Several years ago I had initially planned on building a 440 plus ci. small block for the Sunfire. Well that fell through and ended up with a 412ci. During that time I purchased a stepped header kit from Dynatech that is 21/8 to 2 Stahl pattern for a strut front end. Looking at similar sized engines these are huge in comparison. Am I over thinking this or just leave it be?

im not an expert on headers but that seems about right on size

We have 2" straight on a 418" as said I dont think you will even notice

On my 409 SBC if I remember right I picked up about 40 Hp going to the larger Fabshop headers. I think they are 2.125" to 2.250". Done a lot of dyno testing on different motors carbs, headers etc so little foggy on actual numbers. The larger tube was definitely better on my motor. On my 463" SBC I would like even larger tube but I am not throwing money away again. Your mileage may vary.

Its not going to matter no where near as much as you think. The "right size" header might be 02 quicker. Build the set that will work for future goals, and your current engine.

Primary tube diameter, if large enough , means very little unless you're trying to produce a veeeeery wide, flat power band.
Since most of us are looking for an operational power band of about a 1000 RPM + or -, horsepower is enhanced with the primary length and most importantly, the collectors / extensions.

Send it!

Thank you for the replies, the only thing I will do is have these headers recoated as they are currently showing signs of surface rust. Again thank you all

quote:

Originally posted by 329L:
Its not going to matter no where near as much as you think. The "right size" header might be 02 quicker. Build the set that will work for future goals, and your current engine.

That is a legitimate point. I gained more power on the dyno with the larger tube but smaller my give you more torque at lower RPM so may not make much ET difference. If I remember right I think we got 35 or 40 more Hp with the larger tube headers at 7800 RPM and we tend to look at peak numbers but we are not always at peak RPM.

I can see where a smaller tube with in reason would make more Hp and torque little lower RPM range so would not hurt Et much.

Here is a point of reference. My C4 Corvette car is 3200lbs and I go 220lbs. 582, BBX380 heads and years back when we put in the 555CI motor we were in a scramble because I hurt the SBC I ran. Everything was pretty straight forward except I knew headers would be a nightmare. I ordered several sets of headers from Summit, for chevelles, 70s corvettes in an attempt to get something close that I could modify to just get back to racing. To my amazement the headman 2inch, with 3inch collector with a ball socket on it went right up in on the drivers side. On our 67 chevelle we had to jack the motor around to get it in. On the passengers side the frame needed boxed a little.
My point is these headers were supposed to be temporary. I then have then running thru 4ft of oval 3inch pipe. The collector opening is maybe 2.5inches.

The car consistently runs 5.6-.7s@122 with loose converter. With the tight one it will hit 126MPH, and I have been 159MPH in 1/4.

The headers are downright laughable when you look at them. Had two guys looking at it last night and they know the car and could not believe it. But they perform. Not sure if I would see any gains but it outruns most other cars I know with similar motors. I do have plans this winter to attempt building a set, but it will be nothing more than to challenge myself with the project. My gut tells me I will see zero gains and may even loose performance.

I do not believe bigger is better with most things in this sport. Our brains make sense of bigger is better. As someone pointed out, peak number are what most look at. I look at average and real world performance.

My initial thought regarding the question was "1/8 of an inch in primary tube diameter should not make much difference at all, if any".
After reading the responses, (all good), it got me thinking a bit. So I calculated the surface area of the opening of a tube, (assuming the dimensions are Inside Diameter). Surprisingly, it is a more significant difference than I would have guessed:
2 Inch Primary = 3.14 Square Inches.
2-1/8" Primary = 3.54 Sq In.
These figures are rounded to 2 decimal places, but a difference of 4/10 of an inch, (it is actually 0.45") per tube is definitely meaningful.
Ski, I hope you will share the results after you build your new headers and test them.