My 17:1 engine ran fuel that was designed for that. On a race engine you want the DCR to be as high as practically possible, once you've established valve timing. High duration cam timing reduces DCR.
DCR discussions apply to street engines and fuel characteristics. Race gas is not legal for the street, and even if it was, you have availability and cost problems.
You can calculate the theoretical effect of increasing compression ratio with the math that's presented in any IC engines textbook. The effect trends to asymptotic (further increases have smaller returns), but the slope is always positive.
Part of the lope deal has to do with the opening ramps vs. the duration @.050" lift. A good example is the Duntov 30-30 factory cam for the SBC. It has a fairly moderate duration @.050" but it is a huge cam if you measure duration at .005" lift. This gives you a lot of overlap at idle which makes it lope and idle badly. It also bleeds off a lot of DCR, which means you need a lot of compression with it. Factory engines that used that cam had advertised 11:1 CR. It's also why an aftermarket cam with less duration @.050, particularly a roller, will make more power than a Duntov 30-30. Comp Cams "Thumper" cams are another example. They are ground to intentionally produce a lope.
So if you're looking for bad idling and lope, you probably should be comparing overlap at full advertised duration.
Keep in mind that camshafts that produce lope will require a lot more tuning work at idle and low speed/low load with EFI to get them to idle at all and have driveability. They usually respond to more ignition timing in these areas, but without increasing timing at higher rpms/more throttle opening. Actually this is true of a carb engine also.
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Originally Posted by hutchenc
