Can an original (in great condition) Model A frame be used with a SBC motor? Can it be strong enough with just partial boxing...in a mild HP, full fendered roadster?
My R.P.U. is on a boxed and reinforced model A frame. It is powered by 425 buick nailhead with a st400 trans. No problem so far and Ive had it on the road for 7 years
They must be boxed correctly and custom type crossmembers added. Make the frame as strong as possible, and then stronger. It is the foundation for the hot rod . Partial boxing? I would say no, Full Box.
I can put an A frame on the floor, put the forklift forks down on the rear rails, and twist one rail up with only my strength. I was going to model one and do finite element analysis on it, but after the forklift test, I did not bother to waste my time. When the Model A was designed and built there was no interstate system, and very few paved roads. The frame was designed to flex and act as part of the suspension. That characteristic is not a desirable one now, even with stock drivetrain, let alone an SBC. I box every single Model A frame that comes through my hands. I use c-channel boxing plates now: https://www.code504.com/the-boxer-1928-1931-model-a-inner-channel-boxing-plates/ These are not expensive, and are easier to install than flat ones, as they resist warping, and hold the frame more true during installation. Many of the frames that I have worked with were already in hot rods. Of those, most had cracks near the rivets. As an Engineer I have a lifetime of materials strength analysis. I can tell you with scientific authority that the frame needs to be boxed.
'Can' and 'should' are sometimes two completely different animals. Engine output is not the only consideration. The stiffer and less flexible the frame is, the more your suspension system can do it's job properly, and the more it can be tuned to ride and handle well. I would rate this as just as important, if not more so...............
My 31 roadster has the stock model a frame with a 49 f1 center crossmember. I am just running a flathead and didn't want to box the frame. When I jack the car up from the center of the rear axle and push one rear tire down and back up, you can see the door gaps open and close at the bottom. They twist alot, so you really need to concentrate on keeping it from twisting, such as a good x member set up. You could incorporate it into the frame rails as ford did later if you didn't want to box it.
1928 to 1931 "A"s are same mostly. If not cracked or rusted much,>YES will hold SBC or any V8 with under 400HP< with out boxing. But yes,I did box my own 1928 frame much later,plus some updated cross members for tranny, about 40+ years after first building it into a hotrod in 1959,with a 57 V8 Ford Thunderbird power. I added boxing many years later,as rust had taken some straight out on inside of "C" in two spots.
Partial boxing is a cop out. You want your chassis as ridgid as possible so the suspension does the work and no twist means your sheetmetal body work will not be stressed and hence no chance of little corner tears etc. The other pls IMHO is that the undside of your project looks cleaner finished and easier to wipe down and keep clean...
The model a in my avatar had a sbc,turbo 350 and a pont/old rear end. It was not boxed when I bought. If you parked or drove up a drive way on a slant the car would twist and it had terrible torque steer.We drag raced this @ baylands it ran 14's.We were not smart enough to know better.Now it is boxed and a extra cross member added.
A Model A frame with a V8 engine needs to be boxed full length and another center crossmember added to replace the stock Model A center crossmember. On my latest build we are running a very healthy hopped up 59A flathead and used a 1932 center crossmember inside the Model A rails to make it that much easier to mount the 1939 transmission. The Model A rails were then fully boxed right up to the three crossmembers. The boxing plates were set back inside the rails about 1/2 inch to give a stronger weld and make it that much easier to run and hide the brake lines. We just made up small spacers inside the frame to get the right spacing. The frame is the foundation for any build. Do it right the first time.
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