401 vs. 425

[CyberGolem]

Hi,
I was wondering if the 401 block had any similarities to the 425 or if they were different castings altogether.
~If they are different, then how much more can a 401 be modified in size and power?
~From what I've heard the exhaust ports on the heads are pretty much maxed out and there isn't much that can be done to improve them, so would this be a reason not to bother with beefing up a 401?
I'm not wanting to build a race engine, but when I go to redo this block I'd like to know the best approach while I have it apart.
Cheers,
-Chaz

[Dr. Frankenbuick]

They have the same external casting, but the internal casting is not as thick in the 401 as I understand. They share the same heads and stroke as well. .030" over is all that i would bore a 401 without sonic testing to see if it can go larger.. The engine is very strong and durable as is. You could do a little more cam and a little porting in the heads and call it a day. http://www.fileden.com/files/2006/9/...ing%20List.pdf

[BigRivy]

The 401 and 425 are basically identical except for the bore size. The only thing that does not interchange is the pistons and rings. You cannot bore a 401 to 425 specs as the block was cast with larger cylinders in the 425. The 425 is the thin one. That is the one that needs sonic tested before any large overbore is attempted. Many recommend sonic checking the 425 before any rebuilding is attempted as a few have been found with a lot of core shift which has resulted in some very thin cylinder walls. The 401 is a damn good engine in my opinion. They have some weak spots, but most engines do. As far as performance goes, I don't think one is better than the other. Have you ever driven a good running Nailhead? They don't need a whole lot if you have a street driven vehicle. It's a torque engine not a big horsepower/RPM engine. Yes the heads limit the flow. Not much you are going to be able to do about that. Find a Q-Jet intake, put some headers and good exhaust on it, Maybe a little cam, electronic ignition and tune the hell out of it and it should run great. You'll never look at a small block Chebby again.

[CyberGolem]

Thanks for the replies guys.

Making a 401 into a 425 surely isn't the intent. I just want to squeeze out the most without making it into a needy motor. Hopefully turning my Riv into a strong and reliable luxo-cruiser is the end goal. I'd only bore it if it required it or the improvements were substantial enough without causing other problems.

@Dr. Frankenbuick: Any recommendations on who makes a decent cam for the 401 and who does reliable head porting? Thanks for the checklist!

@BigRivy: I'm not sure if I've driven a decent nailhead. The 401 I own runs okay (I think). There's some pinging and she feels a little sluggish on the throttle at times, but overall I'm a huge fan of this motor. While driving, it sounds like she has an exhaust leak on the passenger side, but when I stand outside the car and rev the motor I don't hear the same sound and don't see any evident leaking... strange. The pinging comes and goes.

[Dr. Frankenbuick]

Thanks for the replies guys.

Dr. Frankenbuick: Any recommendations on who makes a decent cam for the 401 and who does reliable head porting? Thanks for the checklist!

The fuel pump can ping. Listen in that area. I would be most worried about pinging under acceleration. That could mean your timing curve is out. The bushing in the mechanical advance can fall out or disintegrate which allows too much total advance.

I like Carmen Faso for parts 716 693 4090 for parts. He has cams and is very helpful. TA Performance (TAPerformance.com) also has cams and makes a shorty header that will work in your car.

One of the best Buick porting services is done by Greg Gessler in NJ: http://gesslerheadporting.com/gessle...nu-items/ghp00.

The Quadrajet manifold and Quadrajet is a very good idea as stated above. I had Carmen rebuild mine.

[wkillgs]

For camshafts, there are a few suppliers to choose from:
Carmen Faso
TA Performance
Schneider racing Cams
http://schneidercams.com/364401425hy...camshafts.aspx
Accelerared Motion:
http://www.amotion.com/amcams.html#14841
and Comp Cams....while they don't have much listed in their current catalog, they DO offer custom grinds for your specific requirements.

Nailheads do have limited valve-piston clearance, which can become nonexistant if the wrong cam is installed. Ask before purchasing!

[CyberGolem]

Awesome info. Thanks to everyone.