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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.
 
#3 ·
You'e always going to have some back pressure unless you're in outer space. Atmospheric pressure at sea level is 14.7 psi and that will be there no matter what sort of exhaust system you use. Of course, as you go up in altitude, that will decrease, but so will the available atmospheric oxygen and thus the horsepower you develop unless you supercharge.
 
#14 ·
I would have to hazard a guess that your exhaust is too open. Everything needs to be tuned to work together at the same RPM's. If say a 1.75" pipe was ideal, and a 2" was added it wouldn't work as well. If the engine was tuned to run in the 2500-4500 rpm range, and the pipes in the 3500-5500 rpm range performance will suffer. Some of the scavaging effect was lost at lower rpm's. Nothing was gained at the higher rpm's as the engine didn't breath well enough to take advantage of the bigger pipes.

Just adding one thing such as bigger carb, cam, exhaust or what ever usually doesn't help anything. It all needs to work together.
 
#5 ·
Any backpressure?:dunno Really?:think
 
#6 ·
Here is a quote from RB racing.
Our testing has shown that the Harley V-Twin is extremely sensitive to back pressure which is why 2-1's that use small or restrictive collectors don't breathe well, and why restrictive baffles shut the motor down. Disc type baffles are good for spark arrestors on dirt bikes but they have no place on a big inch V-Twin. You don't make power by adding restrictions to your exhaust system whether it's a bunch of stainless discs or some damn piece of aluminum billet machined into a Harley butt plug! Ever wonder why they had to put a hole up the center of the discs? Well, one reason was the discs are so damn restrictive you have to have several pounds of them to get enough flow through their waffle shaped passages. Good mufflers, but they have nothing to do with performance and they sure as hell do not create vacuum as has been claimed.

When you get confused by all the bullshit simply ask the following question.." Do they run them in NASCAR or in Formula One?". Nope, they run straight pipe collector systems without any stupid discs or aluminum butt plugs. It's always funny how, when the money is on the line, all the little things like discs, billet caps, anti-reversion flaps, reverse megaphones and other such nonsense somehow don't make the field. Black Hole technology does not involve stupid eye candy or outdated, or just pain dumb, ginger bread.
 
#37 ·
We need a like button!

:thumbsup
 
#7 ·
Baffles, plugs or discs are a cheaper way to create back pressure for what is normally an UNTUNED exhaust system.

What RB Racing fails to mention, is that although race car exhaust may be straight through, the diameter, the number of bends, and the degree of those bends are all scientifically calculated to create a tuned exhaust, sans muffler.

Street vehicles are REQUIRED to have a muffler, by Federal and local laws.

"Backpressure" is a mechainical force that slows EXHAUST VELOCITY. By maintaining a 300 ft/min exhaust velocity, you maintain optimal power and torque via proper exhaust flow.

Lower that, ans the numbers lower with it. You may gain a small amount of HPs in the top end, but not torque.

Raise it, and the numbers lower again.

There are gimmicks on the market and there are things that are real, and people that know what works and what doesn't. HD uses 1.75" diameter exhaust tubing for a reason.
 
#8 ·
I've fooled arround with a few pipes and exhaust systems on differant vehicles and the effects they have. Adding a baffle to slow the exhaust flow a bit can help in low RPM torque but will usually hurt high end output a bit if the pipe is in the proper tune for your motor. Adding a bit of back pressure is a fix for a pipe out of tune. Pipes are tuned for a torque/HP curve for a purpose so choose a pipe that gives you torque where you want it. 2 into 1s generally run smoother and have more peak torque/HP. You can have loads of bottom end torque and give up some top end or you can loose some bottom end and pick up the top end. It's a trade off that you have to decide.
Billy
 
#17 ·
With just a ev27 cam, S&S shorty/with thunderjet carb tuner ,dyna signle fire and "Python" thunder headers, it takes for ever to warm up, and you cant dog the bike at all. 3&4 gear are a pain, its where its needs the the back pressure the most, and cold start ups.
If you run it like down the strip when riding it runs great. The best way to ride it is to keep the rpm's up. Python thunder headers are tuned exhaust and a good street/drag every day use. Still wish there was $$ to put stock exhaust back on , even trade so some one else can make noise & play with horse power
 
#28 ·
It sounds like your bike is not tuned properly. The EV 27 cam works from 2000 to 5500 RPMs. The thunder jet card tuner adds more fuel at about 4500 RPMs. I think if it was put on the dyno with someone that is good at turning they could fix the problem without change in the exhaust.
 
#23 ·
Harleys are a little tricky in that regard cause of their uneven firing sequence....

Unequal length headers are meant to time the exhaust pulses inorder to maximize the scavenging effect.
 
#25 ·
Performance headers on V8 engines can be tuned , or should I say come in different configurations for a RPM range. The primary's have different lengths and diameters depending on what RPM range you want the most effect. The collector length and diameter are configured to work in the same range as the primary's.

The key is to have the whole motor built for a certain RPM range , that's having the induction system , heads, valve size , cam, lift , duration & timing and exhaust to be designed to work in the same RPM range or power band. That brings you to volumetric efficiency and the ability to flow efficiently is the key to power. An engine is really just a air pump.

Back pressure is a restriction , however you also need to maintain flow so if the stock pipe is only 1.75" then that's what it probaley flows best with for the stock engine configuration and the power band it was built for.


Take your garden hose 5/8" with no nozzle , it pours out with some pressure behind it. If we went to a 2" hose it would still pour out but with little or no pressure behind it thus not sized correctly for the volume of water flowing from the pipe. This would be like putting a 2.25" header pipe in place of a 1.75" pipe on a stock engine because you haven't increased the flow of the rest of the engine.

Now put the nozzle on and you have back pressure , the 2" hose will built up pressure once it fills ,as you have plugged the exit , but your not flowing freely either.
 
#26 ·
ive read most all of this and back pressure is a loose term used by people who dont understand it
as stated before you want the scavanging effect and velocity not "back pressure"

most would say a 3.5 inch pipe would have less back pressure then a 2 inch pipe...well that really isnt the case, everything goes back to RPM, Displacement, and Efficiency

an 80 inch motor at 6k will need a larger exhaust then an 80 inch motor at 4k

if any motor picks up power from adding some sort of "restriction" to the exhaust then its not because it needed back pressure, its because it needed velocity

thats like puting the largest intake valve you can into a motor and expect good results, doesnt happen...you need velocity over volume

and when the exhaust is being pushed down the pipe, any size change will effect how fast the exhaust flows, same thing with a river, and this makes it real simple to think about...when a big body of water moving slow is forced to go through a tighter channel it picks up speed....to an extent...to tight and it jams up, but if its to open, it becomes slow and lazy...this is what happens when you throw an exhaust on a motor that cant utilize the size of the pipe, this is why a motor will gain power by puting somthing up it like a torque cone or baffel
 
#27 ·
Try tuning a single cylinder engine. The exhaust plays a more critical part in tuning than a multi cylinder engine. With out a pulse from another cylinder you have to have a baffle to reduce reversion in the low to mid RPMs or you'll only have power in a very narrow band usually in the upper RPMs. Some times it's much better to have a broader power band than to have a bit more hp in the upper RPMs that you would rarely be in. In mini dragsters using lawn mower motors running at a constant RPM have no use for baffles and are tuned to make peak power in a very narrow RPM range. The torque converter they use is tuned to keep the motor in the peak torque. Dirt bikes can use a bit broader torque curve in the midrange. Super Trapp has been doing tunable pipes for singles for years.
Billy
 
#31 ·


this is my single cylinder quad, tq is pretty flat and i have true dual exhaust...i will also say this is on bad rings :) is smoked the dyno room out hahahaha ill be going back when i install new rings in the spring

but anyways, i have no real baffel system on these pipes, not like what comes in a harley....pipes do play an important role but it is the combination of all the parts that will give the best results....or worst

i will also add that my bike only made 88 hp when i first dyno'd it after a quick cam timing advance, ignition timing retard, and leaner jetting from where i first was, power went up to 97 and the curve improved, so maybe the pipes are not his problem, maybe it just needs to be retuned and perhaps a cam change, i was never fond of the ev-27 anyways
 
#30 ·
I tuned it. The dyno was just to see what it was going. The guy running the dyno had been doing it for ten years and said he hadnt seen one flat line g/a mix with a carb.
Still if some one would trade me for stock exhaust for the thunderheaders I'd be happy too.
 
#32 ·
For what it's worth, the exhaust design has to take into consideration the RPM band where you want the power. Open pipes will produce maximum power at high RPM's and a little restriction will produce more power in the lower and mid-range. For street use, a little restriction is welcomed.
For all out racing the littlest amount of restriction is what it's all about.
 
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