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I'm getting rid of the headers and 3 inch exhaust on my '72 U code Supreme and going back to the factory setup. My understanding is that the down pipes from the exhaust manifolds are 2-1/4 to the mufflers, but then it drops to 2 inch for the tailpipes. Why is that?
I'm getting rid of the headers and 3 inch exhaust on my '72 U code Supreme and going back to the factory setup. My understanding is that the down pipes from the exhaust manifolds are 2-1/4 to the mufflers, but then it drops to 2 inch for the tailpipes. Why is that?
The exhaust gas cools and slows in the muffler. Cooling gas occupies less volume, so smaller pipes at that point are still adequate.
It's been my understanding exhaust pipes are designed to manage exhaust flow velocity with either benefits or lack of benefits in torque/HP produced by a specific engine. I think there exists a trade off between the two relative to engine performance expectations i.e. consistently/routinely operated at high RPMs (e.g. racing) vs. low RPMs (daily driving). Most likely OEM standard exhaust application flow velocity was/is designed for the greater majority of daily use driving conditions which most likely equates to cost savings - why design something larger when it isn't justified?
It's been my understanding exhaust pipes are designed to manage exhaust flow velocity with either benefits or lack of benefits in torque/HP produced by a specific engine. I think there exists a trade off between the two relative to engine performance expectations i.e. consistently/routinely operated at high RPMs (e.g. racing) vs. low RPMs (daily driving). Most likely OEM standard exhaust application flow velocity was/is designed for the greater majority of daily use driving conditions which most likely equates to cost savings - why design something larger when it isn't justified?
Also correct, but frankly production cost savings was the largest influence. Any decision like this is a tradeoff of engineering vs production cost.
I think it relates to the ability to both describe & measure the motion of flow velocity along a continuum. Letting my mind wander I visualize I can see dark black smoke, or maybe another example like measuring dust particles, or electrons along a continuum. I know I can relate to it when I get behind the yoke of an aircraft to increase &/or decrease aerodynamic lift. Flow velocity over/under an aircraft wing foil.
I think Koda is saying. By slightly constricting the exhausts exit. (Reducing the tail pipe diameter slightly.) It helps keep the exhaust gases flowing at closer to maximum pressure. By being more tightly packed. Rather then wafting out from diesel size tail pipes... But of course too small and you kill performance. It has to be a moderate, just right transition...
It's been a couple decades, but, volumetric air flow is equal to the cross sectional area of the pipe times the velocity. In other words, smaller pipe means faster velocity, like putting your thumb over a hose for some spray.
As the exhaust cools down, it becomes denser. Since mass stays the same, more density means less volume. Less volume along the same diameter pipe means it slows down. This makes, counter-intuitively, a little more backpressure, less scavenging, and less efficiency. So, you choke the pipe down a LITTLE to keep the velocity of the stream up so it doesn't slow down getting out the exhaust. Note that this is not the same thing as just ending the pipes early; it can slow down immediately then, but you don't care as it's gone already from the system.
I think Koda is saying. By slightly constricting the exhausts exit. (Reducing the tail pipe diameter slightly.) It helps keep the exhaust gases flowing at closer to maximum pressure. By being more tightly packed. Rather then wafting out from diesel size tail pipes... But of course too small and you kill performance. It has to be a moderate, just right transition...
Ok, so right now I have headers with 3 inch straight pipes going into the mufflers and 2 1/2 tailpipes. When I switch to the factory setup with exhaust manifolds and factory correct 2-1/4 pipes with the 2 inch tailpipes, will I notice the difference in performance?
In other words, too big or too small of a pipe affects performance, correct?
In an ideal flow situation, yes. In a fabricated exhaust system with bends (and not mandrel bends for OEM stuff), after the muffler, not so much. There is no scavenging in an OEM exhaust system, especially aft of the mufflers. Minimizing backpressure is the only goal. GM engineers tried to balance pipe size with production cost. Another consideration was sound into the car. Don't read too much into this.
I believe that the factory dual exhaust have a 2" driver's side head pipe and a 21/4" passenger side head pipe. I remember how weird that was as well.
Rick
PS: In regard to tailpipe size, I agree with Joe. It becomes less important for power as the exhaust cools, loses velocity and exits the mufflers. If you look at a vintage Mopar Hemi, they have 2 1/2" pipes to the mufflers with 21/4" tailpipes, with good performance as well.
Ok, so right now I have headers with 3 inch straight pipes going into the mufflers and 2 1/2 tailpipes. When I switch to the factory setup with exhaust manifolds and factory correct 2-1/4 pipes with the 2 inch tailpipes, will I notice the difference in performance?
Based on the dyno results that cutlassefi posted, yes you will lose around 25 horsepower / 25 ft-lbs torque from just the switch from headers to manifolds. I don’t know how much power may be lost due to the smaller pipes.
I remember some years ago when I wanted to put 2.5" pipes all the way back on my smallblock Camaro, the old school hotrodder guy at the muffler shop said, "Don't do that, you'll ruin your low-end torque," and suggested I limit the diameter to 2.25".
'Course that could also have been because 2.25" is easier to bend.
The 2" tailpipes should also be a tad bit quieter I think, too. When I changed out my stock mufflers and 2 in. tailpipes (that U code 455 was smoooooth and quiet) to Sonic Turbos and 2 1/4 tailpipes, sound level definitely notched up. Probably mostly from the mufflers but I'm sure the bigger pipes added a bit too.
I'm getting rid of the headers and 3 inch exhaust on my '72 U code Supreme and going back to the factory setup. My understanding is that the down pipes from the exhaust manifolds are 2-1/4 to the mufflers, but then it drops to 2 inch for the tailpipes. Why is that?
The more you breath in, the more you need to exhale. If you have a lower compression engine, it doesn't need 3" pipes out the back. Like many have said here, it would gain anything and may very well lose power. 2" tail pipes are easier , cheaper and all that was necessary. However, if you run 11:1, Mondello built 455, 3" pipes work best with 2.5" tail pipes. I guess what I am saying is, the factory built what their engineers deemed necessary, "Day Two" you engineer the exhaust to meet the breathing needs of the air pump up front.
The more you breath in, the more you need to exhale. If you have a lower compression engine, it doesn't need 3" pipes out the back. Like many have said here, it would gain anything and may very well lose power.
While you are correct that a lower performance engine doesn't need a 3" exhaust system, no engine in the world will lose power if the exhaust pipes are too large. This myth refuses to die. I am NOT talking about headers here, but the exhaust system. Open headers have "exhaust pipes" with infinite diameter. No racer in the world would think of putting exhaust pipes on open headers to make more power.
Unless it's in a land yacht, I know nothing about boat motors. I will say that two stroke motors DO take advantage of tuned exhaust systems, but that is essentially what real headers do for a V8. The pipes under your car are not tuned like this.
Actually the first question has been answered (why 2" tailpipes on a 2-1/4" exhaust)....but see #18 for my second question...that's what I really want to know.
Actually the first question has been answered (why 2" tailpipes on a 2-1/4" exhaust)....but see #18 for my second question...that's what I really want to know.
My answer is no, you will not notice any such drop in performance.
Ok, so right now I have headers with 3 inch straight pipes going into the mufflers and 2 1/2 tailpipes. When I switch to the factory setup with exhaust manifolds and factory correct 2-1/4 pipes with the 2 inch tailpipes, will I notice the difference in performance?
headers to manifolds? Yes. Maybe. Smaller pipes? No, maybe. Definitive enough?
