A/F dips real low between 5000 and 5500 RPM, any ideas?

[TR930]

Forum,

I ran the car on a dynojet this morning and noticed that between 5000 and 5500 RPM the A/F ratio dipped dramatically and correspondingly my HP and TQ line dips in the same spot. I have a Paxton S/C on a 2006 coupe and I am at altitude in Denver.

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Any ideas on how to correct this? I am running the Paxton fuel system that came with the kit.

Also, these are corrected numbers.


Thanks

 

[plumcrazy]

Woodhouse built the car. Call jorgensen

 

[TR930]

I sent him a PM in this board.

 

[TR930]

Any recommendations on replacing the Paxton FMU unit? I just need something reliable, no plans on going crazy with the HP.

 

[plumcrazy]

I don't run the paxton fuel system. I can't help. Sorry

 

[rw99]

You do have a bit of a divot there. Looks like that tune just wants to slope AFR down; 5000 may be the point at which you just start going too rich.

Dynojets don't fully load, though... Maybe that tune is written with a "street" load? Do you perceive any loss in power ~5k rpm when driving at WOT?

 

[TR930]

The car is tuned for 8 lbs. Boost but is running 5 lbs at altitude. Think that has anything to do with it?

 

[Dan Cragin]

I would be happy help. Please feel free to send us the dyno files and I will give you a call.

 

[TR930]

Dan, thanks! What specifically do you need?, I will ask the tuner for it on Monday.

 

[TR930]

I would be happy help. Please feel free to send us the dyno files and I will give you a call.

Dan, File sent to your email address today.


Tyler

 

[TR930]

Going to go to the 10lbs Pulley on the Paxton. My theory is the car is tuned for 8lbs of boost, but here at altitude I only get 5.5 lbs, so the car is pumping the right amount of fuel for 8 lbs, not 5. That is my theory anyway. Going to throw on new Harmonic damper, crank seal and then pin the crank at the same time and then make a dyno run.

Any thoughts out there on running a smaller pulley at altitude to equal the stock boost pressure at sea level??

 

[rw99]

I think your AFR, and power, are off by a small amount... and that increasing the boost with the 10 psi pulley (a more significant change) will push you too far lean.

 

[plumcrazy]

what did DC performance say ?

 

[TR930]

Dan said that putting the 10pm pulley on should be okay as long as I don't go back to sea level. I can always put the 8 lb back on if things look bad on A/F.

I am leaving quite a bit of power on the table up here, approx 125+ HP. The car is set up to handle 8lbs, I have to compensate to bring it back to that level.

 

[vpower01]

I think putting the 10lb pulley will get you right on the money.
No air up there so you have to cut back on fuel but the pulley should fix it.
Put the 10 lb pulley on and get it on the dyno.

 

[TR930]

Question, which comes first? the Fuel or the Boost? Does the computer look at the boost created and then raise the fuel based on it? Or is the fuel curve mapped against expected boost and will dump fuel regardless of the actual boost created? I have the Paxton VCU. Also, looking at my dyno again, am I running lean in the beginning and sloping down to rich? I don't know why the "saftey line" on this dyno was set to 13 A/F but on my other dyno graphs from previous owner it is set to 12 A/F. Does the altitude have something to do with this?

 

[ViperGeorge]

You may be running a tad rich above 5K but then it is safer this way. Is your bottom end built? i.e. forged pistons and rods? Unless you'll never drive your car anywhere but in the Mile High City I'd leave it alone. If you decide to take a trip to lower altitudes then your car will be too lean.

 

[Dan Cragin]

Just to clarify, we shoot for a 11.5 AF on supercharged engines, at lower rpms and lighter loads 12.2 can be acceptable and in closed loop operation 14.7 is the target on all engines. In this case, once the pulley change has been made the AF will most likely need to be optimized with a calibration. On these cast piston engines, knock is the enemy (not boost). Knock will break a cast piston. Controling knock is the key. Also, a slightly richer mixture on a superchared engines keeps exhaust valve and piston tempretures down, which reduces chances of scuffing and the catayst from overheating under hard use.

Boost and altitude, on our Viper one mile car. We made 12psi at sea level and 5psi at 6500feet.

 

[plumcrazy]

thanks for the info Dan

 

[TR930]

I told Dan on the phone that I am either going to leave the car as is, or put on the 10lb pulley, dyno it, send the dyno sheet to Dan and get a custom tune from DC to help smooth out the A/F and then call it a day. Oh, and never drive the car to sea level unless I put the 8lb pulley back in. : )

 

[plumcrazy]

ever consider flying out dan or one of the few good tuners to tune it properly to get the best results ? i dont know the cost but id bet in the longrun it would be worth it. piece of mind too

 

[rw99]

ever consider flying out dan or one of the few good tuners to tune it properly to get the best results ? i dont know the cost but id bet in the longrun it would be worth it. piece of mind too

I second that. Do it once, correctly.

 

[ViperGeorge]

When I first looked at your dyno run I only looked at the dip in A/F. On second look it might actually is be a bit lean below 4500. Dan set up my PCM for my 06 SC coupe and it is definitely richer at those RPMs.

 

[TR930]

I think it might be too lean as well, but I am wondering if that is from lack of boost? No boost, no fuel? It looks like the fuel curve is programmed into the ECU to match boost curve.

I talked to Dan about it being lean and he did not express concern about it, he saw the dyno Sheet too. I am up in the air about what to do??