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Compression ratio and air fuel ratio explained

Suck, Squish, Bang, Blow part 3- Compression Ratio and Air Fuel Ratio

By Mike Kojima

In our last edition of our ongoing series on engine tuning and theory, we explained how the 4-stroke cycle works.  Now we are going to break down a couple of common and very important terms that you will hear all the time when discussing engines that you must understand if you are going to successfully tune or select parts for your motor.  This information can also help prevent you from getting ripped off by bogus speed shops.  This can also help prevent you from being steered wrong by SPE's, FFF's and other lameo guru types that inhabit various forums and online resources.

For part one of the series click here!

For part two of this series click here!

Compression Ratio

This is a very important term that gets thrown around a lot when talking about engines. Compression ratio is the ratio of the volume of a cylinder when the piston is at the bottom of its stroke BDC, to the volume of the cylinder when the piston is at the top of its stroke TDC.  The compression ratio is described as a numeric ratio.

compression ratio
Compression ratio is the ratio of the cylinder volumes at top dead center and bottom dead center.

Compression ratios are affected by the volume of the cylinder head's combustion chamber, the piston top design (domed, dished, flat), the head gasket thickness and the deck height of the block (affects the position of the piston in the bore at TDC).

We will show you the effects of various piston dome volumes on compression ratios on these various JE and Arias forged pistons for a Nissan SR20DE engine.  This is a forged copy of the stock Nissan piston.  Its compression ratio is 9.5:1.  It has a slight dish in it.

The higher the compression ratio, the more closely packed the molecules of fuel and air are when the mixture is ignited by the sparkplug, this causes a more powerful explosion by making a more violent reaction  which produces more power.  Higher compression makes the expansion ratio of the exploding hot gas greater which means that more energy is impinged on the piston top, pushing it down harder, making more power. Increasing the compression ratio improves the thermal efficiency of an engine and this is the primary reason why higher compression increases power. Improving thermal efficiency improves fuel economy from getting more power from the same amount of fuel and a reduction of combustion chamber surface area to volume.  This means less wasted combustion heat and more expansion being used to drive the piston down.

A flat top piston with no dish has a compression ratio of 10:1 in an SR20DE.

Increasing compression ratio also increases an engine's volumetric efficiency.  Volumetric efficiency or VE for short is the measurement of how well the engine can move the air fuel mixture into and out of the engine's cylinders. VE is the percentage of charge mixture that dynamically enters the cylinder during the intake stroke compared to the actual displacement of the cylinder.  The higher the percentage, the more volumetrically efficient an engine is said to be. The more air and fuel sucked into an engine the more power it will make.  Increasing the compression increases the amount of suck an engine will have on the intake stroke which will increase VE.  The increase can equate to a few percent which can be significant in the overall increase in power.

This version of the piston with a high dome has a compression ratio of about 13:1

You might hear the term dynamic compression ratio thrown around a lot.  This term is a misnomer as there is no such thing as a dynamic compression ratio except for an odd Saab and research engines where the compression ratio can be changed on the fly.  The engine's dynamic cylinder pressure can be affected by many things but this is not the compression ratio which is a fixed mechanical ratio that doesn't change except for the above mentioned exceptions.

The turbo version of this piston has a deep high volume dish which reduces the compression ratio to around 8.5:1.  The size and shape of the domes or dishes will vary from engine type to engine type but you can get the idea of how the piston top can influence compression ratios by these examples.


 

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Comments
Jeff
Jefflink
Tuesday, July 13, 2010 1:01 AM
Should mention somewhere that your quoted AFRs are for gasoline only.
JDMized
JDMizedlink
Tuesday, July 13, 2010 8:52 AM
@Mike:
Question: I am about to finish my 2RZ engine (from a Tacoma). Everything is balanced and blue printed.
The Arias pistons I'm running have a 12.5:1 compression. I will have the cylinder head mill down some .002, along with a thinner Cometic MLS gasket (thinner than OE). Once it's done, I'm expecting the compression ratio to be close to 13.0:1
The question is: is running 100 octane enough for the compression ratio, or I have to run higher octane?
I know I can run something higher, and it won't hurt.....but I was thinking to getting my engine tuned with 100 octane.
Thanks
Mike Kojima
Mike Kojimalink
Tuesday, July 13, 2010 10:10 AM
I would not run 13:1 with 100 octane on the track but it might be ok for the street.
JDMized
JDMizedlink
Tuesday, July 13, 2010 1:37 PM
I was thinking the other way around.
This engine was not built for the street. the engine will be run mostly at the track (occasionally I might use it on the street, but I doubt it).
You think I should run higher octane then?
Mike Kojima
Mike Kojimalink
Tuesday, July 13, 2010 3:24 PM
Yeah I would run 110 or so for track use. VP C12 is real good NA fuel.
mesin_lumba
mesin_lumbalink
Wednesday, July 14, 2010 8:34 AM
Let say if the compression ratio is set up to be 13:01 and there is overlap in camshaft timing, so the engine will never see 13:01,doesn't it?
JDMized
JDMizedlink
Wednesday, July 14, 2010 11:43 PM
Thanks for the tip Mike. I'll look into that.
Mike Kojima
Mike Kojimalink
Friday, July 16, 2010 10:08 PM
"You might hear the term dynamic compression ratio thrown around a lot. This term is a misnomer as there is no such thing as a dynamic compression ratio except for an odd Saab and research engines where the compression ratio can be changed on the fly. The engine's dynamic cylinder pressure can be affected by many things but this is not the compression ratio which is a fixed mechanical ratio that doesn't change except for the above mentioned exceptions."
Just_Wanna_Drive
Just_Wanna_Drivelink
Monday, July 19, 2010 4:58 PM
A question for the guru's...

"Turbo and supercharged engines usually have lower compression ratios, in the range of 7-9.5:1. "

How can one figure what would be a safe [semi-aggressive] CR, for a given fuel [93 oct street], at desired power levels for a engine?

Very open question right???

Ok... Would a 9.5:1 piston be too high for a responsive ~375 whp SR20det on pump 93?

What are the limits [b4 boost and CR start tripping over each other] and how can we calculate them w/o blowing engines to say "well, that didn't work!"?

I know Mazworx has 90mm 11:1 CP Pistons in their 11k rpm, 1400 whp SR, But I'm guessing that's a little more maintenance than I want to budget... http://www.mazworx.com/index.php?l=page_view&p=racecars

Thanks for the insight!
Mike Kojima
Mike Kojimalink
Monday, July 19, 2010 6:09 PM
Because the SR20 has very poor coolant flow through the head and the head has a lot of trouble getting rid of heat even when you correct the coolant flow, I would not recommend going higher than 8.5:1. I would just run more ignition advance or boost when running 93 octane fuel. The SR20 is detonation prone.
Just_Wanna_Drive
Just_Wanna_Drivelink
Tuesday, July 20, 2010 12:54 AM
Thanks Mike!

I was hoping to run at least 9:1 when I switch to my s14 SR [running stock block 9.5:1 KA-T @ ~300 whp now] to keep the response up through the rev range. SR's seem very sluggish at low to mid rpm range to me... Maybe it's just the lack of tuning in the SR's I've driven/rode in.

It's nice to have a reason 'why' now instead of the typical forum 'just because' answers. ;[]
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