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Necoa #: 7515
Location: 13(TX)
Joined: May 20, 2006
Posts: 117
Year: 1981
Model: El Camino
Posted: Tue Apr 15, 2008 6:38 pm Post subject:
elcamino74guy wrote:
A little backpressure can be a good thing. Helps scavenge the cylinders. Too much is bad but a little is good. Right now when I let off on the gas on the freeway I can actually hear the exhuast burble which is neat. I had the same thing on my 79 Z28 and it was awesome.
Haha, my car does that and I was wondering if it meant something was wrong... I'm glad it doesn't cause I think it sounds cool.
well if you have a air injection pump installed...reasoning being for that after firing is due to a faulty air injection pump! The un burned gas that left teh cylinder is getting burned up _________________
Proud Wyotech Graduate
could be ur exhaust leak but other than that still coo! I liked hearing my after firing puffs when my smog pump was defective! _________________
Proud Wyotech Graduate
Necoa #: 11902
Location: 25(All Other)
Joined: Jun 11, 2008
Posts: 102
Year: 1978
Model: El Camino
Posted: Thu Jul 10, 2008 8:09 am Post subject:
At low rpm's no back pressure will allow more low end torque, because there's no resistance on your engine to breath the air it needs to burn the fuel. At high speeds no back pressure will cause your engine to have less horse power and more torque, that lowers top speed. When you have no back pressure you get more off the line power, when you have a good induction, getting enuf air through your carb, or throttle body. The X-pipe I have heard, with balance the pressure, between dual exaust because each side has different back pressure, & the x-pipe balances the exaust pressure, wich means more power. If you add some good mufflers like Edelbrock's Ti-Tech SDT 304's you will have some back pressure, but not too much. With high end heads, and good headers, you will see a nice gain, with great air flow, now re-tune your carb when your done-(Happy engine=Happy driver).
Necoa #: 11902
Location: 25(All Other)
Joined: Jun 11, 2008
Posts: 102
Year: 1978
Model: El Camino
Posted: Thu Jul 10, 2008 8:46 am Post subject:
I have alittle more tech. info on this subject- & I have a LOT more tech. info.
"Backpressure and scavenging ability can easily get confused. [/U] If you build a system with no backpressure, you'll end up with a system with no scavenging ability. The trick is to find the fine balance between the two. The bottom line is on a normally aspirated engine, too big results in poor performance....practical experience has told me that."
"Scavenging has much to do with size and length of header runners but the size of the system beyond that may have some substantial impact as well."
"Cylinder Scavenging, backpressure etc all has to do with manifold/extractor/header design, once your past that point, you want as much flow as possible.
What some seem to overlook though, is that often header design needs to extend to well under the car depending on where in the rpm band you want to make good power."
In english please, how does a basic system using factory manifolds on a 305 with 2 1/2" pipes with X and flowmaster 40's would work, and will that suffice when a 350 (w/headers) is installed at a later date ? Thanks ! _________________ Chillin in Lauderdale
Your question doesnt speak english as well, but in shorter term if you go to big with a free flowing exhaust you will lose power in theory. So if you have a street engine mild build 1 5/8" headers then a x pipe and flow masters and teh x pipe in the correct location and wiht 2 1/2" exhaust tubing you will have no back pressure existant nor having air being sucked in into the exhaust being the scavinging effect so I belive.
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