Is the ratio higher the better or lower?
like 9:8, 10:0, 10:2:1, 10:5, 11:0
What does it mean?

G

 

Here is how you calculate compression:

VARIABLES DEFINED:

V1 = swept volume of cylinder = [(16.4)*(pi(r*r)h)] where pi = 3.14, r = 1/2 the bore diameter, h = the stroke and 16.4 = the conversion factor to CCs.

V2 = volume of combustion chamber.

V3 = volume of dead space above piston at TDC (deck height) = same formula for V1 where h = piston below deck height. Use a negative number here if the piston is above the deck.

V4 = Volume of head gasket opening at compressed thickness (manufacturer can supply spec)

V5 = volume of valve reliefs and/or any dish in the top of the piston (manufacturer can supply spec)

CALCULATE COMPRESSION:

CR = [V1+V2+V3+V4+V5] / [V2+V3+V4+V5]

With each full point increase in compression ratio, there is typically an increase of 1%-2% in horsepower. Higher compression requires higher octance fuel. Higher compression also places additional load on the bottom half of the rod and main bearings.

The ideal compression ratio depends upon the purpose of the engine. In today's world of computer controlled engines, the stick with the following guidelines:

9.5:1 and lower = regular unleaded fuel
9.5 to 10.5:1 for performance engines burning premium fuel
11:1 and up for racing or street performance with 100+ octane race fuel only


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2002 TL-S Navi
White Diamond/Ebony

 

wow...thanks that helps clear things up for me.
i was looking at NSX 10.2:1
integra gsr 10.0:1
and TL type S 10.5:1

what's the difference there??

G

 

The compression ratio of your combustion chamber is the volume at bottom dead center divided by the volume at top dead center.

Higher compression means more efficiency, but more opportunity for detonation.

Of all the compression ratios, none that are listed are valid for a piston automobile engine. Typical ratios are from 9:1 to 11:1.

9:8 and 10:5 are not reasonable, 10:2:1 would be some kind of strange engine cycle, and 10:0 / 11:0 are impossible unless operating in a vacuum.

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btw...the Honda S2000 has 11:1 ratio....so does that mean it has to actually use 100+ octane gas??? where can u find that???
i haven't seen any gas station with higher than 92.

G

 

oops...the : meant to be .
like 10.2:1
10.5:1
10.0:1

what's the difference in the numbers???

G

 

The higher the first number, the higher the ratio. The last number is always expressed as 1.

A higher compression ratio will require more octane, but this has a lot to do with how the engine manages the spark knock. A car with a knock sensor, direct ignition, EGR (exhaust gas recirculation), etc. will be able to tolerate a higher compression ratio than it would otherwise.

Oh, and higher compression ratio means higher thermodynamic efficiency.

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<FONT COLOR="#800080" SIZE="1" FACE="Verdana, Arial">[This message has been edited by Einstein on June 25, 2001 @ ]</font>