backpressure: what are the facts?

Discussion in '2-Stroke Engines' started by ratty, Jul 10, 2009.

  1. ratty

    ratty New Member

    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.
     
    Last edited: Jul 10, 2009

  2. 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.
     
    Last edited: Jul 10, 2009
  3. Bikewer

    Bikewer New Member

    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.
     
  4. ratty

    ratty New Member

    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.
     
  5. duivendyk

    duivendyk Guest

    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.
     
  6. ratty

    ratty New Member

    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!
     
  7. pdxrhett

    pdxrhett Member

    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.
     
  8. 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
     
  9. nedwreck

    nedwreck New Member

    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!!!!
     
  10. duivendyk

    duivendyk Guest

    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.
     
  11. nedwreck

    nedwreck New Member

    4 cycling

    Thanks for the info. So the 4 cycling I am having at cruising rpm is caused by a lean situation. How do I rejet or get more fuel into the mix? As for pushing the rpm following the guideline in the motor instruction of " avoid sustained full throttle operation or motor life will be shortened" and the fact that bicycle type brakes aren't that great above 12 mph. Should keep our motor together. Also shaving the head .020-.040. Adds some. GuTs to the little beastie!
     
  12. ratty

    ratty New Member

    4 cycling

    Since we are on the topic, I have 2 HT equipped bikes....both are the 80/66 cc versions. both seem to be a little different (I bought both bikes already converted with low hours on engines) but they both pretty much run the same. Both will 4 cycle until a certain RPM.....approx. 3000 by ear. This was with the exhaust unmodified, and later with modification. Thats when they start to run right, like a 2 stroke should, throughout the rest of the RPM band. Unless there is a load on the engine; like climbing a hill, then it 4 cycles again. When I got my first bike I thought something was wrong, until I watched some videos of bikes in action. When I got the second bike, thats when I assumed it probably was 'normal' for these engines. Anybody find a way around this?
     
    Last edited: Jul 11, 2009
  13. nedwreck

    nedwreck New Member

    4 cycling

    Ratty, after further thought I seem to remember model airplane motor beak-in called for a RICH mixture causing 4cycling. As the tank emptied bubbles would enter the fuel line leaning the mixture causing the motor to run normally (2cycling) I'm starting to think that racing carb for sale at some sites might be worth it iF. It allows more adjustability. Anyone tried one of these? How about the low restriction air cleaner from S.B.P.? It looks a lot cooler than the black plastic one too. As I said earlier took the head off both my motors , and using apiece of plate glass with 3m stickit sand paper on it I planed .040 off one head and a more conservative.020 off the other, this increased compression and resulted in a significant power increase. Be carefull of piston/head interferance. If you goof or don't like the change a new head is a whole 12 dollars. Party On!
     
    Last edited: Jul 11, 2009
  14. duivendyk

    duivendyk Guest

    It would appear that way,4 stroking at low throttle would occur esp. in powerful 2 strokes like the Kawasaki 550 for instance, if you were just poking along.If it's at medium throttle you're likely to be too lean (beware of overheating),does it idle fast too ? (intake leaks!).
     
    Last edited by a moderator: Jul 11, 2009
  15. nedwreck

    nedwreck New Member

    4cycling

    We can assume the 550 kaw has a much more refined exhaust intake carb etc. ThAn. Our little china motor. Intake leaks would probably also be accompanied by uneven throttle response and backfiring but the general imperfections in the making of our little mills serve us well as they will run in less than perfect conditions and states of repair, also ride the kaw everywhere in one gear 4th should do. Rich 4cycling will probably occur.
     
  16. duivendyk

    duivendyk Guest

    4 stroking cannot really be prevented,it's part&parcel of the '2 stroke condition',a lean micture would promote it.The carb may be improperly designed.Normally at higher air speeds the micture tends to get richer (Bernouilli's law),so possible the needle taper may not be right and raising the needle position one notch would improve matters.However that may cause rich running at higher throttle,this would usually not be deleterious,but would increase fuel consumption&might cause plug fouling.It is important to ALLWAYS get enough oil into the engine,even at low throttle,that's why I am opposed to higher than 30:1 mictures.The only occasion a 2 stroke began to seize up on me was when had I tanked up with premixed gas once and took off at high speed on a 4 lane road.The engine seemed to lose power all of a sudden for no apparent reason,realizing what was prob,going on I cut the engine,coasted to a stop&let it cool down,luckily it freed up,never ever did that again.
     
    Last edited by a moderator: Jul 11, 2009
  17. duivendyk

    duivendyk Guest

    The engine specs posted by cabinfever77 have mistakes in them.For the 48 cc engine the max power was quoted as 1.61 HP at 6800 rpm.However if one computes the power at 5500 rpm based on the torque of 1.77 lbs.ft ,it turns out to be 1.88 HP,this is hardly credible the output at 5.5 k skould be lower, or the max power at 6.8k higher (more likely the latter.)
    It turns out that the figures for the 68cc engine can not be trusted either.The max torque of
    4.57 lbs.ft is not credible,it can not possibly be 2.5 times as high as the 48 cc engine.That would correspond to a power output of 4.9HP at 5.5 k rpm,The max output quoted at 5.5 k is 2.68HP.The power ratio between the two engines at 5.5k turns out to be 1.42 ,the displacement ratio is 1.39 ,which makes sense.The torque at 5.5 k for the 68cc engine based on 2.68HP output turns out to be 2.52 lbs.ft.For anyone interested in these sort of computations,I can give a handy-dandy formula (not that hard),here goes:
    P= 0.193 x (rpm/1000) x Torque , P is in HP, rpm in rev/min and Torque in lbs.ft.
    For instance in the last case P= 0.193 x 5.5 x 2.52 or P=2.68 HP.
    It would be appropriate to post revised numbers in the specs.
    There happens to be an ballpark approximate empirical relation between engine displacement and max torque: Tmax=D/25 and Pmax=D/20to25. (50/40HP/liter).This applies to typical small engines, not the souped-up variety.
     
    Last edited by a moderator: Jul 11, 2009
  18. nedwreck

    nedwreck New Member

    4cycling

    Thank you DUIVENDYK for increasing our knowledge of 2cycle motors a very great deal. This is precisely why I joined this site. I just went for a ride on my 80cc screamer and it was happy 4cycling at low speed but as you mention too much can lead to plug fouling so I get on it frequently. Performance is so hot with the head planing and reduced back pressure its unbeleivable. I weigh 290 lbs. ANd this thing just flies! Just few more power tweaks and I'll trade my pedals for pegs and a pull starter!
     
  19. duivendyk

    duivendyk Guest

    Yes, the effect of back pressure on performance is quite pronounced,did you shave the head 0.040" ? (1mm),should have raised c.r. by about 10% from 6.5 ? to 7.2 or so,may be more.I would not push the revs too much,if the con. rod bearings croak you can prob. kiss the engine good bye.I am not an advocate for high oil ratios,it has more to do with environmental concerns than promoting engine life exp.The design of these engines goes back 40/50years or so and I assume they are designed for something like 20:1 ratio,as was pretty standard in those days.Good luck with your screamer!
     
  20. nedwreck

    nedwreck New Member

    compression ratios

    From 6.5 to 7.2 thanks again for the math, duivendyk I first tried .020 that was fun so I took off another .020 Do you know at what point higher compression ratios become a detriment to the performance of a 2cycle? Can it interfere with crankcase mixture pressure? Another thing I did when I put the head back on I turned it180 so the spark plug gap faces forward causing the flame front to advance toward the exhaust port I'm not certain if this hAd. Any effect on performance. I to am concerned with the environment and was pleasantly surprised to hear the synthetic oil I bought is bio-degradable.
     
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