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Old 02-27-2009, 06:12 PM   #31 (permalink)
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First of all, I do NOT have a 5.4 liter gas engine. I have a 7.3 liter turbocharged diesel V-8. Yup. 444 cubic inches. The same Interbational T444E engine put in school busses and Class 5 trucks.

Diesels are very different than gas pigs. Diesels almost always operate with more air than stoichiometrically required - "excess air." The engine power output is modulated by regulating the fuel input only. Same air every stroke, but fuel varies. A gas engine with its narrow fuel:air ratio modulates air input (hence fuel input) with the throttle plate.

A normally aspirated engine is inherently a resonant device. The intake/exhaust column has a natural frquency and the valves are tone generators. At the resonant frequency gas passes through the gas flow path at its maximum rate. A turbocharger on a diesel flattens that resonant peak with brute force - the compressor, so air flow varies only proportionately with engine speed. Result: a very flat torque curve. My engine idles at 650 RPM (it will idle, albeit roughly at 500 RPM) and can move the truck nicely at 750 RPM. From some dyno runs (a popular event with diesel truck types) I know that at 800 RPM, my engine make 90% of peak torque. Further with no throttle, a diesel never loses efficincy due to excessive pumping losses.

There really is no "sweet spot" (relative to efficiency) between 800 and 2000 RPM. Above 2000, the engine frictional (mechanical and gas flow) begins to rise and efficiency drops off proportionally, until (with low cetane US diesel) the torque begins dropping off around 3000 RPM.

The flat torque curve and lack of a throttle are the reasons a big displacement diesel can get good fuel economy. For a diesel, the only advantage of a small engine is lighter weight that the vehicle has to tote around. My engine weighs about 1,050 lb. Yup. A half-ton of iron.

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Old 02-27-2009, 09:01 PM   #32 (permalink)
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Originally Posted by some_other_dave View Post
All that said, I still think that the most efficient cruising RPM for any gasoline-powered car out there is "as low as you are comfortable with".
Yup, that's been my experience with every car I've driven with a ScanGauge or factory fuel consumption instrumentation.
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Old 02-27-2009, 10:32 PM   #33 (permalink)
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My xA idles at ~600-650RPM (after warm up, natch).
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Old 02-27-2009, 10:44 PM   #34 (permalink)
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My car is going 50 mph at 2000 rpm. 4 speed auto, in top gear w/locked converter of course.

I'm surprised how short the gearing is in some small cars, like Neil's xA for instance.

As for peak torque, my engine's peak torque is listed as 125 lb/ft at 3750 rpm. I know from experience it takes very aggressive acceleration to reach this rpm. I also know that this engine (single cam 2 liter 4 cylinder) feels pretty responsive at lower rpms compared to some other small cars.

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Old 02-28-2009, 12:31 AM   #35 (permalink)
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yeah diesel are a whole different ball game.
i was gonna say what do u rev out at abou 3.5k

yeah lower is better but under abou 32mph mine sucks. if it is flat i can run 4th at 25 mph, but if it goes bleow 23mph it vibrates.
i guess truck arnt pose to be efficent.
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Old 02-28-2009, 08:14 AM   #36 (permalink)
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You can't just say the engine is going to deliver its best FE at such-and-such RPM. The entire package has to be taken into consideration - how fast the car is going at those revs, what the torque curve is like, etc.

If you've got a ScanGauge, you can do some quick-and-dirty testing and find out for yourself exactly what is the very best possible speed for steady-state mpg, then come back to your friend and tell him that, in your car, Xmph at eXrpm delivers the best fuel efficiency. Then overwhelm him with data to make him go away.

When I get my vacuum gauge installed - I'm looking at it right now! - I'll be able to tell whether my seat o' the pants estimate of 43mph is indeed the perfect happy-happy cruisiing speed for my Toy.

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Old 02-28-2009, 08:16 AM   #37 (permalink)
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Originally Posted by Daox View Post
Check the BSFC charts is all I got to say. They really say it all.
Still looking for one for the Toyota 22R, and another for the Subie EJ25

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Old 02-28-2009, 05:11 PM   #38 (permalink)
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So with a diesel, smaller isn't necessarily better? (Weight not withstanding)... cuz if so, that means I don't necessarily have to look for a utility engine to do a swap with, which was a concern for me.


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