Exhaust/Back pressure

[DJSKJ]

Has anyone experienced loss of power with the installation of a new high flow exhaust system? Talking with a guy at an off-road shop yesterday he told me the Jeeps with the 3.7L are notorious for loosing power when a high flow exhaust system is installed due to the high amount of back pressure needed by these engines. True of false ????

DJ

 

[tjkj2002]

True and false,loose low end torque and gain some high end HP.

 

[honu]

Agreed... all engines need some backpressure in order to develop low end torque. By "overly" reducing the back pressure, the power band moves up the RPM scale.

Biggest problem with the "free flowing" systems is that the flow too free... the smaller the engine, the more noticable the effect on torque.

My opinion, and that's all it is, is to use a mandrel bent exhaust tubing in a stock diameter and go to a freer(sp?) flowing muffler. You'll maintain the low end grunt AND benefit from increased exhaust velocity at higher revs.

 

[RenegadeJay]

Due to laziness I haven't taken pics or posted a video, but I recently had installed a Thrush Weld muffler with all new piping from the stock flange (that was removed) all the way back including a nice chrome tail pipe. I have noticed that the off the line seems to lag a bit, along with the initial take off when I punch it while moving but it does seems to get to speed quicker once you get past the low end. I didn't do it for the speed, I did it for the sound. I really like it. Not as bad ass as it was when I clamped the muffler on and had no piping to the back, but it is still cool. I hopefully will get around to getting some video very soon. I was so hooked on having to have a flowmaster because of the videos I saw but I am very happy with my choice.

 

[honu]

A case in point... several years ago, having purchased a Chevy p/u to tow my boat, I decided it "needed" a "better" exhaust system. I read all the magazines, all touting the improved performance to be gained by going with a larger, cat-back system.

I drove to my friendly, neighborhood exhaust shop and ordered up a nice, big 3 1/2", double throw down, triple step over, hot rod exhaust! The proprieter asked "Why"? I told him about all the research I'd done and the results acheived... he responded, "Bulls--t"!

He patiently explained that by installing such a large diameter system, I'd effectively be acheiving the exact opposite of what I was trying for. That the only thing I'd be doing is converting more gasoline into noise, except if my plans were to be towing my boat at 85 to 90 MPH.

"Oh yeah..." I said... "put it on".

Two weeks later, as I was attempting to maintain 30 MPH, with my right foot shoved past the throttle plate, trying to ascend the Cajon Pass in SoCal, being passed by every vehicle (and a few bicycles), I realized I should have listened to someone who knew what they were talking about. This truck just couldn't get out of it's own way!

I sheepishly returned to the exhaust shop, told the old guy he'd been right all along and asked that he "fix" the problem. He cut out a section of the tubing, behind the cat and ahead of the muffler, installed a 2 3/4" pipe and miraculously, the torque returned.

 

[mmc757]

Well its true that the results you guys are talking about will occur, its not actually backpressure thats making the engine work correctly. Thats a common myth that persists because its an easy way to explain away the phenomenon.

People who discuss velocity have it closer to correct. In the right sized pipe, the exhaust gasses leaving one cylinder (or set of cylinders) at a high speed creates a venturi effect which helps to "pull" out the gasses from the next set of cylinders, thus minimizing the amount of exhaust that goes back into the engine, therefore maximizing the fresh air/fuel mixture entering.
At low RPM's, where we normally operate, this process is critical to proper operation because the velocity is so low to begin with.

If the pipes are too large, these gasses dont travel as far/fast down the pipe, so when the piston hits the intake stroke, it draws the exhaust in, leaving less room for air/fuel, and causing lousy combustion.
If the pipes are too small, then a backpressure really will buildup, which causes the same problem as having pipes that are too large, but its more evident at the higher rpms instead of the lower.

Sorry to geak out in my second post, but I have researched this a lot.

 

[honu]

Well its true that the results you guys are talking about will occur, its not actually backpressure thats making the engine work correctly. Thats a common myth that persists because its an easy way to explain away the phenomenon.

People who discuss velocity have it closer to correct. In the right sized pipe, the exhaust gasses leaving one cylinder (or set of cylinders) at a high speed creates a venturi effect which helps to "pull" out the gasses from the next set of cylinders, thus minimizing the amount of exhaust that goes back into the engine, therefore maximizing the fresh air/fuel mixture entering.
At low RPM's, where we normally operate, this process is critical to proper operation because the velocity is so low to begin with.

If the pipes are too large, these gasses dont travel as far/fast down the pipe, so when the piston hits the intake stroke, it draws the exhaust in, leaving less room for air/fuel, and causing lousy combustion.
If the pipes are too small, then a backpressure really will buildup, which causes the same problem as having pipes that are too large, but its more evident at the higher rpms instead of the lower.

Sorry to geak out in my second post, but I have researched this a lot.

I think you hit the nail on the head... exhaust "velocity" is the issue and properly sized exhaust systems, creating the "right" amount of backpressure, are what control the flow.

There are many other factors at play, only one of which is exhaust valve timing. On many engines, the exhaust valve remains open during the initial phase of the intake stroke, the theory being that the outbound gasses will help "pull" the new intake charge into the combustion chamber. Without proper velocity or scavenging, the opposite occurs, some of the exhaust gasses may actually be pulled back into the combustion chamber, reducing the incoming charge with essentially, inert gasses.

In years past, it wasn't uncommon to see "stepped down" exhaust systems. The concept was that as the exhaust gasses move along the tubing, they cool and condense, thereby reducing mass and slowing the velocity. By progressively reducing the tubing diameter, the effective mass of the exhaust gasses were maintained, along with the exhaust velocity.

This is all theory to me, but it made sense. I'm from the old skool, early 60's through early 70's, and what made sense then, may hold water now.

 

[Clyde Frog]

Due to laziness I haven't taken pics or posted a video, but I recently had installed a Thrush Weld muffler with all new piping from the stock flange (that was removed) all the way back including a nice chrome tail pipe. I have noticed that the off the line seems to lag a bit, along with the initial take off when I punch it while moving but it does seems to get to speed quicker once you get past the low end. I didn't do it for the speed, I did it for the sound. I really like it. Not as bad ass as it was when I clamped the muffler on and had no piping to the back, but it is still cool. I hopefully will get around to getting some video very soon. I was so hooked on having to have a flowmaster because of the videos I saw but I am very happy with my choice.

+1

I noticed a slight lag in the start-from-a-full-stop department but much better pick up when speeding up while already at a 40-50MPH speed. I don't care too much because I'm not trying to jump off the line and race whatever's next to me. I also did it for the sound and am much happier with my Jeep now. The only thing I regret is getting a shiny new chrome tip because it's hard to keep clean when I want it to be!