401 to 425?

[bebop138]

Can a 401 be bored to 425? Is there any other things need to be done if this is possible?

[The Old Guy]

NO a 401 will not bore to 425 specs.

[Guest GSXMEN]

Best to have the block sonic checked to determine how far you can overbore. Money well spent.

[55PackardGuy]

I wouldn't do it. The 401 is an excellent engine in its original displacement. When I had one, I heard on the grapevine that it has some performance characteristics superior to the 425. I believe the torque curve is flatter. If you want to freshen it with an overbore and oversized pistons, that might be a thought. Check with the "performance guys" out there for details. But, it's your car! Have fun whatever you decide. <img src="http://forums.aaca.org/images/graemlins/smile.gif" alt="" />

[Guest BJM]

The stroke I believe is the same for a 401 and a 425. The bore is the difference. Having said that, I see no advantage in going to a "true" 425. The decision for Buick to go to a 425 was due to the ongoing displacement machismo of the era. It was a marketing decision more then anything. The Buick engineers had to monkey with timing and valve duration so much that they lost the smoothness in the motor.

Here is what I would do (I've owned 401's and 425's). Rebuild the 401 and bore it out .030 over making it like a 407 or 408. Buy a higher lift camshaft to add about 10 hp without the lumpity lump at idle. Use a higher flow valve (I think you can get them through TA Peformance) Use a 2 1/2 inch exhaust instead of 2 inch or 2 1/4.

With this, you get the same horsepower as the 425, better torque, flexibility, and smoothness.

[NTX5467]

From my observations of hanging around an automotive machine shop for many years, it generally takes a .030" overbore to get to "fresh metal" to do a good bore job--at a minimum. Many drag racers went past that to .060", but for a street motor with normal wear and hasn't been bored before, the .030" will work just fine and still leave room for another overbore to .060" many years and miles down the road. Staying with a stock-spec piston rather than some domed higher compression ratio piston.

On the cam issue, possibly something close to stock duration but with a little more lift (maybe even something like a CompCams cam with assymetrical lobe configuration--initial lift is quick, stays at max lift for about 10 degrees of crank rotation, then closes slower that it openned--but with the wider lobe centerlines that most stock cams had rather than the aftermarket's common 110 degree centerlines). It might be a 400 cid engine, but you can still overcam it and loose some of the designed-in smooooothness and wide torque curve, hence staying close to stock duration (advertised, not using the newer .050" lift specs).

Port matching the intake and such can work too, just as the undercut stem valves can increase port flow. Finishing it with a 2.25" exhaust pipe dual exhaust system, maybe even 2.5" if you can find somebody to bend it and do a good job in putting it in the factory configuration.

Enjoy!

NTX5467

[55PackardGuy]

Some good observations, especially about that overbore, which could give you a nice boost and "freshen" your engine without getting radical on cubic inch displacement. Just a note on the cam... depending on the year of the engine, I believe the 401 had a pretty healthy cam, even to the point where the idle wasn't so smoooooth! If the "bump-stick" is in good shape, you may be just fine with stock.

[NTX5467]

Sometimes, a "smoooooth idle" can be more a function of the internal "balance" of the rotating assembly plus the eveness of the induction charge each cylinder gets (which gets into intake manifold flow characteristics and possible flow biases) in addition to the accuracy of the spark with relation to TDC for each cylinder.

"Internal balance" is the relationship of the weights of the piston assembly, connecting rod, and bobweights on the crankshaft to each other. There are specs for each of those weights. Some other considerations for these things when you see an engine disassembled (like the size of the balance pads on the connecting rods, near the piston pin bosses on the underside of the pistons). Additionally, there are also variations on the balance operation depending upon what rpm you desire the best smoothness (higher rpm, lower rpm, etc., called "overbalance" and "underbalance").

Of course, cam specs can play a part in that whole situation . . . plus the accuracy of the lobes from cylinder to cylinder. As with almost any machining operation, there can be slight variations from the spec, but sometimes you get a "tolerance build stack" that can make things a little ragged.

Just some thoughts and observations,

NTX5467

[55PackardGuy]

NTX,

That may all be true, but I was only referring to the slight "rump" in the idle that many 401 owners have noticed, and is thought to be attributable to a camshaft that's a bit more radical than is typical for a stock engine. Internal balance, while important, isn't related to this issue.

Since talk of 52er's 401 freshening had turned to camshafts, I thought I'd point out what I've heard of the 401's stock camshaft. When you think about it, it makes sense that the small intake valves of the "nailhead" would benefit from a longer duration and/or higher lift camshaft, so I tend to believe the rumors, but have no specs on the cams. Maybe you could look it up?

[Guest BJM]

Willis, 55 Packardguy,

My information on the 425 smoothness came from a book by Norbye and Dunne (Postwar Buick) where they quoted the Buick engineers at the time mentioning that they had done all they could with the Nailhead size wise and that they had to tinker with timing just to get it civil (as most Buick owners were accustomed to). I think 401 performance is execellent myself. A very torquey motor. I own and have owend the Pontiac 389, Buick 401/425 and Olds 425. Of those the best all around motor is the 389 followed closely by the 401.

[55PackardGuy]

Thanks for confirming that my allegiance to the tried-and-true 401 is not misplaced! I raced a guy with my 401 Electra against his 425 Oldsmobile 98 back in, oh, about 1977, and he beat me. He was crazy though. <img src="http://forums.aaca.org/images/graemlins/grin.gif" alt="" /> Actually, he was a college buddy who died at age 36, not in a car but at the dining room table--from a heart attack. <img src="http://forums.aaca.org/images/graemlins/frown.gif" alt="" /> I really miss Bob and I'm kinda glad he won. But I did beat him to the stop sign afterwards! I shouldn't have stopped though, he didn't expect me to and kinda flattened out his tires on one side. Kids, don't ever ever do this. <img src="http://forums.aaca.org/images/graemlins/smile.gif" alt="" />