cracks in firewall???

m glassburner

m glassburner

New member
I just noticed some cracks in the firewall
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Not good IMHO ! Anyone ever notice something like this before?? The engine is coming out later in the week....so I'll have time and room to fix.I don't currently use a strut bar,Do you think that will help or hurt the condition?? Also the car was in a fairly big accident 3yrs ago that required some frame straighting.p.s. These(cracks) are somewhat new.(it's a fwd dodge omni 89 )
 
Right around the interface to the exhaust tunnel, right? Cracked firewalls are common in Dodge-based race cars. Both my L-body ITA Shelby Charger and my P-body SSA CSX ended up with significant cracks. Most of the IMSA cars from the 80's and early 90's (G-bodies, mostly) were cracked, too. It got so bad at one time that one of the pro teams was caught with having a double firewall!!

Most likely cause is concentration of twisting forces of the chassis localized in that area, due to the installation of a rollcage and inability to tie into the front suspension. The only possible solution is to design a cage such that it will minimize the twisting on the firewall.

The only short-term solution is to suck it up, remove the drivetrain and dash panels, and weld it up. Then, plan on it happening again... GA
 
I don't think a strut bar will help this problem, and in fact it could make it worse. Recognize the problem is a chassis that is very flexible, and is not designed with a rollcage in mind.

If you were to take a rectangular box and twist it with both hands the entire box would twist. However, if you install a bunch of cross-bracing from one end to 4/5 of the way to the other end then all the twisting forces would be concentrated right where that cross-bracing ends. That's the problem with the Dodge chassis: we take a flexible box and stiffen up only 4/5 of the chassis. Where that stiffening ends - the firewall and exhaust tunnel - is where all the torque forces concentrate. That sheet metal (there's no box or frame structure there) can't handle the load and eventually fatigues and cracks.

About the only thing you could do would be to maximize the rollcage rules and build some serious 100 sq-in plates on the firewall to spread out that twisting forces to as wide an area as possible. It'll only delay - or relocate - the inevitable, as the area between the firewall/exhaust tunnel and the front susnesion is not very stiff either. Something's gotta give, and since you cannot stiffen the cage-to-front suspension interface, there will still be a lot of flex.

Just do the best you can, and cross your fingers... - GA
 
Cross brace (ie dash bar)was added last winter...the car is 4+ years old....thanks guys...I guess it's time to go to fp/lp
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Ummmmmm one last question what repair technique w/you recommend (legal and safe)I want to fix it right and once ??

[This message has been edited by m glassburner (edited July 12, 2005).]
 
Fixing stress cracks--Hmm...lot's of welding; grind; weld thicker sheet metal over the area.

You may further want to "seam" or stitch weld a bit out from the repair.

Maybe not perfectly legal, but we are talking about saving a chassis via means of "standard" repair methods. Plus it's an American car.

A new firewall is not practical nor probably available.

Good luck!
 
A really good friend of mine in the performance tire and wheel business once told me that he saw more Shelby GLS and Omni GLH cars with collapsing strut towers and cracks around the fender wells than any car he'd ever seen. Apparently, it's a design flaw that Chrysler Corp. didn't check for. Problem of stiffening a portion of the car to eliminate a weak part only leads to the weak part finding another place to happen. A strut tower brace might have prevented the problem early on, but it may be too late for it now. A possible fix might be to do some really delicate and creative welding on the firewall and anywhere it attaches to the tub. Also some stiffening of the welds at any place under the hood where the suspension components attach to anything. Trouble is, I'd be real careful about how that was done as you're skating on the rules about stiffening the chassis. After you feel confident that your firewall problem is fixed, then go with the strut bar. The cross bar on the cage is good, but its real service is performed in stiffening the roll cage, not the chassis.

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Chris Harris
ITC Honda Civic
 
I don't know if you can call it a design flaw as much as the weak part of the system. Every product has a weak point that you will see failure develop at first, it happens that the GLHS's saw a lot of hard duty but tens of thousands of Omni's and Charger's made it through life without a problem.

The two areas I've found to watch out for on the charger/omni are the firewall/tunnel and the where the inner fender/shock tower attaches to the frame. In both cases there isn't a whole lot to be done under IT rules. Exploiting the cage rules to the max will buy you some time before the new stress point appears for the firewall. As for a strut tower brace, the problem has more to do with the attachment to frame rails so tying the tops of the towers together doesn't buy you a lot. There is still a lot of motion between the strut mount and the rails. It's worth doing but I still make it a point to check the shock tower spot welds and fire wall regularly.

It could be worse, I could have an E36 that tears out rear subframe mounts! We all have to live with the limitations of what we choose to race.
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~Matt Rowe
ITA Shelby Charger
MARRS #96
 
like BMW's overdone "factory authorized" fix, doesnt Dodge have a fix for it?? are there any TSB's on it maybe?? if there are that may help in the tech shed if any protests arise...

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Evan Darling
ITA Integra
 
I haven't been able to locate a TSB on it. That doesn't mean it's not there but I have done some checking. In fact, I have had very little success in finding any factory documentation on frame/body repairs. "Industry best practices" is the MOST definitive statement that seems to pertain.

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~Matt Rowe
ITA Shelby Charger
MARRS #96
 
Better yet, if you're logbook is old enough, the 100 sq in rule doesn't apply. I suddenly envision a VERY, VERY large pair of front mounting plates in my future.
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~Matt Rowe
ITA Shelby Charger
MARRS #96
 
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