Thread: Muffler Help plz

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Hey, I may be wrong and not an expert on exhaust systems but the gases that come out of the engine are a lot hotter than the intake so the volume is greater. Let's assume the fuel is octane, C8H18.

C8H18 + 25/2 O2 --> 8 CO2 + 9 H2O (steam) + heat(!)
moles of O2 = (12.5) gives 17 moles of gas out
and

N2 (25C = 298K) --> N2 (125C = 398K)


First the relatively inert (no reaction) N2 will be merely heated by a factor of the Kelvin temperatures which is (398/298) = 1.336 which assumes that the final temp at the end of the exhust pipe is about 125 C and since the incoming air is about 80% N2 this is the main effect. Also not all the O2 will be used up in the combustion so the unused O2 will also be expanded by that factor and so the 1.336 factor is probably the main effect. We don't see liquid water coming out after the system is warmed up and a hand over the pipe feels a lot of heat so I am guessing the temp is above the boiling point of water at about 125 C as a rough guess, it may actually be hotter since you can't keep your hand there very long! Thus based just on the N2 dragged through the engine, 800 cfm would produce more volume of about 800 x 1.336 =10688 cfm. Then whatever O2 is used in the combustion will be augmented by the CO2 and H2O steam formed by the amount (8 + 9)/12.5 = 1.36 plus it will be heated by the same temperature factor of 1.336, so the O2 used in the combustion will expand by about 1.336 x 1.36 = 1.82. These are just rough calculations and inside the exhaust system the temperature is much higher than 125 degrees C as noted by cherry red manifolds at night after a long hard run; the gas actually cools quite a bit coming through the exhaust system. I don't know what factor automotive engineers used for the cfm requirement but this simple analysis indicates that the intake cfm ahould be multiplied by about 1.4-1.5 to get the exhaust cfm at wide open throttle???? Of course the exhaust pressure is higher than 14.7 psi so the engine "pumps" the gases through. Maybe the factor is lower, something like 1.3, I really do not know the number but it is certainly quite a bit larger than the intake cfm. The dual 410 cfm mufflers with an 800 cfm intake would be on the ragged edge of limiting performance, but only at wide open throttle. At cruise conditions of say only 400 cfm intake the 820 cfm would be adequate for the exhaust. Maybe most of you know this, but I did this little exercise so I myself can understand the muffler ratings. In my case with an Edelbrock 600 cfm it looks like twin 410 cfm mufflers would be adequate. Well maybe this is much ado about nothing, but it looks to me that the exhaust system should be rated at about 1.3 x the maximum intake cfm at wide open throttle, but for cruising I come back to DennyW's answer that (820/800)=1.025 is OK for all but wide open trottle, so DennyW is right for all but wide open throttle. I guess many of you know this from experience, but I had to check it out for my own understanding.

Don Shillady
Retired
Scientist/teen rodder