[6pack] TR5 and TR250

[Ed McGuirk]

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 at comcast.net>
To: Timothy Holbrook<tjh173 at yahoo.com>
Cc: 6Pack List<6pack at autox.team.net>
Subject: Re: [6pack] TR5 and TR250
Message-ID:<1D261D88-AF06-4CA3-A25C-22AC516DF6C4 at comcast.net>
Content-Type: text/plain; charset="us-ascii"

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 at comcast.net


------------------------------