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Old 11-05-2008, 07:59 PM
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Why backpressure?

OK I know
I'm sure this has been discussed and argued and ran over
I just,,,, for some reason,,,,can't get it.
All the drag cars I see run open header,,,,now I can see header primary size beeing important and I understand the scavaging effect.
But how can full exhaust help your motor,,,,mine seems to run alot better w/ the y-pipes uncapped.
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Old 11-05-2008, 08:03 PM
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I don't know who is saying a full exhaust will help, but they're all wet.
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Old 11-05-2008, 08:15 PM
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LOL,,,I've heard people argue over pipe size....2 1/4 or 2 1/2.
Why isn't bigger better after the header?
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Old 11-05-2008, 08:47 PM
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it comes down to Exhaust Gas Velocity. as each cylinder fires it produces a pulse of exhaust gasses which travel down the header tube and meet up with the other cylinders. Cubic Inch, RPM, and load will all affect the size and width of these pulses

As each pulse travels down the tube it creates a low pressure zone behind it. when the pulse exits the exhaust tube atmospheric pressure will try to fill the lower pressure between the pulses, this is why on extremely short exhaust systems you hear allot of popping and banging on de-acceleration. the atmosphere pressure is actually crashing into the following exhaust pulse.

Tuned exhaust will change the length and diameter of the tubing to match the output of the engine, each exhaust pulse should help pull the following pulse into its low pressure zone as they travel down the tubing. this will help to draw the used gasses out of the combustion chamber.

low RPM engines will need a smaller tubing to help it run efficiently, but the small tubing wont have the capacity to fit the larger pulses from a higher rpm engine, therefore you would need bigger tubing.

there is allot of research involved to tune exhaust properly, but my general rule of thumb is engines 360ci and under N/A running under 6500RPM work well with 2 1/2 " tubing, and engines over 360 withe the same specs should run a 3" tube. once you get into forces induction its hit and miss. it is in great debate, but those are my numbers.
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Old 11-05-2008, 08:59 PM
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when backpressure increases torque, it looses hp. The reason it does that has little to do with the actual exhaust set up though, its more to do with excessive exhaust cam timing. A lot of these dual pattern cams were designed for really crappy exhaust ports with mediocre intakes. Newer heads have much higher E/I flow ratios and in general bennefit from a cam closer to a single pattern, and in some cases even a dual pattern with less exhaust timing.
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Old 11-05-2008, 09:59 PM
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Losing torque is not necessarily a required consequence of reducing back pressure. It might be because nobody bothered to recalibrate the fuel system to compensate for the extra air flow.
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Old 11-06-2008, 10:07 AM
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Quote:
Originally Posted by jimfulco
Losing torque is not necessarily a required consequence of reducing back pressure. It might be because nobody bothered to recalibrate the fuel system to compensate for the extra air flow.
I think thats where the "you need some backpressure" thing started, people not re-jetting their carb after changing the system.
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Old 11-06-2008, 09:19 PM
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I agree... Its a big buzzword. Its no different from using a certain intake or carb. There is no reason why you can't choose exhaust components that fill the bill for the full range of the engine's powerband. The secret is just knowing how big is just right.

There will be a point where going smaller will start capping off HP without adding any more low end torque, and there is a point where going bigger will start reducing low end torque while not adding any more HP up top. But there is a pretty generous middle ground that will provide minimal trade offs.
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Old 11-06-2008, 09:36 PM
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I run 2.5" to the super 40's then 3 out.
When I open the headers via cutouts it definetly runs better,,,,or maybe it sounds like it runs better so I think it does
Guess I'll just have to wait till it's tubbed & 9 inched so I can take it down the track both ways
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Old 11-07-2008, 02:24 AM
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Most people can't feel even a 10 hp difference, so there is a very good chance that your "feeling" is just a perception. I have played around with computer tuning, exhaust, intakes, and other modifications on all of my cars, and although the hp shows up on the dyno, its very difficult to actually feel it in your butt.

Even if you gained 10 hp from that exhaust modification (which is significant and feasible) its likely that your gut feeling is just that... a feeling, not an actually noticable increase.
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