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Crankcase Compression Ratio

20K views 23 replies 10 participants last post by  Lixan 
#1 ·
I just spent the last couple hours reading a thread on PS about crankcase compression ratios.(Brain hurts alittle) Awesome read if anyone is interested and beleive some of the members here contrubuted to the thread.



http://www.planetsand.com/forums/ubbthreads.php/topics/322/1



I know the next time I have my motor out I will be giving this a shot and see what mine measures out to. It's amazing how technical our simple 2 stokes can be.



Maybe we can get some discussion around it going here?
 
#4 ·
Calvin at CP seems to agree. Seems like the correct CCP is the key to getting big singles to rev. Calvin actually states the ratio they were shooting for with the 496 saber when designing it. I beleive it was around 1.3:1. From the reading I have done 1.3-1.4 to one seems like the general area you want to be.
 
#5 ·
Seems like the 1.3-1.4 rules are old 2stroke road racing tech based on poor cylinders bad heads and bad pipes. Not saying that it didn't work back then some made it work very well. Just like now days there are some that are really leaps and bounds ahead off the rest and they did it buy good ol elbow grease testing on the dyno and proving it at the hill and track

I'm not saying crankcase pressure isn't important but a good builder can make it work low or high . JMO
 
#6 ·
"I'm not saying crankcase pressure isn't important but a good builder can make it work low or high . JMO"



I'll be the first to tell you that out of everything and the TON of time I have into my R motor, I have not checked the primary compression. There are things about the motor that some would say are not right and shouldnt work, but you keep at it and make it work. Change other areas of the motor to work together and as a whole unit. Most engines will tell you what they want if they are in the ball park to start with, you just have to listen sometimes lol.
 
#7 ·
Crankcase compression is an important part of two-stroke engine design; formulas have been developed and implemented to prove the affects of over or under compressed bottom ends. More important than the compression of the crank area is the overall all crankcase volume including the transfer & boost tunnels.



Most everyone thinks of crankcase compression as the volume available in the crank (lower end) only while neglecting to think of the overall volume including the transfers, once you consider the volume of the tunnels as well, and how they affect total lower end compression, you realize how important crankcase compression really is.



Carlos.
 
#8 ·
Crank case volume is checked in two typical ways. Some people drill a hole in a piston, clue the reeds closed, and fill it with oil. In the alternative, the piston is greased, and placed at tdc, and the liquid is poured in through the reed area until it is full. So all of the volume is taken in consideration.



How important it is, is the question indeed. I tend to believe that you can work with what you have. And if one has any experience in changing cccr, and testing, they will find it takes a lot of material to make a little change in cccr, or effect.



Hondas are a great example. Anyone who has built a Honda in very many configurations will recognize that a stock cylinder 250R has drastically different cccr than a 370, for example. But similar components work on both of them.



This is another topic upon which we can theorize, and claim to be smarter than the other guy, and we don't have to worry about someone penning us down.
 
#15 ·
Well I guess this is where we differ in opinion, however; having done a lot of transfer 7 port work since the mid eighties on two stroke engines, I am more than sure of my results.



On a 250R based BB, you can get the same overall crankcase volume with adjusted transfer volumes; done it many times. It is not necessarily about “2 times more area through the transfer passage than you can get at the port window without messing everything up”…it is more about available transfer volume at the port window verses volume at the bottom crankcase area.



And yes; The proof is on the track, and not on a key board, while some of the fastest ProX’s may not have or may have transfer port work, it means very little to me, my experience and knowledge.



Carlos.
 
#16 ·
I understand what Carlos is saying. But The DMX is an example of a motor that makes great power with very small transfer passages. The cccr is a factor to consider, but I also agree with pipe-ster. We can work with quite a variety of things and make them work. In testing various combination for CP industries, we have had a chance to test many cccr setups. Fortunately, we've been able to get satisfactory results out of each cccr we have worked with. As the bore gets larger, the cccr is very hard to get down to the lower ranges. But it doesn't seem to matter; we have some setups that are good as any on the dyno and in the field, that are on the highest end of the cccr range.


As to transfer volume: consider how little time the transfers are full open. And then compare the size of them to the passages. You will find that the volume of the passage does not need to be much larger than the window for the window to be the limitation of the flow. And if the passage was as small as the flow size of the window, the window is still the limiting factor for most of the transfer-open cycle, since the window is closed to some degree at all but BDC.

Fun discussion, but not as critical as some would think, imho.
 
#17 ·
Simple test for a Honda 250R:



Take a known engine; with documented Dyno numbers that represent the best possible performance achieved for the complete set-up…leave everything the same. Now remove the Tins from the crankshaft, run and document performance.



Take the crank out measure the volume of the overall crankshaft…use the drop in Oil method measuring the amount of Oil is displaced, remove the Tins from both halves. Re-measure the crankshaft volume without the Tins. Now you know the volume the Tins displace.



Match the volumes of the Tins to a quantity of “Setting Epoxy”…spread the epoxy evenly throughout the transfer tunnels, and cure. Use cardboard templates to keep transfer tunnels of equal volume.



Run the same engine with the same set-up as before except without the Tins in place, then with the Tins in place…document results. Now reverse the procedure…add volume to the lower end and increase volume to the transfer tunnels.



Now tell me the results…




PS: I was involved with this type of testing years ago, results were conclusive for a Honda ProX or OEM based engine when adding displacement by increasing bore & stroke over OEM 66MM specifications…by altering transfer & case volume, you change transfer port velocity as well as lower end volume…tuning the crankcase compression to a given cylinder bore/volume.



Carlos.
 
#18 ·
That test would test the opposite of what you had tried to emphasise in you previous post. In the hypothetical test, you would be testing smaller tranfer volume when you epoxied them. And the only larger volume would be the lower crankcase with the tin removal, of cource. Many here have likely tried tin removed, and likely with most unfavorable results. And as some here might also know, thee cccr effects the way the pipe works, and arbitrary changes without optimising/considering the pipe will be hard to derive any conclusive/helpful conclusions. Imho
 
#19 ·
Miss typed this portion as posted above, it should read: "Now reverse the procedure…REDUCE volume to the lower end and increase volume to the transfer tunnels"...



My point was to try both decreasing and increasing transfer volumes while maintaining the overall lower end volume the same, and test to a known engine system taking out as many of the variables as possible.



FYI...the detailed method is basicly a simplified version of how Honda tested lower end volumes verses increases in bore sizes verses transfer tunnel velocity & volume verses EC designs.



Carlos.
 
#23 ·
been working on a engine with cccr of 1.6:1 and thats with a 4mm spacer under the cylinder. ran fine when it was a stock motor. gonna double check all my calculations again to be sure theyre correct but does 1.6:1 seem alittle off to you guys ? its a reed valve engine with boyesen ports that stay open 360* but the main intake port is only open 196*. seems like it should be open longer
 
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