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Old 07-15-2007, 08:06 PM   #1
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Vacuum gauge instead of Scangauge?

I was talking with my friend today and he said a great way to monitor fuel economy is hooking up a vacuum gauge to the intake manifold. Basically, you train yourself to drive in ways that keep the vacuum maximized and stay away from actions which cause it to drop.

Has anyone tried this and been able to train him/herself?
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Old 07-15-2007, 09:08 PM   #2
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On the old cars, that's what you'd use.

I think a TPS reading is much better, though. For example, on my Saturn, I know that TPS 11 will give me about 65 mpg on flat with no wind, so if I'm getting onto an expressway, I give it TPS 17 or so to get into traffic, and when the speed gets to 65 I back off to TPS 11.

Vacuum sorta tells you that, but I'm not sure how well.
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Old 07-15-2007, 09:33 PM   #3
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my dad had a vacuum gauge hooked up to a 1981 buick centure. i was young and only cared about going fast, so i don't know if it worked. i do know that it would bounce up and down a lot.
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Old 07-15-2007, 09:36 PM   #4
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The vacuum goes low (poor mpg) when you first open the throttle then increases some as the engine picks up speed and sucks more air, so it kinda represents mpg better than just the throttle position, but it has a lot of shortcomings:

1. if you are idling at the light, it says you have high manifold vacuum (getting good mpg), if you are engine off coasting, it will say poor mpg
2. Likewise, it will likely be confused anytime the engine is not directly coupled to the drive wheels.
3. It can only show roughly the improvement in mpg. Actual mpg depends on what gear you are in (and a host of other things).
4. it is instantaneous, so for P&G style it is not as much help.

But it is damn cheap and effective.

But look at the " Similar Threads" section below, and definately check out Coyote Xs ride.
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Old 07-16-2007, 03:10 AM   #5
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That's the way they did it years ago. I was thinking of having a switched lighting system to tell you when you are in a certain area of the gauge and to warn you to back off for mpg.
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Old 07-16-2007, 04:27 AM   #6
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OK, so if I understand those shortcomings it can be a benefit to me.

I am still having difficulty understanding, from a physics equation standpoint, how P&G can be better than constant cruising speed. Unless of course it has to do with "tricking" the PCM somehow, or that people think it helps because they spend some amount of time at a lower speed, or that it makes the Scangauge think you're getting good FE.

My gut instinct would be that constant speed trumps ANY amount of acceleration in terms of the amount of fuel to get from A to B.
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Old 07-16-2007, 04:51 AM   #7
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It's been covered a few times, but the basic reason is that your gasoline engine is not real efficient over a wide rpm range. In general, it is most efficient at lower rpm's (say 2-3 thousand) with a health dose of throttle (over half but not full).


So, if you had just enough engine that it barely kept the the car at 55mph at two thirds throttle at 2200rpm in top gear (I'm just throwing numbers out here), would you see any benefit from P&G? (I appreciate the hypotheticalness of this question and the impracticalities it implies)

Electric motors get a far less return from pulsing and gliding, as do diesel engines. Diesel engines do not have a throttle plate and don't have to maintain a substantial vacuum in the intake system. Gasoline cars at higher RPM and/or with the throttle plate partially closed suffer from more pumping losses.
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Old 07-16-2007, 05:44 AM   #8
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So, if you had just enough engine that it barely kept the the car at 55mph at two thirds throttle at 2200rpm in top gear (I'm just throwing numbers out here), would you see any benefit from P&G?
Nope. IMO.
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Old 07-16-2007, 06:37 AM   #9
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My experience has shown that any gas engine starts to work good at 3000rpm. Doesn't matter if I advance or retard the cam. Same results, they just start to smooth out and pull.
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Old 07-16-2007, 07:50 AM   #10
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Quote:
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OK, so if I understand those shortcomings it can be a benefit to me.

I am still having difficulty understanding, from a physics equation standpoint, how P&G can be better than constant cruising speed. Unless of course it has to do with "tricking" the PCM somehow, or that people think it helps because they spend some amount of time at a lower speed, or that it makes the Scangauge think you're getting good FE.

My gut instinct would be that constant speed trumps ANY amount of acceleration in terms of the amount of fuel to get from A to B.
Personally, I think that one of the biggest things that P&G overcomes is ENGINE DRAG. Engine drag is the drag that is associated with the engine being in gear. If you understand manual shiftthen it will be easier to understand. Imagine going 50mph in 2nd gear in your car. Your car actually CAN go 50 in 2nd gear, but its not a good idea because you are making the engine race and getting VERY poor FE. Furthermore, as soon as you let go of the gas pedal the car will quickly slow itself down because the car does not naturally want to rev at a zillion rpm. The only way you can rev that high is by FORCING the car to rev high by flooring the pedal. A speed of 50mph is more suited for 4th or 5th gear, although 2nd gear can attain it, just not as efficiently.
Well now imagine doing 80mph in 5th gear. Just like a car can go 50mph in second gear, a car can go 80mph in 5th gear. Just not a good idea and with poor FE. Speeds of 80mph are more suited for errrrrr, lets say 6,7, or 8th gear. Hence are problem: I dont know about YOUR car but my 98 Camry doesnt have an 8th gear I can only get 80 mph by really pushing the pedal. As soon as I let go of the gas pedal the car immediately slows down trying to reach a more reasonable and comfortable rpm. By going 80mph I am fighting the cars natural inclination to not race and run at an equlibrium rpm.
SOLUTION: Anytime your car is at a speed where the engine is racing, it is trying to slow down but we fight against the cars tendency to slow itself by pushing the pedal and forcing it to go increase rpm. So once you force a car to go 80mph, just free the car from any gear and let it GLIDE. It no longer wants to slow itself down. The only force working against a gliding car is wind resistance, tires, ect, BUT NOT the engine.
No one will go 50 in a stick shift and downgrade into 2nd gear if they want to slow down to 20 mph unless they want to do it quickly. They will go MUCH farther if they go 50 and just remove the car from gear and let it GRADUALLY slow down. Once the car reaches 20mph, they will re-engage the gear. Well thats exactly what P&G is. You reach a speed that the car will not naturally move at and then let it slowly reach a slower speed. Dude, just drive at 60 to top of a gradual incline and then record speed at bottom of hill. Then do the same experiment BUT put the car in neutral just before you start going downhill. I bet any dollar amount that speed at bottom of the hill in neutral will be HIGHER than when car was in gear. Neutral driving is better for car than in-gear driving. Thats big advantage of P&G. If you are gliding for half of your drive then you are subjected to engine braking for only half your commute. Imagine if we could get rid of aero drag for half our drive. . I know this was long explanation, but I hoped it helped. Any oppossing comments welcome. Im here to share and learn just like anyone else. If Im misleading Jim then I wanna know cause Im also misleading myself. Good luck !!!
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