backpressure: what are the facts?

[nedwreck]

rmwDave porting

I was wrong when I said the ports in my jug looked specially they are nasty. DIsc cut off wheels wheel sHaped stones flapper wheels and those little saNding discs work insIde the cylinder with my 400xpr exhaust port used to be 15% blocked by fLAsh.

 

[duivendyk]

Comp.volume, it's simple,66cc displ.engine, 66/6.5=10.15 cc. and 66/7.0=9.43 cc.If you had 0.040 shaved off the cr would go up to 8.43 from 7.0 which is not bad.For 6.5 you get 7.9.

 

[nedwreck]

Thx. For tHe calcs . Duivendyk hAve you run across any upper limits for c.r. In 2strokes, other than twisting off spokes when tRying to start it? Does the efficiency drop off? I hAve ground the top of the cylinder Another .020 to clean up casting garfs to make a better gasket mating surface. I also removed a great deal (!) of flash and caSting error. Also the exhAust port is oval straight through with a round gasket to mAte with a round tube muffleR .

 

[duivendyk]

Not really,I would suspect the limit in principle may well be geometric in nature,apart from load increase on all bearings & pistonrings with overall increased internal pressures.I don't see anything much changed during scavenging.It is conceivable that the combustionchamber residual pressure when the transferport opens would be higher ,delaying the entry of the new charge into the cyl.The crankcase pressures are unaffected of course.Also Preignition would be quite damaging in a 2 stroke due to the inherently high heat load on the piston.

 

[ratty]

I would think that opening up the exhaust pipe and removing that casting flash from the cylinder ports would be very beneficial to the increased compression. Probably the next hinderence would be the junk carburetor and the crankshaft balance.

 

[MikeJ]

Some good ideas here... Casting flash removal should be high on the list. If you remove the cylinder to clean up port casting, just be careful not to scratch the cylinder. I used a round file to clean up and shape the intake port gasket. When you really get into performance mods: One reader took a Dremel and shaped both front and back of the piston skirt into a curve, like part of a circle. That opened up "exposure time" of the carb output going into the crankcase. He claims low-end torque improved. I expect max compression ratio to be around 9:1 and still use bottom-end no-lead gas. (Unmodified air-cooled general aviation airplane engines compression ratios range from mid-7s to mid-8s when using 100 octane gas.) But as mentioned, the tradeoff will be greater stress on the connecting rod roller bearings and a shorter life. I suppose muffler size and shape is to comply with U.S. noise abatement laws. (But if I lived out in the middle of noplace, like eastern Colorado, I'd be tempted to temporarily replace the muffler with a stubby "T". Riding at night would produce yellow flames flying out both sides! And you'd be able to hear it in the next county! Just be sure to wear asbestos pants and shooter's earmuffs. -- The irresponsible child in me.)

 

[duivendyk]

With increasing comp. ratio the eff.and the power(torque) go up for any IC engine,as long as you don't get pre-ignition (pinging) that robs you not only of power but puts destructive loads on all bearings.What's really going on is that the expansion ratio is increased,so you extract more work/cycle from the expanding hot gas.Comp& exp ratio are often close to the same,but also depend on other factors (valve timing).The socalled 'Miller cycle' takes advantage of this to improve eff. (but not output power,I believe).Mazda used it fairly recently in one of their cars (the Millenia ?)

 

[RMWdave]

as far as porting goes, i wont even try it with a dremel..... its a waste of my time and too much of an uncontrollable risk.
i WOULD however like to get myself some carbide cutter bits for the mini air die grinder
if you want a porting tool, they make these specialty bits that are hard to find that are like a deburring head with a 6" long true shaft. they spin perfect and they are uber precise like coloring with a pencil.

a good set of porting bits should last you a long time.

 

[machiasmort]

good point duivendyk

 

[nedwreck]

OK project clean the ports and git her breathing better has progressed to the point where I have broken 2 motor mount bolts and can pop wheelies from a rolling start consistently (my laughter can be heard for blocks) I am at point where I have excellent low speed power and am maintaining 30mph at 3/4 throttle above that I am getting 4cycling or short cicuiting. The jet in the carb is removable does Anyone know where I can get bigger jets to cure my high rpm lean condition? The ports in the cylinder (intake and exhaust) are oval nd much larger than intake or exhaust manifold has anyone tried a larger exhaust or intake or would this drop backpressure too much? It seems wrong to have an oval exhaust port with the area (aprox ) of a quarter with a dime sized pipe bolted to it.