Originally Posted by Fitz
Wow. Lot's of folklore in this comment. My 572 can't take full advantage of a 3 inch exhaust...it actually LOSES low end torque and HP throughout it's RPM range with that size pipe. Here's why.
The concept that maximum power is obtained by zero pressure in the exhaust is only partially true. There should be absolutely no back-pressure from the collector rearward, but the diameter of the system beginning with the exhaust valve is a compromise. The highest efficiency for the system requires a minimum speed for good exhaust gas velocity to insure that gas does not "back up" into the chamber during overlap at low engine speeds, and that the "suction" (negative pressure pulse) effect of a resonant (tuned length) and/or collector (overlapping exhaust pulses) system is optimized.
To predict what primary size will be best for a specific motor, you must know where you want the engine to develop peak torque. If the existing torque peak is at bit lower RPM than you prefer (typical in under-cammed or stock motors), it can be "bumped" a bit by increasing the primary diameter. If the torque peak is too high (motor is "peaky", with no range and poor recovery from gear changes), the peak can be adjusted down by using a smaller pipe. A change of 1/8" in the primary diameter will raise or lower the peak torque RPM by 500 or so.
This factor slightly overlaps the effect of primary pipe length, but the pipe length generally will not change the peak torque or the RPM at which it occurs. A length change has the effect of improving the torque on only 1 side of the peak by "borrowing" it from the other side. A shorter pipe improves the torque after the peak (reduces it at lower RPM), preventing the curve from flattening out so quickly as speed increases.
A longer pipe extends the torque curve backwards to improve the engine's flexibility, at the expense of after-peak torque. Less stall speed is required, and the motor will pull taller gears; this re-tunes a 4-speed motor for better operation with Torqueflite, etc.
For best effect, the gas speed in the primary tube at the peak torque RPM should be about 240 feet per second. The formula to calculate pipe size is:
Area of Primary Pipe = RPM × Cylinder Size ÷ 88,200
This determines the pipe's cross-sectional area, from which we can calculate the ID. Typical exhaust pipes are 18ga. (wall thickness of .049"), so the OD will be .098" larger. From this we can construct a formula for an 8 cylinder motor, and factor in the 18ga. wall thickness:
Area of Primary Pipe = RPM × Motor Size ÷ 705,600
Pipe ID2 = RPM × Motor Size ÷ 705,600 ÷ .7854
Pipe ID2 = RPM × Motor Size ÷ 554,177
ID = (RPM × Motor Size ÷ 554,177).5
OD = (RPM × Motor Size ÷ 554,177).5 + .098"
The following Chart shows exhaust pipe outside diameter, based on this formula, for various motor sizes and speeds. To determine if your pipe size is large enough, search across the top row for your motor size (interpolate if in between 2 sizes), and then down that column for your current pipe OD. If the peak torque RPM in the left column is high enough, your pipe is not a restriction. If the peak torque RPM is lower than where you feel your torque peak is, look for the pipe OD on the line with your torque RPM.
If your primary pipe's wall thickness is 16ga. (.059"), add .020" to the OD figure to compensate.
Here is a table of exhaust Pipe Size @ Peak Torque RPM vs. Motor Size
Engine Displacement (all MOPAR some stroked some not)
273 318 340 360 392 410 383 400 426 440 451 494
RPM Tube size for optimum flow
3000 1.31 1.41 1.45 1.49 1.55 1.59 1.54 1.57 1.62 1.64 1.66 1.73
3250 1.36 1.46 1.51 1.55 1.61 1.65 1.60 1.63 1.68 1.70 1.72 1.80
3500 1.41 1.52 1.56 1.61 1.67 1.71 1.65 1.69 1.74 1.77 1.79 1.86
3750 1.46 1.56 1.61 1.66 1.73 1.76 1.71 1.74 1.80 1.82 1.84 1.93
4000 1.50 1.61 1.66 1.71 1.78 1.82 1.76 1.80 1.85 1.88 1.90 1.99
4250 1.54 1.66 1.71 1.76 1.83 1.87 1.81 1.85 1.91 1.93 1.96 2.04
4500 1.59 1.70 1.76 1.81 1.88 1.92 1.86 1.90 1.96 1.99 2.01 2.10
4750 1.63 1.75 1.81 1.85 1.93 1.97 1.91 1.95 2.01 2.04 2.06 2.16
5000 1.67 1.79 1.85 1.90 1.98 2.02 1.96 2.00 2.06 2.09 2.12 2.21
5250 1.71 1.83 1.89 1.94 2.03 2.07 2.00 2.04 2.11 2.14 2.17 2.26
5500 1.74 1.87 1.93 1.99 2.07 2.12 2.05 2.09 2.15 2.19 2.21 2.31
5750 1.78 1.91 1.98 2.03 2.11 2.16 2.09 2.14 2.20 2.23 2.26 2.36
6000 1.82 1.95 2.02 2.07 2.16 2.20 2.13 2.18 2.25 2.28 2.31 2.41
Note: 392" is 340/360 block stroked with 3.79" crank; 410" is 340/360 block
stroked with 4.00" crank 451" is 400 block stroked with 440 crank; 494" is 400/440 block stroked with 4.15" crank
Remember that your peak torque RPM will always be lower than your peak HP
RPM. The separation between peak torque and peak power is roughly
proportionate to your range of useable power (wider is better). Be realistic in your
estimates and plans - peak torque @ 7000 RPM sounds good, but is almost certainly
beyond the breathing ability of even a professionally-built race motor, and if
true will make the car impossible to launch. Note that 1-1/2" pipe is large
enough for a 273" motor with max torque @ 4000 RPM. A 360" only needs 1-3/4"
for 4200 RPM. A 440" is fine @ 4500 RPM with 2" primaries.
If choosing pipes for a 4WD, van, towing, etc. keep the size small to
improve torque where you need it most - the lower RPM ranges, typically 2500-3500.
One exception where use of a larger pipe (than indicated by the above
formula) will help power is, of course, motors using nitrous oxide, supercharger or turbocharger. In these cases, size the pipe for the expected peak torque, not the motor size.
Another instance where a slightly larger pipe may help is where the
departure angle of the pipe from the flange is very sharp (typically downward). The added cross-sectional area immediately after the flange apparently helps reduce the restrictive effect of a small radius after the port. This partially explains why some header models or brands work better than others with similar dimensions.
So, the bottom line is bigger is not necessarily better AND the average Joe Bag-O-Donuts can't tell the difference in pipe size without getting out on the track for a couple of years!
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12-06-2012, 01:54 PM
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