"bob and john" (bobandjohn)
10/02/2014 at 18:24 • Filed to: None
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"bob and john" (bobandjohn)
10/02/2014 at 18:24 • Filed to: None | 2
| 14 |
UPDATE: ok, that make sense now. thanks for your time and patience everyone!
About compression ratios. I understand it has something to do with the bored and stroke of a motor.
So, how do you change it, I hear all the time in builds : oh, I bumped up the compression ratio) how? from what I understand, it means that they made the bored smaller and the stroke longer....Something isnt adding up.
Class is in session now. What can you teach me?
Clean IS250 (in green!) for your time
Sweet Trav
> bob and john
10/02/2014 at 18:26 |
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So the compression ratio is the ratio of empty space in the cylinder and combustion chamber at the piston's lowest point vs it's highest.
https://www.rbracing-rsr.com/compstaticcalc… this is a good calculator with diagram to show you.
Racescort666
> bob and john
10/02/2014 at 18:27 |
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Your compression ratio is the ratio between the total volume of the cylinder (swept and combustion chamber) to only the combustion chamber volume.
Edit: teaching you more:
You can change your compression ratio several ways. Changing (increasing) the bore and stroke of the engine without changing the combustion chamber volume will change (increase) your compression ratio.
An easy way to change the combustion chamber volume is by changing the thickness of the cylinder head gasket. More complicated ways include piston shapes (domed, taller, etc), milling the head (which reduces the combustion chamber volume), milling the block (also reduces the combustion chamber volume but it's the small gap between the top of the piston at top dead center and the top of the block), and also porting can have an effect on combustion chamber volume.
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bob and john
> Sweet Trav
10/02/2014 at 18:28 |
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ok, I understand that. What I dont get is how to people change the compression ratio without boring/stroking the motor?
and wouldnt that also change the volume (displacement) of the motor?
Zipppy, Mazdurp builder, Probeski owner and former ricerboy
> bob and john
10/02/2014 at 18:30 |
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Compression ratio is the swept volume divided by clearance volume. The higher the compression, higher octane fuel is needed. The design of the combustion chamber + pistons can affect the ratio as well as bore and stroke. Diesel engines usually use a special shaped piston to increase the C/R (Also applies to gasoline engines). (VW's 1.9 TDI has either a 19:1 ratio or 19.5:1, depending on what year, and .5 can make all the difference.)
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bob and john
> Racescort666
10/02/2014 at 18:30 |
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ok, got that...what I'm not understanding is how to change it without bring/stroking the motor.....and wouldnt boreing/stroking the motor ALWAYS change the compression ratio?
twochevrons
> bob and john
10/02/2014 at 18:32 |
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Displacement is swept volume, so, for example, if you reduce just the combustion chamber volume (commonly done by either skimming the head or using thinner head gaskets), you have increased the compression ratio without changing the displacement.
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Racescort666
> bob and john
10/02/2014 at 18:33 |
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Yeah, I updated my comment because I realized I left that part out. Short answer: yes, boring and stroking an engine will always increase the compression ratio if there is no change to the cylinder head/combustion chamber.
camaroboy68ss
> bob and john
10/02/2014 at 18:34 |
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the tops of the pistons also make a difference. A flat top vs a pop up will have different ratios. Milling the heads will also boost compression because you are putting the valves closer to the piston. Cams and rockers also have a part in compression ratios.
I'm basing all of this off of small block Chevy v8s since I only mess with those motors.
Roadster Man
> bob and john
10/02/2014 at 18:34 |
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Even something like a thicker or thinner head gasket than OEM can result in a change in the compression ratio. People take that idea a step further with head spacers. Another way to change compression ratio without changing the bore/stroke would be piston heads: Making the piston head more concave will result in a larger volume combustion chamber, meaning that the compression ratio has changed. That's also a case in which the overall displacement of the engine does not change along with a change in the compression ratio. They don't always need to go hand in hand.
This is all very simplified, though. Motoiq.com is a good source for more in-depth info. Look up their "suck, squish, bang, blow" series and you'll learn a ton.
crowmolly
> bob and john
10/02/2014 at 18:35 |
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Preface, is there's a mistake somebody please let me know and I'll fix it.
There are two types of compression ratio- Static and Dynamic.
Static compression ratio is a calculation that consists of the following:
Bore— The diameter of the cylinder bore
Stroke— The distance the piston travels in the cylinder
Deck Height— The distance between the top of the cylinder bore and the top of the piston, when the piston is at TDC (Top Dead Center, or at its highest position. How far the piston is "in the hole".
Compressed thickness of the head gasket
Piston top volume. Positive for a dome, Neg for a dish or valve reliefs
Combustion chamber volume— How much open space is in the head above the cylinder
Most discussions of compression ratio are talking about static compression.
Dynamic compression ratio is different. Instead of using the full sweep of the piston from BDC (bottom dead center, at the bottom of the hole) it uses the position of the piston at the moment when the intake valve closes. It is a bit more complex of a calculation since you take into account cam timing. It will always be lower than the static compression ratio, and DCR is what is used to estimate pump gas friendliness.
There are online calculators for all of this stuff if you have the data.
Worth noting,
Cranking compression is a way to measure piston ring condition using a compression tester in a spark plug hole. You can't determine SCR or DCR from this as there is no solid relationship. The word compression is used but it's not really related to SCR or DCR.
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Sweet Trav
> bob and john
10/02/2014 at 18:36 |
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Piston compression height, head gaskets and different cylinder head volume.
For instance. http://www.jegs.com/i/Keith+Black/…
http://www.jegs.com/i/Keith+Black/…
You'll notice the 327 piston has a higher compression height, than the 350 piston, the reason is, the 327 has a 3.25 inch stroke and uses a 5.7in rod, the deck height of a small block chevy is just about 9 inches. Half of the Stroke + Rod+Piston compression height should just about equal the deck height of the motor, for the 327 its 3.25+5.7 = 9.003
Make sense? the piston is within a hair of the top of the cylinder block. next you can change your piston with a dome at the top to eat away at the open space in the cylinder head, you can use a smaller volume combustion chamber within the cylinder head or a piston compression height that is a little bit taller. also you can get a minor bump by sometimes using a thinner headgasket.
Drakkon- Most Glorious and Upright Person of Genius
> bob and john
10/02/2014 at 18:39 |
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You normally do not achieve it by changing the bore or stroke. You would change it most commonly by changing the shape of the piston, or choosing a thicker head gasket. Back in the bad old days of 'turbo all the Hondas' shops would move the position of the rings on the sides of the pistons.
Lowering the compression (such as adding forced induction) is generally easier.
Changing the shape of the piston is the most likely route for raising it.
MonkeePuzzle
> bob and john
10/03/2014 at 10:58 |
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wow this is a glorious colour!
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bob and john
> MonkeePuzzle
10/03/2014 at 11:07 |
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it is a REALLY nice color on the IS, not gunna lie. We almost bought a green one for my mom, but it ended up having about 5k worth of crash damage. she ended up with a black one instead