tech questions: compression ratio

[bozothenutter]

Why didn't renault max out the compression ratio on the RS engines?

As I understand it with 98RON you'd be able to run at a maximum of 12.5:1 (combustionchamber design permitting of course)
Is it because of said chamber design?
Because the engines have to be able to use 95RON?

as an aside...won't higher comp also mean lower emissions?

 

[Chip-mk1]

Higher compression means bigger strain on the rings at idle etc which increases wear, it can also lead to WORSE idle emissions as you have to r****d the timing more, then there is the issue that the higher the CR the more chance of failure if some wally goes and bangs 95 RON into it, so quite a few different reasons really.

The 197 chamber shape is a little better which is why they have edged it up a touch on those cars.

 

[bozothenutter]

So for a roadcar, upping the comp may be more trouble that it's worth? (emissions wise) even if you only plan on using 98RON?
say if you want an engine to last 200K between rebuilds, would going to 12.5 from 11 to seriously reduce that mileage?

 

[Chip-mk1]

Yes you will see a drop in how many miles you can do typically if upping the compression, assuming other factors are fairly equal, as your average cylinder pressures acting on the rings are higher.

 

[bozothenutter]

How do Mazda do it then with their SkyAktiv tech......I understand they do 14:1 with 95RON? (It is mostly for emissions I gather)

 

[Chip-mk1]

How do Mazda do it then with their SkyAktiv tech......I understand they do 14:1 with 95RON? (It is mostly for emissions I gather)

By speccing the entire engine from the outset to do that, the design of the cams, the way the ecu controls it, the shape of the top of the piston, the shape of the cylinder head etc.
I dont know the specifics of that technology but I can guarentee there will be more to it than just whacking the CR right up.

 

[Tutuur]

To refer to your first question, a car has to be produced to work in every environment, maybe we have stable ron 98 here but it could be pretty different in eastern europe for example

 

[bozothenutter]

thanks, it is hard to find info on what it exactly entails.

found this:

http://articles.sae.org/10335/