Randall wrote:
>
> Dan Buettner wrote:
> >
> > That being said, my Bentley manual quotes 8.5:1 for the compression
> > ratio on my TR3 and also quotes 120 psi as the compression pressure.
> > If you do the math, those numbers would indicate 96% volumetric
> > efficiency based on 14.7 psi atmospheric pressure. (so maybe I'm
> > full of it)
>
> In addition to the heating affecting the pressure reading, it's entirely
> likely that the VE is higher at cranking speeds than at peak power (for
> a 'normal' engine).
Hmmm, this may require some further explanation and explication. At
cranking speeds, the engine is entirely dependent upon atmospheric
pressure to fill the chambers, and in that context, the rising piston
approaching TDC will expel a fair amount of the mixture before the
intake valve closes, thus considerably reducing the effective pressure
in a single stroke. This is the reason why a compression test requires
several rotations to reach the peak pressure at cranking speeds (that
and eventually obtaining enough gas pressure in the cylinder to make the
rings seat fully).
If it were true that maximum volumetric efficiency were obtained at
cranking speeds, it would also mean that maximum torque would be
obtained at same, since maximum torque is only obtained at the greatest
BMEP (Brake Mean Effective Pressure). Practical experience and
dynamometer testing suggests otherwise. The speed and inertia of the gas
column entering the cylinder actually determine peak volumetric
efficiency, and the optimums for those factors only occur higher in the
rpm range. See Philip Smith's _Design and Tuning of Competition Engines_
for a full explanation of BMEP and its relationship to torque.
One needs only to look at the dynamometer results of some of the early
Jaguar racing engines to see that intake and exhaust tuning and cam
timing are the determining factors in establishing the point of peak
volumetric efficiency. Some tests on such engines with slide-valve
inlets and Lucas PI verified that VEs of approximately 108% could be
achieved in a normally-aspirated engine with the proper length of intake
(adjusted with ram pipes of differing lengths), but those VEs were
established in a range around 4500-4800 rpm.
In short, cranking pressures are an indication of cylinder health, and
are usually empirically derived, not calculated. In contrast to the
figures Dan cites above, a modern engine, with a similar compression
ratio, might have an optimum cranking pressure reading of 150-170 psi,
depending upon its intake and exhaust configuration and upon its valve
timing.
> Just two more reasons why only comparative measurements are meaningful.
On this point, I am in complete agreement.
Cheers.
--
Michael D. Porter
Roswell, NM
mailto: mporter@zianet.com
`70 GT6+ (being refurbished, slowly)
`71 GT6 Mk. III (organ donor)
`72 GT6 Mk. III (daily driver)
`64 TR4 (awaiting intensive care)
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