Exhaust? Who needs one of those?

[Magua]

Pickin' up yet another dumped car an havin' a look at me last school.
1972.

The school has gone down hill man, "tough stance on drugs" so they kick out the users but then let in the Westlake rejects who are known to do exactly the same.

[Frankie]

blame it on ian and louis lol...

recon it started going down hill since i left..

[thehollowmen]

You wont get any more power out of it with no muffler - back pressure is important....

Oh and when you (re-)mount it, use rubber washers, or at least spring washers if you weren't already...

I know this is gonna open up a can of worms, get called a troll, but *WHY* is back pressure important?

People often say 'to stop damaging the valves' but there isn't any backpressure in the intake of a engine, and those valves seem fine... or do they require more frequent replacement?

I'm not trying to be smart, I'm just really curious. A lot of performance car / racers seem to try and reduce back pressure to a minimum with the exhasts...

[Blakamin]

I know this is gonna open up a can of worms, get called a troll, but *WHY* is back pressure important?

People often say 'to stop damaging the valves' but there isn't any backpressure in the intake of a engine, and those valves seem fine... or do they require more frequent replacement?

I'm not trying to be smart, I'm just really curious. A lot of performance car / racers seem to try and reduce back pressure to a minimum with the exhasts...

The pulsations and back-pressure create a scavenging effect on the exhaust gasses and help remove them from the combustion chamber quicker.....
but i could be wrong.... i usually am

[Mr Skid]

The pulsations and back-pressure create a scavenging effect on the exhaust gasses and help remove them from the combustion chamber quicker.....
but i could be wrong.... i usually am

Yeah, what that guy said.

I had a notion that the exhaust gasses also serve to cool the valves, and if the flow rate is too high, then there won't be time to absorb the heat from the valves, thus cooking the valves.

[thehollowmen]

We also say that back pressure slows the piston, robbing the engine of precious horsepower at high RPM... so if we remove back pressure we won't need any scavanging of exhast gasses. There seems to be a lot of missinformation about the subject and maybe some field trials are needed to help us investigate :cool2:

And my question stands about valve damage: if they stop the valves from cooking, then why don't the intake valves cook too?

[Madmax]

the intake valves are cooled by the incoming fuel are mix

[Madmax]

some info
http://www.enginehistory.org/air-cooled_cylinders_3.htm

[thehollowmen]

some info
http://www.enginehistory.org/air-cooled_cylinders_3.htm

As much as I hate to say it that article didn't even mention back pressure.

I keep seeing articles like this around:
http://www.nsxprime.com/FAQ/Miscella...austtheory.htm
and
http://www.miata.net/garage/KnowYourCar/S4_Back.html

Then again I also find this article:
http://web.tampabay.rr.com/redroby/3liter/exhaust.html
but a correctly tuned engine won't need back pressure to allow scavanging, correctly setting an overlap in the valve timing would do it just the same...

[Magua]

Well either way my exhaust has been retached, I went out during lunch and bought a bolt. Reattached it myself after school . Went for a ride, seems to be holding. I'll buy some washers tommorow and I'll keep an eye on it for a while.

[Mongoose]

Just remembered this story while I read the thread.
Mate decided his Ducati just NEEDED new cans, so off with the old ones, "Hmmmm, hang on, forgot to pick up the new ones from the shop" First
Simple solution, ride bike as is to the shop. Second
Arrive at shop AFTER finding out the zorst finished exactly where max noise will travel up-inside helmet. Complaints of blurred vision not to mention impaired hearing on removing helmet.

[Blakamin]

Just remembered this story while I read the thread.
Mate decided his Ducati just NEEDED new cans, so off with the old ones, "Hmmmm, hang on, forgot to pick up the new ones from the shop" First
Simple solution, ride bike as is to the shop. Second
Arrive at shop AFTER finding out the zorst finished exactly where max noise will travel up-inside helmet. Complaints of blurred vision not to mention impaired hearing on removing helmet.

jeeebus... i'd hate to hear that!..

[Mongoose]

WHAT???

[thehollowmen]

Many thanks for the backpressure help guys, understand it it a bit more, and can now understand how it could be detrimental to a bike that isn't tuned for it

[Posh Tourer :P]

I know this is gonna open up a can of worms, get called a troll, but *WHY* is back pressure important?

People often say 'to stop damaging the valves' but there isn't any backpressure in the intake of a engine, and those valves seem fine... or do they require more frequent replacement?

I'm not trying to be smart, I'm just really curious. A lot of performance car / racers seem to try and reduce back pressure to a minimum with the exhasts...

A couple of things. Firstly is the valve issue... Dont know the mechanics of that one sorry...

Paparazzi - it wont be speed of air, unless there is a finite volume of air - speed over the same time increases volume therefore equal cooling power.

Secondly, at a certain amount of revs, you get the muffler/exhaust system working for you. If it is the right size etc, it will start to pull the air through the engine as there is a vacuum after each output of exhaust gas. This creates some of your power peaks (horsepower).

Yes, many people put on "free-flowing" exhausts, and they are quite correct, as most factory systems are too restrictive, in the interests of long term reliability and noise reduction. However, you can go overboard. Milky's exhaust mods are a case in point (do a search for it). Put in a washer to restrict it, and it eliminated a power flat spot at about mid-range.

Dont really know if it reduced top end power, but it may have.

My theory tends to suggest that the less restrictive your exhaust/muffler, the higher your revs have to be to get the use of the vacuum effect I referred to above. Sometimes it just sounds cooler/faster, but really isnt.