Twin Turbo power falling off at 4800 RPM

[AndyMac]

Car is a 1995 Viper, completely stock motor with MLS head gaskets. Fuel management is a VEC 3, T4 turbos(1.05 A/R turbine), 4" exhaust no mufflers or cats, 60mm gates vented to atmosphere, NGK plugs, AB's wires.

The car pulls like crazy to 4800, then power drops right off, and if I try to stay in it, it will backfire around 5200. Does the exact same thing at 5psi as it does at 15psi. A/F ratio is good, but leans out from 11:1 to 12:1 when it starts loosing power. I have 20 datalogs of different gears/boost levels/etc, and they all show HP dropping off at 4800. It's not gradual either, it's like someone threw an anchor overboard.

I pulled the plugs I could easily get to, and they are nice and golden.

I'm thinking that since 4800 is where I'm making my peak torque, the cylinder pressure is going to be highest at that point. This leans me towards an ignition problem. (plus a misfire would show up lean on my WB)

Since it's so consistent though, I wonder if maybe I'm floating the valves?

Lastly, I thought I read somewhere that the VEC had a firmware issue that resulted in high RPM misfires?

Anyhow, any thoughts/ideas/comments?

Thanks,

 

[drlee50]

What are the plugs gapped at? Did you upgrade the valve springs?

 

[AndyMac]

Stock springs androckers, plugs gapped at 0.025

 

[plumcrazy]

springs are a weak point in these. maybe there ?

 

[martyb]

Andy,

Whats your fuel pressure??

 

[cfiiman]

Andy,

Whats your fuel pressure??

That is what I was thinking, maybe the pump can't keep up with the demand after 4800

 

[plumcrazy]

more i think about it, id look hard at those plugs and wires

 

[AndyMac]

Thanks for all the help guys. I scrambled this morning and found some new plugs to throw in it. I had a dyno apt, so I had to give something a try. I found the car ran and drove better after the plug change, but power was still dropping off.

I decided to put it on the dyno anyway. It pulled good to 5000rpm, but went lean. I think I have some fuel issues, and my WB is reading way too rich compared to the dyno WB. Methanol saved my ass for sure.

 

[Jack B]

what injectors.

 

[Red Snake]

Are you still running the stock fuel pump? The Roe cars max out the stock fuel system at around 660-670 rwhp. Maybe you need to upgrade the pump?

 

[AndyMac]

Side feed 600cc/min injectors, and twin inline walbro pumps with a FMU. (stock pump still in tank) This is the same setup that the paxton kit uses for 800whp. My fuel pressure was down to 58psi by 5000 rpm. I am almost positive that I'm out of fuel. I am going to build a proper system this winter.

 

[Jack B]

Side feed 600cc/min injectors, and twin inline walbro pumps with a FMU. (stock pump still in tank) This is the same setup that the paxton kit uses for 800whp. My fuel pressure was down to 58psi by 5000 rpm. I am almost positive that I'm out of fuel. I am going to build a proper system this winter.

What do you mean down to 58 psi?? That VEC misfire was fixed with a firmware upgrade and it appeared as a V notch on the hp trace. The power would pick back up after 5000.

 

[AndyMac]

What do you mean down to 58 psi??

I had my FMU dialed in for 6:1 to run 16-17 psi, but I should have been more aggressive. When my pumps turned on, fuel pressure came up to 68 I think, but dropped to 56-58 by 5000rpm. I could probably change the ratio of the FMU and get a bit more, but I think I'll go with a different fuel setup now.

 

[Jack B]

That power fall cannot be only associated with a moderate change in a/f.Oi

 

[Jack B]

That power fall cannot be only associated with a moderate change in a/f.[/yQUOTE]

 

[Viper Specialty]

I had my FMU dialed in for 6:1 to run 16-17 psi, but I should have been more aggressive. When my pumps turned on, fuel pressure came up to 68 I think, but dropped to 56-58 by 5000rpm. I could probably change the ratio of the FMU and get a bit more, but I think I'll go with a different fuel setup now.

FMU's work by producing a very high head pressure increase. You should be seeing a HELL of a lot more than 68 PSI unless you are only running 1 lb of boost, lol.

My guess is that you have a major problem in FMU plumbing or design, probably on top of other issues.