I'm going to be building my own set of fenderwell headers with 1 7/8 primary tubes should I use a 3 or 3 1/2" collector? Race only engine, Big block 440 see's 6500 rpm max, est 525-550 engine HP.

I've been down that road more than once, on your deal with that motor combination in my A body cars liked the Hooker A body fenderwell headers with the 2.0-inch primary pipes with 3.5 inch or larger collectors for all out drag racing better than the 1 7/8 inch set with the same collector size: up:
You know you're going to want to make more power later so build the big pipe set now and not have to do it over later

Though I've never tried a larger collector mine has made plenty of power using 3.5". And that's with a 580" motor that sees 7300rpm. I doubt if you need 3.5" with your combo.
Doug

I agree w Cab on the tube size. Also on the collector, start with at least18 inches in length and shorten from there if you need to.

I was told by my father and his buddies who raced S/S back in the 60's to start with a Longer (24"?) Collector - paint it a light cheap black color and after 5-6 runs the paint will burn off to where we should cut them off.
Does anyone still use this theory?
I happen to have a new set waiting to use on my Demon that I just (lightly) painted.
Not trying to hijack - just asking to hopefully help.
Thanks to the OP for this post!!

Wally.

I would run 3 1/2 if it were me....1 7/8 is kind of on the border line though. Anything smaller I would run the 3 inch.

Somewhere I have my Headers by Ed design documents that go into details on tube sizes, collector size ratios vs tube size, etc.

What I recall is based on Ed's formula, a 1-7/8" with a 3.0" collector is pretty much right in the middle of his acceptable range; a 3.5" is pretty big for that tube size, unless you're turning a lot of RPM and/or narrow RPM range (stick car vs auto).

With 2" tubes, a 3.5" is also pretty much right in the middle of his acceptable range. I've only seen one prod header that I remember 2" tubes and 3.0" collector, and that was a Hedman Header model which I never considered.

A 3.0" for a 2" tube is definitely on the small side, although in a merge collector it could taper down to 3.0" before expanding back to 3.5". But everything I've heard says you should design that from the start, not just slap a merge collector on a header not designed for one... even though I see that's what a number of header companies do now cuz it's simpler that way.

if running a 3 1/2" collector into a full exhaust system, and the collector is 3-6" long on the header, would it be of benefit to run 3 1/2" pipe to the x, then reduce to 3" [at the x pipe] for the rest of the exhaust system ?

Respectfully, I wouldn't build fenderwell headers until you NEED to poke them out the fender. e.g.- OVER 1000HP Plenty of room to accommodate 2" or larger in a b eng a-body and do it clean and 'correct'. Also, if you're building custom just use 2" x 3.5" IMO OR BUY the TTI. While I dont like them, they seem to work pretty OK!

I was told to use a crayon to mark the collector extensions and run the car several runs and to cut the collector off where it melts the crayon
I've never done that but every Mopar, stick or automatic car, I've added collector length to went faster and quicker so test, test and test some more

Car had 2" hooker fenderwell on it for years. We than added snout bars and for whatever reason the way we did it interfered with the headers, but at the same time I bought a set of TTI 2" headers as I wanted to run a larger front tires after years of red lighting. Used them for a good many years, but absolutely HATED working on the car with them. Yes they fit great once on, but man they were miserable especially the trans bell bolts. I ended up putting TTI 1 3/4" headers on the car as I had them and was over working with the 2" ones. Used them for a few years but felt they were a little small for my engine, so I modified the old Hooker 2" fenderwells so they would clear the snout bars. I did the swap over the winter but didn't change anything else in the process. Comparing time slips for the 1 3/4" TTI vs Hooker 2" it seems the 60fts are the same, but MPH may actually been slightly better with the TTIs. The TTI's did have a 18" glasspack muffler vs the Hooker 6" Schoenfeld Race Mufflers. I ran each for atleast 2 seasons, so have a decent data base to compare to, but didn't make a ton of passes (only made about 30 this year). I'm to the point that I don't want to mess with underchassis headers as I've been spoiled by the ease of fenderwell headers, but also I'm tired of my front tires rubbing. The header flanges have been welded up and need replaced on the hooker as they are about 25 years old.

In the late 70s we ran a 413 stocker with 2 inch adjustable hookers and 3.5 collectors.One weekend we substituted 3 inch Stahl collectors just because we had them and were curious. The results were stunning. Can't remember exactly how much better but we never went back. Of course the craftsmanship was much better in the Stahl collectors. The collectors on my current motor are junk compared to those old Stahl pieces so I will only suggest you look for guality as well as size in today's parts market