EC 300 cylinder pressure

[iancp5]

Originally Posted by kendunn

I am not sure what a GG is, but I looked up a Honda CR250 2t and it was at 8.5:1. I never understood these high compressions.

Compression ratio on a 2 stroke is always much lower than 4 stroke and I am never sure if they quote some virtual ratio using the stroke or the real ratio from when the exhaust port is closed. Additionally the working compression is hugely effected by the pipe and rpm although I don't suppose this has any effect when you're kicking it to get a test reading. That's why I assume the advice is test when it's known to be good so you have a reference with your own guage and speed of kick.

[Fred1956]

Originally Posted by iancp5

Compression ratio on a 2 stroke is always much lower than 4 stroke and I am never sure if they quote some virtual ratio using the stroke or the real ratio from when the exhaust port is closed. Additionally the working compression is hugely effected by the pipe and rpm although I don't suppose this has any effect when you're kicking it to get a test reading. That's why I assume the advice is test when it's known to be good so you have a reference with your own guage and speed of kick.

Not to hijack but here is some theory from a very technical friend of mine:

Compression ratio is measured in two ways in the two-stroke engine, the "standard" one better suits a four-stroke and that is to take the bore/stroke volume of the cylinder in relation to the volume of the combustion chamber with the piston at TDC.

The more accurate reading is "Corrected Compression Ratio", which is measured as the volume of the cylinder with the piston just closing off the exhaust port.

The two-stroke essentially has TWO compression ratios to consider. The one mentioned above is the "secondary compression ratio" but there also is the "primary compression ratio", which is the displaced crankcase volume TDC/BTC vs. the total volume of the crankcase cavity. This has a direct affect of how power is developed and a Supercross engine, for instance, will have a high primary compression ratio to develop power in the low/mid ranges but the SX engine will "flatten out" at high RPMs due to "pumping loss" as it takes energy to compress the air/fuel mixture in the case. An outdoor MX bike will have a lower PCR to help develop HP at the mid/upper range.

Porting and ring seal have an effect on CR and if the engine has a exhaust port power valve, this will have a big effect on reading depending on if it is open or closed (and how well the valve seals when closed).

Mechanical "compression ratio" (the mathematical computation) is quite different from "compression pressure" (the reading in PSI on the pressure gauge).

[iancp5]

Fred, that prompts a couple of questions:

If smaller crankcase volume promotes low down power how are engines modified from MX to SX? How do they shrink the crankcase volume? I might like some of that.

Do reed block spacers increase top end?

How do the GasGas PV chambers work?

Thanks

[kendunn]

I guess we have totally hijacked this thread now, but have a look here
http://www.sacoriver.net/~red/uccr.html

[HuskyDude]

Wow...
Now that's some good reading right there. Might have to read it twice though to let it all sink in...

[Fred1956]

Originally Posted by iancp5

Fred, that prompts a couple of questions:

If smaller crankcase volume promotes low down power how are engines modified from MX to SX? How do they shrink the crankcase volume? I might like some of that.

Do reed block spacers increase top end?

How do the GasGas PV chambers work?

Thanks

I will pass that question on, I do know that it is crankcase pressure that helps low end, I believe that is part of what 'stuffing the crank' does. The less openings in the crankcase ( ie balancing holes, crankpin holes etc) when filled will increase the crankcase pressure. There is a point when too much can be done as Jon said, if too much primary pressure is developed it causes a 'slowdown' of the piston trying to compress the higher pressure. I think this is what the GYT 'longrod kit' did for the YZ250 back in the day. I performed that mod on my '93 YZ and the increase in low end was very noticeable. It added a 5mm spacer under the cylinder along with a 5mm longer rod and powervalve rod. 5mm is not a lot but it did make low power

[Fred1956]

Originally Posted by iancp5

Fred, that prompts a couple of questions:

If smaller crankcase volume promotes low down power how are engines modified from MX to SX? How do they shrink the crankcase volume? I might like some of that.

Do reed block spacers increase top end?

How do the GasGas PV chambers work?

Thanks

Specific answers from Jon; BTW Jon Stoodley is who I ask these things to, he loves this kind of thing and rides a GG trials bike. Very very knowledgeable and enjoys it immensely & especially sharing & teaching. When we get a 'first timer' at the trials club, that guy is assigned to Jon and they go one-to-one. Jon takes the time to teach them how to walk a section and read the lines. He is a true promoter of the motorcycle world and one of our best ambassadors to the sport of motorcycling, I believe he has had tech articles published more than a few times

Answers:

Q) If smaller crankcase volume promotes low down power how are engines
modified from MX to SX? How do they shrink the crankcase volume? I
might like some of that.

A) Epoxy is used to decrease the volume, but also to change the flow characteristics of the A/F mass in the case cavity. Most new engines are not modified in this way for SX (especially now that SX is dominated by four-strokes) as it depends on the engine tuner's goals and how a higher PCR would work with the other design modifications. I do not recommend this modification unless you have the background and experience to understand exactly how you want to modify the passages as, like porting modifications, you can easily (and most likely) make matters worse.



Q) Do reed block spacers increase top end?

A) It depends. They generally tend to move the torque curve lower in the RPM range by changing the resonance of the intake tract, but they also lower the PCR so, in theory, they raise the torque curve. They are used for two other purposes, to space the carb bowl off the case to reduce possibly vibration-caused fuel frothing (some spacers are a wedge-shaped for more "kick up" of the carb body) and to move back the reedblock slightly to unschroud the boost ports on either side of the main intake port (these are actually called "Boyesen Ports" after the well-known engineer who developed them).


Q) How do the GasGas PV chambers work?

A) I haven't looked at the GG chambers specifically but I would imagine they are a "resonance chamber" that increases the volume of the "head pipe area"
(the extremely
critical 7-8 inches from the piston crown edge out into the pipe) at lower RPMs. The first one's I dealt with were the Honda "ATAC" chambers on the MX bikes that were a good idea but suffered from technical design problems. The resonance chamber is a very common device on the two-stroke MX bikes over the years.

Send me any questions you want, I'll share whatever I know. Jon

[kendunn]

Even if the fuel mixture is getting pre-compressed in the crank, why does the motor not predetonate at those pressures? I thought anything over 11:1 would need race gas, and at the pressures of 190 or whatever I don't know how it would run on anything other than diesel without predetonation.

[GMP]

Per Ron at RBD, 200 PSI is about the limit for pump premium. If you have had head work done there you would know about choosing compression/fuel requirements.

[iancp5]

I thought you needed about 15:1 true to get diesel detonation (with diesel fuel)? Thought the issues with octane on high comp petrol engines is the speed of the flame at high compression when ignited by the spark not detonation through compression alone? I have no idea at what compression petrol ignites though.