Sunday, May 01, 2005

 

Dynamic Compression Ratio

While static compression ratio (calculated by dividing the cylinder swept volume plus actual combustion chamber volume by the actual combustion chamber volume) gets all the attention, the dynamic compression ratio ends up being vastly more informative when attempted to discern whether a given combination will be usable with "pump gas" (typically defined as unleaded with an octane rating of 92 or 93).

The single biggest factor influencing dynamic compression ratio, or DCR, is the closing time of the intake valve. This, along with the same variables used to compute static compression ratio (SCR), can yield a useful approximation of DCR.

Let's examine the engine in my '96 Impala SS, a 396 stroker based on the GenII LT1. With 55 cc LT4 heads, flat-top (5 cc valve relief) pistons, and a 4.030" bore combined with a 3.875" stroke, a static compression ratio of about 11.7:1 is the result. Conventional wisdom would have it being much too wild for pump gas, even with aluminum heads and reverse-flow cooling (both of which combine to not just allow but require about one point of additional static compression ratio).

But taking into account the intake valve closing point results in some amount of compression being bled-off, as the valve stays open After Bottom Dead Center (ABDC), the nominal endpoint of the intake stroke. As the piston starts coming up, the intake remains open, and some cylinder pressure is lost at low engine speeds (at higher speeds, this results in improved cylinder filling due to the momentum of the intake air charge, but with most street engines we're well past the point of optimum cylinder filling when this starts to become a major factor). With that "bleed-off" effect taken into account in a compression ratio measurement, shooting for a absolute maximum of 9:1 DCR will typically yield marginally-streetable results on pump gas, allowing reasonable amount of timing to be used if the quench/squish height (distance from the piston top to the bottom surface of the head, forming a volume where air/fuel charge likes to accumulate and detonate) isn't too large (0.040" being an absolute minimum for safety, 0.060-0.080" being adequate, and much beyond 0.080" likely to be too large).

With a cam that's based on Comp Cam's XR282HR specifications, I've got 282 degrees of "advertised" duration (0.006" lift at the lobe, or 0.009" at the valve with 1.5:1 rockers) on the intake side, 230 degrees of duration at 0.050" lift (once again at the lobe), 113 degrees of lobe seperation, and 4 degrees of "ground-in" advance.

The intake valve closing point ABDC is calculated in the following manner. First, divide the intake duration by 2, and add that to the lobe seperation. Next, subtract out any ground-in advance. Finally, subtract 180 degrees. Confused? Check out this helpful tutorial from Comp Cams, and especially this graph, and keep in mind that the camshaft turns at half the speed of the crank (since it takes two revolutions of the crank for one complete combustion cycle in a four-stroke), so that a given number of degrees at the cam is twice that at the crank.

With the cam specs above, I get an intake valve closing point of 70 degrees ABDC: 113 + (282 /2) - 4 - 180. We're using advertised duration here, since air in the cylinder is still being bled-off with a valve opening of only 0.009".

Punching those calculations into Keith Black's DCR calculator, along with misc. other engine specs (a 4.125" gasket bore, 0.039" gasket thickness, 0.034" deck height, and a 6" rod length), I get a DCR of 8.97:1 for my engine. Given its pickiness about fuel brand, that's a very believable number. Another calculator is available here - it requires fewer variables so it's likely a bit less accurate, but it does allow one to account for the effects of boost (via forced-induction) and altittude, if that's of interest.

Note that the calculator asks for the intake closing point at 0.050", plus an additional 15 degrees. I guess that's a somewhat decent substitute for knowing the "exact" valve closing point (which is, at best, an educated guess anyways - who's to say that the intake valve is leaking-down cylinder pressure at 0.009" of lift but not at, say, 0.007"?), but I'd rather use the "advertised" number. The 15 degrees that the calculator asks for is too small - adding in about 25 degrees would work for a relatively "steep" hydraulic roller cam like my Comp Cams Xtreme Energy (the difference in advertised and 0.050" duration being 52 degrees in this case), and something more like 30 degrees would be better for a traditional flat-tappet cam.

The bottom line here is that there's more - much more - to detonation resistance than the SCR, and understanding the DCR and its influences is a good start towards determining an engine's usefulness with real-world gasoline. If you've completely screwed-up your component selection and ended up with a motor that's simply not happy with pump gas, perhaps a change in camshaft timing (either retarding the cam via an adjustable timing set or selecting a different cam with less advance, more lobe center angle, or more duration) might just result in an engine that tolerates pump gas - but with a likely loss in performance, if you selected the correct cam in the first place!

Comments:
Well, Duh!

But that said, at what point do you move from pump gas to specialty fuel?
 
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