Looking at these, I've always seen them around on other cars and what not. But I'm not actually sure what they do.
I guess they help by keeping the strut towers from flexing on hard cornering, but again, I don't know how that will affect the handling of the car.
Can someone explain when and why someone would need one? And what do they fix.
Thanks,
Charlie
They will keep the towers from flexing under load. If there were to flex things like toe, caster, camber, etc will change for an instant and the car will not have the geometry that you set it up with. In addition to that if one side were to flex more than the other (very likely) then not only would the settings change they would be different from one side to the other. That's double bad.
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So this basically keeps your wheels and how your car is handling more inline so you aren't getting any surprises and a overall more stable car then right?
True, They do not make a huge difference in everyday driving though, it is more of a subtle difference in the spirited driving. I think they are a cheap and easy part and are worth it to put in.
I'm actually looking into converting one into a Monte Carlo bar. ONly difference is it will also bolt on to the fire wall. Dont know if I want a rear one or not though.
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I have a flexy 65, and after I put in some engine compartment bracing, it seemed to drive straighter than before. However, you will want to install subframe connectors as well, because now you're moving the flex from the engine compartment to the center of the car. This has the effect of imbalancing the car front to rear, making it skittish. Sure, you might drive straighter through the corners with the monte carlo bar, but now you're a bit more likely to spin. Not much, though. Subframe connectors are always a good idea, though.
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Fox bodies are notorius for being thin and flexy...You need to do every thing you can to stiffen it . Sub frame connectors, 3point strut tower brace. If your gonna do track days at least a 4pt roll bar that bolts thru the sub frames or is welded, a 6 pt is better. The 05+ cars are already very stiff....you need very little for these cars. GA
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Also consider bracing the lower rear points of the K member. It has the rear mounting points of the A arms, and has some flex to it during cornering. A simple straight bar across the chassis will suffice. Some aftermarket sources market the bar.
Incidentally, the shock towers don't get as much distortion as people expect. The common use of the "strut bar" is in McPherson Strut designs where the spring and shock are mounted together (Mustang is not a true McPherson Strut due to the spring placement). In the McPherson, ALL of the forces are transmitted to the towers, where in the Mustang, only the shock forces are. The chassis resistance to the front spring is provided by the K member, which is not directly attached to the towers.
Early Mustang ('64-'73) used the "Import Bar" to distribute the loads from the springs and shocks that were mounted from the upper arms to the towers. Therefore, in their design, they were suffering from the same problem as the McPherson Strut designs.
Strut Tower Brace - How does it work? 
