Backpressure?

[warriorspeed350]

yes it does have disks...and if you put a piece of paper on them it will stick unless you rev it obviously.....i am not retarted and you need to chill the **** out and not be so quick to critisize

 

[griff]

"Back Pressure" is pretty much always bad for "performance", and the term is misunderstood and used incorrectly quite often.

You basicially always want to keep the exhaust flow/pulse moving (i.e. velocity). One of the things that will cause the exhaust flow/pulse to slow down is a rapid drop in temperature. Hot exhaust gases expand immediately when given the chance, cooler gases are denser and slower (you start losing velocity).

For example, going from a small hole such as the exhaust port in the head of a Raptor 350, to say a huge 4" diameter header causes a few problems:

* Drops the exhaust velocity almost immediately
* Rapid expansion of hot exhaust gases
* Super rapid cooling caused by the above
* Substantial turbulence caused by the above

A properly sized header will generally keep the flow rate up, but it will still lose heat quickly, the difference is in the way in which it loses heat. Remember, the second the exhaust gas leave the cylinder, the gas starts cooling, its getting denser, and slower by nature (this is why keeping the velocity up from the get-go is important).

A lot of people call the above properties "back pressure", which is essentially incorrect, theres a piece missing, read on:

One of the side effects to a properly made header, thats spec'd to the engine its on, is keeping that exhaust pulse moving quickly (it has velocity and momentum), as it travels away from the exhaust port/valve it creates a low-pressure area behind it - i.e. creating a slight vacuum in the header right in front of the exhaust valve.

So what happens when the exhaust valve opens again? Theres a slight vacuum there that actually helps suck out the exhaust gases from the cylinder. Theres the performance

That is called "scavenging". Scavenging must be tuned properly, pulse frequency, temperature, header/exhaust length (and other stuff) - all play factors in doing this correctly. Thats why time and testing is necessary to get it right.

Do it wrong, and you'll have exhaust pulses doing all kinds of weird stuff, piling on top of each other, causing massive turbulence, etc etc. That can potentially cause restrictions in the exhaust, i.e. loosely "back pressure"

Back to the term "back pressure" it does also exist in the sense that manufacturers do intentionally restrict the engines performance by placing a "cork" on it (usually a muffler with super small diameter exit), then they'll restrict the intake to match the flow characteristics of the other end. They do this for any number of lame reasons - noise being the big one, and also making sure that model doesnt have a chance of competing against a higher model stock to stock, etc etc etc.

Thats the basics... it can and does get much more complicated than that though, but for the purposes of this conversation, it'll do

Hope that makes sense

 

[warriorkid]

wow...thats almost exactly what DSWwarrior said in our AIM convo. lol. But what your saying is that as long as you have the right header, then you will gain power. What about the silencer? Could you run without it?

 

[griff]

wow...thats almost exactly what DSWwarrior said in our AIM convo. lol. But what your saying is that as long as you have the right header, then you will gain power. What about the silencer? Could you run without it?

Well, it is a little more complicated than that. For the sake of the Raptor 350's, I would say no. The reason being is that full systems headers (and slip-ons with the stock header) were tuned/designed around the use of a muffler.

The tuned length of the system has a lot to do with the timing in between exhaust pulses, so that a low-pressure differential is maintained until the next exhaust valve cycle.

If you shorten that length too much, the exhaust pulse will leave the exhaust system before the next time the exhaust valve opens, and it will effectively kill the low-pressure differential at the exhaust port at the header (you will lose power).

For example, on drag bike, most will use a straight pipe off the header, but it still has some length to it. Yes, its loud as hell, and yes its a bit shorter than say a normal street system (with a muffler), and they do get a performance gain - BUT, it comes at an expense, they lose low and mid-range power. BUT, they dont care about that, its a drag bike, the system is designed to operate at max rpm's for that run.

i.e. the tuned length can be a shorter because at high rpm the exhaust pulses are very very close together. The "frequency" (of exhaust pulses) fits within the tuned length of that system.

 

[warriorkid]

so then if i did shorten the muffler. I would gain on top-end, and just lose some mid and low end power?

 

[griff]

so then if i did shorten the muffler. I would gain on top-end, and just lose some mid and low end power?

You have no way of knowing. Without testing that specific configuration on a dyno, with baseline runs and test runs, youre just throw'n the dice.

In theory, yes its possible, but without the proper testing, you may do more harm than good. And IMHO, with a 350cc single cylinder engine, any possible gain would be so small, it'd be a complete waste of time.

 

[dave76]

Hey Griff don't you also have the possibility of burning or bending a valve with a strait pipe, because we do drive these things all year round. I've heard this being a problem with Harleys, because of guys reving the motors up real high and then let it wind down, basically sucking in all that cold air on a 1300° to 1400° F exhaust valve????

 

[griff]

Hey Griff don't you also have the possibility of burning or bending a valve with a strait pipe, because we do drive these things all year round. I've heard this being a problem with Harleys, because of guys reving the motors up real high and then let it wind down, basically sucking in all that cold air on a 1300° to 1400° F exhaust valve????

Bend a valve? Piston would have to hit it

Burned valves I've heard of on some bikes before, and even some cars. And in every case, its because it was made way lean. On some harleys, guys goto systems with full straight pipes (as well as an opened-up intake), and never bother to rejet or remap the damn thing.

 

[dave76]

Oh that would explain why so many have blue marks in the pipes.

 

[dswwarrior350]

Oh that would explain why so many have blue marks in the pipes.

This is true but it can also be caused by cheap pipes... most cheap pipes are singe layer tubing that is near impossible to keep from bluing.... the high end pipes... such as vance and hines and etc use 2 layers of metal.. keeping the pipe from bluing.

Jetting plays a major role however its near impossible to keep the cheaper pipes from bluing to come extent.... even the stock pipes on my dads have some bluing.

 

[dave76]

Is it possible that FMF makes a thinner walled header pipe to? because my pipe is like purplish blue, and the jetting is on, at least that what my plug reads?

 

[griff]

All stainless steel (SS) is gonna discolor to some degree. Massive blue'ing will certainly indicate a lean condition, yellowing is just heat discoloration (also depends on which type of SS too). A little blue'ing is not a big deal, its gonna happen even on top systems.

How fast it changes color can tell you something.... some manufacturers use different gauges of steel, some thinner than others (saves weight). 304 and 316 SS are the most common, particularly 304 for exhaust systems.

Several current V&H systems are double-walled, they have V&H's BP design, if thats what DSW means. They dont make those for sportbikes AFAIK, primarily cruisers. You wouldnt want that on a sport-bike (or anything else where weight is more important than looks), double-walled systems weigh a frik'n ton.

If you wanna save weight, and gain performance, theres absolutely nothing wrong with a single-walled system (almost all sportbike systems are single-walled). Single walled doesnt necessarily mean cheap (dont get me wrong, there are some shitty ass systems out there for everything).

As long as you get a major brand, FMF, Yosh', etc, you dont have worries.

 

[dave76]

The first couple times I rode with the new carb I didn't have it jetted right, and the pipe actually glowed bright orangey red, so I rejetted and it's been good since.

 

[warriorkid]

i juse cut holes in my airbox and did a few other little things to help it breathe better. Do i need to re-jet? Or can i get away with tweaking the air/fuel for now?

 

[dave76]

Probably so, the stock jetting is conservative in stock form, let alone doing anything to free it up.

 

[warriorkid]

ive already jetted it dude, it has an HMF with the other mods i just talked about.

 

[schppppeeiiddiiiee]

i didn't think u had to repack HMF

 

[yamarider]

i didn't think u had to repack HMF

Oh yes, any exhaust with packing should be repacked from time to time.