Can I cut the rear firewall?

Kroggers · Kroggers's Garage

Probably a really dumb question, but with the engine out, I was considering the option of creating an access panel in the firewall for easy future maintenance at the front of the engine whilst in the car. But before I attack the firewall, I want to know if it is a structural part of the car and will require me to do some additional strengthening? I will be installing a role bar or full cage at the same time if that makes any difference?

Has anyone done this? I know some have created access panels to get to the transmission, but not seen any examples of access panels to get to the engine?

real550A · real550A's Garage

While almost every panel in a unit body is structural, your addition
of a roll bar/cage could be the substitute, depending greatly upon how/
where it is mounted.

Gene Wilkes · 11-28-2007, 06:00 AM

Check Jack Olsens thread. I'm sure he did that with a removeable panel on BB!

Kroggers · Kroggers's Garage

Thank you Mike. I was considering welding in extra strengthening bars above and under the access panel as well.

I think Jack only did a panel down to the transmission on BB where the bottom of the rear seats are, not directly back to the top of the engine?

AvonGil · 11-28-2007, 07:24 AM

Hey that is a great idea. You can cut it out, and make provisions to bolt it in, with two screws per side, this way it wont reduce the structural rigidity at all.

Kroggers · Kroggers's Garage

Quote:

Originally Posted by AvonGil

this way it wont reduce the structural rigidity at all.

Are you sure

tcar · 11-28-2007, 08:56 AM

Quote:

Originally Posted by AvonGil

Hey that is a great idea. You can cut it out, and make provisions to bolt it in, with two screws per side, this way it wont reduce the structural rigidity at all.

I don't agree. Of course it would depend on size, but, any usable hole would, even with a 'patch' screwed on. There are dynamic loads applied to the tub from the suspension.

Also... rust at the cut?

AvonGil · 11-28-2007, 09:05 AM

Well, everything is relative! I don't really think it would be that huge of a loss considering its a low stress section of the unibody...

TechnoViking · TechnoViking's Garage

I'm not a mechanical engineer, but I'm guessing with a sufficiently small hole and a sufficiently rigid frame (angle iron?) welded around the hole this can be done without compromising the structure.

The question is, what is sufficient?

I would not cut a large hole in the body and then just screw/bolt the patch back on, though.

AvonGil · 11-28-2007, 09:12 AM

A circular hole right smack in the middle of the panel would be the best idea. Losses would be minimal, even if not reinforced...

cstreit · cstreit's Garage

I've done it, but then again I also added a lot of cage and cross-member bracing as well. As someone said above, I think a smaller cicular hole would be the least problematic.

Some pictures:

78targa · 78targa's Garage

Here are a couple of pictures of a panel I put in my last hotrod/street car.

The look was a rear seat delete, but the access panel was big enough to get to the transmission/trans oil pump and cooler. The opening was braced with a gridwork of 1/2" square tubing. I thought it came out pretty well.

I would think that the rear seat area offers very little in structural rigidity. The rear shock mount is curved...frame rails are boxed. The sheet metal ther is extremely curvy for the 2 seat indentations....It can't be doing much as is.

porsche930dude · porsche930dude's Garage

I get angry at the porsche gods for not doing this from the factory everytime i have to fiddle with the damn cis!

haycait911 · haycait911's Garage

Quote:

Originally Posted by AvonGil

A circular hole right smack in the middle of the panel would be the best idea. Losses would be minimal, even if not reinforced...

right-on! a round hole in the middle of a panel, or even a few of them, will have very little affect on strength/rigidity.

Walt Fricke · 11-28-2007, 09:53 PM

Kroggers

I shared your worry - how are ordinary folks going to know which panels contribute what to the rigidity of the car? Of course, you can make some fairly good guesses as to what is critical. Any double panels, for instance, as they are functionally like a tube.

Many many cars which have been turned into race cars have lost much or all of the horizontal sheet metal in the rear seat area. You have to replace it so you have a metal fire wall, but that can be done with aluminum, and with panels that are fairly easy to remove.

I'm with those who suspect that the extra roll cage bracing that tends to get welded in serves to replace what is lost. In my car (a Targa tub) I cut out all the horizontal stuff from the top of the vertical panel in front of the rear torsion tube back to the structure at the top of the rear deck lid. The car already had a full cage, with an X brace going back to the rear shock mounts. If my cutting out of sheet metal had weakened something significantly I think I would have seen fatigue cracking somewhere back there in the 10+ years I've had this configuration.

As a Targa, access is great. The flat panel I installed over the front of the engine and rear of the tranny just Dzuses on, so I can get in there really easily, and without having to crawl into a back seat through roll cage tubing.

But for you guys with roofs and rear windows, having an access panel still makes some jobs a lot easier.

Walt Fricke