Exhaust question

[Changin Gears]

I don't understand why you would want in restrict the exhaust flow out of your engine.  I strive for zero back pressure. 

Matching the primary tube size of headers to engine size to keep velocity up is one thing but clogging up your exhaust system is another.

[Strawdawg]

You don't.  That link above on the Pontiac shows the benefit of reducing it at all speeds. 

[tommyg29]

I always thought that very specially designed "scavenging" exhaust systems can help an engine put out more than even open headers. Which would mean that properly designed or applied backpressure can enhance power.
Or is that just bs?

[Changin Gears]

I always thought that very specially designed "scavenging" exhaust systems can help an engine put out more than even open headers. Which would mean that properly designed or applied backpressure can enhance power.
Or is that just bs?

That sounds like Flowmaster BS.  Ever seen a Pro Stocker (or NASCAR etc.) with mufflers?

[tommyg29]

i didnt necessarily mean mufflers. I meant the shape, size and length of the tubing, and especially of the manifold/headers. For instance, do headers themselves help the engine run a little more efficiently than an engine that doesnt even have an exhaust manifold of any kind?... straight out the side of the heads? which would obviously provide the least amount of backpressure possible, but I dont think I've ever seen that either.

[Changin Gears]

Diameter/length of header primary tubes and collectors need to match engine size/RPM.  Pass that less=more.

[Strawdawg]

I always thought that very specially designed "scavenging" exhaust systems can help an engine put out more than even open headers. Which would mean that properly designed or applied backpressure can enhance power.
Or is that just bs?

scavenging systems pull exhaust gases out of the engine in excess of what they would normally flow.  Two strokes take the idea to the extreme when they come into "tune".  You can hear the rpm jump.  Four strokes are not as sensitive to the effect, but, it still works.  Diameter, length, and configuration all come into play in the design and affect where the effect is the strongest.

Back pressure is akin to sticking a potato in the tail pipe....   A properly designed system after the headers does whatever it can to not negate the positive benefits of the headers.

The X pipe tries to reduce backpressure by giving each pulse the benefit of both pipes rather than constraining the flow to a single outlet.  I would think this is more apparent at low rpm than high when the pulse frequency is high.  Drop the system after the collectors and it will usually pick up unless the  rest of the system flows very well.

[Strawdawg]

i didnt necessarily mean mufflers. I meant the shape, size and length of the tubing, and especially of the manifold/headers. For instance, do headers themselves help the engine run a little more efficiently than an engine that doesnt even have an exhaust manifold of any kind?... straight out the side of the heads? which would obviously provide the least amount of backpressure possible, but I dont think I've ever seen that either.

Well designed headers do precisely that

[Strawdawg]

Here is a hemi comparison   http://www.moparmusclemagazine.com/techarticles/engine/mopp_0301_dodge_challenger_x_pipe_exhaust/index.html

[Aussie Challenger]

  All street cars need a little back pressure to reduce flat spots etc. You can have too large a dia. pipe though even on a race motor.   
  The headers help scavange the exhaust and for out right performance the rest of the system is usually not necessary.   
  The better muffler manufacturers have done some testing and they do offer mufflers that don't hinder performance, usually the customer wants a good sounding system as well as performance and sometime that is not possible to have both.   

[Strawdawg]

  All street cars need a little back pressure to reduce flat spots etc. You can have too large a dia. pipe though even on a race motor.  
  The headers help scavange the exhaust and for out right performance the rest of the system is usually not necessary.  
  The better muffler manufacturers have done some testing and they do offer mufflers that don't hinder performance, usually the customer wants a good sounding system as well as performance and sometime that is not possible to have both.  

You can have too large a pipe diameter and that causes a velocity drop which attenuates the tuning effect of a properly matched header (at low rpm).  That does not have anything to do with back pressure, however


Street cars don't need a little backpressure...they benefit from a better flowing exhaust system as well.  I think that belief goes back to the days when short exhaust stacks were reputed to suck cold air back in and warp the exhaust valves.

[dodge freak 2]

 Old race man Smokey said in his speed book that it is like supercharging in a way to place the end of the exhaust at a low pressure area. Guess there are gauges you can place and then drive the car at race speeds to see where air pressure is low.

Then at high speeds its like a vacuum helping to suck the exhaust gasses out.

On speedtalk some race guys laugh how some believe exhaust pressure is a good thing. Only time it may is if the carb is too lean but then why not adjust the carb?

[Changin Gears]

On speedtalk some race guys laugh how some believe exhaust pressure is a good thing. Only time it may is if the carb is too lean but then why not adjust the carb?

I've always thought this is how muffler manufactures show a power gain when going from open headers to their mufflers.