When does a GTP stop pulling?

[louvered97gtp]

I had the opportunity to go really fast in my 99GTP this weekend and noticed on a couple of pulls, my car didn't want to accelerate much and seemed to just flop after 116mph in 3rd gear. My RPM curve also seems more like horizontal line than a power curve during 3rd. Is this typical or should i be expecting more power here? I've got the 3.29 gears in my trans but don't think these should affect this much do they?
Any thoughts? Thank you,

[JerryH]

Depends a lot on the car condition, tune, mods and more than anything else, the cam and torque converter. The lower the stall speed, the more HP/TQ you will have up top.

Though there is a gradual decrease in accelleration from 120PH, I have no real sharp slowdown till I hit about 145-150MPH, then aerodynamics becomes my enemy and the car fights for every MPH increase.

The 3.29 gears will make a pretty big difference, IMHO. I have the 2.93s and my car loves to run above 100mph without effort. Mind you, above 110mph, I am running in 4th gear too. With the 3.29 gears, what is your RPM at 116MPH?

[louvered97gtp]

Well at about 110mph it's about 4530RPM. At this high speed, my 99GTP just seems to not want to rev anymore than that and kind of just sits at that rpm. My car reads 10mph faster than it really is. Have not been able to calibrate it for the higher 3.29 gears. I just ordered a fuel pressure guage adaptor which is on it's way, to find out what the reading is. Although not really sure what kind of fuel pressure i should be looking for.

[JerryH]

Between 45-55 PSI I believe.
A stock cam goes to sleep above 4500. Also valve float on factory or weak valve springs will do that too even if you have a cam that breathes well above 5000 RPM

[louvered97gtp]

Ok, so finally hooked up fuel pressure guage, and my psi on cruise is around 47ish and dips to about 38ish at WOT. New filter so i'm assuming this means the pump is not keeping up? So should I be looking at boost a pump or switch out to a racetronix warlbro?

[JerryH]

If you want to try to experiment, you can make your own fuel pump rewire mod for about $15.

- 1 X 20 amp relay
- 2 X 20 amp fuse holders
- 2 X 20 amp fuses
- 15 ft of 12 gauge wire

Thats what I did on my car. I felt and scanned no differences before/after, but for $15, its cheap insurance.

[louvered97gtp]

The car already has a fuel pump rewire but otherwise stock pump. But thanks for the tip Jerry, I would like to do this on my other GTP. I did notice that my fuel pressure dropped 10psi like previoiusly stated. However the psi is consistent throughout the PE AFR, and towards the end of my runs, my Wideband is suddenly going lean and spiking pretty bad. Although my PSI seems steady, does this indicate fuel delivery problem not getting into the cylinders? Kink in line, clogged filter, etc?

[streetcar]

Go to a Racetronix Walbro and the GP Sorsesen FP Regulator, base pressure 50-52, WOT should be base + boost pressure. You want to always have X amount of pressure differential.

[louvered97gtp]

So my problem is the pump in your opinion? My fuel PSI should be increasing with boost not dipping as it shows on the scan correct? My confusion is.. why does my WB show eratic behavior only towards the tail end of the WOT run whereas it's steady at start of the run? I know where to buy the racetronix, where do I look for this FP regulator? And is it still needed with the racetronix?

[JerryH]

It seems that the FP is fine and then petering out after a few seconds. Could be some amount of fuel starvation due to a restriction somewhere.

Before spending money on fuel pumps and what not, I would make sure that:

- the fuel pump filter (sock) in the gas tank is clean
- the lines from the FP to the fuel railing is not kinked anywhere
- that the fuel filter is not clogged (replace if 1 year or older)
- that there are no kinks in the line between the fuel tank and the injectors (fuel lines to fuel rail)

Hold off on getting a new FPR (fuel pressure regulator). Zooomer got to over 450HP on the stock FPR, so unless yours is damaged, there is no reason I see from the scans to change it. Whatever you do, don't raise pressures above 60 PSI. GM documentation states that injector pintle damage can occur at pressures over 62 PSI. Last thing you want potentially is an injector to fail while at WOT.

[streetcar]

There should ALWAYS be a CONSTANT pressure differential between the intake manifold and the fuel rail. The factory regulator is 3.5 Bar, its stamped right on it, clear as day. 3.5 Bar = 50~51 psi. If there is 10PSI of positive intake manifold pressure that means fuel rail pressure should be increased 10PSI to net the same effective pressure differential. If the pressure doesn't increase in a one : one ratio, the injector becomes "smaller"

All port fuel injectors are rated at 3 bar, 43.5psi pressure. Car companies will use varying rail pressure, Ford uses 38psi, Late model GM uses 58psi, e.g. 60lb injector @ 50psi = 65lb/hr injector - or what the 3800 community sells them as. In your case, 38psi of rail pressure, 60lb injector = 56lb/hr injector, with boost in the equation, 48lb/hr effective injector size.

Pressure, when working with non-compressible fluids, is a sign that there is not enough volume to keep up with engine demand. When moving to a fuel pump that outputs a higher amount of volume, there needs to be a consideration of both supply and return line paths. The GP Sorenson regulator has a larger diameter return port, allowing it to better regulate rail pressure at idle and cruise. The stock regulators get sloppy with higher volume pumps and precise control of the air fuel ratio becomes difficult.

FYI - Mototron / Siemens Injectors (and most aftermarket) are balls to the walls statically flow tested at rail pressure of 600KPA, 87PSI, http://www.racetronix.com/images/injectors/L107.pdf

Regarding why the sudden lean condition -- looking at the RPMs - that looks similar to misfiring / detonation.

[louvered97gtp]

Thanks, this is all very usefull info. Due to my higher compression, I'm comanding only 18* of timing. The lean condition begins when there isn't any much apparent detonation happening, hence.. the steady 15-16* , meaning no more than 2* timing being pulled. My timing runs pretty steady at mid 15ish for most of the run in the scan. When the WB readings jump to lean, if this was due to KR, you'd expect to see my timing dip even lower wouldn't you? My timing does not dip further until a bit later where the KR begins to actually show.
So isn't is more plausible that lack of volume, or flow restriction could be the culprit? Lack of fuel causing me to go lean would also induce detonation would it not? Just thinking out loud here. you guys have more experience, i'm just trying to learn to diagnose these things.
Thanks again for all your help.

[streetcar]

Keep in mind:

• The wideband is collectively measuring the output from all cylinders
• The knock sensor is a fail safe system, not a definite tuning tool

Here's what I'm thinking: pressure is staying constant, even though it is low, it is still consistent through out the WOT run. Since you have large injectors, even when cut down to 48lbs/hr, they should still provide enough to show a gradual taper towards the lean side.

Try this, remove 3 degrees of timing, set your Add vs RPM and Base AFR vs ECT to stock. Log a run with car at operating temp. Post back the scan.

[louvered97gtp]

Well, I've changed the add vs rpm and base AFR back to original tune file per your suggestion. I don't understand what you will find from these changes, in my inexperience, this seems useless to me only because now I have no control of my WOT AFR, and it's producing unreliable and varied results. My WOT AFR is no longer what I want it to be because of the adder table. So could you explain if this new scan indicates anything? It appears i'm still having same problem
File attached.
Sorry, file's a bit long, I did the scan earlier in day so i had to drive out to where there's less traffic.
Thank you.

[johnh]

I thought there was a Supercharger setting to bleed off boost at higher 100+ speeds...

[streetcar]

MAP and MAF continue to increase with RPM, and limitation starts at 116mph, spark cut at 120mph...

[streetcar]

As long as commanded AFR = actual AFR, you are in 100% in control. By adding additional fuel demand to the fuel system (injector duty is +10% in this scan) we should see the rail pressure drop, but it didn't. If it were a fuel supply problem, the fuel rail pressure would slowly bleed down and AFR would drift lean.

Where is the fuel pressure sensor mounted?

Would you mind posting up your tune?

[louvered97gtp]

I thought there was a Supercharger setting to bleed off boost at higher 100+ speeds...

Boost bleed off? Interesting, hadn't thought of this, but don't know how the boost timer tables even work. I've attached image of this below.

As long as commanded AFR = actual AFR, you are in 100% in control. By adding additional fuel demand to the fuel system (injector duty is +10% in this scan) we should see the rail pressure drop, but it didn't. If it were a fuel supply problem, the fuel rail pressure would slowly bleed down and AFR would drift lean.

Where is the fuel pressure sensor mounted?

Would you mind posting up your tune?

Fuel pressure sensor mounted in side port of fuel pressure regulator block. I have a PRJ unit that came with fuel rails.
I've attached the following

Post 00 bin file: thas adder tables filled back in per recomendation
Post 02 bin file: my standard tune with 11.5 WOT AFR
wot-scan3: scan of the post 02 file attached.

[JerryH]

I thought there was a Supercharger setting to bleed off boost at higher 100+ speeds...

You are thinking the catalytic converter over temp protection table. If set too low, at speeds around 103-105 MPH boost drops.

[streetcar]

Few things:

Is there any way to put the 3.4" pulley from the 97 onto this car?

What is your spark plug gap?