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Backpressure

18K views 75 replies 30 participants last post by  Dave Harryman 
#1 ·
Before I was a bike guy I was a car guy. One thing I learned over the years is that ANY backpressure is a bad thing. I truely believe that many people get it confused with a syphoning effect.

Backpressure is pressure that is working against the flow. Anything that is against the flow will hurt performance. Sort of like the stupid stock air deflecter on the new bikes.

I understand I'm a new guy on here and all but I keep reading on here about how backpressure is a good thing. IT'S NOT A GOOD THING!

Syphoning of exhaust is a good thing. It's where the pulse of the last stroke pulls the air from the combustion chamber as it flows down the pipe. BACKPRESSURE would slow that flow.

OK. I'm off my rant.
 
#36 ·
Exhaust flow

Exhaust flow is not constant. As a cylinder fires, the pulse or wave passes thru the exhaust as a pressure wave. Like a big fart. When the pulse leaves the exhaust pipe, a reverse wave passes back up the pipe towards the cylinder and back into the cylinder if the exhaust valve is still open. Backpressure. The wave can reverse again - scavenging the cylinder. The timing of the alternating pulses is very important and depends on the length of the exhaust pipe and the cam timing. The length of the pipe is more important than the diameter. Pipes too big tend to slow the waves and may make less power. The torque peak of an engine is where the cam timing and pipe length are ideally matched. My experience is that straight pipes really suck for power - they result in good power only at very high RPM. My low rider with super trapp exhaust flat runs away from a friends low rider with straight pipes and I out weigh him by 100 lbs.
 
#39 ·
Back Pressure

You guys are missing a serious element of motorcycle tuning and I see it a lot. Especially when people have common "super big" exhaust problems.

There is also another reason to have back pressure.. (Or more restrictive exhaust). Keeping positive pressure in the exhaust keeps the pressure high enough to allow the exhaust gas to make it through the pipe without atmospheric pressure being allowed to mix with the exhaust gasses that are exiting the pipe. Air from outside the pipe cannot mix with the exhaust gasses, this causes backfires. Less pressure... more air and when it gets enough air to detonate it backfires. Remember you "fire" triangle... it applies here as well.

What happens to the idiot that decides to put big exhaust on his bike to make it "loud"? He gets pops and backfires when letting off the throttle. Then some "genius" decides that he is "lean" and rejets or remaps his bike to "adjust out" the backfire problem. The issue here being that the bike was already at optimum Stoichiometric air/fuel ratio... Yes, there is only ONE optimum air fuel mixture... 14.7:1

so you have a bike running down the road with gas almost pouring out of the tailpipe to stop the bike from popping and backfiring when the PROBLEM is the pipe that was installed, NOT the A/F ratio at all.

Too many times I hear people say "The bike runs lean from factory for emissions" - If you think that, you need a lesson on internal combustion theory severely. A bike runs best at 14.7:1... PERIOD.
 
#41 ·
Too many times I hear people say "The bike runs lean from factory for emissions" - If you think that, you need a lesson on internal combustion theory severely. A bike runs best at 14.7:1... PERIOD.
You couldn't be any more wrong.

Although 14.7:1 is Stoich, that doesn't mean it runs "best".

That simply means it runs compliant with EPA regulations.

Dyno tuning and heat related issues all prove that HDs run at their "best" in the low to mid 13's range afr, running cooler and producing more horsepwer.

14.7:1 IS LEAN by performance standards.

14.7:1 is just the target range for any EPA compliant vehicle to keep big brother happy.

Don't get it twisted. Lesson over...........
 
#40 ·
:ducking
 
#42 ·
Off

No! It's not for emission compliance, it's a chemistry FACT! You can tell me all your "beer swillin" bench racing theories but I'd rather stick to the science of internal combustion efficiency in theory and practice.

I have a degree in internal combustion theory and the science behind it has never let me down. I have over 20 years automotive experience and even designed an alternative fuel system that is in production and running down the road on THOUSANDS of vehicles which were both EPA and CARB certified to run in ALL 48 continuous states. I know what I'm talking about.
 
#43 ·
No! It's not for emission compliance, it's a chemistry FACT! You can tell me all your "beer swillin" bench racing theories but I'd rather stick to the science of internal combustion efficiency in theory and practice.

I have a degree in internal combustion theory and the science behind it has never let me down. I have over 20 years automotive experience and even designed an alternative fuel system that is in production and running down the road on THOUSANDS of vehicles which were both EPA and CARB certified to run in ALL 48 continuous states. I know what I'm talking about.
Impressive that a man with such credentials could be so wrong. :laugh

I guess Dyno shops and engine builders are all wrong, and you're right.

Good luck with that degree. Must look nice hanging on the wall.
 
#47 ·
14.7:1 of what? A compressed charge? If he's burning pistons, he's lean and NOT 14.7:1..

Guys, this is a chemistry FACT.

Additionally, there is spark advance AND the ability of the fuel system to adapt at a particular RPM..

Pre-detonation is a spark issue, not a fuel mix issue.

You DON'T put fuel in the cylinder just to pump it out the exhaust, you put it there to burn at the highest BTU you possibly can. 14.7:1 is that ratio.
 
#48 ·
Your chemistry class is confusing you with building and tuning performance engines.

14.7:1 does run fine, but it's not an optimal horsepower producing air fuel ratio.

If you want to argue with the entire Performance world, have at it.

People like myself have real world, Dyno results from building bikes and tuning them, showing that Harley Davidsons produce more horsepower and run cooler in the low 13:1 ratios (13.1-13.5) than at 14.7:1 ratio.

You can tell us all about your degrees, college courses and studies, but these results are based on real world experience.

Harley is REQUIRED to run their bikes at 14.7:1, as is any manufacturer, in order to meet EPA standards. It's NOT for performance.

Pre-detonation will be seen more over with 14.7:1, than with 13.2:1. This is why HD incorporated the ION sensing knock detection into their ignition, over the 2000-previous models, that didn't need it.
 
#49 ·
Your chemistry class is confusing you with building and tuning performance engines.

14.7:1 does run fine, but it's not an optimal horsepower producing air fuel ratio.

If you want to argue with the entire Performance world, have at it.

People like myself have real world, Dyno results from building bikes and tuning them, showing that Harley Davidsons produce more horsepower and run cooler in the low 13:1 ratios (13.1-13.5) than at 14.7:1 ratio.

You can tell us all about your degrees, college courses and studies, but these results are based on real worl experience.

Harley is REQUIRED to run their bikes at 14.7:1, as is any manufacturer, in order to meet EPA standards. It's NOT for performance.

OK, if you say so. When you put a set of "free flowing" exhaust on a bike and it back fires on decel... Keep thinking it's lean and pump massive amounts of fuel through the engine so you have so much left over hydrocarbon in the exhaust that it's too rich to backfire... remember this!

Put it on a dyno, and the bike that is "backfiring" will outrun the one that is tuned bad. I know it and I've seen it and I hear it perpetuated over and over. Like I said, I know what I'm talking about. Where do you get your info?
 
#50 ·
Pre detonation is not always a spark issue, to low of octane or bad fuel can cause it as well, and any forced induction vehicle requires more fuel as the air is being compressed as with compressing brings heat which the fuel will keep some of the intake charge temps down.
 
#52 ·
Thanks for ruining a good thread.

Biker Bry said:
Like I said... Science produces FACTS! Bar room talk between bikers = Goober's garage results and bikes that run and sound like crap.
And apparently, you think less of bikers who work on their own bikes. Good to know.



Argue all you want.


From the factory all EFI Harley models are tuned based on several objectives. This includes emissions standards and the calibration itself is based on the stock components. The Delphi ECM is a speed density system which is based on modeled airflow. While the system is calibrated based on load and air mass the closed loop area of the calibration is configured to operate at or near 14.68 AFR. In most calibrations closed loop is from idle up to approx 80 kPa (say approx 60% throttle) which during this time the ECM uses feedback from various sensors including the stock narrowband O2 sensors to maintain the tune. The NB O2 sensors can only measure a small area near stoich which is 14.6:1 (+/- about .3) Besides the EFI systems ability to adjust for conditions such as ambient temperature, barometric pressure, etc... the system is designed to adjust for small variations such as inconsistencies in fuels and production tolerances based on the stock components, but the primary function is to keep the AFR at (or near)14.6:1 in closed loop. Although there is a window of adjustment and the system does have the ability to adapt to a certain extent, It is simply not in its strategy or is it capable of adjusting for free flowing components When you change these parts you can easily put the fuel requirements outside of the EFI's window of adjustment as it is calibrated for the stock components. This is a problem, especially considering the closed loop AFR target in the stock calibration is 14.6:1 which is already leaner than optimum. If your fuel requirements are beyond the ECM's adjustment the mixture goes dangerously lean, it can easily go leaner than 15:1. This is where you will feel poor throttle response, surging, detonation and in extreme cases engine damage. The higher RPM regions are generally not as much of a problem as the stock AFR targets have to be richer to prevent detonation, but they are certainly not optimum. For these reasons EFI tuning is one of the best improvements you can make to your bike. This is our specialty and we offer 100% tuning support.

http://www.fuelmotousa.com/harleymain.htm
 
#54 ·
Hi to all of you. I have a sportster low 2006 model and i have a bit of upgrades on it.
1-vans and hines straight shots
2-mesh open air filter
3-carburetor jet kit
4-performance iridium sparks
5-Kevlar clutch kit
and many more
Now backpressure to na engines is needed on high rpms when the engine power is starting to go down.
I have check on the highway with and with out exhaust baffles and the conclusion was that without baffles i get more power on start but i got only 183 km/h.
Now with the baffles i got a bit less noise almost the same at start but i got 209 km/h and stop cause of no down force and it was a bit dangerous at cornering.
So as I see small baffles they do make a different if you have all of the thinks change and upgrade the bike correct.
 
#59 ·
Well, there have been quite a few single cylinder engines with twin exhaust ports (Rudge and Panther spring to mind); not exactly true-duals but only because the designers had more sense.

Now I'm going to get controversial and say I agree that 14.7:1 is the ideal ratio. However I'm not going to insult all those who have learned the hard way that you do get more power by adding more fuel. The science is correct - at 14.7:1 you get complete combustion of the mixture and therefore the maximum conversion of fuel to energy. Maybe the difference in the real world is that it's extremely difficult to get a true 14.7:1 throughout the combustion chamber at all revs. As has been discussed at great length in other threads, the valve timing, exhaust design, intake system and a host of other things only work really well at a narrow rev range. At other times it's a compromise. The result is that the fill rate of the chamber varies and the fuel/air ratio varies. My guess is that it's necessary to pump more than the ideal ratio in there to ensure that you get as close to the ideal as possible over the desired rev range. Those of you who remember a previous back-pressure thread will remember that Dave and I agreed to disagree about exhaust systems. I still believe most of them make a lot of noise but are badly designed in relation to the engine they're fitted to. dave preferred to believe that the engines were not properly tuned for the exhaust. The end result is the same, just looking at it from different ends.
Of course it all gets even more complicated when we think about stratified charge engines with direct fuel injection. The engine in my Honda Jazz (I think it's called the FIT across the pond) has a 14.7:1 ratio immediately around the ignitor but most of the combustion chamber is at 25:1. It has multiple valves, some of which only open under certain conditions and is one hell of a lot more efficient (and cleaner) than my Harley engine. It's also slightly smaller!
However, pre-ignition is a different story. Way back in the 70s (?) Honda made some experimental engines of about 30cc if my memory serves. They could run these in excess of 30,000 rpm. The most staggering discovery was that they could move the ignition point forward and back as much as they could and nothing would make the things pre-ignite. To me that says there's more going on than just ignition. It's all about the rate that fuel burns and when the maximum pressure/temperature occurs relative to TDC. If the fuel burns too fast (i.e. explodes) due to weak mixture or the ignition point is too advanced then the max. pressure will occur at TDC instead of just after it. That's what makes the pinging sound and blows holes in pistons. If the cooling is inefficient it will cause the fuel to burn faster - which is why my iron-head Triumph used to burn through 9.5:1 pistons in about 100 truly awesome miles. In my old Messerschmidt 3-wheeler I had a switch which advanced the ignition so far that the 2-stroke engine ran in reverse. Simple but dangerous - as I discovered the only time I got into second gear going backwards.
So lets not get insulting - science has it's place but sometimes the world isn't the perfect place that scientists wish it was.
 
#62 ·
Techlusion TFI FAQs

Q: I hear your product only adds fuel. Some people say you need to remove fuel?

A: It is true that our product only adds fuel. We designed it that way to
keep it simple and affordable. Our belief is that in almost every
situation, addition of fuel is what fuel injected bikes need.

Fuel systems are mapped by the factory to be lean in cruise and everyday driving
conditions in order to meet emission standards
. If you decide to change
your fuel system, most aftermarket exhaust systems tend to reduce back
pressure, which leans out the fuel even more
.

There are a few exceptions to this, and one that comes to mind is the Honda CBR929/959 when the exhaust power valve is removed. Honda designed their engine specs around that feature (specifically the exhaust cam timing) and when you remove that valve, volumetric efficiency in the motor is lost causing a bottom end bog.

You can mask that bog by pulling out fuel around that area, and for that, you may have to go to another product. However, our product performs well with all aftermarket exhausts. We keep in mind that the exhaust must at least match the flow of the stock system that is being removed or modified in order to maintain maximum volumetric efficiency.
 
#63 ·
Being this was a good discussion I'm going to get it back on the subject it started on. I had some Honda dirt bikes with single cylinder motors that had two separate pipes comming from the head. They were 4 valve heads that had a small pipe comming from each exhaust port going into a single muffler with good low to midrange torque. No doubt the 2 smaller pipes had higher exhaust gas velocity than a larger single. There was a removable baffle in the muffler but I don't believe it made much differance other than sound when removed. We had to leave them in to ride in the National Forrest as they were part of the spark arrester. It looks like a baffle in a pipe decreases the size of the pipe, forcing the exhaust through a smaller area and increasing the gas velocity. This would increase scavenging. I said in a earlier post a baffle slowes down the exhaust but this may not be correct. It may slow down reversion a bit and give the exhaust valve more time to close.
Billy
 
#66 ·
I build pipes for 4 stroke MX bikes etc. and have done a lot of dyno development on them and one of the problems that come from baffles is not so much that they hurt flow , but they cause a sonic wave to reflect back up the pipe and it plugs up the flow on overlap and causes a power loss , I actualy designed a special muffler core that steps out in size where the baffle is located that is designed in a way that the sound waves reflecting off the baffle hit another perimeter baffle and cant travel back up the pipe so flow on overlap is not effected, and even though the flow is restricted with the baffle there is no power loss to speak of
 
#70 ·
yes it is ,and a good tunned meg is the ultimate, but some cant handle the noise they put out , and the timing of the sonic wave is the factor , and when you just stick a baffle in a tunned system ( weather it is a meg or regular tunned exhaust system )it changes the sonic wave timing resulting in loss of power , so if you can design a baffle that wont otherwise change the sonic wave timing of a good tuned system then you have something , I was just trying to explain to the other member why baffles often hurt power
 
#71 · (Edited)
I know Im new here so let me tell you guys that Im not new to building engines for more power , I am a engine builder / machinist / fabricator , here are a couple pics of some VW engines I built back in the 80's ,you cant buy these parts , they are hand made , not trying to brag ,just trying to let you guys know I have a lot of experience when it comes to hot rodding stuff
this one had Porsche Hemi heads and overhead cams fitted
 
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