backpressure: what are the facts?

ratty

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I had always been under the impression that backpressure was vital to engines, especially mildy tuned ones. In fact, all the data i have ever read suggests it is required at least on car engines and proven to add power in dyno tests. That is on car engines. This is certainly not a car engine. Why then, when my muffler (a hollow one at that) blew off at the head pipe on my HT motor during a ride, did the bike take off like a rocket? Well, you know what I mean. Talk about poor welding/inferior metal. Substantial power gain with NO backpressure . I dont get it. All of a sudden the bike picked up a few MPH or so and got louder, although it wasnt too much louder than the hollow muffler. Actually it took me about 15 seconds of riding before I noticed it was gone, no BS. It seems like I read a similar thread a while back but I cant find it now. Does anyone have a plausible explanation for this, because I am at a loss.
 
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when my exhaust gasket blew out,my bike got a lot louder ,the bike really took off and got much faster,like 10mph faster.it felt like i was riding a motorcycle. it had a lot more power and accelration.
sadly i had to put a new gasket on it because it was too loud.

how come some small airplane 2-stroke engines only have straight pipes and a silencer for mufflers.
 
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There are a lot of factors... Are we talking two-stroke or four-stroke here?
On a four-cycle engine, the sudden loss of backpressure may well result in more power, at the expense of a very lean mixture that might well cause the engine to overheat.
Unless, of course, it was running rich to begin with....

Two-stroke exhaust is part science and part art, as far as I can tell. A properly set up system not only scavenges the burnt charge from the combustion chamber, it directs a "reverse" pressure pulse to keep the incoming charge in place.
(At some point in the cycle, both intake and exhaust ports are open.)

We used to pay very close attention to jet sizes, carburetor needle sizes and settings, and plug temperatures during my rather brief racing fling.
 
My engine is, like it says in my post, a HT....Happy Time, i.e. cheap-o Chinese 2 stroke bike kit engine, the 66/80cc version. I have quite a bit of IC engine know how and understanding. I don't claim to know everything, but I know enough, having owned, built, raced, etc. plenty of carbed and injected 4 stroke gasoline engines from 1940's model vehicles on up into the 2000's. I have worked as a professional mechanic for many years, but not anymore. Plus....none of that stuff really matters anyway, because my 2 stroke knowledge is limited to R/C aircraft engines, lawn equipment, a four wheeler, a yamaha scooter, and now this. None of which I have really ever modified. So 2 stroke engines are still like a black art to me. Like cabinfever said, R/C engines petty much have a straight pipe with a silencer, and they dont burn up. Those engines even run nitro! Im not planning on keeping my bike motor with just the 'straight' pipe because the neighbors would more than likely complain, and i dont want to draw any negative attention to the sport. Its too loud to really enjoy riding anyway. At this point I'm thinking that A DIY expansion chamber couldnt hurt this THING plus parts are dirt cheap. I really cant believe how much power I gained when the stock exhaust gave up, enough to make me come up with an expansion pipe one way or the other.
 
2strokes are sensitive to back pressure since there is only about half the crank angle available for the exhaust cycle compared with 4 strokes.This makes low or even negative back pressure desirable in these engines.Exhaust tuning, by creating an under-pressure from the reflexion of the exhaust gases at a discontinuityin the tubing can help in pulling the burnt charge out and thus improve operation at high speed,This is rpm dependent so it is not a cure-all.In 4 strokes back pressure can be helpful in protecting the exhaust valve from temp. shock I believe.
 
I understand that a good expansion chamber pipe is 'tuned' to a narrow rpm band, the 'power band' of said engine. I wonder what the best power band of these chinese engines is? They do not like to rev high. I dont have a tach for any of these engines, it sounds like I need one tho. I suppose I should email the SBP guys and get more info on their pipe, maybe that is the best route. Time is money, and their kit is relatively inexpensive. I think more research is in order. Thanks for all the replies so far!
 
Well, if you find the torque specs for the engine at what rpm, then you could use jpilot's gear ratio calc to find what speed = that power band, then buy a bike computer (mph/kmh), and try to stay at that speed to maintain maximum torque.
 
48cc max 1.2kw(1.61hp)/6800rpms
max speed 40kph(24.8mph)(this is just a estimate the engine kit site gives)
max engine torque 2.4nm/4500rpms(1.77lbs/ft)
max no-load speed 8500rpms
torque at rear wheel:
50t 8.9lbs/ft
44t 7.8lbs/ft
36t 6.4lbs/ft
34t 6.0lbs/ft
32t 5.7lbs/ft

66cc max 2.0kw(2.68hp)/5500rpms
max speed 40kph(24.8mph)(this is just a estimate the engine kit site gives)
max engine torque 6.2nm/5500rpms(4.57lbs/ft)
max no-load speed ????rpms
torque at rear wheel:
50t 22.9lbs/ft
44t 20.1lbs/ft
36t 16.5lbs/ft
34t 15.5lbs/ft
32t 14.6lbs/ft
 
straight pipe power

I too noticed a huge power gain when the baffle fell out, so much so that the 2nd time it happened I didn't go back for it. I have read that a 2cycle has a "secondary" compression ratio, that of the fuel air mixture in the crankcase. When backpressure is too low the mixture can exit the cylinder w/o burning called short circuiting or in the case of an r/c engine "4 cycling". An expansion chamber takes advantage sound waves which travel through the exhaust pipe faster than the spent gases and both pull the gases out and when they return back toward the exhaust port help hold the next fresh charge in the cylinder. Having said all that don't you love the power these motors put out YEE HA!!!!
 
High rpm is a strain on bearings (rpm squared).I would not push my luck with the Chinese 2 strokes (or redo the bearings).4 cycling occurs at low throttle opening when the incoming charge is too small to replace the old one and the micture fails to ignite.
 
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