Backpressure is a myth???

[Fizzyhair]

So I was lucky enough to spend an hour talking to one of suzukis race mechanics at a local open day. He's worked on suzukis British superbike for years.

He was explaining that backpressure from a baffle is irrelevant to an exhausts performance. It's only an issue if you can't adjust the fuel side. Open pipes (assuming you get them right are the best way if only the race rules would allow it. No engine NEEDS backpressure to work. Actually a simple google search explains this very quickly.
Suzuki use different pipe diameters to balance between velocity of flow and expansion volume to make the exhaust more efficient. This is why the header pipe is actually the most important part.the muffler can be removed with no adverse affects. Yes you will need to rejet etc to allow the balance to be restored but backpressure Has sweet f all to do with it.

Just thought I'd share what I'd learned with the group.

[Agemax]

different story with 2 strokes though,
the Harris race exhaust on my GS is completely void of back pressure and it runs sweet

[Steve]

Yes you will need to rejet etc to allow the balance to be restored but backpressure as sweet fa to do with it.

I sorta agree with the general idea, but there seems to be a few letters missing from the highlighted area, so I don't really know for sure what you meant.

.

[Fizzyhair]

Physically Speaking the pipes need to offer as little back pressure as possible but offer a small enough space to force the exhaust gases to move as quickly as possible.

[Steve]

Physically Speaking the pipes need to offer as little back pressure as possible but offer a small enough space to force the exhaust gases to move as quickly as possible.

True enough, but my question remains.

I am presuming there is an "h" missing from "as", making "has", but I don't understand the "fa".

If it is a texting shortcut for something vulgar, re-word it to something that the local nanny filters will let through.
(I don't speak "text", I speak "Amurrican". )

.

[Agemax]

FA is a very polite way of meaning "nothing"

[Fizzyhair]

Edited it to f all lol

[Tom R]

Then the next thing your know you're gonna try to tell us that open headers don't burn exhaust valves.

[Flyboy]

Then the next thing your know you're gonna try to tell us that open headers don't burn exhaust valves.

No they don't, over heated valves from the resultant lean mixture is what burns them.

[Agemax]

Then the next thing your know you're gonna try to tell us that open headers don't burn exhaust valves.

how come drag bikes get away with it then?

[Stahlgrau333]

I usually keep my mouth shut on things like this because they always end up being long-winded internet arguments (ie., 'Will the plane fly'?). The truth is, I've never understood how backpressure could possibly 'help' anything, with the exception of some scavenging advantages in a narrow set of situations. Now, properly-sized pipes, on the other hand, DO make a huge amount of difference. Too big of a pipe right outside of the exhaust port will create backpressuer, whereas a properly sized pipe will keep exhaust velocities up, lowering backpressure and even creating a scavenging vacuum behind exhaust pulses when tuned right.

And while I refuse to have the argument with anyone...no, it doesn't make any sense that open headers/pipes would burn exhaust valves. Unless, of course, you throw on free-flowing pipes that lean out your mixture to the point of burning valves, but that's a tuning issue, not a pipe issue.




Edit: I see that the others beat me to the punch on the burnt valve issue.

[GSexpress]

For a race bike on a race track where max power through the powerband is all you care about, that's true. For a street bike that also needs to carburate well at low and midrange rpms some back pressure helps to even out the exhaust flow over a wider range and extend the torque curve for streetability. Both backpressure and pipe diameter can be used to control the speed of exhaust gas flow. It depends on the rpm range and powerband reqiremnets of intended use as to how much of which you would want to use. Too large a pipe diameter and no back pressure will make for a bike that will only run spot on at peak horsepower rpms and fall on it's face at lower rpms cause the gasses are moving to slow and prevent the combution chaimber fom scavenging efficietly. This makes for a bike that is very hard to jet correctly.

[Stahlgrau333]

The idea is for exhaust gas to flow away from the port, at all RPMs, as quickly as possible. Backpressure does absolutely nothing to increase exhaust gas velocity...only heat, wall friction, and pipe diameter can.

[Agemax]

The idea is for exhaust gas to flow away from the port, at all RPMs, as quickly as possible. Backpressure does absolutely nothing to increase exhaust gas velocity...only heat, wall friction, and pipe diameter can.

unless its a 2stroke, then its a whole different pot of kippers

[GSexpress]

The idea is for exhaust gas to flow away from the port, at all RPMs, as quickly as possible. Backpressure does absolutely nothing to increase exhaust gas velocity...only heat, wall friction, and pipe diameter can.

You wouldn't think so just from the term backpressure ... but it does.

By creating a restriction in the tailpipe you force the gases to speed up. Though it's not always obvious the gas flow in the pipe is not at a steady speed. the increased flow speed through that slightly restricted tailpipe, actually causes a vacuum in the header inbetween exhaust pulses at lower than max rpms and can actually boost midrange performance over a nonrestricted tailpipe. I've seen it work on the dyno, it's a fact.