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1 3/4" headers too much for a GM 350?

3244 Views 20 Replies 8 Participants Last post by  67elko
hi all,

I have a GM 350 crate engine in my '67. I currently have 1 3/4" headers going back to dual 2 1/2" exhaust. I think I have too much flow, robbing me of low end torque/performance. Should I replace the headers with 1.5" shorties into dual 2" exhaust?
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Factory cast exhaust manifolds or factory headers such as the shorties they put on small block Corvettes in the 80's were 1 1/4" to 1 3/8" I.D. tubes in general. 1 3/4" is more than your engine needs, especially if it is a crate motor or similar. What kind of cam is in it? Carb size and intake? Rear end gear ratio? It is important to tune all the parts to the application. The headers may be robbing you of a little low-end torque but the combination of parts will contribute to the issue.
The headers you have will never warrant the primary size of 1 3/4". I would go with the 1 1/2". As JLcustoms pointed out, Tri-Y headers are also better for off the line (low-end torque) due to the unequal length of the runners. "Tuned" headers - long, equal length tubes are better for mid-hi range RPM's. You may even find eventually that the 2 1/2" exhaust is too free-flowing and might take that down to 2 1/4" or even 2" for the combination you have. One step at a time . . .

One of my original Elkys - an 82 - I set up with a crate 350, THM 350 trans. Edelbrock and 600 Holley carb. I notched the frame to accept early 70's Chevy pickup ram-horn exhaust manifolds and had 2 1/4" duals. Off the line it was great - gas mileage was pretty decent for a daily driver.

For a street car, a 750 CFM carb is usually more than you need. If you are worried about gas mileage there will be a big difference from a 600 to a 750. A 650 might be a good compromise - all depends upon the use. Mild cam will be an improvement over the crate cam.
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