So, I know the conventional wisdom on this: NO! But I was on the phone with Price Motorsport, about their 4V flanges, and the guy I was talking to mentioned that those flanges had provisions for 1 3/4" tubing at each port. I said that that was great, because I had a set of 1 3/4" 2V headers that I could just swap flanges on, but then it dawned on me, if I'm using the same diameter tubing, and the same mounting bolt locations as a 2V head, how different can the ports be?

As I said, I had always been told that they were extremely different, which they are, but that they were so different that 2V flanges wouldn't cover 4V ports. This is not the case. I just went out and did all sort of measurments, back and forth and back and forth, and the flanges are plenty large enough, bigger in some places than the 4V manifolds in fact. The only part that is questionable as far as fit is the raised section around each port on the header that actually does the sealing, but I could grind that off, and either weld a new, wider one, in the shape of a 4V port, or just mount it all up flush, flange to head.

Does anyone see any reson why this won't work?

AleX

 

Performance will certainly be down, way down with 1 3/4 tubing. It is way too small for the big 4v ports. I think you would be a lot better off to just run the stock 4v manifolds.

 

Well that's one thing that I thought of, but after doing research, I found that one of the most common sizes for a 4v header is 1 3/4". The flanges that are my other option are premachined to accept that size tube, and on summit and jegs 1 3/4 is one of the top two most common sizes for these engines (although I know I have 50 inches on a 351), and that's for cars, where the engine is going to rev way up as high as it's meant to, not in a truck like my application. I'm running a 2V intake to get the low end tq up some, I'm thinking same idea on the other side. Am I wrong?

Even the Sanderson headers, what I've turned up as basically theee name in headers for a 4V, are only 1 7/8".

 

Are your heads open or closed chamber ? Have you considered a after market dual plane intake with a 4v ?

 

Closed chamber, and I'm putting a 2V Edelbrock air gap on today. And I looked into dual plane 4V's, but I'm not trying to build a 7000 RPM engine for a Mustang, this is going in my 4 wheel drive truck. The truck is used aaalmost exclusively for street driving, and alot of hiway at that, so I'm actually trying to get the resistance in the runners up a little, to get the fuel to blend better with the higher velocities of a 2V runner, even at low speed.

 

Here's a very interesting little page I dug up, for all aspects of Cleveland building, but there is some cool info on 4V 2V.

This guy says that on a 351 4V a 1 7/8" header is ideal, but he is also talking about a 351 in a Pantera, revving close to 7000 RPM. At 7000, each cylinder in a 351 moves 88 cfm (I know that is assuming 100% VE), at 6000, the MOST my engine will ever see, my engine moves (wow, this is a strange coincidence, I am just doing this math as you read, this is a suprise) 88 cfm also.

The cross sectional area of a 1 7/8" header is 2.76"sq. For a 1 3/4" header it is 2.4"sq.

That's actually more of a difference than I thought, but the smaller tube still moves 87% as much volume as the larger. Do I want to settle for 87% performance, no, but I am so rarely going to be pushing 6000 RPM's, and at the lower speeds, I feel like the scavenging ability of a slightly smaller tube is really going to help me out.

Keep in mind also what my actual options are: Shorty 1 7/8" headers into 2.25" duals soon, 4V manifolds into the main tubes even sooner, and less smoothly, or long tube 1 3/4" headers with big fat collecters that reduce nicely into the main exhaust pipes.

I have not been able to find a full length 4V header that will work on my truck, just some very short ones, that basically pattern directly off the shape of the stock manifolds, and I feel like those will just be so free flowing, that with a tall cam like mine, with a decent amount of overlap, my exhaust gasses could just get sucked right back into my cylinders.

What I do think I'd need to figure out if I went with 1 3/4" primary tubes is the shape of the port. The 4V head ports are square, corners and all, and the 2V ones are tall ovals. I don't really like how the square ports flow in my head, with their corners, but I can't just have them slamming into the sides of the thinner oval 2V header ports. For that I could cut the tubes off right behind the flange, reshape the flange ports, and then expand the tubes to fit with some heat, but that would leave me with smooth, yet flow reducing headers, with the resistance to flow pointing directly back into the ports, and then we're back to the same problem, bad scavenging.

Hmmmmm. Sorry to be longwinded, but I've got my thinking cap on right now. Interested to hear what you (or anybody else) think.

AleX