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Old February 25th, 2013, 05:37 AM   #1
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What is compression

There seems to be some confusion about compression ratios and how to calculate them. I'm offering up my years of experience for your perusal. Keep in mind, this is my opinion, and that there is some disagreement among engine professionals about what is, or what is not, important.

1/ STATIC Compression Ratio (also known simply as Compression Ratio)

This is defined as: clearance volume + swept volume, divided by the clearance volume. This relationship is easily calculated from measurements the average enthusiast should be able to determine. This is a theoretical number, useful for discussions during beverage consumption. It is also useful as a comparitive guideline for possible changes in C/R. It has no place in attempting to evaluate performance or output between differing engine types or displacements, although these comparisons MAY be useful in determining who pays the bar tab........


2/ EFFECTIVE Compression Ratio

This is typically defined as: clearance volume + effective volume, divided by the clearance volume. This relationship is more difficult to determine. The effective volume of the cylinder is whatever volume the engine "sees" when the intake valve closes. The effective volume is determined by relationship of the build geometry, (such as: rod length/stroke ratio; wrist pin offset; etc.) the intake valve actual closing point (in crankshaft degrees) and the volumes calculated for the static compression ratio. For normally aspirated racing engines with a long duration camshaft (or camshafts), this number is always lower than the static compression ratio.

There are a number of programs & websites out there, that are available to help calculate effective compression ratio. You must know your camshaft specs to calculate effective compression ratio. Although also theoretical, this number is more useful, in my opinion. This is the number you want to use to judge basic octane requirement for fuels, changes to camshaft timing(s), valve events and rocker ratios, and changes to build geometry such as rod length, wrist pin offset, stroke, bore diameter Vs stroke for displacement limitations, etc.

Keep in mind that typical performance camshafts delay the intake valve closing point until well after BDC. This is done to maximize inlet charge inflow, also known as "ramming" or "ram tuning". This is a compromise between the "ram tuning" and "maximizing" the swept volume. It is important to realize that once the crankshaft passes BDC, swept volume is lost at a rate dependent on the build geometry, thereby lowering "effective" C/R. Trade-offs of this nature can be negligible or severe, depending on how long the piston is effectively "parked" at or around BDC. Statements that long(er) rod/stroke ratios are always of benefit, without specifying an engine type and rpm range, are clearly simplistic. And yes, it is possible to have a rod which is too long. Good engine engineers/designers/tuners evaluate these variables during the design phase.


3/ DYNAMIC Compression Ratio

This is typically defined as the effective compression ratio multiplied by the volumetric efficiency% @ whatever rpm range (usually the peak bhp range) is being studied or optimized. This is usually modeled based on dyno bhp output results, or back calculated from the resultant BMEP figures from dyno output numbers. Typically, volumetric efficiency percentage can exceed 100% on well tuned race engine combinations. You need to be aware of whether your engine is capable of 115% V/E (Cosworth 4 valve x 2 ohc) or whether it can only achieve 50% V/E (Hello Briggs & Stratton flathead.....)

Modeling your engine from hard dyno data is the best way to accurately determine dynamic compression ratio, V/E% and BMEP. However, the programs which model these types of data are much more expensive, much more subjective and require much more experience to utilize properly.

4/ To make things more confusing, sometimes the definitions of effective C/R and dynamic C/R are interchanged........


So what does all this mean? Some examples:

Well, if you are only calculating "static" C/R, you are only giving yourself part of the picture. For instance: 12.5/1 "static" C/R with 320 degree cams may only be 8/1 "effective" C/R. But, combined with 110% V/E = 8.8/1 "dynamic" C/R.

OR: 14/1 "static" C/R with 240 degree cam(s) may give 11/1 "effective" C/R. Combined with 90% V/E = 9.9/1 "dynamic" C/R.

A common complaint I encounter is: My C/R is 14/1. Why are my bhp numbers so low? What's wrong with your dyno? When I calculate the BMEP for poorly performing engines, it invariably points to either:

A/ A parts selection error in the design phase. (ie: too much "camshaft", carb, etc. And difficult to correct quickly & cheaply.)
B/ A parts selection error in the tuning phase. (ie: exhaust too large/long/short, etc. Easier, possibly cheaper to correct.)
C/ A huge opportunity to improve state of engine's tune. (ie: "These are the jets/timing/etc, my friend runs." Easiest & cheapest to fix.)
D/ The "Triumvirate". A combination of all 3 of the above......... (Don't laugh. This is more common than you might think.)

The bottom line is: If you actually have 14/1 C/R, (and usually when I calculate/measure your C/R, you don't......), and your BMEP is 165psi, there is a problem somewhere.......

Well, we haven't even talked about supercharging yet.....and yes, it is possible to have 65% V/E, bolt on a huffer, and have a BMEP of 235psi. I'm going to leave this for a separate topic at some later date.

So, it turns out that the 4-stoke Otto cycle of internal combustion, which we were taught in high school & college, is a gross oversimplification. Depending on who you want to believe, performance internal combustion engines have as few as 5 separate "events" or as many as 7 significant "events"...........


Confused? Then I suggest this:

Determining "effective" & "dynamic" compression ratios is an area where "modeling" build geometry specs, ahead of ordering expensive parts, can payoff big-time. BUT, you must know what you are doing as opposed to just punching numbers into a program. I find that modeling helps to avoid poor decision making, due to lack of information. And, without hard numbers you are just guessing. The net result is: If you are just guessing, that makes you: Inspector Clouseau.

For the best results I suggest you:
A/ Model your engine during the design stage,
B/ Be realistic about your output goals, depending on engine type,
C/ then dyno your engine, to optimize output,
D/ then analyse the results of the dyno session,
E/ Make changes accordingly.

Final thought. Modeling programs that cost $39.95/$99.95 or even $199.95 are useful for the limited functions they calculate. Programs that cost $499.95 to $999.95 have more features and are therefore more complex and perhaps more useful, IF, they are used properly. Big-time professional race teams, aftermarket Mfg's and OEM's spend hundreds of thousands, if not millions, on their computer simulations, models and their engineers......... Caveat emptor......
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Old February 25th, 2013, 05:53 AM   #2
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That's very detailed, and useful in calculating compression ratio!

But, in a nut shell for anybody that's scratching their heads, the compression ratio is volume of the cylinder at the top of a stoke, usually expressed as 1, to the volume at the bottom of a stroke expressed as a multiple of the volume at the top of the stroke... Well it's expressed the other way round, but sure.

In the ninja case I believe it's 12.4:1

Last futzed with by Joshorilla; February 25th, 2013 at 10:33 AM. Reason: Clarified
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Old February 25th, 2013, 07:26 AM   #3
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Had me scratching my head but I did find it very informative.
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Old February 25th, 2013, 07:50 AM   #4
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Old February 25th, 2013, 08:03 AM   #5
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For the record I did not wright this. I found it informative and wanted to share. We are getting pretty deep with these motors.
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Old February 25th, 2013, 08:39 AM   #6
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For the record I did not wright this. I found it informative and wanted to share. We are getting pretty deep with these motors.
So was it Frank Lloyd Wright, Steven Wright, or Wilbur and Orville?
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Old February 25th, 2013, 09:09 AM   #7
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Damn auto correct
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Old February 25th, 2013, 10:09 AM   #8
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whats that... its a bird... no, its a plane! ... no! its a topic completely over my head! congratulations eric, i have no idea what you're talking about
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Old February 25th, 2013, 10:11 AM   #9
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hey eric, can i just have you build the motor, and i'll buy it when you're done? i'll take one without nos please!
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Old February 25th, 2013, 02:25 PM   #10
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Very interesting explanation, Eric !!!

Why is compression important to the point of being carefully and expensively calculated?
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Old February 25th, 2013, 02:32 PM   #11
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For the record I did not wright this. I found it informative and wanted to share. We are getting pretty deep with these motors.
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Very interesting explanation, Eric !!!

Why is compression important to the point of being carefully and expensively calculated?

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Old February 25th, 2013, 04:40 PM   #12
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When you build engines for real. Like to set land speed records. You have to know the compression ratio so you can tune it for what ever fuel and use the engine will see. Like for a turbo to set the boost level. You need to have all the information before you start.

The magic stuff happens between Suck ,squeeze ,bang and blow. Things like scavenging. Or cam over lap and cam center.

I have a set of adjustable cam sprockets. I spent weeks fooling around with them. I took them off because it was just one more thing to deal with. And I don't understand it enough.

I found that moving the cams either direction I could make the compression gauge read from 100 to 300 psi. There is a lot of power locked up in the cylinder head.

I really can't build motors. I put junk together for myself and hope it does not blow up. I have some one else building me a real motor.
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Old February 25th, 2013, 06:44 PM   #13
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whats that... its a bird... no, its a plane! ... no! its a topic completely over my head! congratulations eric, i have no idea what you're talking about
wait. how do you know more about carbon nano tubes than you do compression ratio?

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For the record I did not wright this. I found it informative and wanted to share. We are getting pretty deep with these motors.
Very informative x. Didn't know VE effected CR. But then again, it makes complete sense.
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Old February 25th, 2013, 07:17 PM   #14
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wait. how do you know more about carbon nano tubes than you do compression ratio?
electronics and fluid dynamics are VERY different. compression ratio is fluid and thermal dynamics.... this is a topic i know next to nothing about. electronics, electrodynamics, computers... i've been building robots since i was 5 and programming computers since i was 7... these are things i understand. the closest thing to fluid dynamics i know about is like... particle physics and fourier transforms for wave signal processing... which if you dont know, those topics are like nowhere near fluid dynamics.
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Old February 25th, 2013, 09:26 PM   #15
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electronics and fluid dynamics are VERY different. compression ratio is fluid and thermal dynamics.... this is a topic i know next to nothing about. electronics, electrodynamics, computers... i've been building robots since i was 5 and programming computers since i was 7... these are things i understand. the closest thing to fluid dynamics i know about is like... particle physics and fourier transforms for wave signal processing... which if you dont know, those topics are like nowhere near fluid dynamics.
Good to know you know lots about stuff that won't help me go faster. This has nothing to do with understanding thermal and fluid dynamics. All you need to know about this it that you can go has high as 15:1 before you lose efficiency.

To measure CR you need a dial indicator and a crankshaft degree wheel. The calculating of CR in respect to VE and BMEP is slightly more complicated but not needed unless you have had cylinder head work done.

Aluminium heads can handle more compression both static and dynamic than cast iron heads. Usually around 1 to 1.5 points higher. Without experiencing detonation.

E.g A cast iron head with a dynamic CR of 8.6:1 will have the same resistance to knock as an aluminium head does with a dynamic CR of 9.5:1 to 10:1.

fyi, advancing the IVC by only 5 degrees will enable 13:1 static to give very similar dynamic cr to 12.5:1 with stock cam timing, 56 degress ABDC.

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Old February 26th, 2013, 06:33 AM   #16
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Why is compression important?

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.......Aluminium heads can handle more compression both static and dynamic than cast iron heads. Usually around 1 to 1.5 points higher. Without experiencing detonation.........
Why?
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Old February 26th, 2013, 07:20 AM   #17
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The aluminum will dissapate heat MUCH better than iron. And with water cooling you can run very high compression. Air cooled engines are limited to under 10 to 1
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Old February 26th, 2013, 10:36 AM   #18
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Good to know you know lots about stuff that won't help me go faster.
lol.
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Old February 26th, 2013, 11:20 AM   #19
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When you build engines for real. Like to set land speed records. You have to know the compression ratio so you can tune it for what ever fuel and use the engine will see. Like for a turbo to set the boost level. You need to have all the information before you start.

The magic stuff happens between Suck ,squeeze ,bang and blow. Things like scavenging. Or cam over lap and cam center.

I have a set of adjustable cam sprockets. I spent weeks fooling around with them. I took them off because it was just one more thing to deal with. And I don't understand it enough.

I found that moving the cams either direction I could make the compression gauge read from 100 to 300 psi.
There is a lot of power locked up in the cylinder head.

I really can't build motors. I put junk together for myself and hope it does not blow up. I have some one else building me a real motor.
Cam timing is pretty important.. Adavance the cam you get better bottom end power/torque, Retard it for better top end horsepower.
Allot of manufactures are using variable cam timing in there motors these days... get the best of both worlds.
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Old February 26th, 2013, 11:30 AM   #20
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What I have done is remove the top of the block. I milled it down .010 . That retards the cam timing. I need my own dyno to test it though.
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Old March 14th, 2013, 08:05 AM   #21
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Why is compression important to the point of being carefully and expensively calculated?
http://en.wikipedia.org/wiki/Compression_ratio

http://en.wikipedia.org/wiki/Otto_cycle

http://en.wikipedia.org/wiki/Thermal_efficiency

"A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency. This occurs because internal combustion engines are heat engines, and higher efficiency is created because higher compression ratios permit the same combustion temperature to be reached with less fuel, while giving a longer expansion cycle, creating more mechanical power output and lowering the exhaust temperature.

Thermal efficiency is the quotient of the net work to the heat addition into system. From analyzing the equation it is evident that the Otto cycle efficiency depends directly upon the compression ratio.

Ignition Process: The piston is momentarily at rest at TDC and heat is added to the working fluid at constant volume from an external heat source which is brought into contact with the cylinder head. The pressure rises and the ratio is called the "explosion ratio". At this instant the air/fuel mixture is compressed at the top of the compression stroke with the volume essentially held constant, also known as ignition phase.

Expansion Process: The increased high pressure exerts a greater amount of force on the piston and pushes it towards the BDC. Expansion of working fluid takes place isentropically and work is done by the system. The volume ratio is called "isentropic expansion ratio". Mechanically this is the adiabatic expansion of the hot gaseous mixture in the cylinder head, also known as expansion (power) stroke.

When expressed as a percentage, the thermal efficiency must be between 0% and 100%. Due to inefficiencies such as friction, heat loss, and other factors, thermal engines' efficiencies are typically much less than 100%. For example, a typical gasoline automobile engine operates at around 25% efficiency, and a large coal-fueled electrical generating plant peaks at about 46%. The largest diesel engine in the world peaks at 51.7%. In a combined cycle plant, thermal efficiencies are approaching 60%.[2] Such a real-world value may be used as a figure of merit for the device."
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Old March 14th, 2013, 09:03 PM   #22
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If your interested in calculating your compression ratio you need to measure the cc's of your combustion chamber.

**Valves in and a old spark plug installed to create a water tight seal.

To do this you need 2 things.

#A piece of perspex (hard clear plastic) big enough to cover combustion chamber.

# Something to measure cc's of liquid. A syringe, a Burette for example.

You need to drill a small hole in the clear plastic so that the hole will line up with the edge of the combustion chamber (highest point preferably).

Push down on the clear plastic so it creates a water tight seal.

Then fill chamber through the hole will water until full using your burette or syringe. Record the cc's of liquid it takes to fill chamber.

Mine is 12 cc's. A stock ninja 250 will be 10 or 11 cc's.

Now place your head on block with a head gasket and torque head bolts to spec.

Get out your dial indictor with the spark plug attachment.

Now you can take measurement using a dial indicator to measure your change in piston height from BDC to TDC.

Next you need the exact measurements of the top of your piston so your can accurately calculate the area of the dome. J&E send you these. If you got stock pistons your going to have to measure.

Now with some simple mathematics you can calculate your actual compression ratio. You will most likely find that 12.5:1 is actually 12.1:1.
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Old March 15th, 2013, 02:44 AM   #23
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They have big ones for 180
Bucks . But this kit for 25 dollars will work great.
http://bit.ly/XDrr52
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