If you look into the innards of the PI and the CV carb, you'll see that the PI
only knows about engine vacuum while the CV carb only knows about the volume of
air ingested.
The PI will give you the same amount of fuel per engine revolution at 10 inches
of vacuum for 1000 RPM and also 10 inches of vacuum at 3000 RPM. If you look at
how modern fuel injectors work, you will see that an engine wants very
different amounts of fuel per engine revolution for those two situations due to
better breathing (volumetric efficiency) at 3000 RPM.
Meanwhile the CV carburetor will try to give you the same amount of fuel for
light load high RPM and heavy load low RPM. The engine wants different mixtures
for these two situations but the differences are usually smaller.
In other words, the PI assumes a dead flat torque curve while the CV carburetor
assumes that the engine has the same torque curve shape with the throttle
closed and throttle open.
Variations in altitude will also be covered better on the CV carb by simply
adjusting the needle while the PI control unit has to have all its concentric
spring perches individually re-calibrated (a guess on my part).
Until better fuel injection came along that could meter fuel based on both
vacuum and RPM (3d fuel map instead of 2d map), the CV carb just had better
all-around drivability at the cost of worse ultimate power.
I would not be surprised if triple Strombergs could actually be better than the
PI that came stock.
ed
Message: 2
Date: Thu, 17 Jan 2013 13:38:35 -0500
From: Ashford Little<70tr6@comcast.net>
To: Timothy Holbrook<tjh173@yahoo.com>
Cc: 6Pack List<6pack@autox.team.net>
Subject: Re: [6pack] TR5 and TR250
Message-ID:<1D261D88-AF06-4CA3-A25C-22AC516DF6C4@comcast.net>
Hmm, which brings up another question to the uninformed (me). How did the PI
system meter fuel? Or why was it the higher altitudes caused issues?
Obviously the air gets thinner, and it didn't compensate, so what did the PI
lack that the carbs had to compensate for altitude?
Ashford Little
70tr6@comcast.net
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