I have a custom set of headers on my 401 nailhead. The tubes are 1& 1/2 ins diameter on the primaries with a transition section from the the square exhaust ports. These tubes feed a 3 ' collector. I'm replacing the 401 with a 425 bored .040 over. I'm told that my primary tubes should be 1&5/8 ins. I see that Sanderson headers for the 401-425 primary tubes are 1&1/2 ins. My question is " can I run my exhisting headers on the 425 or should I builds a new set using 1&5/8ins. tubing? Any advice you can give would be greatly appreciated .I don't want to burn the valves.
I wouldn't spend the money if its not necessary. Run what you have for now. The 401 and 425 all have the same size exhaust valves (1.5''). My personal opinion, the exhaust valve diameter combined with duration and lift (based off cam lift) of opening wont flow more air than what can fill a round 1.5'' pipe.
Keep what you have and enjoy it . If you at full throttle all the time you may or may notice a difference . They type of headers you are running are really not noted for performance gains . Nice car and I love Nail Heads keep it alive and enjoy it
1.5" is totally unacceptable, we'll take the 425 out next week and put the 401 back. I'll take the 425 off your hands in payment for having my hoist for so long..
Last thing you want to do is choke the engine with too small a diameter primaries, I looked at a Falcon build the other day with a Boss 429 and it had 2 3/8" primaries
Is there something I should know about Fram filters? And what brand should I use? I have a 62 Electra 225, 401 nailhead, two AFB's. Thanks in advance.
The good thing about Fram filters is the color. After that everything kinda goes downhill. I have 1.5 inch tubes on my 425 nailhead.
Your headers will work fine as is for street use. If you ever do have another set built, I'd have them use 1 5/8" minimum (I'll be using 1 3/4" on the 425 in my '33 pu) and make sure the finished ports in the header are the same or slightly larger than the ports in the head.
There are multiple threads that attached some pretty conclusive videos about the filter construction of several brands. Use Wix or NAPA gold. I mean damn, if you saw the videos you would not waste money on those Fram filters.
The only 2 filters that you should ever run are WIX or Napa Gold. Fram filters are restrictive and are built with zero quality control.
Keep what you have your not going to gain anything by going to a 1-5/8" tube. The 401 and the 425 both use the same heads and you can only pump so much air through those heads. X2 because NAPA gold is WIX.
In late because I have an opinion... lol 1 1/2" is absolutely fine for a 425, they don't spin a lot of R's so it's fine. And Fram filters suck. Ha I said it.
You will get better low and mid range flow, so they should help you where you do most of your driving. You would have to go to redline pretty much all the time to really utilize a 1 5/9" pipe. 1 1/5 inch primaries will support up to 325 HP reliably. Are you making that much power?
Any headers are better than the stock manifolds. With the small valves in Nailheads, those should work out fine. Those heads do not flow at full-out competition rates. I have custom made 1.5 inch primaries on the headers on the Nailhead in my '35 Pick-up and they are fine for all around driving......other than at WOT how would you know any different????
There is a recent EngineMasters episode that swaps header sizes on a 500 ish HP small block Chevy. I forget it 1 5/8" were the bigger ones or the smaller ones. Not much change below 5000 rpm or so I think I recall. I'm thinking any under car exhaust is likely to be a weaker link. Details like how tight the pipes turn off the cylinder head is mighty important too. Over on Speed talk there is a kool thread about "is your exhaust small enough" https://www.speed-talk.com/forum/viewtopic.php?f=15&t=55767
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