The Stovebolt Site Forums The Shop Area Engine Shop 401M intake

does anyone know of a way to machine a GMC 401M intake to accept a 4bbl carburetor? i have an adapter on it right now but isn't the smaller hole going to restrict the flow a bunch.

I was still playing with the GMC inliners when the v6 came out, but I don't think that much was done for performance with them. It was designed for a heavy truck, and I'm not sure what you can do with it. What are your intentions? Is it still in a truck?

i was thinking that without the bottle neck between the 4bbl on it right now and the intake it would have a little bit more power for hills, and the the extra 2 barrels might be a little more effective. yes it's still in a truck a GMC 7500 6X6 wrecker.

By the way it's a edelbrock 1406 i think it's a 600 CFM

I like four barrels because they have the progressive linkage. 2 small holes for cruising and 4 open for power. It would be better to have a 4 barrel manifold, but I'm not sure they made one. I would think the CFM should be about right. Not many 401's were produced which makes it hard to find speed equipment.

thats why i wanted to machine it. i was looking at some pictures and saw this http://rides.webshots.com/photo/1378765044033798696CEeydR do you think it would work?

That would certainly help, I'm sure. How much is hard to estimate.

If you over-rev a 401 by getting it to breathe better, have a big basket handy to take all the parts home. The big GMC's were designed for low-RPM torque, and with their inherent balance difficulties that required all sorts of special procedures during manufacture to keep them from shaking themselves to pieces, getting one overspeeded even a little is flirting with disaster.

I worked for a machine shop where we did a lot of big-six reconditioning, and most of them required incredibly accurate rebalancing after a bore job and piston replacement to have any chance at staying together.
Jerry

what's the max rpm for it?

I'd have to look up the RPM where it was rated for its maximum HP, but I definitely wouldn't exceed the factory-rated max RPM by much, if at all. If the carb helps it breathe for better torque without over-revving, no problem, but the lower end simply won't stand high speed without stretching rods, or other catastrophic failures. The pistons are extremely heavy, and put a lot of stress on the rod bearings at speed. If the engine doesn't have some sort of governor, I'd suggest fitting an electronic rev limiter to it once the carb size is increased.
Jerry

Edit:
I don't have specs on engines prior to 1974 handy, and by then, the 401 was not an option. The 379 V-6 that year was 170 HP @ 3600 RPM, with max torque of 280 foot-pounds @ 1600. The 478 made 192 HP @3200, with 371 Ft/Lb @ 1400. They're definitely low-rev engines, with absolutely no advantage to increasing the operating speed.
Jerry

401 horsepower/rpm specs, '61-'64; 182 @ 3400, '65; 210 @ 3400, '66-'70; 237 @ 4000. Torque specs are 351 @ 1400, 377 @ 1400, and 372 @ 1600, respectively.

The 401 used the same stroke as the 305 and the 351 which made it more tolerant to high rpm. My first truck tractor was a 550 GMc with a 370 V8 and I ran along side quite a few 401's while hauling grain from the Texas panhandle. I could run with them in the rolling country, but they could out pull me in the hills. They kept their tach as close to 4000 as possible just like I did with my V8 and their Hoof governor cut them off about there. They could go a lot longer between over hauls than I could and they were not throw aways like the 409 Chev. If you drive it sensibly, I don't think you could hurt it. Many of them pulled 3 loads a week from Plainview to Corpus.

I've been following this but didn't comment till now. If you need more power in your 7500 6X6, you may just want to bump up to a 478. I drove a '62 6500 with a 478 and it had no problem pulling 35,000, even up hill. Might not have been the fastest, but it sure had power.

Side note: How high to rev it and when to shift/downshift played a pretty big part in producing the power of it. (You don't need to max out the revs like you would a Detroit).

Bill

i actually have a freshly rebuilt 427 tall deck in the garage that i want to put in it but the 401 runs good and only has 16,000 miles on it. also my dad says I've got too many trucks torn apart in the driveway already.

i started working on the intake and ground off most of the carb mounting pad and it's only about 3/16 of an inch thick when it's level with the rest of the intake do you think it this could warp enough to make it leak?