Backpressure

[Biker Bry]

Originally Posted by Josh48

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.

Again.. a compressed charge... lets stick to natural aspiration. A compressed charge is a whole different subject BUT still the same chemical rules apply.

[Dave63]

Thanks for ruining a good thread.

Originally Posted by Biker Bry

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

[Josh48]

Well that said it all, also just for fun Biker Bry I have seen dyno pulls for naturally aspirated motors with an air fuel ratio as much as 12.5:1 and it was making well over 500 hp.

[Yiangos]

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.

[XL1200R]

Originally Posted by Biker Bry

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.

Yes and no...

Any internal combustion gasoline engine runs most EFFICIENTLY at 14:7:1. This means that per gram of fuel burnt you get the most miles out of that gram of fuel..
That being said, 14:7:1 is TOO LEAN to make the MOST POWER an engine can make. For the most power generally you'll be in the 13:2-9:1 range.

At 13..... whatever ratio you won't have gas pouring out the tail pipe. In fact, the bike will run and start much better in the cold, especially if it is carbureted, don't believe that, try it for yourself.

And, for an air cooled engine, 13.... a/f ratios prolong engine life by burning core, therefore reducing engine temps.

Adding an open exhaust DOES NOT in any way hurt the engine if the engine is tuned correctly after that. Also, if you tuned it back to 14:7:1 the bike would run virtually the same as it did stock, but with more power, likely. I would argue that there is no OPTIMAL fuel ratio, only one that suits the needs of the vehicle, riding style, and mechanical setup the best.

Talk to the professionals on here, Hillsidecycleco, dave63, schmidtys and they will all tell you the same things I said, or close to it. I am no professional but I have taken the combined 100+ years of experience they have and try to make worthy posts based on fact and good reasoning, not hearsay and disgust at fellow riders and their exhaust choices.

[toad94]

I'd still like to see that single cylinder engine with the true dual exhaust.

[granpita]

Wow! This has been educational and entertaining! Thanks, guys.

[Kainam]

Originally Posted by granpita

Wow! This has been educational and entertaining! Thanks, guys.

And no small animals were injured in the process!

[Baldrick]

Originally Posted by toad94

I'd still like to see that single cylinder engine with the true dual exhaust.

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.

[Leetek]

I am not going racing, If I wanted a fast bike I could have bought one of those twin turbo jap. things
I want my bike to sound really good and not get a ticket every 20 miles.
Lee