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Old 01-25-2012, 09:25 AM   #1 (permalink)
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Back Pressure

Just recently I read a few posts suggesting that some exhaust "upgrades" don't provide enough back-pressure (reversion as some people call it) to prevent popping on a closed throttle. I always understood that back-pressure was necessary to ensure that unburnt fuel wasn't sucked down the exhaust when the inlet valve opens before the exhaust valve has shut. If there isn't enough to prevent the popping, doesn't that imply that the so-called upgrade exhaust is actually badly designed? If the fuel is going down the exhaust port then you won't get optimum filling of the combustion chamber. So not only do you get popping but you get less power than you should.
Way back when I was sensible and rode an FXR, you could buy cones that fitted up near the exhaust port - they claimed to cure popping by increasing the back pressure which makes sense to me.
Or am I missing something here?
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Old 01-25-2012, 09:33 AM   #2 (permalink)
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It's not all about backpressure. By reducing backpressure, you slow exhaust velocity. Slowing exhaust velocity, you reduce scavenging, and that's what causes all the popping and reversion loss.

It's not that the pipes are poorly made, it's because any time you change air intake volume, and or exhaust velocity, you need to change the engine's tune to match the newly added mod(s).

It's tuning-101.

If you Google "Reducing or slowing exhaust velocity" you'll see exactly what I mean.

Backpressure is the physical change inside the exhaust.

Exhaust velocity is the effected physical property from the cylinder to the exhaust tip, and what needs to be adjusted for, when ever changed.

Reversion cones increase backpressure exiting the cylinder, which increase exhaust velocity. However, they also effect exhaust pulse width and aren't the best for performnce. They're a bandaid to me, for a much larger issue.
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Old 01-25-2012, 09:52 AM   #3 (permalink)
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It's not all about backpressure. By reducing backpressure, you slow exhaust velocity.
Interesting. I can understand that exhaust velocity will increase the volume of fresh mixture - the momentum of the exhaust gas will "drag" more new mixture in behind it. What I was wondering about was the reverse wave that is propagated when the exhaust hits the open end of the pipe. I understood that this should be timed to arrive back at the exhaust port just after the inlet valve opens (or just before the exhaust valve closes) to seal off the exhaust from the frech mixture. The momentum created by the exhaust velocity will continue to push fresh mixture in and effectively "supercharge" the chamber. I imagined that mistiming this pulse would allow unburnt fuel into the hot exhaust, hence the pop. Understanding this certainly allowed MZ to transform 2-strokes from pathetic weaklings to power-monsters!
When I get home I'll dig out some of my old books and follow up on your suggestion
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Old 01-25-2012, 10:04 AM   #4 (permalink)
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What I was wondering about was the reverse wave that is propagated when the exhaust hits the open end of the pipe.
I believe this is directly effected by the exhaust's velocity, which needs to be at or around the prime number 280-300 fps on most vehicles, including Harley-Davidson. This is why the 1-3/4" exhaust tubing works best with stock-slightly modded engines, and 2:1 pipes make the best torque. HD's crossover pipe helps to equalize the exhaust pulses as well, considering how short the exhaust is. Longer exhaust produces more torque as well.....

As exhaust velocity drops, so does torque and scavenging.
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Old 01-25-2012, 10:46 AM   #5 (permalink)
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Learning from this thread guys, thanks. Dave, I'm with you except for scavenging? Is that the little bit of unspent fuel lost to exhaust before the exhaust valve has closed? Seeking definition.
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Last edited by Chief Illiniwek; 01-25-2012 at 10:47 AM. Reason: Typo
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Old 01-25-2012, 11:04 AM   #6 (permalink)
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Learning from this thread guys, thanks. Dave, I'm with you except for scavenging? Is that the little bit of unspent fuel lost to exhaust before the exhaust valve has closed? Seeking definition.
Scavenging is the process of pushing an exhaust and combustion pressure out of a cylinder into the vehicle's exhaust system and the drawing in of air and fuel, for the next cycle and combustion process.

Changing your exhaust directly effects scavenging, your tune and performance. Not all changes can be felt, but if significant enough, most definitely.
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Old 01-25-2012, 11:20 AM   #7 (permalink)
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So more fuel to the engine would help reduce some of the popping ? Not a wrench just a bolt on specialist. Like adding a type of fueler like a Rev-Tech DFO ? Or did I miss the whole point?
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Old 01-25-2012, 11:26 AM   #8 (permalink)
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So more fuel to the engine would help reduce some of the popping ? Not a wrench just a bolt on specialist. Like adding a type of fueler like a Rev-Tech DFO ? Or did I miss the whole point?
If exhaust isn't leaving quick enough, then fresh air and fuel isn't coming into the cylinder fast enough, or without the correct intake volume of air and fuel, it creats a lean condition. The mixture needs to be richened.

This is why you see cam specs including "Duration". This is the time, marked in degrees, that a cam lobe holds a valve open. The more lean (EPA compliant) a bike runs, the less duration you'll see for the intake and exhaust valves. That duration is a direct reflection of the narrowness of the cam lobe, at its highest point of lift. The cam LIFT, is also another factor for air in, exhaust out, as is the diameter of the valve/seat, and flow characteristics of the cylinder head.

Richen mixture= more exhaust=more exhaust velocity=more intake air brought in for combustion = more torque and HP.

You're correct. Adding fuel to the mixture, creates a richer, more powerful mixture and explosion, replacing exhaust velocity, increasing the intake air and fuel intake, that was lost with decreased backpressure.

Extreme example: Dragsters. Straight pipes, but short and one bend.

That BEND, creates resistence (backpressure) to speed up exhaust velocity. The mixture is soooo rich, to produce sooo much HPs with such advanced timing and tremendous amounts of scavenging, that the fuel is STILL BURNING in the exhaust, as it exits the end of the exhaust pipe.

The EPA doesn't like this, because unburned fuel from the exhaust with a rich, powerful mixture is called "Hydrocarbons". Unburned fuel. That's barely noticable in an EPA complicant 14.7:1 (Stoich) mixture.

Of course, like with anything, your total tuning and HPs are directly related to the timing, cams, head flow, fuel delivery and ignition.

It all works together, but one change to one thing can cause a drop in Hps, Torque and total performance.
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Old 01-25-2012, 12:16 PM   #9 (permalink)
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This thread is fricken gold.
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Old 01-25-2012, 02:15 PM   #10 (permalink)
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Thanks Dave, Love reading your posts. You lay things out in such a way that even the laymen can grasp the concept. I am learning everyday.
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