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Old 05-12-2004, 08:05   #1 (permalink)
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To little back pressure

i have a question, i maybe laughed at but is there such thing as to little back pressure, like if u install say like 2 inch (each cylinder) extractors with a 5 inch exhaust pipe, would you lose power or would you get about the same power as normal extractors and exhaust?? not planning on doing something like that but just wondering...
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Old 05-12-2004, 08:12   #2 (permalink)
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Re: To little back pressure

Theres some information midway through this autospeed article that should be of help to you mate

http://www.autospeed.com/cms/A_46/article.html
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Old 05-13-2004, 05:19   #3 (permalink)
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Re: To little back pressure

its not a lack of back pressure as such, but if the pipes are too big you loose the "extractor" effect of the pipes.

if they are the right size each pulse of exhaust gas out of the head creates a low pressure zone behind it, which "sucks" the exhaust gas out of the next cylinder to dump its load into the system.

as the gas is sucked out, it sucks fresh air/fuel mixture into the cylinder at the same time - this happens during a short "overlap" period, when both the intake and exhaust valves are open at the same time.

if the pipes are too big you loose power because there is no low pressure area in the pipes after each exhaust pulse.
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Old 05-13-2004, 07:53   #4 (permalink)
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Re: To little back pressure

On the other hand, higher revs require less scavenging and more flow. Therefore bigger pipes are better for top-end power. A set of extractors is a trade-off, really, between low-end and top-end power setups.
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Old 05-13-2004, 09:43   #5 (permalink)
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Re: To little back pressure

we've seen variable length inlet to maximize intake raming, when will we see asjustable exhaust primarys? Difficult, but a thaught
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Old 05-13-2004, 09:52   #6 (permalink)
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Re: To little back pressure

Soon as I get some time/money capability mate! I've been planning this for a while, was going to see about hiring a day with a dyno and taking a few manifolds down that I'd made up to determine favourite top and low end runners, then take it home and weld them together. I had it all planned out but now it'll have to go on hold for a while :(
Also decided there's not much point me doing it until I finish working up the AU motor which will take a few k more than I have right now
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