Need advice on a TT408 tune problem..

[Redline Motorsports]

I have been playing with the EFI Live for a couple months and are starting to get the hang of the tuning process...at least on a NA motor!

I have a friends car in the shop that is a F-body that has a 408 motor with twin turbos. The problem I am having is that as soon as the motor hits 4400 rpm's it just lay's over and drops off in power all the way to 6500. On the dyno it feels like the load has been turned up.

As a novice I am trying to get a good handle on a tune for this car. Someone had tuned the car before in which it ran worse then what I have gotten it to run. I think I have gone through all the basics and feel that I am either missing something stupid or I am WAY out in left field!

I started with a stock 99 bin and worked through every table. I have been told that the MAF may be the problem and the scaling could be off. The guy had a ZO6 MAF in which the scaling was incorrect. I loaded a stock ZO6 scale and no changes. I also checked to make sure that the PCM was not taking timing away for some odd reason and don't see it happening. There is also no KR and the WOT timing is a solid 21 degrees. The A/F is still quite rich but it should still not drop off in power as bad as it is. I have seen plenty of 10:1 AF that still spun up. The AF was 10.5:1.

On the dyno the car is pulling 7-8 psi of boost, all the way through the pull. I am looking for some direction as to what I should be looking for with a boosted application. So far it has been a hell of a learning experience and I have learned a ton. Now I want to get it running strong.

Any direction would be appreciated!

Howard

[Delco]

I have been playing with the EFI Live for a couple months and are starting to get the hang of the tuning process...at least on a NA motor!

I have a friends car in the shop that is a F-body that has a 408 motor with twin turbos. The problem I am having is that as soon as the motor hits 4400 rpm's it just lay's over and drops off in power all the way to 6500. On the dyno it feels like the load has been turned up.

As a novice I am trying to get a good handle on a tune for this car. Someone had tuned the car before in which it ran worse then what I have gotten it to run. I think I have gone through all the basics and feel that I am either missing something stupid or I am WAY out in left field!

I started with a stock 99 bin and worked through every table. I have been told that the MAF may be the problem and the scaling could be off. The guy had a ZO6 MAF in which the scaling was incorrect. I loaded a stock ZO6 scale and no changes. I also checked to make sure that the PCM was not taking timing away for some odd reason and don't see it happening. There is also no KR and the WOT timing is a solid 21 degrees. The A/F is still quite rich but it should still not drop off in power as bad as it is. I have seen plenty of 10:1 AF that still spun up. The AF was 10.5:1.

On the dyno the car is pulling 7-8 psi of boost, all the way through the pull. I am looking for some direction as to what I should be looking for with a boosted application. So far it has been a hell of a learning experience and I have learned a ton. Now I want to get it running strong.

Any direction would be appreciated!

Howard

21 deg timing a good stocker will not like that much timing , has this motor been seriously decompressed ?

Pull some timing out and lots of fuel , aim to get it to the 12:1 region as a starting point with min timing , something with 8 psi should see you in the 10-15 deg timing area on stock compression.

[MCG]

One thing that I've found when tuning boosted vehicles is that people underestimate the required seat pressure on springs. If you cut the seat pressure close to what the camshaft required, the extra push from the boost may possibly be opening them up.

I know that I can immediately tell when it is happening. As boost starts to come on, the car just lays down, somewhere around 4500 rpm as you describe and it almost feels like it is hitting a rev limiter without the noise.

I'd be willing to bet what you are describing is a valve spring issue. What springs did you install?

[edit] Also, when this occurs, the A/F ratio is exactly as you describe. You can't seem to get it to budge no matter what you do to the commanded fuel curves. It all seems to fit with what you are seeing.
[/edit]

[Redline Motorsports]

I started the timing out at 17 degrees and kept stepping it up. The motor picked up some good power with the timing increase and no KR. I have been suspecting valve float but here is the argument; the motor has less ten 1200 miles on it. The heads where specifically setup for a high boost application (15+). The first 500 miles the guy had front mounted twins and now its a custom rear mount setup, same motor. If the valves should have floated the first time around but didn't and they car pulled to 6500 all day long. I think I need to bump the valve spring theory down a notch and check some other things out first even though it is acting as such.

I just started a new file using a stock A4 file. I changed the VE table at 400-1200 rpm for the camshaft. I left the rest stock.

Changed the IFR table for the correct injector size.
Changed the usual, cat protection, abuse, engine torque, rev limiter and idle settings as needed.

I used a 02' ZO6 MAF calibration as he is using a stock 85mm MAF.
I was told that another approach to deal with the boost was to change the C6101 ETC predicted air flow table by adding 200 % to the entire table.

I haven't got a chance to upload these changes to see whats up. Any other feedback would be appreciated!

Thanks

Howard

[joecar]

Howard,
When you get a chance, please post the following (pictures' worth 1000's words):
a. images of logged data (charts),
b. images of changed tables.

Also, I thought the predicted airlflow table was used only for setting a DTC if actual airflow was too far away from this table...

[hpcubed]

Isn't ~4.5K where the maf usually maxes out (@8lbs) and the PCM dumps a bunch of fuel in? Are you sure the afr is straight all through the run? What is the maf reading at 4.4K?

[joecar]

hpcubed, good point; is the MAF restricting airflow above 4500...?

[Redline Motorsports]

hp,

The AF is going pig rich! 10:0-10.5 at that spot. I am still trying to get a grip on the MAFS operational parameters. As a matter of fact, the motor is pulling 8 lbs of boost when this is happening. Heres the thing though; I believe when this is happening, I have had the MAF disabled..... I assume that if the FLASHSCAN is not ready MAF that it is inop.

If the MAF is going to be a problem with the boost, shouldn't I focus on tuning in SD?

[turbolou]

A suggestion would be to run a tighter plug gap. I have seen many times on the boosted engines I work on tightening the gaps have resolved issues like this. The cars that I'm reffering to are Buick grand nationals. I
would imagine that the principles are simular. Platinum plugs also don't seem to work well with boosted applications. Just a thought.

[Redline Motorsports]

I agree with the plat. plugs not being used...which are not. I believe the problems are more tuning related and I must not having something configured properly. I have already got burned, as a newbee, in forgeting one minor tweak in a table that caused all kinds of headaches :lol:

The MAF sounds like it is the first thing I need to rule out. If I just unplug the MAF won't the car be forced into SD? If not what other parameters can I disable or adjust to take the MAF out of the picture. This motor will be running upwards of 15+ lbs and if the MAF is crying over 8, may it should go.

The power drop off is right between 4400-4500 all the way to redline.