Cubic Inch Displacement And Compression

Hi everybody!
Here comes a silly question, been a long time since HS auto shop.
I was looking through the Dauntless section of the FSM and noticed that CA Jeeps (with V6) have a lower compression ratio (7.4:1 CA, 9:1 everywhere else).

If CID is bore x stroke, neither of those can change to reduce compression ratio without a relative reduction of CID. Is that correct?

Question is, how is compression ratio reduced?
Concavity in the heads combustion area, or the piston? Very curious as to how you guys explain this.

Another question, I’ll pick on @truckee4x4 for this one since he has a fresh rebuild, did you or your rebuilder address this?

As usual, I look forward to your answers!

Andy

 

I understand it's the piston top geometry that defines the final compression ratio for the Dauntless. Chime in if something else is a factor as bore & stroke are fixed; not sure about head combustion chamber.
When I ordered my remanufactured 225 from S&J, I asked this very question. They assured me the engine I was purchasing has 9.0:1 compression and should use a mid grade fuel of at least 87 octane.

 

This also brings another question to my mind.
Not meant to be political, but if you Moderators see it that way, I will not be offended if it is moderated.

If a higher compression ratio creates a more efficient combustion, why did CA decide a lower ratio would be better for emissions? Something to do with the smog pump pushing fresh air into the exhaust?
Again, just curious.

Andy

 

As best I recall it was roughly during the late sixties and early seventies when CA. was having smog issues.
Especially so around the LA area.
When higher compression ratios are used in engines the amount of nitrous oxides being produced increases exponentially.
That said I would not consider anything at 9/1 or lower ratio to be a high compression ratio engine.
Personally I feel 9/1 ratio is a great compromise between nitrous oxides and max power per octane required.

 

The combustion chamber volume, the volume displaced by the piston, and the thickness of the head gasket are the only variables. Typically manufactures increased the head combustion chamber volumes and/or used dished pistons to lower the CR.

 

In answer to your intial post, The lower compression ratio for California vehicles was achieved though a deeper "dish" in the top crown of the piston. That would be a more cost-effective method than casting a cylinder head with a larger combustion chamber.
There are three part numbers for Dauntless 225 standard size pistons, #1399524 for 9:1 ratio, #1395513 for 7.6 ratio, and #998242 for 7.4 ratio. There are corresponding different part numbers for the 4 to 5 oversizes.
-Donny

 

Yes, different pistons.

 

Higher compression increases efficiency, but it also makes the combustion temperature higher. Higher temperatures make more side-products of combustion, like "burning" some of the nitrogen in the air, making nitrous oxides. Lowering the combustion temperature by using a lower compression ratio was a stop-gap measure to reduce emissions NOx, a significant component in photochemical smog. It was a reaction to the legal mandate outpacing the technology. Later, measures like exhaust gas recirculation (EGR) and variable spark timing were used to additionally address the formation of NOx. Modern cars have higher compression ratios, but they also have computer systems that can address these issues via more modern systems.