[GTOKIDRH]

Originally Posted by amcmike

Actually it is, and today's OEMs do.

Modern engines use much higher compression ratios, enabled by direct injection. The higher SCR increases efficiency even at part throttle, thus increasing fuel economy (in addition to increasing torque under wide open throttle). They can get away with these compression ratios due to the charge cooling as the fuel is injected directly into the combustion chamber (especially with multiple injection strategy at under medium loads and low-mid rpm). This then allows them to downsize the engine displacement (if desired) to further improve fuel economy. This is why OEMs use DI even on small economy cars, not just performance vehicles.

So correct me if I'm wrong. The LS engines before DI were mostly if not all in the 10+ CR range. I understand this was largely due to the use of aluminum as well as the heart shaped combustion chamber with a centrally positioned spark plug tip. Yes DI helps immensely.

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[amcmike]

Originally Posted by GTOKIDRH

So correct me if I'm wrong. The LS engines before DI were mostly if not all in the 10+ CR range. I understand this was largely due to the use of aluminum as well as the heart shaped combustion chamber with a centrally positioned spark plug tip. Yes DI helps immensely.

It's a bit complicated, but I'll try to simplify.

The GENIII passenger vehicles (Corvette, Firebird, etc.) were in the low-mid 10s with aluminum block/heads premium. The small engine trucks 4.8/5.3 were mid-9s, the exception was the 5.3L HO which used an aluminum block and 9.9:1. The 6.0 LQ4 was only 9:1 but was meant for HD trucks that weighed more and would be used for heavier loads and 87 octane. There was a 6.0L LQ9 which was a 10:1 variant of the LQ4 but was used in Escalades and required premium.


The GENIV LS (debuting with the LS2) were still PFI, and saw a bump in compression with passenger cars adding about 1/2 point. For the GENIV trucks, the iron blocks (and some aluminum) were typically 9.5:1, but often had VVT which bumped up the dynamic compression below 3500rpm. Some larger aluminum blocks (like the L92) were as high as 10.5:1 but recommended premium.

With the GENV having DI, compression even with VVT, is typically 11:1 on trucks, and higher for passenger cars (but with premium recommended). And we haven't even talked about Super Charged cars, but the LT4 runs 10:1 with 9.4 lbs of boost.

[PunchT37]

One can’t talk about modern engine specs as a reference to old car building without realizing the fast and powerful computers running them.