While reading through some posts im noticing most questions are about parts and problems. I have a question about EXHAUST and im looking for the most technical answer possible. Most people push off answers onto something like "it depends on your combo". Can someone tell me.. WHY do people think/say exhaust needs "back pressure"? Why do bigger primary/collectors move the power UP the rpm range? If you have smaller primarys less flows...period, why doesn't it take power from 305's? If you have a small cam or small valves i can see not NEEDING 1 3/4 to 3", BUT i dont see it HARMING, or at least why it harms, not saying anybody is wrong, and **** i even agree you can see a loss, but whats it all about???? thanks guys.

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1985 IROC-Z, nuff said :P

Headers work because the inertia of each exhaust pulse creates a slight vaccuum behind the pulse that helps pull the next pulse out of the cylinder. If the primaries are too large then at low speed the exhaust pulse moves too slowly to create enough vaccuum.

i always thought of it as this way: the more quickly and efficiently you remove exhuast gases, the more quickly and efficiently you bring in FRESH fuel and air mixture......apeiron is right about the back pressure part....also the power moves up in the rpm range because of the engine just moving too slow to have the full effect of the headers.....

Well that last response is why it seems to me larger EVERYTHING will get exhaust out of the way quicker..... I mean why do "race" cars always either run open headers or HUGE pipe/no muffler

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1985 IROC-Z, nuff said :P

Ok you want technical here goes. Most race headers are designed to work with the exhaust pulses of the engine. The goal is to get the pulses to follow each other one after the other into the collector this makes a vaccum effect in the collector which helps to evacuate the cylinder. The reason smaller tubes work better on smaller engines or low RPM engines is that in a small tube exhaust gases are forced to flow faster due to Berneuli's principle. It is true that a smaller tube will not flow as much as a large tube for the same fluid velocity. By increasing the velocity you can increase flow. If the pipe is too small you begin to build pressure inside the pipe which slows down exhaust flow. The same thing applies for larger pipes. The larger the pipe the slower the exhaust gas moves, but since it is so large you can move more CFM to the point where flow is so slow that you lose CFM. That is why big block headers are larger than 305 headers because of the amount of air they have to flow at the proper velocity. The other thing that will assist in header flow is wrapping or coating them. This keeps the heat in the exhaust gas, which raises the energy of the exhaust. A fluid with higher energy will have a higer velocity than a fluid with lower energy. It will also have a lower pressure which results in better cylinder scavenging. This is also why exhaust valves are smaller than intake valves. The longer your exhaust is the more fluid flow losses you incur. That is why racers run open headers instead of full exhaust. The only reason to run pipe off of the headers is if it is specially tuned to increase exhaust velocity. I hope this helps. If you really want to know more study thermal and fluid dynamics and Heat Transfer.

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'87 Bird 350 .060 over 700R4 + Lots more

You kick @$$



So if I understand this right..... In theory if you could put a barometer or some "pressure sensor" in your collector like an 02 sensor and adjust and tune your exhaust sizes and lengths you should have less pressure (because of vacum) at the collector than outside of the exaust. Or at least the MOST power would be obtained from the least possible pressure at the collector? What im wondering if there is a possitve pressure (because of backed up gasses due to mufflers etc) or a negative pressure (from vacum of the "pulses"). Any ideas? Any studies on this done?


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1985 IROC-Z, nuff said :P

[This message has been edited by IROC-Z_85 (edited July 12, 2001).]

because race cars (especially circle track cars) operate in the high RPMs for extended amounts of time (ei the whole race).