Backpressure

gobucks · Post by **gobucks** » Sat Aug 30, 2008 11:20 pm

Hi, just wondering how back pressure affects running, is it only necessary for two stroke engines or for four stroke engines too, what does back pressure do to the engine?

Dac · Post by **Dac** » Sat Aug 30, 2008 11:33 pm

this shows more the basics of a expansion chamber exhaust, but the same concept works.

its needed so when the exhaust gases leave the cylinder, the gas needed for the next blast dont go with it.

its needed more for 2 strokes, but 4 strokes kinda need it too.

Kenny_McCormic · Post by **Kenny_McCormic** » Sun Aug 31, 2008 1:24 am

I have seen 2 strokes run for years with exhaust just pouring out the port and I have heard pistons shattering in under a minute. I have heard from a very reputable source that no harm will be done to a 4 stoke running without an exhaust, another source, tells me the exhaust valve will melt(strangely enough this person is also the owner of the chainsaw that runs with no exhaust system the muffler bolt threads are stripped).

In any event its very cool to look into the exhaust port of a running 2 stroke and see the fire inside change colors under different loads.

gobucks · Post by **gobucks** » Sun Aug 31, 2008 12:22 pm

so it just keeps the new fuel/air mixture from rushing out or being pushed out of the cylinder by the piston through the exhaust port since the exhaust port is higher than the intake?

So then with a four stroke since the exhaust valve is closed when the mixture is being compressed it should be ok to have an open exhaust?

Kenny_McCormic · Post by **Kenny_McCormic** » Sun Aug 31, 2008 12:31 pm

Open like a straight pipe yea, but not without anything.

justinnielsen01 · Post by **justinnielsen01** » Sun Aug 31, 2008 5:00 pm

2 stroke needs back pressure. a four stroke has valves

Kenny_McCormic · Post by **Kenny_McCormic** » Sun Aug 31, 2008 7:17 pm

justinnielsen01 wrote:2 stroke needs back pressure. a four stroke has valves

Have you tried running a 2 stroke without a pipe?

gobucks · Post by **gobucks** » Mon Sep 01, 2008 12:17 pm

no, i haven't, so if you do run a two stroke without a pipe part of the mixture runs out in the exhaust making the mixture that is ignited lean?

Kenny_McCormic · Post by **Kenny_McCormic** » Mon Sep 01, 2008 1:08 pm

You would want to adjust the carb.

gobucks · Post by **gobucks** » Tue Sep 02, 2008 7:33 am

wouldn't that be giving you worse gas mileage though since some of your mixture is going out in the exhaust, which would require more gas to be taken into the intake to get enough mixture to cause enough
compression, but a small portion of it not burned?

Ziraya · Post by **Ziraya** » Tue Sep 02, 2008 7:46 am

yes, plus you run the risk of the escaped gases igniting inside your body panels, melting s***, maybe even blowing them off while you're at speed in traffic

Bear45-70 · Post by **Bear45-70** » Tue Sep 02, 2008 11:53 am

gobucks wrote:wouldn't that be giving you worse gas mileage though since some of your mixture is going out in the exhaust, which would require more gas to be taken into the intake to get enough mixture to cause enough
compression, but a small portion of it not burned?

Exactly, The unburned mixture caused real high NOX numbers and that is why there are no 2 strokes available anymore (thanks EPA) in lawn mowers, motorcycles, ATVs and outboards, except for the 2 stroke direct injection into the cylinder after the exhaust port is closed type in the big outboard engines.