LS3 427ci Blueprint w/ Harrop ITBs - Backfiring issue

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11-17-2019
first350

LS3 427ci Blueprint w/ Harrop ITBs - Backfiring issue

Started up my new crate motor for the first time and am getting fairly bad back-firing (~3 backfires per second).

https://photos.app.goo.gl/zn3X4ZhXeVt2AkG98

Details:
Engine: New Crate motor from Blueprint, LS3 427ci, EFI w/ full wiring harness and GM LS327/480 ECU
Intake: Harrop ITB setup, w/ custom intake manifold that joins both Left & Right banks into a common 4" intake tube so that I can use the MAF. Harrop integrates the OEM Electronic Trottle body into the ITB set-up, so that's what drives the ITBs.
Fuel pressure:confirmed to be ~60psi, as expected

My thoughts:
-Data says the ECU thinks TPS (Throttle position) is 30% open...but that's at idle. I can see the ITB throttles and they are all closed. Maybe it's dumping in extra fuel b/c it thinks the throttle is at 30%? (I blipped the throttle a few times, which shows up in the TPS data,...so I know it works).
-Intake: Maybe I have a leak, and it's running lean?
-ITBs: maybe it's un-even (should be all pre-set & flow confirmed from Harrop)...I measured the temp at each exhaust runner and most were at 120F, but a few ~85F. Not a big difference, but I didn't let it run very long.
-Fuel Injector wiring harness: I had to lengthen all of the wires...fairly confident that I did it correctly and confirmed good crimps. Each exhaust runner is hot, so I assume that IF I get fuel, it won't be intermittent...so this seems like a lower likelihood.
-Spark Plug wires: I had to make custom length spark plug wires and had to be much more firm when installing the boots. Each exhaust runner is hot, so I assume that IF I get spark, it won't be intermittent...so this seems like a lower likelihood.

Any help would be greatly appreciated...I've done some tuning before (not a lot), but this is my first time w/ HP Tuners. Thanks!!

Link to video: https://photos.app.goo.gl/zn3X4ZhXeVt2AkG98
11-17-2019
WAK

"Data says the ECU thinks TPS (Throttle position) is 30% open" thats normal, and not accurate. You can look at the throttle commanded rate and see how much the throttle is actually open.
How long did you let it run? If its a returnless fuel system it will take a few minutes to get all the air out of the fuel rail and injectors. All the exhaust runners should come up to temp right away if the cylinder is firing.
11-17-2019
first350

Quote:

Originally Posted by WAK

"Data says the ECU thinks TPS (Throttle position) is 30% open" thats normal, and not accurate. You can look at the throttle commanded rate and see how much the throttle is actually open.
How long did you let it run? If its a returnless fuel system it will take a few minutes to get all the air out of the fuel rail and injectors. All the exhaust runners should come up to temp right away if the cylinder is firing.

TPS: gotcha...thanks. (I plugged back in my OEM Throttle, and when I compared the idle & WOT % & Volts were nearly identical).

I let it run only ~30 seconds due to the large amount of back firing before I shut it off. It's a standard LS3 fuel system (return-less)...for a motor swap, I'm accustomed to having to crank it longer before it fires up (waiting for the fuel to fill the lines), but I've never had backfiring before.

thanks for the help!
11-17-2019
kevin87turbot

Is it backfiring out of the intake or the exhaust? Is it one bank or both?
It's easy to have one bank of TB blades out of sync.
If it's only on one bank, it could be the blades out of sync or one of your coil harnesses could be flipped. I've seen that several times now.
Edit...looks like it's just backfiring out of the exhaust. Check the coil harnesses and the plug wires carefully.
11-18-2019
first350

1 Attachment(s)

success (sorta)!! I switched to Speed Density (turned off the MAF), and updated the intake manifold volume from ~10,000cm3 to 6,000cm3 (a rough guess as my ITBs have a very small volume after the butterfly valves). Results:
-a few backfires (exhaust) on start-up, but after that it was clean
-Black smoke through the exhaust, so likely running rich
-High idle at 2,700rpms...when I blipped the throttle, the idle stayed high
-I only ran for ~30 seconds as the exhaust started to smoke (new exhaust), and I was keeping my garage closed b/c this thing is loud and I have neighbors. I'll need to do a prolonged run on a weekend w/ garage door open.

Up next:
-Need to lower the Idle
-Would like to figure out how to make the MAF work