Compression ratios

[blowit]

In my previous post, I sounded rather snippy. I did not intend that. I was merely stating what I needed to answer the question accurately. If your engine is apart y chance, we can probably get to the bottom of this. If not, I can still probably get close if you are willing to cc test your combustion chamber.

 

You will have to remove a plug and fill the combustion chamber with test fluid at true TDC. With that volume and I go off your stated duration, I can work it from that angle. Any thing else really is just assumptions and since I cannot agree with your stated displacement vs ratio, I would not know which might be closer to accurate.

 

 

 

Brandon

[fastrthnu]

The higher the compression ratio, the less fuel the engine will use to produce the same horsepower. Why diesels ar emore fuel efficient. more is gained by a longer duration. most common mechanical compression ratios are based on fixed volumes. The Static Compression RATIO is defined as the Volume of the Combustion Chamber when the piston is at the very bottom of it's travel (called "bottom dead center" or BDC) DIVIDED BY the Volume of the Combustion Chamber when the piston is at the very top of it's travel (called "top dead center" or TDC). To calculate static compression ratio.....

 

CR=(D + PV + DC + G + CC) / (PV + DC + G + CC)

 

CR = Compression Ratio

 

D = Displacement

 

PV = Piston Volume

 

DC = Deck Clearance Volume

 

G = Gasket Volume

 

CC = Combustion Chamber Volume

 

Because of the scavaging/timing in a 2 stroke motor some air preasure as mentioned above can alter the preasure from the time the piston leaves bdc and pushes out of the cylinder. Some positive/negative waves bounce preasure back and fourth from the expansion chamber before the exaust port is fully covered, so pipe timing may also affect this. This is more on the way of dynamic compression ratio. Some motors of equivalent build with different timings will end up with different dynimic ratios. Static with the same dome, stroke , bore, ect would remain the same. The dynamic may alter depending on timing durations. The longer the ports are open, the more static CR you can run.

 

Of course other factors play a role, such as the burn rate/efficiency of the oil you use, what temp your motor runs, piston and dome material/shape, atmospheric preasure and temperature, spark advance/retard, ect in what CR is safe for your motor to run on.

 

Thats my .02 Im not writing a book.

[elwilliams13]

Well I have my head off at this moment. What can I do with that?

[deckheight]

Get a decent caliper, establish an accurate exhaust port timing ATDC and bore diameter. Put the head back on, get a buret, dial indicator/spark plug fixture and establish an accurate trapped volume and proceed to the next level of tuning.

If you are going to be modifying you own engines and want to do it correctly anyway...