Questions about Limp Home Mode

[DavidBraley]

I've been reading about the Netres chip and how it's a vital part of the limp home mode system. My present understanding of the Netres chip is it's a unique network of resistances based on engine size and cylinder count, and with the input of the TPS, MAP and other sensors, it can create an emergency fuel map that will allow the engine to continue to run under a control system that has failed in some way. I've also read it's important to choose a memcal that's the same cylinder count and as close to the cubic inch size of engine you are using. It makes sense.

Some of you know this, but I'll repeat for clarity. The engine I'm collecting parts for is a BBC that will be bored and stroked to 489 cubic inches. This engine will conservatively produce 550 hp and something around 550 foot pounds of torque (That's carbureted. Hopefully more with EFI). These numbers are approximately double the original output for this engine. The PCM I've chosen is the 7427, and I've modified it for use in MPFI mode. I'm also building my own port injected intake for the project. The transmission will be a 4L80e.

It's been a comfort to read about others having success using the 7427 for heavily modified engines. What I'm not understanding is how limp home mode will deal with an engine that needs a different fuel map at the same TPS position and MAP output from the original engine, even at low power needs.

I've read a couple of threads where others have mapped out the resistance network of the Netres chip for different engines. Is anyone out there fiddling with these Netres chips in a way that would allow someone to modify the resistance network to create a different emergency fuel map when the PCM defaults to limp home mode? The idea being if someone is experimenting with a kind of "universal" Netres circuit. A board of some kind that would allow you to create your own unique resistor network. I hope that made sense.

My last question is if the limp home mode is transmission type dependent. Can a memcal from a 7.4L truck that used a manual transmission be used in a truck that uses a 4L80e?

Again, thanks so much for your thoughts on my questions.

David

[myburb]

I only have tbi but my testing might be of interest. First off when in full limp your 80e has no input from the pcm. When you select drive it is second which is both solenoids off and it stays there. My setup up is 454/4l80e lightly modified and my chip is from a truck 7.4/4l80e. In limp and in second at 2600 rpm I do 43 mph. On my test at that point I was a little lean so I upped my fuel pressure until I had 14.7 and made a note. I have since had a chip fail about 40 miles from home, tweaked fuel pressure and went home. It felt like I could have gone quite away like that although a pain but fooling with the netre is beyong my pay grade.

[DavidBraley]

Interesting. When I get this thing running, I will definitely do some testing in limp mode and see what's up. So far my research suggests a 42 lb/hour injector for this thing. You mention a fuel pressure increase to richen the mixture in limp mode. I'm wondering if I could adjust fuel supply in a similar way to keep limp mode happy by simply playing with injector sizing. Then tune the whole system to that injector size when it's not in limp mode. I've been reading it's best to keep the injectors no larger than needed... But it might be a possible compromise.

I asked that last question about transmission types because I'm having trouble finding a 7427 compatible memcal from a 7.4L/4L80e truck. I was hoping I could expand my search to trucks that came originally from the factory with a manual transmission. Because this vehicle will be used on some cross country trips, I'm hoping to have some kind of limp mode available.

Thanks myburb!

[myburb]

I suppose you could modify limp with different injectors but if it were me I would put in injectors that were right for the motor and worry about limp later. I did my 40 miles to see how it went and it was fine for the 43 cruise but not great across the board as idle was pretty rich. As a back up on trips, I carry another pcm and spare chips sealed in a container in the back.

[DavidBraley]

Man, that's good advice. With a spare PCM and programmed eeprom, limp mode home isn't really necessary. I'll find a location for the PCM in this project so changing it out is a super easy thing to do. Maybe under the bench seat. Under the seat could be a great place to store the backup PCM. The bottom cushion lifts out for easy access. There really isn't much to this 2-Ton 53 GMC. No accessories or power anything!

If limp mode home is not as necessary, does anyone know if I could just use the 5.7L memcals that came with the 7427's I presently have? According to Rock Auto and O'Reilly Auto Parts, the knock sensors for both the 5.7L and 7.4L with automatic transmission are the same. I'm hoping if the knock sensor in the block is the same, maybe the knock board that sits over the netres chip on the memcal is the same. I've got four of these 7427's with 5.7L memcals. If so, then I can stop my search for a 7.4L/4L80e memcal...

David

[myburb]

You could use the 5.7 memcal and burn a $OE chip. I have a friend who uses a 5.7 memcal and has 454/4l60 and $OD bin modded for the 454. He has had it in limp and says it runs suprisingly well.

[DavidBraley]

You have been so helpful myburb! Thank You!

[myburb]

You're welcome. Have fun with the project.