Henry floored,
On a stock flathead engine I don't think the exhaust manifolds are a restriction. If you look at the size of the port at its start right next to the guide, it's tiny this obviously hurts flow. Now look at the manifold end of the port, the exhaust gas slams into a wall at right angle, this must play havoc with any flow, let alone the violent exhaust gas bouncing a shock wave back up the port. It's gota turn 90 degrees towards the front or back of the engine (weather it's the rear or front port) then it hits another flat wall were the port comes out. Meanwhile trying to get through still very small opening through this series of turns, I can't get much knuckle through that gap.
After all this hurt through the block? Even the early smaller exhaust manifolds look like the wide open world to the exhaust gas.
This is why headers don't make a difference on a stock port flathead in my opinion.
Proper porting changes everything I believe, if you can get the exhaust gas out of the block nicely without all these 90 degree turns, then the manifold plays it's part, properly tuned for length would make a difference, ONLY if you get the exhaust out the block nicely.
On a stock flathead the porting in the block is the restriction not the manifolds.
I do agree with you on the W shape header being the best shape, but they are far to short to play a roll in tuned length.
Martin.
Scooter it sounds like we agree. I have given all this quite a bit of thought over the years.
I would like to add a little bit more on this subject so here goes:
The Flathead Ford, to my knowledge is the only side valve "vee" type engine that runs exhaust ports through the block. If you look at early prototypes of Henry's venerable V8 you would notice that the end exhaust ports exited the block up high and were carried down by a bolt together exhaust manifold. All of the exhaust ports were connected down in the normal location by this muti piece exhaust manifold. In a very interesting side note these early prototypes had very graceful looking exhaust manifolds that featured gentle curves for good flow.
Now I have not had a chance to look at too many early blocks but I have had a couple apart. I noticed the edge of the end exhaust ports as they exit the block has a nice radius to it. That was not put there by accident. In later engines that became a sharp edge which hurts the cause. I can only assume as the demand for more production became evident, some attention for detail went by the wayside.
Looking at this even further still we come to the Lincoln 337 engine. Now that is a late design premium engine with no where near the need for high production numbers that the basic Flathead Ford had to adhere to. If you have one to look at take a gander down into the end exhaust ports. You will find beautifully design end port exhaust passages. They are curved and flow gently away all the way from the valve seat to the port exit. There are no sharp turns or harmful pockets of "no flow".
Next if you will, look at the French Flathead. This truly is a Flathead Ford engine built under license by the French Military. There were however many small or not so small differences in these engines. Looking into the end exhaust ports it's the same thing as the Lincoln. Smooth gently curved and efficient. I highly doubt the engineers responsible for making a military spec engine would make any change whatsoever if they didn't see some merit to it.
I am not claiming to be 100% correct I'm just saying that common sense has pushed me in a direction that engineers were doing long ago and not so long ago in the case of the French Flatty's. When I started doing the exhaust port mods I did not know of the early V8 exhaust port differences. Nor had I ever seen a Lincoln 337 apart or even hear of a French Flatty's. It's only when I discovered all these things that I had some outside satisfaction that I was on the right track.
Going back to the question. Do headers make a difference on a stock block? Well I can't argue with someone's dyno tests if I don't have numbers to back up my case. I do believe there has been tons of testing done on how exhaust back pressure affects engine behavior. I have never seen a heavy truck or auto based boat engine designed without high volume exhaust manifolds.
In closing let me say this. If you only drive your Ford on and off the trailer, around the fairgrounds or two miles away to the lawn chair car show then I would agree, stock manifolds are fine. If you would like to actually drive your car on primary roads, pull grades or maximize that sporty Flathead Ford feeling then I think decently designed exhaust headers are a plus. Unless the dyno can and has measured Flathead Ford performance under all different conditions and for waaay more than 15 sec dyno bursts I will remain skeptical.