Whether one sees gains or not is really going to be determined by your camshaft's valve events.
With a camshaft that has less and later exhaust activity, as opposed to "vice versa," you may see some gains. With later exhaust timing, and adding larger exhaust to what already exist, it can decrease cylinder pressure, and there-by relieving you of some torque you once had.
HP = Torque x RPM/5252. What happens to the end product of horsepower, if your "plugged-in" torque is decreased (loss of cylinder pressure)?
I would only upgrade the exhaust, if the camshaft intake/exhaust valve events have been constructed around the larger exhaust point. This is why choosing the camshaft should be done last and with the long-term goal of the car is going to be.
This is why you see gains with some small to large swaps, and losses from some small to large swaps. It is the entire package, mainly the camshaft events.
That is correct, there is only an 1/8 of an inch difference between each; 7.7% difference in o.d.
I am going to take the outside diameter as the inside area, hypothetically.
I get 2.072" (squared) for the 1.625" long tube.
I get 2.404" (squared) for the 1.750" long tube.
Now, long tube headers range in many different lengths: 30-38".
Using a 36" tube, no taper, and assuming uniformity you are getting near 1420cc for the 1.750" pipe.
Using a 36" tube, no taper, and assuming uniformity you are getting near 1225cc for the 1.635" pipe.
Could this have an effect? Possibly...
The O.D. is not as drastic as one may seem, as Adam pointed out, just like a cylinder bore. A 4.030" bore is .015" on each side. Now it suddenly does not seem so big, does it?
But run that down the entire length of the pipe and it begins to seem to matter.
2.404" - 2.072" = .332" of area difference. Now multiply this difference down the length of a tube, and you see the difference increase all the way down the tube.
Now factor in the different collector lengths, styles, lengths, design, and you see how not all headers are the same in power.
Either way, I would rather have a "small" diameter tubing, but a long pipe, than a large diameter tubing, and a short tube header.
Spinning it another way...
Hooker - 18 gauge
- 18-16 gauge
Mac - 16 gauge
- 14 gauge
Bassani - 14 gauge
JBA - 14 guage
All are rated at 1.75" outside diameters right? Does a higher gauge (least thick) header have a larger cross section for exhaust flow?
We all know that a 1.625" header works just fine on an average to higher performing 302. So...
302 - 1.625" header
A 347 displaces 14.9% more cubic inches than a 302. So should we increase the header to a 1.875" header (15.3% larger than a 1.625" header).
Or, what about a daily 408, which is 35% larger than a 302. So should we increase the header diameter to 2.2" (35% larger than a 1.625" header).
Then, we have to keep in mind that many of the exhaust ports are the same size or very close throughout a variance of smaller and larger cylinder heads.
AFR 165 - 68cc
AFR 205 - 70cc
Just more questions to ponder over
In the end, all we can give someone is a place to start, and let the custom cam guys give you an idea that works for them and your goals.
I plan on investing in PipeMax soon.
Looking for thoughts...