Compression ratio question

[my1st911]

While waiting for a haircut today, i was thumbing through car and driver. They had a mitsu evo, taurus sho, and on another page a 2010 911 turbo. All of these turbo version had 10.5 to 1 or higher compression ratio. How can newer cars have such high ratio, and the older cars had to have their compression lowered. I was stumped

[mark houghton]

Let me venture a guess...the newer cars have higer comp ratios to support low and mid range performance, but at the expense of running lower boost. Run that kind of compression in an old 930 with .8 bar or more boost and she'll blow sky high. Plus, the newer cars with electronic fuel management and timing management can better deal with the boost and compression combinations.

Just a total non-expert opinion.

[midlife]

They are also water cooled, so their head temperature is MUCH cooler.

[JFairman]

Originally Posted by midlife

They are also water cooled, so their head temperature is MUCH cooler.

and they have 4 valve heads and twin cams with variable valve timing.

The 4 valve head has a spark plug in the middle instead of off to the side like a 2 valve head.
This lights off the mixture from the middle of the combustion chamber similar to dual plugging and the flame front spreads out evenly to the outer edges of the combustion chamber burning more evenly which allows a higher compression ratio before detonation starts in.

The digital electronic ignition and fuel injection along with twin turbos and much better intercoolers also helps them run higher static compression without detonation.

[drmatera]

not to mention most are direct injection now which pretty much gets rid of pre-ignition as the fuel only sprays in when it's time to light off. The Evo is not direct inj'ed but the reasons given above cover that.

[my1st911]

Those are all answers that make alot of sense. I thought about the direct injected theory, but since evo didnt have it I wasnt sure if that played a part. I was always under the impression that turbo=low compression ratio. Its pretty cool to see its not always the case.

[911st]

8/1 compression with 1 bar (14.7lbs) is close to the same effective compression ratio as 10/1 running about 7.4 lbs of boost. Both are on the agressive side for a 930.

Doing this requires close management / listening for knock and motor temps. They are quick to pull back spark, add fuel and or pull back boost. Often boost is managed by rpm. I belive higher boost can be run early in the rpm range and has to be reduced near TQ peak.

Most OEM turbos also run AFR's on boost in the 10's.

And all the other stuff mentioned.

[Speedy Squirrel]

+1 on water cooling. All the other stuff is just gravy. Our exhaust valves are too hot.