RE below:
You are correct. I am guilty of shooting from the hip on that one! I had
completely overlooked the role good breathing plays in producing
horsepower. This comment of mine mostly likely came from recalling the
days of my youth when one would stick a set of triple carbs on an otherwise
stock V8 and call it a hot rod engine. What they ended up having was no
increase in horsepower and an engine that flooded easily.
It is obvious when looking at the PI intakes - more so when sitting an old
carb setup next to them - what a greater volumne of air can be inhaled and
that is travels through a straight path to the intake port.
I still think the PI could have met the 1969 smog levels with some minor
adjustments. However, it may have had trouble with the 74 and later
standards.
I love learning so much about the PI system, thanks to you and others on
the list.
Dave
San Diego, Ca
1970 TR6 PI RHD CP51649
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On 17 Dec 1997, dstauffa@csc.com wrote:
> After all, its not the PI per se that determines the hp, its the cam and
> compression ratio.
Not true. Not being choked by a venturi, and having essentially six
open pipes at WOT, the PI system has a significantly increased inlet area
(95 cm2 compared to 23 for a 150 CD, 31 for a 175 CD or even 51 for 3 DCOE
40s). This definitely has a significant impact on power output, given a
suitable cam profile.
> The PI can be
> calibrated to deliver a wide range of fuel to the cylinder. It would be
> easy to tone it down to meet US emission standards.
The Lucas PI fuel metering is based on two simple principles:
1. Proportional to RPM (goes without saying for injection systems).
2. Governed by engine vacuum, translated via a fuel curve (the
mechanism for which involves a couple of springs and a cam and
roller, with a number of adjustment points).
The problem with #2 is that engine vacuum is not a very good way of
determining air volume - the relationship between vacuum reading compared
to air pumped per stroke is not constant throughout the RPM range.
This simple approach is good enough if it is max power and good
driveability one is seeking, but probably not good enough to fulfill the
requirements for a de-smoged car.
See also:
http://home.sn.no/home/egilk/t_pi.html
Egil
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