Hello Bob,
Actually modern stock engines have a volumetric efficiency of about 80%-85%,
mildly built (your engine) will approach 100%, and a fully tuned race engine
can get to 105%-108%. Those higher percentages occur at narrow specific rpm
bands that the engine is tuned for. The way that you get over 100% is by tuning
the intake and exhaust pulses to resonate at a frequency that causes a slight
increase in pressure at next opening intake valve. My engine was 97% at 6000
rpm.
Other differences between the dyno and the chassis: The exhaust is open with
large, long headers; they use a large velocity stack on the carburetor; no air
cleaner, and of course no drive train. My car "lost" 100 hp out of 440. It went
to
1. Transmission - even at straight through there is a lot of viscosity lose
through the cluster gears. I know guys that race Spec racer Fords and the
serious ones use 0 weight oil in the transmission and measure 10 more hp at the
rear wheel. (Of course they replace the transmission every third race!)
2. Drive shaft - not much here
3. Rear end - Jag rear ends are known to be in efficient, Ford 8.8 and 9" rears
are known to be strong and efficient.
4. Tires - even on the chassis dyno, I would guess that adding 15 lbs to the
tires would reduce the rolling resistance.
5. Tight, untuned headers - 10 - 20 hp? maybe.
6. Glass pack muffler - 20 hp?
7. no velocity stack - ?? some
Just my thoughts.
Craig Wright
PS Are you going on the 200 mile tour of San diego county this weekend? Meet at
the auto museum in Balboa park for an 8:30 departure. Ends up in Escondido
about 3:00 after taking the back roads to Julian for lunch. There should be
some Cobras, Jaguars, Mustangs, Miatas, etc. A Tiger would look good.
Craig
Bob Palmer wrote:
> Theo,
>
> It's interesting to also compare the pressure drop to the flow needed by
> engine. Just starting with round numbers, a 300 cu. in. engine turning
> 6,000 rpm would pull in 521 cu. ft. per minute; that's IF it had 100%
> volumetric efficiency, which it never does without a blower or turbo. I'm
> not sure how much to knock off for a typical or even hot rodded engine, but
> somewhere between 80% and 90% I would think. For 85% efficiency, that's 443
> cu. ft. per minute. I think these numbers just support your contention that
> the filter per se isn't the problem. It's probably just this line of
> thinking that led to velocity stacks and the like.
>
> Brgds,
>
> Bob
>
> At 10:10 PM 4/25/00 -0600, Theo Smit wrote:
> >Hi all (all of you that haven't abandoned this discussion)
> >
> >I was reading the K&N website info some more and they mention that they
> >compute the rated flow capability of their filters based on 1.5" H2O
> >pressure drop. What does that mean in the big scheme of things? If you
> >run the filter at double the rated flow (like Larry), you will get four
> >times the pressure drop - six inches of water. That is equivalent to
> >less than half an inch of mercury, which is about the difference between
> >a cool spring morning and a hot afternoon with a thunderstorm brewing.
> >
> >The bottom line is, the air filter element itself ain't making 20
> >horsepower difference. What would be interesting is to run the engine on
> >a dyno, with only the air cleaner housing in place - top and bottom
> >(using some kind of spacer), no element, and compare that to the same
> >thing with the element. I think the lion's share of the loss will be due
> >to the proximity of the air cleaner top to the carb throat.
> >
> >Regards,
> >Theo
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