[BADDASSC6]

Let's not get bogged down too far over vaccum. It's effects are insignificant compared to the larger forces acting on the vehicle. If you want to save gas put the car in the highest gear available and run it at the lowest speed that it will operate (preferably smoothly) in that gear.

[94cobra69ss396]

Originally Posted by BADDASSC6

Let's not get bogged down too far over vaccum. It's effects are insignificant compared to the larger forces acting on the vehicle. If you want to save gas put the car in the highest gear available and run it at the lowest speed that it will operate (preferably smoothly) in that gear.

That didn't work for me. I got 27 mpg driving at 60 mph and only 22 mpg driving at 55.

[BADDASSC6]

I'd like to preface what I'm writing with a statement. I'm drunk as shit right now.

O.k. If ou got a five mile per gallon difference by increasing you speed by 5 miles per hour then there is something unique at play with your car. Vaccum is a measure of headloss cause by the butterfly valve as a flow resitriction. In an ideal situation to maximize efficiency wyou would design a system were it would hit peak efficieny at wide open throttle. That's why they sell the barrell valve type throttle bodies. because at WOT they are more efficient. You have a blower on your car. it's possible that it can mask some of the losses at low RPMs. the difference between 55 mph to 60 mph is negligable when you compare 85 mph to 90 mph based on the fact that air resistance is an exponential force.

I'm still super loaded by the way.

[Vettezuki]

Originally Posted by BADDASSC6

I'd like to preface what I'm writing with a statement. I'm drunk as shit right now.

O.k. If ou got a five mile per gallon difference by increasing you speed by 5 miles per hour then there is something unique at play with your car. Vaccum is a measure of headloss cause by the butterfly valve as a flow resitriction. In an ideal situation to maximize efficiency wyou would design a system were it would hit peak efficieny at wide open throttle. That's why they sell the barrell valve type throttle bodies. because at WOT they are more efficient. You have a blower on your car. it's possible that it can mask some of the losses at low RPMs. the difference between 55 mph to 60 mph is negligable when you compare 85 mph to 90 mph based on the fact that air resistance is an exponential force.

I'm still super loaded by the way.

I'd like to preface my response with I'm drunker than most, not as drunk as some (you). Which really doesn't seem to have been such a swell idea considering I'll be on the bike in six hours. Oh well, I digress.

What are you referring to when you say "peak efficiency" in this case?

[BADDASSC6]

I'm no longer drunk, but I am hungover.

If I were going to try to design a vehicle that got maximum fuel effiency. I would pick a resonable speed and I would design the motor to operate at a low constant RPM where it made enough torque to maintain that speed at wide open throttle.

[enkeivette]

Originally Posted by 94cobra69ss396

I'd did some testing today because I had to drive out to L.A. to visit a customer. I filled up at a gas station right before I got on the freeway and set the cruise control for 60. After visiting my customer I stopped and filled up the tank before getting on the freeway to head home. I had gone 67 miles and pulled 27.202 mpg. On the way home I set the cruise control at 55 and filled up when I got off the freeway. At 55 I only pulled 22.218 mpg. This was in my Cobra which has 3.55 gears, 245/45/17 and a T56 with a .72 overdrive.

Originally Posted by BADDASSC6

Let's not get bogged down too far over vaccum. It's effects are insignificant compared to the larger forces acting on the vehicle. If you want to save gas put the car in the highest gear available and run it at the lowest speed that it will operate (preferably smoothly) in that gear.

Define smoothly. That's what I'm trying to do.

Originally Posted by BADDASSC6

I'd like to preface what I'm writing with a statement. I'm drunk as shit right now.

O.k. If ou got a five mile per gallon difference by increasing you speed by 5 miles per hour then there is something unique at play with your car. Vaccum is a measure of headloss cause by the butterfly valve as a flow resitriction. In an ideal situation to maximize efficiency wyou would design a system were it would hit peak efficieny at wide open throttle. That's why they sell the barrell valve type throttle bodies. because at WOT they are more efficient. You have a blower on your car. it's possible that it can mask some of the losses at low RPMs. the difference between 55 mph to 60 mph is negligable when you compare 85 mph to 90 mph based on the fact that air resistance is an exponential force.

I'm still super loaded by the way.

Very true, but you need to read the first few pages. Went over this already.

[enkeivette]

See:

Originally Posted by enkeivette

Sean, yes there will be less pumping loss at WOT. But you're confusing volumetric efficiency with fuel consumption. Obviously gunning the motor is not the best way to save money at the pump. If your Cobra made 30hp, then this would work... or if it were a diesel. With a motor that makes enough power, with more throttle the computer will increase pulse width to prevent the motor from going crazy lean, right?

And what you said about wind resistance is true, yes. The power required to propel the vehicle increases at the cube of the speed while aerodynamic resistance increases at the square. Which is why I specified that this is assuming the engine produces enough power to pull the highest vacuum in OD. And once again, vacuum readings will take this into account.

Not trying to argue with you Sean, I know you're right. Most vehicles will do best at 55mph. This is why I avoided the words best mpg and added so many conditions. Just trying to observe the highest vacuum pulled at the lowest speeds in top gear for different vehicles, assuming my theory is correct (which can neither be confirmed nor denied).

Originally Posted by enkeivette

Wanted to post another clarifying thought about the vacuum theory.

As Sean mentioned, a throttle blade is a restriction. When it comes to efficiency it would be best to be at WOT, this is one of the reasons diesel engines get such good mileage. But, for a gas engine, more throttle equals higher cylinder pressure (due to more air flow) which causes a higher demand for fuel... So although the engine is less efficient with the throttle closed, there is less fuel consumption, and that's primarily what we're concerned with here.

So how do we measure fuel consumption without doing it directly? We look at the vacuum guage, the higher the vacuum reading, the less the throttle is open and therefore the less fuel is being metered with the air. Because the vacuum guage is actually reading the restriction, the negative pressure which relates consequently to lower cylinder pressures and once again, less fuel consumption necessary to match the air being sucked in.

So, if the vacuum guage does not read the highest number possible while cruising, this means that the throttle is open more feeding the engine more air, and yes more fuel, to maintain speed. There is a greater need at lower rpms, (where the engine is making less power) for increased cylinder pressures to maintain speed.

Was that good, did that all make sense? I think I've said most of this before, just trying to be more clear.

[94cobra69ss396]

Sorry, my 6th gear is .62, not .72. Also, I have to go out to the same customer this Thursday so I'm going to do the same test again only I will drive 55 there and 60 back.

[enkeivette]

Originally Posted by 94cobra69ss396

Sorry, my 6th gear is .62, not .72. Also, I have to go out to the same customer this Thursday so I'm going to do the same test again only I will drive 55 there and 60 back.

I'm willing to bet that it will be even worse this time at 55mph. Approaching LA the 5 and the 101 start to incline. I'll bet that 55 going up hill with a cammed motor will bog the piss out of it.

[94cobra69ss396]

Actually, I think it will do better at 55mph next time. The route I drove at 60mph was the 15 South to the 210 West to the 605 South to the 105 West. On the way home at 55mph I went the 105 East to the 605 North to the 210 East to the 15 North.

On Thrusday I will drive to my customer at 55mph and home at 60mph and see what the difference is.