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I was perusing the pamphlet on the kdx 200's, and I happened upon the compression ratios for these bikes. It listed 9.4:1 low speed, and 7.9:1 high speed. I'm guessing the KIPS valve manipulates the ratio in some manner at high rpm's, but the point is: Do all 2 cycle engines use such a low compression ratio? I was of a mind that a relatively high performance engine would have a good bit higher ratio...or perhaps 2 cycle engines don't rely on high compression to make power?....I dunno. From building high performance small block Chevy engines, 11:1 and up was kind of the norm for making good power. And w/ such low compression, why would race gas or 93 octane fuel be necessary? Confusing.

Mr. Black? Paging Ron! Let's go straight to the encyclopedia, the professor, the man.

Re: 'I'm guessing....'

Yes, it does. You have an idea of what the KIPS is and how it operates? Any change in exhaust port position (height) will effect static compression ratio.

Re: 'Do all use such a low...'

No. Not such a low, but low. Comparing compression ratios of a 4-stroke engine (what works) to the same numbers on a 2-stroke engine is, well, no comparison. That fact does not need further explanation...right? Or, no?

Besides all that, STATIC, mechanical compression ratios don't mean anything. 2-strokes work under a dynamic compression idea brought about by the supercharging effect of a tuned exhuast.

Did that CSB have a Roots huffer strapped on running at 120% at 11:1?

Doubt it.

Re: 'w/such low...why..?'

Answered above...right?

It's combustion pressure that matters, NOT compression ratio.

That's why any engine builder will specify such (combustion pressure) when listing cam choices in a 4-stroke. The fact that the static ratio is whatever number has pretty much squat to do with pressure.

Stuff a 100º overlap cam into that 11:1 CSB and your combustion pressures will likely be pathetic..and so will the engine's performance!

Cheers!

CC hit the nail on the head - as usual!! I've read where a correctly tuned exhaust pipe for a 2-cycle motor will stuff approximately 40% more fuel mixture into the cylinder.

Unfortunately...my thumb is usually firmly supplying opposing force to the fingers and therefore holding any such nail firmly by its head..then comes the hitting part.......

The screams of pain and anguish only slightly muffled by the foot planted most often quite firmly in my mouth..and my head being stuck...

...never mind.


What's that smell!!??

I think it's about to rain............

I guess I need to read up on the 2 cycle engine theory. I figured compression ratio would be set in stone, a result of the stroke length, piston dome height, and cylinder head/combustion chamber volume, and not something that could be altered. Intersting to see that the ratio can be manipulated at different rpm's. I was thinking w/ the piston at tdc, the exhaust port would be sealed off by the piston/rings, and compression would be consistent regardless of rpm. Apparently I don't know squat about 2 strokes. Now i'm beginning to think the expansion chamber has something more to do w/ this than just scavenging exhaust gases.

And no, no superchargers w/ 11:1 compression ratios. Just naturally aspirated engines. I never did bolt a roots type blower on anything.

Ahh, but there's nothing like a roots blower at that (or higher) compression! Generally reserved for Alky though

CC hit it on the head....It's all about 'effective/dynamic' compression in a 2T. A powervalve changes the effective/dynamic compression by altering the pressure of the compression wave. High effective compressions are good for low end power, but will not allow the engine to rev out...Like the 'old school' enduro 2-strokes. Low effective compression will allow the engine to rev out, but will make no low end power, and have a very abrupt power curve....Like the old school MX bikes. A powervalve mechanically alters the effective compression by changing the exhaust port dimentions on the fly. Static compression ratio still governs the fuel requirement, and has en effect on overall power. Altering the porting(especially the exhaust) in a 2T is sorta like a camshaft change in a 4-stroke. I'm certainely no expert, but that's about the gist of it.

I've mentioned this on here before, but here's a rough quote from a very famous 2T kart engine builder, Kermit Buller: "A 4-stroke is like an air pump so much that the same laws of physics apply...A 2-stroke has more phycically in common with a brass or woodwind instrument." This guy gets more HP out of the non-powervalve air cooled kart motors than anyone in the business(when open rules apply)....So much that his port mappings/head profile/etc have been copied and undercut his 2-cycle building business into near extinction. Good thing he is also a chassis/part manufacturer. Real cool guy....Ex motocrosser turned MX engine builder....Then found there was more money in karts.

I recall the Porsche 930 Turbo (circa 1977) was running about 7:1.

Ahhhh, the Carter Years.

Rick

StringB wrote:I figured compression ratio would be set in stone, a result of the stroke length, piston dome height, and cylinder head/combustion chamber volume, and not something that could be altered. Intersting to see that the ratio can be manipulated at different rpm's. I was thinking w/ the piston at tdc, the exhaust port would be sealed off by the piston/rings, and compression would be consistent regardless of rpm.

A correct statement: Static compression ratio VARIES in a 2-stroke engine equipped with a power valve system.

A 'power valve system' changes the port timing of the engine through some sort of sensing mechanism. In the KDX it's called the KIPS or Kawasaki Integrated Powervalve System (m0rie, please correct me again if that's wrong...again). The KIPS consists of a spring loaded assembly that when spun to approximately 6000rpm (engine speed sensing, then) through centrifigal force rotates a shaft that moves a number of components in the cylinder including the main power valve.

There are other valve timing systems. Yamaha used one run by servo motors on their RZs (their twin 2T rice burning screamers) fer example.

This part: 'I was thinking w/ the piston at tdc....'

This simply a misspeak I presume. Not to pick, but you'd better hope the combustion chamber is 'closed' some time before the piston reaches TDC, else there won't be nuthin' goin' on!

This part: 'Now i'm beginning to think the expansion chamber has something more to do w/ this than just scavenging exhaust gases.'

Inda posted this on another thread.

The simple answer is, yes. A 2-stroke pipe does scavenge exhuast gasses, meaning it pulls the charge out of the cylinder.

Note I said 'charge'. That applies to spent or new. Air molecules being good buddies (they like to stick together), 'ya pull on one and others close to it follow. The stream is 'pulled' to the point that part of the fresh charge is in the pipe. The fun happens when the returning pressure wave crams that charge back into the cylinder when the transfer ports are closed!

Where did'ja think wet-line jetting came from! Admit it!! You always wondered about that, huh!!??

Dang, that's amazing. I never realized the intake charge was scavenged into the expansion chamber header, and then sucked back into the combustion chamber due to the negative pulses.....reeds keep the charge from exiting back into the air intake tract.....That's about the coolest thing I ever locked eyeballs on. I'm sure glad there's people smarter than me that thought up stuff like this, or we'd still be riding hayburners around.

Well, almost.

It's pushed back in, not sucked back in...but you get the idea.

The reeds do more keeping the charge from exiting due to pressure from the downward movement of the piston that pushes the charge to the 'top' end via the transfer ports. The transfer ports are closed by the time the pipe gets to stuffing. The point is to increase pressure in the combustion chamber, NOT the crankshaft cavity.

Yeah...it's all pretty neat!

You probably know this, but a turbo charged engine is not really running at nearly the low compression quoted in the spec sheet. The real compression is so high even on a lightly boosted motor that they need stronger rods and pistons. Which is partly why they drop the compression ratio in the first place. They also used to add knock sensors to the EMS, but that seems to be pretty normal today.

bradf wrote:Mr. Black? Paging Ron! Let's go straight to the encyclopedia, the professor, the man.

Ok here goes,

Two strokes have had two different types of compression ratios that manufactorys have used in years past till now.

Mostly in the earlier days of two strokes they used the full stroke of the piston (not taking in the fact that the open ports will not produce trapped volume) to mathematically compute the compression ratio. This gave a relative high compression ratio, like 13.5 ~ 14 to 1.

But if you take the actual effective trapped stroke volume (where all ports are closed off by the piston) the compression will be less, like 6.5 ~ 9 to 1.

It is all subjective and in this day a compression ratio is not used to claim HP any more.

Hope this helps

Ron

So, if you were to measure the actual mechanical compression ratio, w/ the piston at tdc, it would indeed be higher than what's listed in the specs. Am I at least semi-correct in that statement?

Sounds like it.

And if you just made up a number that you like, THAT would work just about as well!

MY KDX is 13.2:1!!

stringburner wrote:So, if you were to measure the actual mechanical compression ratio, w/ the piston at tdc, it would indeed be higher than what's listed in the specs. Am I at least semi-correct in that statement?

I think the "static" compression ratio is listed usually as something like ~7.3 - 9.1 on typical dirt bikes which lists the actual mechanical compression ratio based upon whether or not the power valve is open/closed. The "dynamic" compression ratio is engine running with the expansion chamber stuffing charge effect thus boosting the running compression ratio to ~ 11 - 14.

I think the technique of extending the extrusion of charge all the way into the pipe and then compressing it back into the combustion chamber using tuned sound waves is utterly cool. The 2T version of "lift and duration" with a little sonic Roots blower to finish the job.

Kind of like pulling taffy and then squishing it into a bite-sized morsel of explosive goodness.

Kinda like making a "Bit O Honey" with every revolution.

Rick