back pressure?

[gs500f-gaz]

as a newbie asks, sorry if sounds a bit dumb;

with the after market slip ons and cutting the stock pipe has any one found that thier bike runs a bit lumpy. just asking before i buy a R6 can. cause i thought the little GS would of relied on back pressure alot.

[solosociety]

pretty much all gas eneings run on back pressure. but i had straight pipes on my gs and it ran just fine. i probably should have rejetted it but never got around to it and decided to get rid of the pipes. basically as long as you rejet the bike you really shouldnt have any problems

[gs500f-gaz]

so it wont really matter if i dont rejet. cause it will make it alot easier. for now any way.

[solosociety]

well i dont really know if you have to with a stock exhuast from another bike but if you put an aftermarket one on its suggested by pretty much everyone on here.

[gs500f-gaz]

your a star mate;  ill post when i find out if it works or not.ill probably be putting a stock r6 exhaust on it any way. just thought id be a bit F***ed if i sawed off the standard and then i got to jets or if it wouldnt run any good.

[Nikolas]

Back pressure is always bad and to be avoided.

[solosociety]

Back pressure is always bad and to be avoided.

you might want to do alittle bit of research before you say something like that

[Nikolas]

Back pressure is always bad and to be avoided.

you might want to do alittle bit of research before you say something like that

Maybe you should.  How can restricting exhaust flow be good?  It can't, plain and simple.  You want the engine power to reach the wheel/s, not go into forcing gases out of the exhaust.

Maybe you're confusing backpressure with maintaining proper exhaust velocity.  A properly sized exhaust will have no backpressure.  To big and the exhaust gases are slow, causing them to cool down too fast, increase in density, and cause a restriction, too small and the pipe can't flow the necessary volume of gases which results in a restriction.  The ideal exhaust would not even be of a constant diameter, but would be a megaphone to keep the gases at a uniform speed and temperature, requiring minimal work by the engine.  Get the length right and the sonic pulses will suck the gases right out, and be quieter without a restrictive muffler too.

For a single cylinder you want the exhaust pipe to have twice the volume as its displacement.  The ideal length is the exhaust duration times the wave speed (1700ft/s at MSL) over the engine rpm you're tuning for.  I don't recall the formulas for multiple cylinders off the top of my head but they're floating around on the internet somewhere.

I build post cat exhausts for Toyota MR2s that run in SCCA solo ES which win national events and drive around on the street with no muffler using this.  If I ever get this damn GS registered I'll give a go at a better slip-on and possibly full system.

[ben2go]

Go get your leaf blower,start it up,then cover the end just enough to cause pressure to back up in the tube.If you have an older one it'll choke the engine.This is the same thing that happens with a restrictive exhaust.If you remove that restriction,both on the intake and exhaust,you can get more fuel/air mixture through it.Then the only thing restricting the engine is the cylinder(s) volume.

[Gisser]

How can restricting exhaust flow be good?  It can't, plain and simple. 

Ever hear of the Yamaha EXUP valve?

[spc]

ok.......saw off the pipes before the 2-1 and smear shaZam! on the inside of the tube...........no backpressure.....it'll figuratively and literally be a shitty ride

[ben2go]

crucialval's bike has been rejetted with a lunchbox airfilter and his ran wonderful with no muffler.Loud as hell though.Much louder than a Harley.

[solosociety]

alright the only time you dont need back pressure is when you are running a turbocharger on an engine. the gs is not a turbo charged enine so it will be needing back pressure. no i never said that it has to have a tremendous amount of back pressure but it needs some. if you are so confident that the bike needs no back pressure the get yourself a camera, go take the exhaust off your bikle from the engine back. then fire up the bike and tape yourself rididng it around getting on it and shaZam!. then put your exhaust back on and do the same exact shaZam!. when your done post your video so we can all see it. well that is if your valves dont bend and lock up the motor. i really dont care about your job making cat back systems because if you knew alot about catalytic convertors you would know they are involved in creating back pressure along with having headers and manifolds that go from 4 into 1. so im pretty sure i have done my research and unless gas engine designs have changed since i graduated from UTI then well i guess im right.

[slowinthestraights]

alright the only time you dont need back pressure is when you are running a turbocharger on an engine. the gs is not a turbo charged enine so it will be needing back pressure. no i never said that it has to have a tremendous amount of back pressure but it needs some. if you are so confident that the bike needs no back pressure the get yourself a camera, go take the exhaust off your bikle from the engine back. then fire up the bike and tape yourself rididng it around getting on it and shaZam!. then put your exhaust back on and do the same exact shaZam!. when your done post your video so we can all see it. well that is if your valves dont bend and lock up the motor. i really dont care about your job making cat back systems because if you knew alot about catalytic convertors you would know they are involved in creating back pressure along with having headers and manifolds that go from 4 into 1. so im pretty sure i have done my research and unless gas engine designs have changed since i graduated from UTI then well i guess im right.

Hahahah graduated from UTI, I'm sure you are an expert. Running with NO exhaust at all IS bad for the motor, however it has nothing to do with "backpressure". Running with no exhaust AT ALL is bad because the valves are constantly being exposed to hot air (exhaust) and then cold air (surrounding atmosphere air).

[Nikolas]

alright the only time you dont need back pressure is when you are running a turbocharger on an engine. the gs is not a turbo charged enine so it will be needing back pressure. no i never said that it has to have a tremendous amount of back pressure but it needs some. if you are so confident that the bike needs no back pressure the get yourself a camera, go take the exhaust off your bikle from the engine back. then fire up the bike and tape yourself rididng it around getting on it and shaZam!. then put your exhaust back on and do the same exact shaZam!. when your done post your video so we can all see it. well that is if your valves dont bend and lock up the motor. i really dont care about your job making cat back systems because if you knew alot about catalytic convertors you would know they are involved in creating back pressure along with having headers and manifolds that go from 4 into 1. so im pretty sure i have done my research and unless gas engine designs have changed since i graduated from UTI then well i guess im right.

Turbochargers cause immense backpressure and pumping losses.  If you pull the exhaust off an engine, you will be forcing the engine to do work forcing exhaust gases out because you will have destroyed scavenging.  Read up on this guy named Bernoulli, he wrote the book on moving fluids through tubes.  An engine is just a glorified airpump; the more air you move through it, the more power you'll make.  A restriction is never a good thing.

But I guess you're right since you went to UTI.  My University of California, San Diego engineering education and Society of Automotive Engineers membership can't match a tech school.

[spc]

Yeah well my non-complete geology degree and various combat skill from the army say smear shaZam! on it and blow it up

[Nikolas]

How can restricting exhaust flow be good?  It can't, plain and simple. 

Ever hear of the Yamaha EXUP valve?

Yes, it improves scavenging at all rpms by changing the exhausts sonic properties.  Without such a system you can only tune an exhaust for one rpm.  Ideally you'd have a long narrow exhaust that got short and big as the revs increased, but that's not really possible...  Although there are intakes with multiple runners that work off that principle.  It's all a compromise.

[ben2go]

I guess 12 years of building car engines taught me nothing.All internal combustion engines work on the same principles.So I guess all that back pressure is what made the 1100 hp my 65 mustang had.

[Jay_wolf]

Off topic , but i bet that Mustang was a Beast , i want to run my bike withno muffler , just a slash cut pipe poking out? ben , u sed some1 run it fine with it ? didnt mess the engine up? 

[ben2go]

Off topic , but i bet that Mustang was a Beast , i want to run my bike withno muffler , just a slash cut pipe poking out? ben , u sed some1 run it fine with it ? didnt mess the engine up? 

Crucialval ran his bike with the exhaust cut off right below the can.Didn't seem to effect it,but his engine has been heavily rejeted with a K&N lunchbox filter.As long as the engine doesn't lean out through the rpm range,I don't see a problem with strieght slash cut pipes.