Randall wrote:
>
> Michael :
>
> I think that, if you re-read what I said, you will see that I did not
> say that peak VE was reached at cranking speed, only that it was likely
> higher than at max power for a 'normal' engine (which I specifically
> intended to exclude tuned runner racing engines). Since max power is
> reached well above max torque, and usually both the VE and BMEP have
> fallen off significantly, I still feel that is a true statement.
Yup... true, but power is irrelevant to the topic, since power was not
the issue at hand, but rather, the topic was the compression pressure at
cranking speed for testing purposes.
> For example, the factory curve for the TR3 engine shows a BMEP of about
> 120 at peak power (4700 rpm), but about 125 at 1000 rpm. Peak is 143,
> at around 3300.
None of which are at cranking speed, which is typically 100-150 rpm.
> It also is not true that VE and BMEP have a fixed relationship (although
> they are usually pretty closely related).
Not to make a pissing match out of this, but, read the book. Max torque
is directly proportional to max BMEP, period, and volumetric efficiency
determines cylinder filling, which determines BMEP. Less filling means
less pressure and torque. More means more. The bore and stroke and
combustion chamber volume of the engine are fixed. The only variable
related to BMEP and torque is the effective volume in the cylinder. Why
fight physics? <smile> It doesn't matter whether the engine is tuned or
not. Every engine has a peak torque, regardless of state of tune, which
is directly related to peak BMEP.
The only conceivable instance of BMEP, torque and volumetric efficiency
not being absolutely related is when the camshaft timing pushes the
torque peak so high in the rpm range that piston-to-bore friction
overcomes the absolute effects of volumetric efficiency. That occurs
only in extremely high-speed engines, and, again, is irrelevant to the
original topic.
> And BTW the reason it takes several revolutions to get the full reading
> is because the cylinder must pump the inside of the gauge full of
> compressed air. Until that happens, the volume inside the gauge
> effectively adds to the combustion chamber volume, lowering the
> compression ratio.
True, but very marginally so. Calculate the volume of a typical gauge
and compare it to the volume of the combustion chamber at TDC. It's not
several times the 45 cc (2.7 cu. in.) or so of the typical Triumph
combustion chamber (as your reply suggests, in the need to crank several
times), and, in most cases, is much, much smaller. (!) For example, I
have a compression tester with an 8" flexible hose. The volume within
the gauge itself is perhaps 0.1 cu. in. The hose has an ID of 1/8"
(0.125"). For an 8" length, the hose has a total volume of 0.098 cu. in.
Total volume of the gauge and hose, therefore, is approximately 0.2 cu.
in., or, about 7% of the volume of the chamber at TDC. Nice try, but no
cigar.
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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