Mopar Action On-line – Tech Questions


Tech Question

Travis Vela, Stafford, TX, 1974 Plymouth Roadrunner 318

Hey Rick, how’s it hangin’? I know I don’t have the mostpopular and lightest B-body in the world, but I’m having trouble findingcurb weight info on my 74 Roadrunner. Also I’m wondering if there’s anyeffective ways to put her on a major diet, as I hear they’re pretty damnheavy! I plan to make a handler out of it. But all I can do is search forstuff that they actually MAKE for my car, like the light Wilwood brakes,tube control arms or with a little more saving, a tubular k-member, lighterbucket seats (I still have a bench), aluminum heads, Sanden A/C compressor,etc. Will fiberglass make a difference (all I can find is a hood)? But whatreally makes my late B-body heavy? Thanks a bunch, Rick. I always lookforward to your wisdom.


First, a correction: Your car is a Plymouth Road Runner. Not a Roadrunner.

The crux of the problem is the rubber-isolated front suspension andK-member. In a pre-’73 B-body, the K-member tied together the frontlongitudinals, making for a rigid, lightweight structure. When the designbecame isolated (for NVH), each front rail became a floating (cantilevered,actually) “prong” that needed to be amply stiff independent of any otherpart of the car. To accomplish this, lots of mass was added. The bumpersystem, to meet Federal regulations being phased in, also added a LOT ofweight.

Even more unfortunately, most of the added weight was on the nose, trashingthe early car’s near-neutral balance.

The chassis can easily be stiffened to the point where it is actually betterthan ’62-’72 B-bodies, by the installation of aluminum blocks in place ofthe rubber isolators ( The improvement is startling. Butweight reduction is another matter, and, in practical terms, impossible. Youcan take weight out, of course, (aluminum heads, fiberglass, etc.) but youwill always be at a disadvantage vs. the earlier cars. That, however,doesn’t mean you shouldn’t do it — quite the contrary! You could, once theK is no longer floating on rubber, Swiss cheese it like mad with a good holesaw (and lots of time). You could do the same to the bumper reinforcementand brackets, dump the guards, etc. Every pound helps!

Tubular K-members, as a rule, are not recommended for street use. MostWilwood calipers are also not really streetable, having no environmentalseals (“boots”) on the pistons.

Wild guess: Your car is 3800-3900 pounds. Why not scale it though?



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