I hate that I'm on such an offset schedule as you all!!

BUT, considering that I now don't get an SES for overboost or defuel at ~22 or 25psi respecively, the OBA is staying. Now, those of you with trucks that can get 30+psi out of the stock turbo (I never could), how many of you have the OBA installed? I'm guessing all of you because the truck will simply kill the fuel supply when you hit that 25psi number, right? So that being said, how can the PCM posibly control fuel correctly with that limitation? I fully understand what you're saying, Gene. The issue I have is that when I NEED that boost & fueling at ~25psi, I would rather sacrifice some slight amount of performance (that has yet to be determined) than have the truck completely fall on its face when I'm trying to show a CTD or D-Max that 7.3s aren't the "slow POS" that they think they are.

I'm still not convinced that the regulator in place makes that much of a difference prior to hitting the regulation point which I agee probably isn't accurate to a few hundreths of a volt, but does it really make that much of a difference? I'm still pondering the idea of moving the pickup to the pre-I/C side of the spider. I can see how that would certainly have a faster response, see more boost, and cause the PCM to call for more fuel, but that also means you hit the regulation point faster as well. It's what happens after that point that interests me. Somehow, we still get strong performance even after the PCM sees that plateau caused be the OBA at 21psi. Could it be that at that level of boost, the PCM doesn't care that much anymore and the program simply calls for tons of fuel anyway?

Lastly, how is that handled on HIGHLY modified trucks with H2Es or compounds that attain 50+lbs of boost?? Wouldn't you think our trucks would peter out pretty quickly if there was a fueling issue above 21psi on these 50-60+psi trucks?? It's somehow working, and I know we're all just doing some heavy postulating here, but that fact alone makes me not the least bit concerned with it. It would be good to know so I'll just monitor for now, but I would like to hear some theories from you all on how fueling works on big-boost trucks if the PCM is apparently totally blind to those boost levels.

 

Hopefully I can help with some of this....

When I had the stock turbo, I was able to hit 30 psi of boost, and also had the OBA installed. However, before I purchased the OBA, I did have just a blowoff valve that sometimes worked and sometimes didn't. There were times I would set off an overboost code.

Not exactly. There are many factors that contribute to fueling, and MAP readings are only one of those many. When you set an overboost code, a "defueling" effect does happen. However the effect of that can vary a bit. For most people, they hardly notice the defueling and on higher chip settings it's not going to affect overall power and drag strip times by much at all. The fuel is still there, but the amount that is pulled back by the PCM might not be as much as you think. It definately won't kill the fuel. On my truck when I was messing with the OBA a few months ago, I could feel the defueling a bit, but it was only a slight decrease in power at the upper RPM band, kinda similiar to running a hot tune on a single stock hpop, just not as drastic of a power "dropoff". That's how it feels on my truck, yours can vary a bit, many times it depends on what mods you have.

The PCM delivers max fueling at much lower boost pressures (14-16 psi), only assuming that other factors such as throttle position are also at maximum. Since diesels don't have a set air/fuel ratio, the rest of your fueling above 22 or 24 psi of boost will be determined by your chip, injectors, turbo, etc. Properly matching your injectors with the correct turbo, then get the tuning right and you'll have a better running truck. Even though the PCM isn't really seeing those upper boost numbers, all other tuning parameters can still be monitored and controlled with some degree of accuracy.

Hopefully I've answered this in some part. There are many variables that account for fueling, and the MAP is only one of many. Once you reach that initial threshold for max fueling at the 14-16 psi boost levels, it's up to the programming of your chip to determine the rest of the fuel at the upper boost levels.

Those guys are also running OBA's or some type of release valve.

 

Here's some dyno data to support your explanation. I had back-to-back dyno runs at Browns Valley RV before and after installing the boost relief valve shown in the pic. I got 267 HP without it and had a solid SES light from 22 psi on up to my max boost of 27 psi. With it I didn't get an SES light and made 12 HP more.

I was running a 70 HP plug in type Superchip, big head, K&N cone, and Banks DP and tailpipe modified for a side exit. Keep in mind that my early 99 only has 212 RWHP in stock configuration, so these HP numbers seem about right to me. The reason I wound up removing the relief valve was that it made a loud whistle noise when venting, and that got to be very annoying when pulling long grades. I also think the slight defuel helps to save wear and tear on the stock turbo.

 

It seems to me that since chip programmers are able to "fix" the newer computers to override the SES light due to removing the DPF, disabling the ERG valve, etc..., that someone should be clever enough to override the defuel SES light on the 7.3L PCM! Like I've posted, when I ask those who should know I either get a no comment or explanations that vary from it's too hard, can't be done, we're afraid of getting sued, etc..., but considering what they're able to do with the newer computers that have many new OEM anti-tampering safeguards, you'd sure think our PCM could be fixed as well.

 

When Jody was tuning my truck he actually talked about tuning the 6.4L and comparing a few things with the 7.3L. It was a short discussion, but in essence I got the idea that there is very little that can be done with programming and the 7.3L PCM vs. the 6.4L. Too many hardware and software limitations.

 

Agreed. While the 6.4 is a lot more technical, that also means there are more parameters for the tuners to be able to manipulate. With our 7.3's and their limited computer, that also would limit the paramaters a tuner can manipulate.

Think of it like this - how many more things are personal computers today capable of doing when compared to units that are over 10 years old? It's similar with the computers on our trucks. You can do more with the newer trucks and computers because the programming and hardware is much more advanced. Remember that even though our engines were made up until '03, the computer technology used is much older than that. The same basic type of ECM's on our SD trucks is what was used on the OBS trucks. It's old technology, even for a 2000+ model year truck. The electronics that controlled my old '96 Mustang Cobra is years ahead of what's controlling my '00 SD.

 

You sure did, and that's kind of what I expected to hear. Otherwise there would be no way to take advantage of big boost levels, which there obvously is... Thanks Pocket!!

 

How did you do this test?