I'm fairly new to the board and have a lot of questions about the nail heads. I have access to a lot of 401 stuff , but if i go with a nail head in my 65 skylarks i would rather spend the money on the bigger 425. my biggest question is what is different on the 401 vs 425?
I've always liked the way the number "401" sounds myself. Never driven them back to back, but other members have and said apples to apples the difference is hardly noticeable. I'd believe it, as the factory valves/heads have a tough enough time feeding the smaller bore 401.
that is the one of the biggest false hoods about the 401 vs 425. 425 is the way to go, way more bottom end, closer to 25 tq, rule is every cubic inch equals one ft lbs of tq. the heads only hurt at higher rpms over 5000 rpms or with cams over 500 lift. after that you will need head work then you can support 5800 rpms and 600 lift cams and at least 446 cubic inches.
Cam is a bit stronger too. If you can make these pics out the stock 401 in a 66 Electra ran a 8.5 sec to 60mph and the 425 option ran a 8.1 1/4 mile was .2 faster also. All else like rear gear ratio being equal.
did the 66 Electra with 425 have the Q-jet carb and intake or was that just the Riviera? because thats a big difference in performance right there
Yes, they got the q-jet is my understanding. Cool air cleaner too- this one is 66 a-jet- I was checking How it looked on a 430.
that could easily account for the performance difference or at least a good portion of it. A local builder who has a dyno and tests every engine before it goes out the door says the 425 peak HP and TQ comes in at a lower rpm vs the 401 but theres not a big difference in peak numbers in similar builds. the stock heads are very limiting
a 401 @ 5000 rpm needs 160 cfm air flow,a 425 @ 5000 rpm needs 179 cfm air flow. a stock head flows 189-191 cfm. so why would the heads hurt a 425 ? 425 peaks at less rpms so it needs even less air flow
They way he explained it was something to do with volumetric efficiency. Kind of like when a nail head runs better with way more carburetor than it should need.
wonder if both were dead stock or he put a bigger cam in, bigger cam would need better flowing heads on a 425 ,more so than a 401.
That's because the intake tract and heads are the choke point, not the carburetor. If a good flowing heads and intake ever gets made for a NH, then that will change.
