| Author | Message | ||
     Pogue_mahone |
so whats the deal then? i see EXPENSIVE chrome for the HD's and are straight pipes.then i hear about cones(to restrict the flow...give back torque) i see buell pipes are all mandrel bent appear to be equal length with a 2 pipe collector. i ASSUME i4's, v2's and car engines all benefit from good flowing exhaust.it bothers me that the buell "can" is a tube with a pipe to feed it in and a pipe to let it out. and that is why i did my exhaust like i did.a mandrel bent 90 degree elbow trimmed to fit a streetrod muffler underneath.the baffle in my muffler has a gentle curve(like an s curve) and it is all directed out with a tube all the way. i never have understood with any vehicle why the sound is so important.i could care less if it is totally quiet,as long as it performs well.and straight pipes? they let it rev faster,and we all know that is not the v twin's forte. discuss please | ||
| Iamarchangel |
Pics please. Exhaust has three purpose: guide waste gases out; assist valves in air flow: and quiet engine. Noise is energy wasted. Rolling Stones is noise energy not wasted. Somewhere in between is what we're after. There is an aesthetic to the sound of a V-twin. I want that so I'm going to trade something. I'm more surprised that there is little "science" to this after all the years. We're still going mostly "seat dyno". Even when it involves a dyno, it's still by gosh and by golly. My bike's first owner did some exhaust mod. I'm not sure I really like the tone, compared to Xopti's for example, but it rocks and runs well. I'll take that. American Sport Bike has an older dyno chart but the race magazine chose and won with gbalias'. There's no stats on that except that winning is what it's all about. | ||
| Pogue_mahone |
no pics right now....brb for more discussion in a bit | ||
| Blake |
There is plenty of science, fairly well settled science on exhaust tract design. A google search of BadWeB will turn up some good information. I'm too beat to repost it or search for it right now. I'll say one thing, increased backpressure has nothing to do with optimum torque. | ||
| Pogue_mahone |
whatever backpressure really is. and i disagree. exhaust design can change the torque characteristics.exhaust design can can change if your engine revs to max faster. | ||
| Iamarchangel |
I had a look a while ago for some "science" but I didn't have much success. I have quite a bit on two strokes but very little on these engines. I imagine there's lots out there. I couldn't find much in regard to these engines. Backpressure is the air flow wave that "bounces" back to the exhaust port. It's critical in two strokes where it acts as a valve. It's not so critical in a four stroke since they have valves and an exhaust stroke. So, go with Blake on the optimum torque statement. Also, torque is not rpm dependent the way horsepower is. Max revs give you max horse but torque has already levelled before that. Don't get too techy on the last statements, I'm being general. | ||
| Andymnelson |
Some info I posted a while back on back pressure. Some non-scientific science, if you will.  ah the old backpressure is good myth. here's some facts about exhaust systems: - the point of an exhaust system is to quickly and efficiently evacuate exhaust gases - backpressure is not necessary to get the most power out of an engine, but may be necessary to getting the most power out of an otherwise stock engine because the fueling system often can not keep up - burned valves do not happen because of a lack of backpressure, they happen because of a too-lean condition - larger, more open pipes do NOT equal less backpressure because a smaller pipe will carry the exhaust out at a higher velocity (think of water through a pipe- the same amount of water through a large pipe flows slower than when forced through a small pipe) - the trick is finding the gentle balance in between, where velocity and flow are maximized, but free enough that there is no restriction - with more exhaust flow must come more fuel and air flow - without flow meters and lots of bench testing and software modeling, the only other way to find that balance is LOTS of trial and error moral of the story: backpressure is bad! but our motors were not designed to have no backpressure- the fuel system can't inject fuel fast enough, and the throttle body isn't large enough to flow enough air. the system was designed with a certain amount of backpressure in mind. so eliminating backpressure would require redesigning the motor if you want it run at optimum. oh, and my factory designed race pipe, ecm and filter all function together fantastically. : ) | ||
| Rydberg |
Good post. I especily like: - larger, more open pipes do NOT equal less backpressure because a smaller pipe will carry the exhaust out at a higher velocity (think of water through a pipe- the same amount of water through a large pipe flows slower than when forced through a small pipe) makes perfect sence to me. to much back pressure would be like a spray nosle on the end of the hose. No good for a cheep hose. Good thing Buell/Harley dont make cheep hoses. the trilogy of tech i have alwase been taught have been the Exhaust,Filter,ECM combo (esentially de-smoging it)on every thing. But it sounds like the things that are missing there for optimal perfomance out of a stock motor is the larger throttle body to help out the high flow filter and larger jets to go with the Race ECM. | ||
| Pogue_mahone |
and the 3 (or so)things of airflow in an engine: carb/throttle body intake design heads(good,bad,whatever) exhaust. airflow is horsepower and peak hp and peak torque can be had below redline of rpms | ||
| Bromanowski |
You can also add cam profiles to that mix. They determine a lot of the flow characteristics. | ||
| Forerunner |
As most have already posted, backpressure is often confused as a requirement for torque. What is typically required is proper tubing diameter to promote scavenging through the exhaust pulses. One instance of where the biggest diameter tubing is desireable is in turbocharged applications where the inline obstruction is the exhaust side turbine and you want as little backpressure as your ears can tolerate downstream. YMMV, Nels | ||
| Iamarchangel |
airflow is horsepower No. That's only part of it. Always remember the Blessed Trinity: fuel, air (oxygen), and a spark at the right time. The balanced/desired combination of the three is what gives you horse/torque. Airflow by itself is just like a plug sparking away by itself: you're not going anywhere until the other two kick in. | ||
| Slaughter |
If you are serious about seeing just what factors should be considered in an exhaust deign - consider looking at Burns Stainless exhaust design page (as used used on XBRR and others) Burns Link | ||
| Iamarchangel |
Slaughter: thanks for the link. If I remove the back slashes to the home site, there's a tech section on the menu. That has a lot of the answers we're looking for here. http://www.burnsstainless.com/ | ||
| Blake |
"Backpressure is the air flow wave that "bounces" back to the exhaust port." Using that definition of "backpressure" I'd agree that it is important. But that is not how I've understood the meaning of backpressure. People most often talk about "increasing" the backpressure. That indicates to me that they are talking about increasing the resistance to flow. That is never advantageous to performance that I know. Crafting the geometry of the exhaust tract to achieve optimum (best combustion chamber scavenging effect) reflection of the sonic pressure pulses is not what I've ever understood backpressure to be. To say that you are increasing backpressure in that light makes no sense. The trick is crafting the exhaust tract geometry to achieve good scavenging throughout the rev range and not just at one portion. Straight pipes will make the most power, but only at a narrow range of engine speed. So one method is to add a step in the pipe size. Each step added creates another reflection point for the sonic pressure pulses. More later? | ||
| Forerunner |
Leave it to Blake to articulate my thoughts more clearly than I can. Nels | ||
| Iamarchangel |
From the link that Slaughter provided: This reflecting, negative pulse energy is the basis of wave action tuning. The basic idea is to time the negative wave pulse reflection to coincide with the period of overlap - this low pressure helps to pull in a fresh intake charge as the intake valve is opening and helps to remove the residual exhaust gases before the exhaust valve closes. Typically this phenomenon is controlled by the length of the primary header pipe. Due to the 'critical timing' aspect of this tuning technique, there may be parts of the power curve where more harm than good is done. Tons more info on that link. | ||
| Pogue_mahone |
i agree arch. my comments were more of my own thinking LOL in the sense that from the air filter thru the head to the exhaust tip,it all has to be components that work together including the cams. with my vw stuff(mentioned here as basic engine knowledge) i matched the ports and minimised all turbulent areas and made the exhaust flow smoother.so it could rev better and it promptly swallowed a valve LOL.so then to better heads and stiffer springs and better adjusters and stout mounts for the rocker arms. ahh this is fun---better than fantasy football eeeeeewwwwwwwwwww | ||
| Blake |
I understand that at one time, decades ago, before improvements to valve and valve seat alloys allowed them to reliably endure the high temperatures possible in high performance engines, there was indeed a need for increased backpressure wihin the exhaust tract of an engine. For some backyard efforts especially if using the stock cylinder heads, increased resistance to flow was needed to reduce the flow rate of hot exhaust gasses over the exhaust valve and valve seats. To little backpressure would allow the exhaust valves and seats to overheat and deform. Not good. But these days our materials technology has advanced to the point that usually something has to be horribly wrong in the engine tuning for valves or their seats to suffer problems due to excessive heat. |
