Hey guys, look, I'm not looking for one answer, I'm mostly trying to get a feel for the range of opinions out there. Maybe I'm the only one that's noticed, but it seems like companies are talking about compression ratios more often in press releases, and I was wondering if that would hurt companies that aren't going down that path. From the educated answers I've seen, it sounds like there's multiple ways to raise or lower the CR. Mazda's got a new line of SkyActive engines with a CR of 14.0:1, GM's next gen small block PR talked about higher CR, and all these new DI engines are talking about cooler intake charges and higher CR's. So for companies like Ford, BMW, and MINI, how do they market their turbo engines that have lower CR's but similar fuel efficiency and power gains?
For the most part the average consumer doesn't care about Compression Ratio's, Bore and Stroke and all that more esoteric information. It seems like they mainly want to know if their iPhone will connect and if they can text while driving. Companies that have pushed the envelope with more efficient engines will use that in marketing but it is far from the focus of the marketing efforts. That's my take on the matter.
I didn't read all the knowledgeable answers above. My though is it is the amount of squish before you get a bang. Suck, squish, bang & blow 4 strokes of fun!
Back in my day we'd let the pistons get all carbon'd up to raise the compression ratio. Oh the sound of that powerful pinging. Back on topic: direct injection donuts.... :donut1::donut1::donut1:
Some people want products that have 'better numbers', indeed they may base their buying decision only on those numbers - like a 200hp car must be better than a 150hp one, a 12Mp camera better than a 8Mp, or a 6-airbag car safer than a 4-airbag one. Sadly there are many characteristics that are not amenable to simple measurement - just look at the (particularly American) obsession with using g on a skid pan as a measure of cornering, which tells us nothing at all about handling. Similarly a 8Mp camera with a good lens will generally produce better photos than a 12Mp one with a mediocre lens. So there isn't a compression ratio that's 'better' than another - get a diesel and that will have a compression ratio over 15:1 and maybe over 20:1. Them's real man's numbers....
I think the point in the ads is that a higher compression ratio gives more efficiency - it also can lead to higher N0X emmissions and the aforementioned pinging (or pinking, as it used to be called) to get there the mfr's are using all sorts of tech including variable valve timing, forced induction, direct injection, exhaust gas recircualtion and on and on..... And I thank and applaud them for it! Doing so has given more HP and a corresponding huge bump in fuel economy - two things that in the past did not go hand in hand. However, compression ratio is just one peice of the puzzle, and the one they've chosen to highlight. Back in the day, if you could run 10:1 on the street (using ethyl of course) you were doing pretty good. But the rich mixtures required made gas mileage in the single digits - and the car ran so rich you couldn't stand behind it for the fumes. But she went like the clappers! Now adays 10:1 is "normal"....... But it all may become moot if MBZ gets their variable cylinder size engines in production, as the comp ratio will change with the cylinder volume.....