull data as
14.7*lambda(RPM) and see whether they track or
diverge there. If they track then massage up the PE
table. If they diverge then you may have some other
problem like a MAF calibration "nonlinearity" (not that
it's a linear instrument) or a divot in your VE table or
like that. But right off it looks like fueling is the issue
to me.

 

Thanks, I will let ya know if your on the track as we play! Feels like fueling too.

 

Trannys shot...more delays...2-3 are okay, but in 1st I have to go 1/2 throttle to move, and when I let off its like hitting a brake!

 

Jimmy, a variety of A/F ratios have been thrown at this thing with no changes. I don't see where the A/F's shown on that poor pull would "stall" the motor from revving between 4200-5000 rpm. I have pulled cars with a flat 10.0 A/F that would still pull through the rpm range. This car has been given enough timing and A/F that it still should not been laying over at those rpms. I still think its a mechanical issue (valve springs) that we are chasing.

Now that I have the new 7.3 version of EFILive with there new 2/3 BAR MAP boost capabilities, we are going to ditch the MAF and go SD with better fueling control during boost.

During those early pulls I noticed the MAF was not reading at all despite it still being hooked up. It was my understanding that the MAF doesn't start being used until around 4300 rpm to help control fueling. SD will stop the questioning.

 

What size AR on the turbo's?

I have my money on a stalled hot side.

 

holy ****... never thought of that. i wonder....
You were gonna tune this if wheels ever called back too! LOL
They are A/R 60 each

But car winds out at 3/4 throttle, then i can matt it at 5k...but, this may be with mirrors...hmmm ..keep talking Kev

 

A stalled setup should actually spool pretty good at part-throttle conditions. The amount of work that you demand from the turbo compressor equals Boost * CFM. If the CFM is low, the compressor load is not that high and the hot side has no trouble keeping up. Once you open the throttle though the CFM goes above what the hot side energy is able to provide, the turbo slows down, motor VE goes waaay down, and then the waste gate opens up at a fakey inlet manifold pressure that is only elevated because the VE of the motor sucks because the inducer barely spins. The motor stays in this non-HP producing limbo until you get out of it. I'm just speculating, so don't go swapping your hot sides just yet. If possible, route a temporary exhaust pipe to only one of the turbo's (nothing pretty, just for testing), and strap the thing down. If she spools like a champ, change your hot side to something smaller. For the record though 0.60 AR on a set of twins is pretty sizey for a rear mount setup. Most rear mount SINGLE setups are only 0.6 to 0.7 AR.

Good Luck

 

Kev, When I had the turbos designed , they asked me how much boost I intended to run. I wanted 15-20 if they could make it happen. I called the maker today who said "Why are you running 7...you wanted more, those turbos will not be working well at seven...crank it up". We had not tried this, and will do that 1st. I think your onto it. If that doesnt work, we will route all exhaust to one turbo or swap for different A/Rs.
Your idea makes sence, and something we hadnt considered, you dont hear about stalling turbos much. I'll keep ya updated. Thanks for the thought you put into this .
As Howard mentioned above, hes run alot of different combos and tables with almost NO CHANGE in how it acted. Thats why I think this is a real possability.

 

We started at 7 psi to start out slow. No sense pumping 20+ before we saw that all the other parameters where close. I would be interested in knowing if you run a turbo "designed" for 15+ and run at 7-8, how it effects this "stalling" idea...

 

Turning the boost up will hold the waste gate closed until enough exhaust energy builds up to overcome the compressor load. When the waste gate does finally open up the ratio of available exhaust energy to required compressor energy is much higher and you don't end up stalled.
If this is your problem, the dyno graph should clean up alot when you turn up the boost. It may not look perfect with that size twins in the back, but it should clean up substantially.

If it doesn't, I would seriously look into that MAFless tuneup. The computer flips fueling at ~4k. If the MAFless tune is not disabling the MAF properly it will be like hitting a wall at 4k everytime. Personally, with as many tuners that that computer has been through, I would just take it down to the Chevy dealership and flash that bitch back to stock. After several nightmares like the one you guys are going through, if the car has been to more than 1 tuner before me, I flash it back to stock before getting started. There is all kinds of crap that can trip you up from prior "tuners".

 

Kevin,

Makes alot of sense at this point. We keep trying to compare his setup last year with the twins up front vs. this years rear setup. The car ran fairly good last year and the changes consisted of;

rear mount units
LS2 intake (which still had the stock MAP!)
I also found that the fuel system was setup with a fuel pressure regulator that was boost sensing. The first time we pulled it the fuel pressure was starting at the correct 58-60 and then going up to over 80! I disconnected the vacuum line to keep the fuel at a solid 60.

Your explaination of the turbo stalling makes a ton of sense, especially considering the turbos where specifically designed to be used at double boost we have been playing with.

As far as the PCM programming is concerned; I just got the new updated version on EFI Live 7.3 which has the custom operating systems and boost parameters. I have the workshop version which allows a full re-write of the OS. The new custom OS will give us a fresh starting point. The MAF is coming out as with the new 2/3 BAR capabilities its SD all the way.

Thanks for your input!

Howard