Compression ratio



Hey everyone it's been awhile but I have a question that's been answered before BUT I'll ask it again.... :LOL:

If I run a CR of say.....9.5 : 1 or even 10.0 : 1 I should always use premium grade gas, right (high octane rating, I mean)?

I understand it has a lot to do with pre-detenation but what I don't understand is why newer engines (i.e. my 2.4L 4 banger with 10.0 :1 CR running on 87) works that much differently than these older motors. Computer control I guess? EFI?


Exactly. It's not just EFI, it's "engine management".

By controlling fuel, spark, air, and temperatures, you can make millisecond by millisecond adjustments with the ECU to allow the engine to run on lower octane fuel while still maintaining emmissions.
The main reason that they can run this compression ratio is due to electronics but not on the engine. It's the computer they use to design the head and the combustion chamber dynamics. A properly designed head can allow a great deal more compression to work with low octane fuel than they thought during the 60s, 70s and 80s. Proper engine management does play a part but it's mostly the engine design.
There has been bulk work in flame front properagation models, wet mixture flow, and swirl propergation. The early May headed V12 HE Jags ran 11.5:1 compression on a engine running 89 MON octane. The British spec engine ran 12.5:1...a huge squeeze, even on there leaded 97 RON. These engines had Noahs Ark electronic injection, no advanced mapping.

Aussie Falcons started getting higher comprssion engines in the late 70's, and by 1982 the low-octane Regular options was no longer available...the 250 cross-flow carby engine had a mamoth 9.65:1 compression ratio on 97RON leaded. The EFI was down to 8.8:1 with the first 1983 engines, and stayed that way. Then the HSC-style XF in 1985 got the High-swirl thing going so it could swallow 91 RON...carby compression was still hiher than the injected engine.

Run through the calculations on the Tech Section, and see that the compression ratio raises performance. It's clear that the first break into pre 1972 comprssion ratios was as a result of chamber design. Since the mid 1980's the electronic management has ensured a lower compression ratio to make emmission engines easier to get through the feds sniffer tests. The advance curves a darn aggressive. A GM3800 has a unbelivable amount of ignition lead (advance) with a low copmression, and iron heads.

US auto makers know higher compression engines need more work to get through the federal tests for oxides of nitrogen, hydrogen, and carbon. The electronics allow them to buy back performance without detonation.