exhaust backpressure myths?

nuckingfuts

nuckingfuts

nuckingfuts
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This might have been on here before but i couldnt really find anything to deal with it, if so i apologize and i will delete the thread.

Motor backpressure...! i know a bit of backpressure is needed to improve torque among other things. i see these shorty style exhaust with no baffles that dump straight out with maximum flow but, with no backflow its possible to get valve burning or reduction in torque. has anyone inquired about this before or ran a dyno to see possible improvements between styles? you see all these aftermarket exhaust and all this R&D going into their technology, why waste so much money when you can just slap on a straight pipe and call it a day.. all that money cant be spent just trying to figure out how to make a set of pipes meet the legal decibel level. the million dollar question would be the longer the better, the shorter the better...? To baffle, or not to baffle ( blah blah shakespear quote or something:shrug: )

i would like some feedback or peoples experiences ( possible dyno proof kind of stuff ) between exhaust systems because the time will come soon when i need to hash out my exhaust system.. i love the look of the shorty but after throwing some serious coin at this motor i dont want my power and torque to piss away over my esthetic prefferences.:bike:
 
Old myths dire hard. You can tune your carbs to run short pipes easily. The reason for the misconception is that short pipes with low restriction cause a lean condition because the stock carbs cannot adjust to the increased airflow. If you increase your jetting sizes to match the exhaust, it will run perfectly.
 
Here is something to think about. An open pipe will resonate. The acoustic wave bouncing back and forth. The speed of sound in teh pipe is given so at any one rpm, a negative wave will pull / scavange the exhaust just before the valve closes. This wave goes through the intake due to valve over lap where it changes to a Positive wave, comes back, and forces in that last bit of mixture before the intake valve closes. This is the Holy Grail of free HP! At half rpm the exact opposite occures and the engine runs like a skunk. The myth is that an engine needs back pressure but teh reality is a non muffled street engine with open exhaust has too much back pressure due to reversion.

Best running street engine = small diameter long exhaust pipe with a free flowing muffler.

Tom
 
I think I believe mufflers & performance go together really well (there is enough evidence to suggest this is a reality). But because I like to scare small children & set off car alarms in the parking lots I run straight pipes.
 
been away from the site for a couple days, thanks for all the info! i went to 650central.com and i was reading about their Exhaust Peak Optimizers & Torque Peak Optimizers.. anyone have experience with these? so that little weld on item is theoretically supposed to take place or be an alternative of a Port & Polish? interesting..
 
I don't think it's an alternative to porting, you'd still want that too. What the head insert does is fool your motor into thinking it has smaller headpipes, which these engines like up near the head. They perform better though with a larger headpipe downstream a few inches. With the inserts, you can have your cake and eat it too. You get the small exit at the head and the larger headpipe downstream. The stock headpipes had a version of these built into them. You can cut the ends off of old stock headpipes and make your own inserts .....

HomemadeARs.jpg


Inserts.jpg
 
I'm so fucking sick of "backpressure", it's a made up term to explain things to slow minds. Your pipes have to be small enough to give highest velocity possible, but just big enough to flow all the exhaust without choking. Resonance is another factor, square-cut pipes resonate, slash cut pipes don't resonate nearly as well. The longer the pipe, the longer the resonance time, the lower the RPM the exhaust will tune. Way back, NACA and the military found an 8" pipe to be 'neutral'. Season to taste.
 
May be, but I'd still like to see that report! I love reading these old things! Hell, they are mostly straight forward. No flashy power point presentation to water down the subject with graphics!

Also, I may need to change my search engine filters! I think they are DIRTY!
 
There are formulas out there for proper pipe dimensions. Cylinder CC, port sizes and Valve sizes, determine Exhaust length and recommended Pipe ID... Need a shorter pipe? Go smaller ID... Need a longer Pipe, go Larger ID... All the formulas are out there. I make tuned pipes for all of my performance engines and bikes. It's a timely process, not just "1 3/4" and done....

And Baffles can help a ton, along with proper mufflers...
 
The only NACA report I have on hand is about water injection in air cooled engines. some real neat diagrams relating to BMEP and fuel ratio in hemispherical air-cooled engines. I can dig that one up when I get home. There might be another one about turbo-compounding as well...

Diggign into the crannies of my memory, they either considered the 8" pipe to be 'performance neutral' or the minimum necessary to keep from burning exhaust valves. Possibly both.
 

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