Where to cut the hole for Harness in firewall

[1sixty8]

Hey guys, I have a 68 Camaro and I just purchased an LS3 stroker 427 from blueprint engines. It comes with a stand alone harness that uses a GM ECU and fuse box. My question is, where should I cut the firewall to feed the harness through? I plan on mounting the fuse box and ECU in the glove compartment so what would be the best location for a hole to feed the harness through and what size should I make it? I was going to use the old throttle pedal hole and just make it bigger but the LS3 is DBW, so I bought the bracket to mount the pedal and it utilizes the original throttle pedal hole. Any help and pictures would be greatly appreciated! I'm new to this forum but I can see it's got a ton of great info and enthusiasts alike.

[Mark69bird]

Hey guys, I have a 68 Camaro and I just purchased an LS3 stroker 427 from blueprint engines. It comes with a stand alone harness that uses a GM ECU and fuse box. My question is, where should I cut the firewall to feed the harness through? I plan on mounting the fuse box and ECU in the glove compartment so what would be the best location for a hole to feed the harness through and what size should I make it? I was going to use the old throttle pedal hole and just make it bigger but the LS3 is DBW, so I bought the bracket to mount the pedal and it utilizes the original throttle pedal hole. Any help and pictures would be greatly appreciated! I'm new to this forum but I can see it's got a ton of great info and enthusiasts alike.

I have a 69 Firebird and basically doing the same thing. I plan on going through behind the motor just above the trans. Want to try and hid as much of the harness as I can. I don’t have this completed but if you want a clean look I believe that’s what most people do.
Welcome and cheers!

[1sixty8]

I have a 69 Firebird and basically doing the same thing. I plan on going through behind the motor just above the trans. Want to try and hid as much of the harness as I can. I don’t have this completed but if you want a clean look I believe that’s what most people do.
Welcome and cheers!

I'm wanting to do the same thing and hide as much of the harness as possible. I'm just really hoping to see some examples and what kind of gromets were used in photos. It makes me nervous to cut the firewall. it's all painted and clean already.

[dhutton]

These bulkheads are pretty slick.

https://sharpeperformance.com/collections/billet-bulkheads

Don

[robbbc]

I'm wanting to do the same thing and hide as much of the harness as possible. I'm just really hoping to see some examples and what kind of gromets were used in photos. It makes me nervous to cut the firewall. it's all painted and clean already.


I ordered my harness through PSI and it came with the grommet. If i recall correctly, it is 2" diameter.

[Hotwire]

As with everyone else behind the intake. I pulled a grommet from the driver's side firewall of an 05 Tahoe:

https://static1.pt-content.com/images/pt/2023/01/52409897042_cfabb95f73_c-1.jpg

https://static1.pt-content.com/images/pt/2021/06/50436508482_7d5161f1bb_c-1.jpg

[Z06vet]

204856

I have used a spot just inside the tunnel on the pass side on several builds. On a first gen F body, there is a small flat area that is the perfect size. But I have used it on a second gen f body as well. Speartech offers a nice grommet with a thick flange, and you can trim the inside diameter to fit the harness. I mount the ecm above the glove box on the upper cowl, and the pass thru spot works well with that location.

[robbbc]

Here is where I drilled my hole. It seems to be working well. I was trying to attach a picture of the motor in it but the computer just isnt having it. Hope this helps.
204879

[1sixty8]

Thanks guys! all the pictures are really what I was after. You always see the car with the motor in and you can never tell where they cut the firewall for the gromet. Any pictures of how you mounted the computer on the inside??

[1sixty8]

204856

I have used a spot just inside the tunnel on the pass side on several builds. On a first gen F body, there is a small flat area that is the perfect size. But I have used it on a second gen f body as well. Speartech offers a nice grommet with a thick flange, and you can trim the inside diameter to fit the harness. I mount the ecm above the glove box on the upper cowl, and the pass thru spot works well with that location.



What ECU did you use? I feel like that defiantly dictates where the thing gets mounted. My motor came with the GM ECU with the 3 big connectors

[Z06vet]

I use a GM E38 ECM. Same size at the E67 it sounds like you are using, but 2 large plugs instead of 3. I used the E67 on my 67 camaro i'm currently working on. I used the same access hole location, and mounted it above the glove box as I have done with others. However, I use a stand alone harness from a vendor, not the GM harness. The way the gm harness is designed, and the length of the harness, really limits when you can mount everything.

[robbbc]

i have e38 as well. i mounted it in the glove box. it took some trial and error but i (cheaply) made a bracket that utilizes the glove compartment mounting holes. I do not have a glove box now, so thats the downside of it.

[F-Body International]

The spot I would say is closest to what you want is the area where GM put the computer harness hole on ‘81 Firebirds and Camaros. ‘81 was the first year GM installed a computer and that is the closest to a 1st Gen. GM put it on the firewall behind the intake just offset to the passenger side.

[79 Camaro]

On my 80 Camaro LS3/4L60E swap I bought a harness and ECM/TCM from Howell. My car doesn't have AC. I punched a hole in the firewall low enough just to clear the stock heater box. Salvage yard grommet just big enough to get the connectors thru along with the oil, water and DBW wires.
I sheet metal screwed the OBD port, fuse block and relays to the inside heater box. You can't see them from the outside of the car. I struggled a bit mounting the ECM and TCM. They aren't big so I thought how about Velcro? On the stock 80 Camaro inside heater box there is a nice flat spot big enough for for both the ECM and the TCM. Industrial strength Velcro holds both quite nicely. Still have a full glove box.
Pic of my 80 Camaro.205141

[brickyard69]

I put mine where the distributor would be. The hole is directly behind the intake so you barley see it. I used two of the large universal grommets from Summit. One on each side.

205162

[robbbc]

I finally got the pic to upload. This shows the location of the grommet. A clear picture of the hole without the engine in is a few posts above. 205166

[TANKMASTERJ]

Wish I would have done right behind intake above trans as you suggested. I ran mine through fresh air vent hole passenger side with a fabbed up cover plate.
It looks good but would have been better straight of back of engine. Unused glove box area for ECM and mounted LS fuse panel pointing down under glove box. Doesn't show and easy access.

[robbbc]

It seems to be just right for my setup. After I got the grommet seated, I used some of that butyl tape they used to use to put windshields in, to seal around the grommet.

[Hopperj7]

I have a 67 Camaro with an LSA. I mounted my PCM in the glove box and drilled a 2" hole in the fire wall above the trans tunnel.

205322

[Hotwire]

Wish I would have done right behind intake above trans as you suggested. I ran mine through fresh air vent hole passenger side with a fabbed up cover plate.
It looks good but would have been better straight of back of engine. Unused glove box area for ECM and mounted LS fuse panel pointing down under glove box. Doesn't show and easy access.

I did exactly that, but ignorantly used a silverado underhood fuse box. Pretty much all battery power, not a lot of switched circuits, had to get creative. Was also fun turning all the +12v triggers (headlight switch, etc) to ground triggers for the relays in the fuse box. It was a pain but is working really well though. I just threw my ECM on top of my then new glovebox.. box? and it's now starting to deform, have to get in there and fab up some kind of shelf. Thought it was temporary until I installed vintage air, but it's been about 5 years now. :nopity:

This was my mockup, left a little wiggle room once it went into the car.

https://static1.pt-content.com/images/pt/2022/11/52410688244_8b7cba9225_c-1.jpg