Tuning a GMPP 383

[cuervo]

I was so curious, even though I have never done this I asked on Thirdgen and yes it can be done. reason not to is motor could go so low on fuel it will not run.
http://www.thirdgen.org/techboard/diy-prom/629533-can-you-change-blm.html#post5055976

I just don't see any point in it...

If your max rich at 108 why change to 100? Just fix VE!

I max rich at 105 which is the what is in the min BLM. I wonder why in my bin it's at 105 but 7477s are 108.

Also, I know air temp and humidity play a roll in logging. That said, I went out at about 8 am when the temp was a lot cooler and it had a pretty big impact on the log; not nearly as lean as it was yesterday at 15+ degrees warmer.

 

[eagle mark]

I guess a differance in the ECM? Your 8746 or 8747? Wer'nt both HD truck ECMs? Some did not even have a knock sensor... I know one of them was HD for 7.4L no knock... HD trucks have a milder spark curve than a car because they can not accelerate as fast and sometimes need 100% duty cycle... like the IH engines were rated at 100% duty cycle at 3600 RPM all day long, but man do the Scouts come alive with more spark advance.

Change in temps and humidity do change VE then BLM. Hence the impossibility to be at a perfect 128 year round and I guess the reason for min and max BLM

 

[dave w]

Sometimes I get an engine that is running super rich, so I lower the minimum BLM to 98 or so. Lowering the BLM allows data log information to be more accurate. I've had to keep adjusting the BPW / VE because the BLM's are always data logging at 108. Changing the BLM to 98 will allow the data log to show numbers lower than 108. The INT will work better for a rich running engine when the minimum BLM is lowered to 98 or so.

dave w

 

[cuervo]

I'm pretty sure the ECM is 7606.

This is with BPW at 96, VE tables untouch, current air temp is 70*.

 

[dave w]

I'm pretty sure the ECM is 7606.

This is with BPW at 96, VE tables untouch, current air temp is 70*.

When I see a histogram like that, I'm happy with the BPW setting. I then start adjusting the VE table.

dave w

 

[eagle mark]

Sometimes I get an engine that is running super rich, so I lower the minimum BLM to 98 or so. Lowering the BLM allows data log information to be more accurate. I've had to keep adjusting the BPW / VE because the BLM's are always data logging at 108. Changing the BLM to 98 will allow the data log to show numbers lower than 108. The INT will work better for a rich running engine when the minimum BLM is lowered to 98 or so.

dave w

Makes sense.

I'm pretty sure the ECM is 7606.

This is with BPW at 96, VE tables untouch, current air temp is 70*.

The cell statistics don't say anything about the red/rich boxs?

 

[cuervo]

The box that you are seeing is the 2800 100%, it just happen to be the box that is click at the time of screen shot.

This is my adjustment to the current log, I will try to get a log with the updated VE table tonight.

This is my first attempt at adjusting the table so we'll see how it goes.

 

[cuervo]

Here is what the new log looks like. I'm starting to think the only way I'm going to keep the low RPMs cells above 45 is to lower the fuel pressure.

It's a lot closer. Most of those cells are at ave 122 or so and the leans are 133s.

 

[dave w]

Going below 45 might be a cause low idle / low RPM Cruise to surge. Try going no lower than 40 and see if you get surging? I like to keep the fuel pressure up around 14 ~ 15 PSI.

dave w

 

[cuervo]

Going below 45 might be a cause low idle / low RPM Cruise to surge. Try going no lower than 40 and see if you get surging? I like to keep the fuel pressure up around 14 ~ 15 PSI.

dave w

Well, with 90lb injectors and 18 PSI I don't think I'm going to get there. I can't believe there is no after market 63 lb injectors available.

I'm sure I'd be better off with 63s and 16 psi. More often then not I have surging at idle already.

Second pass on the VE table.

 

[dave w]

Well, with 90lb injectors and 18 PSI I don't think I'm going to get there. I can't believe there is no after market 63 lb injectors available.

I'm sure I'd be better off with 63s and 16 psi. More often then not I have surging at idle already.

Second pass on the VE table.

For my TPI roller cam 350, I use the Police Caprice 65 lb injectors with 14 PSI. The 65 lb injectors are easy to find, or low cost. I think the option of dropping the fuel pressure from 18 PSI to 16 PSI is worth trying. Then log again.

dave w

 

[cuervo]

For my TPI roller cam 350, I use the Police Caprice 65 lb injectors with 14 PSI. The 65 lb injectors are easy to find, or low cost. I think the option of dropping the fuel pressure from 18 PSI to 16 PSI is worth trying. Then log again.

dave w

Dave, if you can find me some 65s I'd buy them right now!

 

[dave w]

Dave, if you can find me some 65s I'd buy them right now!

I should proof read my posts, I wanted to say the 65 lb injectors are NOT easy to find, and if you find a set of 65 lb injectors the price is not low cost.

I'll send you a PM if I find a set of 65 lb injectors.

 

[eagle mark]

Sounds familar...

That's as low as the spring will go even with adjustable regulator? If you have an adjustable regulator you could put a stock spring back in, I don't know how far a stock spring will go but I know for sure it will hit 16 PSI, maybe more...

There's always more than one way to skin a cat! I like K.I.S.S.!

Once you start changing fuel pressure there are other things to consider like Injector Bias measured in milliseconds like 0.396 and then AE and PE and probably things we don't even know of? It's like the ECM can adjust itself right? Well how come we have to tune it then? Because it can only go so far, it was only designed for a certain range!

If you stay close to 13 then your in the KISS. I just used the injector sizing excel spread sheet for a 383 with 90 pound injectors at 18 PSI and that would support 424 HP with a BPW of 88.1... pretty big change in BPW and I'm not sure of all the consequences... plus your motor is probably closer to 350 HP.

Now at 13 PSI on your 90 pound injectors will support 360 HP at a BPW of 103.7 so your a lot closer to stock BPW and have plenty of room to go up to? What was yours? 126? I've found the spread sheet to be a little on the low side for BPW... so at 13 PSI you may be spot on.

Plus of all the mail order chips available you have one from a guy who has done this a long time and has a great reputation. So if you gave him all the correct specs to begin with then your chip is probably real close and only God could tune it better by mail!

Just my .02 cents...

Link to injector sizing spreadsheet.
http://eagle-mark.com/pictures/displayimage.php?pid=1188

 

[cuervo]

Sounds familar...

You gotta run what you bring to the track.

 

[cuervo]

I guess I could still go lower on the BPW. My highest number in the 90/100 cells is 88. If I go a couple more points down the BPW I should be able to raise the lower RPM cells some more.

I'm sure I'm just putting off the spring, injector and possible a pump change.

On a side note: The 383 has never ran as good as it is running now. It will pull hard from idle to WOT at 75 mph and would still keep pulling but I'm not looking for the law to ask me why I'm tuning on the street.

I do want to thank you guys for all the help.

 

[eagle mark]

Pump should be fine, injectors could be fine... but lowering pressure is what calculations say... your still getting closer each time even though you have WAY to much fuel...

Your welcome! I enjoy helping others with my obsession!

 

[dave w]

I went online to RockAuto.com.

The OEM numbers and Vehicle applications are correct for the 65 lb injectors.

dave w

 

[eagle mark]

I think my calculations are off? You said you had 90 lb injectors? I just looked at a list of TBI injectors and Big Block came with
80 lb injectors (13psi) in 88 to 90.
76 lb injectors (13psi) in 91 to 93
46 lb injectors (13psi) in 94 to 95 but upped the fuel pressure to 26 to 32 PSI for 67lb to 72lb...

So where did you get 90 lb injectors from? And if aftermarket what pressure was 90 lb at?

Injector listings.
http://www.eagle-mark.com/Forums/viewtopic.php?f=9&t=94

 

[cuervo]

I think my calculations are off? You said you had 90 lb injectors? I just looked at a list of TBI injectors and Big Block came with
80 lb injectors (13psi) in 88 to 90.
76 lb injectors (13psi) in 91 to 93
46 lb injectors (13psi) in 94 to 95 but upped the fuel pressure to 26 to 32 PSI for 67lb to 72lb...

So where did you get 90 lb injectors from? And if aftermarket what pressure was 90 lb at?

Injector listings.
http://www.eagle-mark.com/Forums/viewtopic.php?f=9&t=94

I'll have to go through my links and information. There are both 80 and 90 lb injectors for sure. Or should I say they were said to be 90s. At this point, I'd be happy if they were not.

Delphi injector# FJ10098

The pump I have is a 60 psi 155 LPH