Exhaust Diameter Formula
Does anyone know any formulas (is there any) to find an ideal exhaust tubing diameter; in general.
Not unlike that "in general" carb size: engine c.i.Xmax RPMX3456=ideal carb size assuming 100% volumetric effeciency.
Luv the smell of NITRO in the morning.
Once you use that formula to find diameter, you can use this one for the best length.
1bad80 sent me this in a PM... I thought it'd be helpful for other sheeple here.
Exhaust Design Comments:
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.
LA Motors: Pipe Size @ Peak Torque RPM vs. Motor Size
RPMPrimary Pipe Outside Diameter, Inches
Note: 392" is 340/360 block stroked with 3.79" crank; 410" is 340/360 block
stroked with 4.00" crank
B/RB Motors: Pipe Size @ Peak Torque RPM vs. Motor Size
RPMPrimary Pipe Outside Diameter, Inches
Note: 451" is 400 block stroked with 440 crank; 494" is 400/440 block stroker
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
If possible, slightly enlarge the inside of the flange opening in the
header itself to produce a sharp step (be careful of grinding off the tube weld).
Do not radius the edge. A 1/16" bevel is generally possible and will help. If
there is not much room, don't grind all the way into the flange - a 45° angle
is fine. This has a minor anti-reversion effect, helping to prevent back-flow
at low engine speeds; helps clean up idle quality, etc.
If the primary pipe inside diameter is more than 1/8" larger than the
actual port opening in the head, the header flange bolt pattern can be slotted
slightly to raise the centerline of the primary pipe above the center of the
port, until the bottom of the pipe just matches. This puts the pipe's effective
center closer to the most active area of gas flow, and the mis-alignment at the
roof allows the highest-pressure gas an easier path away from the port; also
adds some degree of anti-reversion.
If controlling reversion is more important than maximum port flow (e.g.
primary diameter is very large), Vizard suggests the mis-match should be at the
bottom of the port where gas flow is slowest, and therefore most likely to
reverse-flow at low engine speed.
An easy way to effect a small increase in exhaust efficiency, if space
permits, is to move the header out away from the port by 1/2" or more (using an
extra gasket and longer studs or bolts). This moves the restrictive angle
farther away from the port - every little bit helps here. It also reduces local
exhaust port temperature in the head slightly. An easy spacer is an extra header
flange or 2, but be sure that the transition from the extra flange to the
header is not restricted. The extra flange may be slightly larger than the port
opening in the head, but must not step down entering the header. Do not taper,
blend, or bevel the extra flange to act as a transition between the port and
If the header in use is substantially larger than other available choices
for the same motor (i.e., you're using 2" OD, but headers are also available
in 1-5/8", etc.), an inexpensive anti-reversion plate can be made from a bare
flange with the smaller size (1-5/8"). Do these steps in order:
» Match the extra flange openings to the exhaust ports.
» Lay it over your actual 2" header flange.
» Spray paint, mark etc. the 2" openings around the 1-5/8" openings (they
be concentric circles).
» Remove the marked area as shown, using ball end carbide, etc.
I would suggest the following rules for best effect:
» The extra, smaller flange be no smaller than your motor's exhaust port.
» The extra flange be at least 1/4" smaller than the header in use (if
header with 2" primaries, the extra flange should be 1-3/4" maximum).
» Make sure there's enough room to move both sides of the header away
from the motor by 3/8" or so (the header flange thickness).
» You may need an extra gasket between the flanges, or sealer may do
(Vizard uses silicone here)
» Longer bolts or studs will probably be needed; remember to use sealant on
B/RB motors due to water in holes.
Just remember you can crunch all the numbers you want but your still stuck buying whats available on the market, for a small block that usually means 1 1/2", 1 5/8", 1 3/4" and sometimes 2" primary diameter.
Pick one according to the duty, build of the motor and type of cam you intend to run, most people would be better off running one size smaller than they pick typically.
Outlawed tunes from outlawed pipes
As far as ability of an exhaust system to flow enough air for a given amount of horsepower without causing significant back pressure, this is from one of David Vizard's books:
1. For avoiding significant restriction from back pressure, the pipe should flow at least 2.2CFM per horsepower produced.
2. A straight pipe will flow ~115CFM per square inch of area (using inside diameter of the pipe)
Mufflers usually won't flow as much as a straight pipe of the same inlet/outlet diameter, so you have to factor that in, also.
I'm looking for the exhaust pipes, not header pipe diameters, but I'm sure the header info will come in useful so I get every bit of torque and horsepower I can for the $$$ when I get to my project car
But for now, it's for the good ofo the general population of the forum.
This should be a sticky, I found this thread from a lot of searching on primary pipe sizing.
Not sticky material. Maybe the author will make a wiki out of it.
Horsepower is how fast you hit the wall.
Torque is how far you take the wall with you.
Midnight Sun Street Rod Association
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