calculating required exhaust pipe size


The Grumpy Grease Monkey mechanical engineer.
Staff member
more numbers for ya'll who like numbers.

All tests via an independent lab
All tests @ 15” wc

2 “ Straight Pipe 283 CFM
2 ¼ “ Straight Pipe 365 CFM
2 ½ “ Straight Pipe 521 CFM

2 ¼” Typical Bent tailpipe 268 CFM
2 ½” Typical Bent Tailpipe 417 CFM

2 ¼” Inlet/Outlet_ Glass Pack Tips- No Louvers- Smooth 274 CFM
2 ¼” Inlet/Outlet- Glass Pack Tips-Louvered 133 CFM
Same as above set for reverse flow 141 CFM
2 ¼” Cherry Bomb 239 CFM
2 ½” Cherry Bomb 294 CFM

2 ½” Inlet/Outlet Dynomax Super Turbo 278 CFM
2 ½” Inlet/Outlet Ultraflow Bullet 512 CFM
2 ½” Inlet/Outlet Gibson Superflow 267 CFM
2 ½” Inlet/Outlet Flowmaster ( 2 Chamber) 249 CFM
2 ½” Inlet Outlet Flowmaster ( 3 Chamber) 229 CFM
2 ¼” Inlet/Outlet Thrush CVX 260 CFM
2 ½” Inlet/Outlet Maremount Cherry Bomb 298 CFM
2 ½” Inlet/Outlet Hooker Aero Chamber 324 CFM
2 ½” Inlet/Outlet Hooker Max Flow 521 CFM
2 ½” Inlet/Outlet Borla Turbo 373 CFM
2 1/2" Inlet/Outlet Magnaflow 284 CFM


Standard OEM 2 ¼” Inlet/Outlet 138-152 CFM
Standard OEM 2 ½” Inlet/Outlet 161-197 CFM

this may provide some perspective when designing your exhaust setups and choosing muflers.
allen said:
OK I'm a bit confused I'll certainly admit that!
Im trying to figure out if I need to upgrade the exhaust pipe size on my 440 dodge now that I have added several performance parts and probably added about 120 more hp?
now keep in mind thats on the engine builders estimate , the original engine was rated at 390hp / 490 ft lbs, so in theory I have 500 hp or a bit more, but it was originally a 3x2 bbl engine but when I got the car it had a 440 engine block from a different car and the 3x2 intakes long gone also.
on the plus side, its a forged crank,better connecting rods,and I now have 11:1 compression, better cylinder heads,than the original engine had, and a radical hydraulic roller cam....any ideas ?

I don,t remember if the 440 six pack road runners came with a 2.5" or a 3" dual exhaust, (IM THINKING 2.5" was OEM installed)
but a well designed 3" and correctly installed dual exhaust , mounted behind a set of decent brand name long tube headers ,will certainly provide you with a low restriction option that won,t restrict your new increased power.
ID point out that adding a 3" (X) pipe, rather than the (H) pipe and carefully selecting low restriction mufflers may help reduce the flow restriction.

any automotive engine will run best with little or no back pressure on the exhaust system ,
back pressure ALWAYS tends to reduce the cylinder fill and scavenging efficiency, this is not a guessing game.
you can simply measure back pressure in the exhaust every500 rpm from about 3000rpm to where ever your upper rpm limit lies, and if its ever over about 2 psi your losing power.
in fact in a properly designed exhaust system you'll ideally run a negative pressure behind the exhaust valves as they open,
helping the exhaust scavenge the cylinders, over most of the peak rpm range.

thus any significant back pressure is always going to reduce the potential power.
a correctly designed exhaust header can induce a low or negative pressure wave at the exhaust port as the exhaust valve opens to greatly increase cylinder scavenging and cylinder fill efficiency over a carefully designed in rpm band of a correctly tuned engine, provided the other factors like compression,cam timing and a low restriction exhaust system past the header collectors are used in a well matched combo. you can get a very good idea


in the ideal exhaust system youll want to have less than .5
(thats 1/2 a PSI of back pressure) at the engines peak rpm


H pipe X pipe (2).JPG


h-pipes are no where near as efficient or effective at equalizing or reducing individual, bank to bank exhaust pulse strength,or reducing the restriction to exhaust flow, as a similarly placed (X-pipe )
if you have the room two (X-PIPES in sequence have been repeatedly been tested,
and repeatedly found to provide a minor but measurably better result in increasing cylinder scavenging and a mellower exhaust sound
your not going to see any huge gains from either configuration, but a single (X-pipe generally produces a bit better and a wider torque curve ,
and two x-pipes in series is not all that rare recently even on the higher end performance cars


3" o.d = 2.87" diam. inside--approximately 6.5 sq inches of area
2.75 o.d = 2.62"diam. inside--approximately 5.4 sq inches of area
2.5 o.d = 2.37" diam. inside--approximately 4.4 sq inches of area
2.25 o.d = 2.12" diam. inside--approximately 3.5 sq inches of area



well worth reading ... ndex1.html




keep in mind installing an (X) almost increases the effective cross sectional area, of the dual exhaust ,or collector cross sectional area to double what it had been behind a single header collector, by doubling the area that the exhaust flow sees, dropping the restriction to flow almost in half

adding an anti-reversion baffle to a 18"-24" longer collector on open headers with the internal pipe about 1/2 the collector length tends to help scavenging on some engines

these threads go into more details, READING THE LINKED INFO HELPS
exhaust flow is never constant it comes in pulses that vary in pressure , some what in volume and timing or duration as the rpms change, thus the ideal cross sectional area required in the exhaust header and exhaust pipe,
for a given engine displacement and rpm also varies
your also dealing in most cases with an un-equalized firing order where each cylinder bank does not always alternate equally.
thus two properly sized exhaust headers blending and flowing into an x-pipe to dual exhaust beyond will generally provide a slight improvement over a single merged exhaust.
read related threads,
as they contain a great deal more related and useful info
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there are plenty of times that I look at some guys car build and think to my self
,(" wow! why didn,t I think of that")
, but theres lots of other times when I can,t help thinking,
(" What the $%^& what this guy thinking about when he did that?")
I went to a local car show and a guy had a yellow, 1966 Pontiac GTO,
as I walked around the rear I spotted one larger than normal 4" exhaust pipe,
I walk around to the open hood and theres what appears to be a rather stock Pontiac v8 with stock exhaust manifolds , so I get down on my knees'
("no small effort in this old geezers case") to look under the car and there was, two 2.5" exhaust pipes feeding to a ( 2.5" Y pipe) that that then feeds too a long section of what looks like 2.5" exhaust that flares out to a 4" stainless pipe after the rear axle??? (no mufflers at all)... all I can think IS
(" What the $%^& what this guy thinking about when he though that modification up?")
within a few minutes the owner shows up and starts the engine, from the sound it became all to obvious,
the guy had installed a long duration cam, in a low compression engine, it had none of the pulsed rumble of a high compression engine with a well matched combo, it sounded more like he had selected the largest cam he could find, in a low compression engine and it might have bad ignition related issues,or perhaps the plugs were original equipment,
(from the engines appearance that might have been an accurate guess)
a few questions and he told me it was a basically stock, Pontiac 350, he had purchased , from a local salvage yard, to replace the 389 that had thrown a rod several years earlier, with this cam, (below) installed ,that his brother-in-law had installed, for him,and he ,and his brother-in-law, had recently installed a 4 speed muncie trans and a 4.56:1 rear gear
because intake ports work at far lower pressure levels and far lower temps,
as a piston drops in the bore outside air pressure tries to fill the resulting lower pressure void, even at sea level thats only about 14.7 psi
on the exhaust the gases are physically forced out the exhaust port by the piston on the exhaust stroke reaching TDC and the inertia of the gases expelled dragging the remainder out of the header primary, and if the cam timings correct the inrush of intake port flow reduces any significant reversion.
every test I've seen indicates a smooth surface and an exhaust tube internal size about 90% of the exhaust valve size comes close to ideal, of course theres calculators that help calculate the best length and diameter, of both intake runners and header primary and collectors and matching cam timing.


Id also point out that cam timing matched to the exhaust scavenging has a huge effect on potential intake flow rates



I just smiled and ask if he was thinking about adding headers and he assured me that adding headers would make it sound like crap........
wow you can learn so much from some people, its amazing:confused::rolleyes:
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Dynomax makes Killer nice Street and Full Drag race mufflers Grumpy.
Surprised Borla XR-1 don't flow more.
The Supercharged 5.0 guys talk about XR-1 & Like.

I have to put thought into my 1963 Grand Prix exhaust.
There is current dual exhaust.
Stone Age old.
GP sits low like my C4 Corvette & my 70-1/2 T/A.
And the Floor pan is largely flat.
Small trans tunnel.
Hard to Tuck dual pipes up like my Trans Am with Dual 3.00".
2-1/2 Duals on my GP my plan.
Like to do 3 or 4 inch duals.
Take Ovalized tubing.
On a budget with that car.
Pontiacs Love ultra high compression.
Pump gas not really.
But They Really Haul Ass.
Spank nearly all but a Full Race Big Block Chevy.

LS forget about it unless Twin Turbo.
I had a recent discussion with one of the younger local guys,
and after breaking down his request for a detailed list of parts,
and his intention to have me potentially build him an engine.
I got the distinct impression,that his most pressing concern was not good mileage,
having a really fast car, running a 10 second quarter mile time slip.
or making lets say some arbitrary horse power like 600 hp.
Nope......he wanted an engine that rumbled at idle and screamed under full throttle!
NOW he wanted a fast car, but fast is a relative term,
and to him a throaty exhaust rumble and the ability to smoke,
the tires endlessly was the key to success

Now in my opinion,thats a rather odd approach,, but I've heard similar concerns in the past.
hes got a 1967 Pontiac lemans and the current 326 engine,
his car looks very similar to this picture I found on the internet

and I think a th350, and it is not anything close to what he really wants,
now I can easily see that he might want to seriously upgrade the cars potential power and I suggested,
he think through his options, and take a good long look at his bank balance and consider,
what he might realistically want to put into the project!
the basic fact is that the RUMBLE and LOPE that the old school muscle car guys seem to think "sounded so cool" was mostly due to a combo of high compression ratios and selecting a cam with low speed REVERSION, that effectively makes the car idle very inefficiently, and an engine that basically only starts to smooth out and run in its effective power band once the engine speed increases a bit so the exhaust and intake runner reversion pulse strength and duration does not have the TIME between cycles to effect the exhaust sound , so you start to get that crisper deep throat-ed, but gradually sharper, and higher pitch, roar as rpms increase, rapidly.
now if I had access to a 400-421-428-455 pontiac engine I would go that route, but his 326 is not in great condition and finding a much better pontiac engine dirt cheap and the required parts locally would not be as easy as using a bbc engine that basically interchanges rather easily in that car.
I suggested he might want to consider building a high compression 427-454-468 or 496 BBC as parts are easy to locate, and any of those choices would provide both considerably more power , than the current well worn 326 Pontiac engine has, and the exhaust sound he was looking to have once we decided on a budget and a set of components.
but to build a decent combo wont be dirt cheap, but we can certainly accomplish reasonable results, in a 12 second car build with a nasty sounding engine and keep reasonable street manors, for less money that building a 10 second car might cost.
  1. Torque ( = 63,025 x Power (HP) / Speed (RPM)
  2. Power (HP) = Torque ( x Speed (RPM) / 63,025.
  3. Torque (N.m) = 9.5488 x Power (kW) / Speed (RPM)
  4. Power (kW) = Torque (N.m) x Speed (RPM) / 9.5488.

or go the simple route
5. The equation to calculate horsepower is simple:
Horsepower = Torque x RPM / 5,252.

so as an example

350 ft lbs at 3000 rpm/5252=200 hp
350 ft lbs at 4000 rpm/5252= 266 hp
350 ft lbs at 5000 rpm/5252=333 hp

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