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**sunilg** · 01-31-2012, 04:40 PM

Discussion approved.

**ROCKRZ** · 01-31-2012, 04:50 PM

the explaination from wikipedia:-
in short, if you compress 10 cc (cubic centimeter) of air to 1 cc, you have compressed the air by ratio of 10:1 this is exactly what the engine compression ratio means..

in ideal condition, the relation between Pressure (P) and volume is P*V=nRT

**tusharsmoily** · 01-31-2012, 09:55 PM

The textbook defintion of Compression Ratio-

It is the ratio of the volume of cylinder when the pistion is at the end of suction stroke (BDC) to the volume of cylinder when the piston is at the end of compression stroke (TDC). Higher the CR, more is the fuel efficiency and power. But it should kept upto a critical value, beyond which, will lead to unpleasant explosion called 'knocking'. It damages the engine.

**Honda_CBF** · 02-01-2012, 02:50 PM

Originally posted by sunilg View Post

Discussion approved.

Thanks for approval sir

@ROCKRZ and tusharmoily,
Thanks for the explanations and definations guys.
But my concern was (and still is)

Originally posted by Honda_CBF View Post

But I want know its effects when altered (increased or decreased) with the respect to torque/horsepower/fuel efficiency??

for instance I'm asking what will happen to ZMR if its compression is increased to 10:1 or say, what will happen to R15 or P220 if their compression is lowered to 7:1

I know doing so can kill these bikes but I want to know how it changes its power delivery & mileage and why?
I also searched about playing with compression ratio to increase mileage and power but again results were unsatisfactory.. few sites/articles said increasing compression will help gain power & mileage both but other said it will kill mileage and provide only power gain.

**aargee** · 02-01-2012, 03:18 PM

Originally posted by Honda_CBF View Post

for instance I'm asking what will happen to ZMR if its compression is increased to 10:1

Think...when CR increases so does pressure, so does temperature, so does power.

Originally posted by Honda_CBF View Post

what will happen to R15 or P220 if their compression is lowered to 7:1

The reverse of what I said above

Originally posted by Honda_CBF View Post

how it changes its power delivery & mileage and why?

How? When pressure drops, power drops, so for the same volume of fuel, the power is less & hence FE drops

Originally posted by Honda_CBF View Post

other said it will kill mileage and provide only power gain.

True to an extent, unless they're not an overkill. Too much CR causes pre ignition (fuel burns even before the spark due to pressure & rise in temperature).

Give a thought on this...
The 848 Evo has a V-Twin (rather L-Twin) with a high CR around 13:1 & less than 1200cc, while MT01 is a 1800 motor with a CR around 8:1; consider which is faster, has more power & why's evo a LC engine while other is not. You can find most answers by comparing both of these.

**Honda_CBF** · 02-01-2012, 03:32 PM

^^ okay, got an idea of power and pressure. Any thing regarding increasing FE via altering CR??
Any sure way for landing onto win-win position?? Any calculation?

**aargee** · 02-01-2012, 03:51 PM

Originally posted by Honda_CBF View Post

Any thing regarding increasing FE via altering CR??

Would you need more on...

How? When pressure drops, power drops, so for the same volume of fuel, the power is less & hence FE drops

Originally posted by Honda_CBF View Post

Any sure way for landing onto win-win position?? Any calculation?

Win-win position of what? And what calculation are we talking about here?

**Honda_CBF** · 02-01-2012, 04:12 PM

Originally posted by aargee View Post

Think...when CR increases so does pressure, so does temperature, so does power.

The reverse of what I said above

When pressure drops, power drops, so for the same volume of fuel, the power is less & hence FE drops

that's what i thought/understood,
but some articles say decrease in CR = gain FE & loose power.!! This 'increased FE' part is what I'm not able to get.

And I was talking about the calculations to choose best CR for increasing power & mileage & durability (also considering 90 octain fuel; taking care of detonation.)

**Drifty** · 02-01-2012, 07:26 PM

Originally posted by Honda_CBF View Post

that's what i thought/understood,
but some articles say decrease in CR = gain FE & loose power.!! This 'increased FE' part is what I'm not able to get.

And I was talking about the calculations to choose best CR for increasing power & mileage & durability (also considering 90 octain fuel; taking care of detonation.)

You have a good example of the Joel tuned bikes which by altering compression ratio also gives good fuel efficiency as reported by many users.
I think this query will be best answered by Joel!

**vineshp** · 02-01-2012, 09:37 PM

Doesn't high compression ratio

need a better quality material for engine and hence cost
more wear& tear
require high octane petrol (increased cost for user)

So, why do Manufacturers opt for a lesser CC + High compression ratio rather than higher CC + less compression ratio for achieving required power output? (new Pulsar 200NS engine Vs Pulsar 220 engine)

**Honda_CBF** · 02-01-2012, 10:03 PM

Originally posted by Drifty View Post

I think this query will be best answered by Joel!

hope the legend is going through..

@vineshp,
may be the reason behind is more power via increasing efficiency (engine's working efficiency)

hopping to get clarifications from the experts

**Joel** · 02-02-2012, 02:22 AM

Compression ratio has a lot of relative factors. An engine could gain a couple of horses with a single unit change of the compression ratio (ex bump from 10:1 to 11:1), and some not much. All depends purely on the volumetric efficiency of the engine.

**Honda_CBF** · 02-02-2012, 08:27 AM

Originally posted by Joel View Post

Compression ratio has a lot of relative factors. An engine could gain a couple of horses with a single unit change of the compression ratio (ex bump from 10:1 to 11:1), and some not much. All depends purely on the volumetric efficiency of the engine.

so what factors make an engine volumetri-cally (if the term is correct) efficient??

**Andante** · 02-02-2012, 06:55 PM

Originally posted by Joel View Post

Compression ratio has a lot of relative factors. An engine could gain a couple of horses with a single unit change of the compression ratio (ex bump from 10:1 to 11:1), and some not much. All depends purely on the volumetric efficiency of the engine.

Originally posted by Honda_CBF View Post

so what factors make an engine volumetri-cally (if the term is correct) efficient??

The thing that Joel is referring to is that CR is just one part of the equation. Volumetric efficiency is how well an engine can fill and empty it's cylinder.
If there is a large restriction on the intake, it will take effort to suck the air/fuel mixture into the cylinder. So by just upping the compression ratio you will not gain much. (You're compressing a partial vacuum, so the final pressure will be less than if you had more air to start with.)
Same for restrictions on the exhaust, it will take effort to push the burnt fuel out. (Yes, I'm skipping over the detail of burnt mixture being left behind in the cylinder.)
Pumping losses, as they are called, cause loss of power. That's why, without ever touching the CR, you can get more power by putting a less restrictive airfilter and exhaust and retuning your AFR.

I'm sure this is be no means a full explanation and probably grossly oversimplified, but I think this covers at least the basic idea.

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