tmak26b

Is it true that you archieve the best gas milage running at peak vacuum?

Grimlock

never heard that before

archieve--hahaha someone dropped out early


im only joking! lol

who told you this?

tmak26b

forgot, a while back someone said something like that. He just said your car is the most efficient when it has the most vacuum

blu_gxl

true my instructor at auto trade school said the same thing but with out an acurate vacume gauge inside the **** pit it's impossible, even then very difficult since you have to watch the gauge all the time. it's not worth worrying about (could get more tech. but that's to long right now)

tmak26b

Well on a turbo car, they canmonitor whats the most efficient RPM to run at. Like at what cruising speed it saves the most gas. I drive 40K a year, every mpg helps

Wankel7

Well, in the airplanes they have a manifold pressure gauge. When you cruise in a N/A aircraft you can set the engine to say 24" of HG. You adjust this by opening or closing the throttle. Don't see why you cant do the same in the car with a boost/vac gauge.

But what is the mystery numbeR?

tmak26b

I have no idea, i see want to see at what rpm my car would archieve the best mpg

Bambam7

I'm sure everyone has seen the computers in cars that tell you your MPG?
Like in Jeep Cherokees, etc.
Also, BMW's and some other European cars have an analog gauge that shows economy- From high to low.
These gauges don't measure fuel flow, they measure manifold vacuum.
The higher the vacuum, the higher the MPG- BUT this it isn't a function of efficiency. Effeciency factors in work applied- and the highest MPG is simply the least throttle that can be applied to maintain a steady speed.
If less vacuum is present, than the thotlle butterflies are open more, therefore more air going through the engine, therefore the MAF signals the ECU to put more fuel in. (or on a carb, venturi effect draws more fuel)

So- more vacuum = better MPG, because of closed thottle plates.

tmak26b

uh huh, interesting. So technially teh slower i rev, the better mpg i get, no exception?

NZConvertible

Nope. An engine's volumetric, thermal and mechanical efficiencies all vary with engine speed. There is a particular engine speed where these combined efficiencies will be maximised. That is the point where the engine will produce peak torque. For a particular road speed, whichever gear puts engine speed closest to peak torque is the most fuel efficient gear to be in.

tmak26b

my peak torque is 4500, does it mean i run to that rpm. See if thats the case, i really would like to see what RPM is the most efficient in 5th gear

Eric89GTU

Is that always true? I find that hard to believe... is there an explanation for that somewhere? 'Cause if my peak torque (theorectically) came at 7000 rpm, that would just burn a -ton- of fuel, wouldn't it?

Bambam7

This is MAX power- where the engine is acheiving maximum volumetric effeciency....
The ratio betwen fuel used and power produced is the highest.

This isn't max fuel economy.

NZConvertible

No. Peak torque is a result of the engine running at its most efficient speed. At that point you're getting the most power/litre of fuel consumed.
This discussion started out without reference to road speed, but now that it's come in, road speed plays a huge part in the equation, because the amount of power required rises to the third power as road speed rises. So 70mph requires 60% more power than 60mph. Have a read of this, it'll explain it better:
http://www.howstuffworks.com/question477.htm

NZConvertible

I was thinking about power produced per litre of fuel, not mpg. Sorry that was a bit misleading, but it is correct. See the link above regarding road speed for maximum mpg.

RETed

If you're talking strictly about fuel consumption, then yes, higher the vacuum, better the fuel economy.&nbsp This is how all those "fuel mileage" gauges tell if you're in "economy" or in "power" mode.&nbsp This means take it to really high revs and lift off the throttle - this is when you'll see the highest vacuum, and because the fuel injectors shut off when you lift off the throttle, you get infinite gas mileage cause the car is still rolling.&nbsp Anyone with a late-model vehicle with those trip mileage digital read-outs can confirm this.

If you're talking about how to get the best mileage from your FC driving around, they key is to NOT engage the secondary fuel injectors and keep the ECU in closed-loop with just the primary fuel injectors.&nbsp For a Turbo II, we're talking about 75mph right under 4,000RPM.&nbsp I think it's a little faster for the NA...



-Ted

tmak26b

i think secondary kicks in at 3800, well i never bring it over 3500 on the highway, but i just want to know whats the exact best speed thats all