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Old 02-11-2004, 04:58 PM
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Is 2 1/2 pipes big enough?

How much motor is too much for a 2 1/2 pipe on a 350 sb engine. I'm trying to figure out if the engine i have built will need a larger pipe. I'm running 882 heads, 202 160 valves, ex. ports d-ported, solid cam 530" lift, single plain team g, holley 750, looking at 3200 to 6800 rpm range. Any suggestions on what size pipe to use. I already have 2 1/2 with flows but just worried that it may be to small with my set up.
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Old 02-11-2004, 05:06 PM
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2 1/2" is plenty big enough for 0-4000rpm. Wouldn't you be looking at some sort of exhaust cut out above that? Anything above 4000 would be at a race track anyway so just unbolt 'em and fly!!
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Old 02-11-2004, 05:09 PM
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Problem is i run the radial street class that requires mufflers
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Old 02-11-2004, 05:09 PM
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IMO, most SBC's do well with 2.5's. I like the sound of 3"ers better, just becuase they usually are deeper, but it doesn't allways make the performance better. With your rpm range, you may benefit with a larger diameter exhaust, but it will hurt your lower r's even more.
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Old 02-11-2004, 05:12 PM
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I agree. I dont think you'll benefit from larger exhaust. Im sure it wouldnt hurt either. I just dont think the cost will be worth the few HP you may OR may not see. As a general rule dual 2 1/2 pipes will support 400HP+.
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Old 02-11-2004, 06:54 PM
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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 88,200This 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.
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.
Minor Improvements
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), 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.
Go with the 2 1/2" pipe with a cross over.
Put the mufflers as rear as possible to keep the exhaust flowing at its warmest temp.
Do not use black paint on any of the pipes, it draws out the heat and slows the flow.
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Old 02-11-2004, 09:33 PM
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Quote:
Originally posted by 1BAD80
The concept that maximum power is obtained by zero pressure in the exhaust is only partially true., etc.
The reference for this article is

http://victorylibrary.com/mopar/header-tech-c.htm
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Old 02-12-2004, 08:04 AM
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Thats where I copied the info from.
I put that info in the KB almost 2 years ago.
Thats why it helps to check the Knowledge Base before posting, plus check the lower right hand icon for search this site is a big plus for anyone looking for info. before posting the same question over and over every week.
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