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Old 02-03-2010, 11:43 AM   #103 (permalink)
Aero Wannabe
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Join Date: Dec 2007
Location: NW Colo
Posts: 717

TDi - '04 VW Golf
90 day: 55.78 mpg (US)
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Diesels are different

Hi Piwoslaw,

The biggest difference is that diesels typically do not have a throttle plate that is choking off the air to the engine at all but wide open throttle. Gassers require a fixed ratio of air to fuel so at part throttle, the plate closes and cuts off air. This creates a vacuum that the engine has to fight against at light loads. This is why it can be more efficient to step on the gas and "shift early, shift often" with a gasser and why a "warm air intake" mod helps some cars get better mpg. WAI feeds warmed, less dense air to the engine and the throttle can be opened more to maintain the same highway speeds with less restriction and engine vacuum from the throttle plate.

My buddy has two VWs, one gas and a newer diesel. He can't understand why the old gas car makes heat in the cabin in two blocks and the diesel takes two miles and a hill before it warms up. Diesels are very efficient at idle but they suck in unrestricted volumes of frigid air while adding very little fuel. I have read that diesels can run at 50/1 air to fuel ratios. VW guys (Deezler, Tasdrouille, and myself) on this site use grill blocking to help the engine get up to temperature sooner where diesels are most efficient. Also, newer VW diesels have three additional glow plugs in the coolant stream that stay on after start-up to get the engine up to temp. I also use as tank heater to preheat the coolant. The engine starts and runs better and hopefully the glow-plugs shut off sooner and quit making a high amp draw on the alternator.

I have a Scanguage on my car and can monitor intake air temps. I do a little intercooler grill blocking in frigid temps that I can easily remove before climbing mountain passes, to feed cool air to the intercooler. As for hard acceleration, lots of guys say to drive a TDI hard for best mileage. However, I have gotten my best tanks by driving under light load on the Scanguage after the engine is warmed up. That said, the occasional "Italian tune-up" doesn't seem to destroy tank mpg.
60 mpg hwy highest, 50+mpg lifetime
TDi=fast frugal fun

Originally Posted by freebeard View Post
The power needed to push an object through a fluid increases as the cube of the velocity. Mechanical friction increases as the square, so increasing speed requires progressively more power.

Last edited by COcyclist; 02-03-2010 at 01:40 PM..
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