I received an email the other day about improving spoolup time and getting the truck out of the hole.

I hope he doesn't mind me quoting him. It's a common question and probably warrants it's own thread.

"Hey nick, had a few quick ?'s: I was doing a little research on trying to get rid of the turbo lag from a dead stop, say if i floor it the truck just belches smoke and lolligags its way through 1st and then takes off in 2nd once the turbo spools up. I read that pulling a little timing out will help it spool quicker, tried it with no noticeable difference in the log files. just wanted to get your opinion and experiences with it."

My experience is this:

It is most certainly possible to overfuel the truck from a dead stop. The best strategy for getting a truck out of the hole quickly from a dead stop without bringing it onto the converter is to give it just enough fuel to to grey out the pipe, and to listen to the motor for timing and fuel pressure direction.

Most of the time a truck is puking smoke or fighting itself out of the hole it has the MAF limited injection quantity tables maxxed out. This allows full fuel and corresponding timing and rail pressure. You can hear the motor rattle when you nail the throttle. If the MAF were still operational, the engine would see a more appropriate fuel level, coming up onto the turbo cleaner. Use only enough fuel as necessary, blacker isn't better.

If you must max out your MAF limiting tables, plan on lowering rail pressure in low boost conditiions. Also, I advise running very low (0-5, or even negative) timing when the truck is off the turbo and heavily fueled. Remember, the GM engineer who wrote the calibration never tuned to the truck for full fuel at 1600 RPM, so we can't assume he dialed fuel pressue and timing in at that part of the map. If it's rattling and spitting, continue to pull rail pressure and/or timing.

Lastly, first gear in these trucks is short. Turbos love a constanst RPM for spool, understand that's not going to happen from a dead hit in first gear. The next best thing is RPM. If you're not getting on the charger until the middle of second, try winding first out a little higher. The extra few hundred RPM will move more exhaust through the turbine housing and bring boost up that much quicker.

Of course, VVT trucks add another dimension to this game, but this should get the wheels turning for now.

Nick

thanks nick, gives me a good place to start playin around with

Dang it Nick, you like to keep me thinking.

Maybe a little off track but...

I have a fuel pressure curve that I use for all of my tunes. I alter the pulse width curves under 160MPa instead of playing with the pressure map; am I missing something?

On to spool up; any ideas for us sledpullers that are looking for life to come together right off the starting blocks without the rapid jump in RPMs seen without 30,000lbs in tow.

Dusty,

If you don't have RPM, you'll need heat. Adding heat to a constant mass of exhaust gas will increase it's volume and thus it's velocity through the turbine housing.

Heat can be introduced into the exhaust system by spraying fuel in late in the combustion cycle and allowing for some part of combustion to occur in the manifold. This can be done by lengthening the pulsewidth and lowering timing in the part of map you're using to stall the truck up against the sled on the starting line. Look at your pulsewidth tables to determine what rail pressure to command during this time as well.

Nick

Nick, don't know if this is what is causing mine to ping or rattle under heavy accleration when its cold. Does not to seem to do this when it is hot. Is this the area I should start to look at to help with that.
Also, is this harmful to the motor, other wise everything else is seems real good with my tune

Thanks, Brian

Nick, don't know if this is what is causing mine to ping or rattle under heavy accleration when its cold. Does not to seem to do this when it is hot. Is this the area I should start to look at to help with that.
Also, is this harmful to the motor, other wise everything else is seems real good with my tune

Thanks, Brian Probably ECT or IAT multiplier tables adding extra timing and/or rail pressure.

Dusty,

If you don't have RPM, you'll need heat. Adding heat to a constant mass of exhaust gas will increase it's volume and thus it's velocity through the turbine housing.

Heat can be introduced into the exhaust system by spraying fuel in late in the combustion cycle and allowing for some part of combustion to occur in the manifold. This can be done by lengthening the pulsewidth and lowering timing in the part of map you're using to stall the truck up against the sled on the starting line. Look at your pulsewidth tables to determine what rail pressure to command during this time as well.

Nick

That's exactly what I've been playing with. Tuning is a pain though; on the street it's hard to consistantly represent the load case applied while on track. I loaded in a tune this year that was absolutely wild on the street, came off the line line a animal; took it to the track and it was flat as a board until deep into second gear. I did some reading; then played around and built a tune with fuel "later and bigger" early in the RPM and low in command fuel range. The on the street the tune was a little sluggish off the line, but she smoked like a freighttrain then cleared up around 2,500RPMs; never got a chance to try it on a sled. I'm guessing the smoke is a good sign for lighting a turbo under load; EGTs came on fast... there's that heat you're talking about ;)

Probably ECT or IAT multiplier tables adding extra timing and/or rail pressure.

They are the same as stock.

They are the same as stock. Yeah, but when you increase base timing tables, the overall amount of timing is more with the stock multipliers.. I lowered the multipliers by 50% on modified tunes, and that really helped to quite the engine while cold.

Yeah, but when you increase base timing tables, the overall amount of timing is more with the stock multipliers.. I lowered the multipliers by 50% on modified tunes, and that really helped to quite the engine while cold.

exactly!

Thanks for the info. I tried it tonight and it seem to help. Will see how it sound in the morning wehn it is colder.

Monitoring this :)

I had ran a tune that blew smoke off idle and the dyno sheet said I had roughly 15-30hp less all the way till the turbo lit. Good advice Nick.

Being new to efi live I would be lost if it wasn't for guys willing to share their experience. thanks