In 1999 Keith Thomas, IZCC Member #5622, did some header comparisons and here are his results. I admire what Keith did because this is the kind of information I love, and I'm reproducing it here to give you a better idea of what a header can do for your car.
It's easy for manufacturers and resellers to make claims about horsepower gains, but the only way to really know is by direct comparisons using the same car on the same dyno on the same day using the same test procedures.
I did not change his charts or his text. I merely made a cleaner, easier to read page with some of the spelling and formatting errors corrected.
From this point, everything is his.
Here is the baseline configuration:
1971 L24, stock bottom end, approximately 150k miles
E88 cylinder head, fresh valve job, shaved 0.025 inches, otherwise stock
MSD 6AL ignition, Pertronix Ignitor, MSD 8mm wires
Stock rebuild distributor - stock advance curve
Stock SU carbs, N-27 needles
Car was driven to the dyno facility to ensure all fluids were at operating temperature. Most of the runs presented here were done back to back to minimize the influence of atmospheric changes; however, during the entire 5 hour test the air temperature vaired less than 4 degrees, and barometric pressure less than .02 in-hg. Temperatures were monitored for the transmission, diff, oil and water to ensure consistent drivetrain losses.
Header swaps were performed within 30 minutes. Car was run at varying speed on the dyno until fluid temps were back up to baseline.
Some graphs are shown with SAE corrected data, a few are not. This is because I had no control over the scaling that the viewing software chose and some graphics were simply easier to read as uncorrected. Regardless, the trends are the same.
The more modified your engine is, the more you have to gain with headers. These tests are for a basically stock L-24. If you have a 3.1L with a cam and a blower, all bets are off.
Most comparisons are without muffler and air cleaner, unless otherwise noted.
What are headers buying you? Not much.
Robello Modified vs Nissan Comp (same results as above)
2.5" vs. 3.0" Exhaust Pipes (no gains - 3" drops off peak power sooner)
It costs you nothing (in terms of HP) to run a DynoMax Super Turbo muffler
The air cleaner can cost you about 4 HP in the peak power band (but well worth it)
(From Ace: this is the same number for my slightly-modified L26 and I've kept the stock filter)
It's easy to over-spend on ignition with little to no gain HP
All Images copyright 1999 by Keith Thomas
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