[enkeivette]

I'm starting this thread to collect results. Drove the SRT4 last night, and I was surprised to find that it pulls highest vacuum at the lowest speed of 65mph in 5th gear, on flat road. Thought it would have been closer to 55 mph seeing as how it's a 4 cylinder. It pulled 10" at 60mph, 12" at 65mph, and 12" at 70mph. Meaning, 65mph would be the most economical speed to cruise at... if my theory about pumping loss is correct, and assuming the engine has enough power to develop highest cruising vacuum in the top gear.

List (To be edited):

Year / Vehicle / Slowest speed at highest vacuum in OD

2003 / SRT4 / 65mph

2006 / Mod Evo (E85) / 65mph

1996 / Mod Cobra (C SC) / 60mph

Feel free to add info and I'll add it to the list. Mods feel free to edit the list. Going to try this in my friends car that has a vacuum guage when we get a chance. His motor/trans setup was NA from the factory and has much higher gearing than the SRT, so I suspect his best will be around 50 - 55mph.

[Vettezuki]

Originally Posted by enkeivette

I'm starting this thread to collect results. Drove the SRT4 last night, and I was surprised to find that it pulls highest vacuum at the lowest speed of 65mph in 5th gear, on flat road. Thought it would have been closer to 55 mph seeing as how it's a 4 cylinder. It pulled 10" at 60mph, 12" at 65mph, and 12" at 70mph. Meaning, 65mph would be the most economical speed to cruise at... if my theory about pumping loss is correct, and assuming the engine has enough power to develop highest cruising vacuum in the top gear.

List (To be edited):

Year / Vehicle / Slowest speed at highest vacuum in OD

2003 / SRT4 / 65mph

Feel free to add info and I'll add it to the list. Mods feel free to edit the list. Going to try this in my friends car that has a vacuum guage when we get a chance. His motor/trans setup was NA from the factory and has much higher gearing than the SRT, so I suspect his best will be around 50 - 55mph.

I love empirical observation. But, uh, don't we need mpgs at the theoretically most effecient speed and compare to some other base point, like 55mph? This also means you need a fixed moderate length test loop and vaguely similar conditions (temprature, humidity). Take it up to this level and I'll participate with my 5 very different cars.

[SeanPlunk]

According to this post from the other thread, the highest vacuum is not the only thing you take into effect when figuring out the best mileage:

"Way back at the top of the post, someone made the statement about the highest vacuum producing the best mileage- that may be how you are advised to drive to get the best mileage, but you are loosing volumetric efficiency on the engine- working too hard just to suck air-

The best mileage comes from having your throttle open very far, usually in a high gear at low RPM's- of course, our vehicles are typically so powerful, that, when you open the throttle, even at 1500 RPM's in high gear, they begin to accelerat- think of like a 1 liter Opel or something similar, running along at 50-60 mph, in overdrive, and you will get the most efficiency holding the throttle open-

Wind resistance, which is the primary thing you are burning fuel to overcome at highway speeds, is logarithmic- if speed doubles, resitance quadruples- things like rolling resistance and the drag of the axles and gearbox pretty much remain constant at a wide range of speeds- This is why small aero mods can make large percent differences in mpg's"

Here is an example from someone in that thread with a C5 Vette (an aerodynamically efficient vehicle).

"My Corvette definitely does not get better gas mileage at 90mph! You can clearly see it on the instant MPG readout. Mine seems to get it's best mileage somewhere around 50-55mph. At that point the instant MPG will read around 34-35MPG if you're on perfectly flat road."

[SeanPlunk]

I think the highest vacuum thing is where the problem is coming in for you. It appears that is not the only thing to take into account when figuring out best MPG. I believe the basic premise of what you're saying isn't entirely correct and a lot of factors come into play.

[SeanPlunk]

That Vette is an A4 with 3.15 gears for the record.

[Vettezuki]

Your points on theory are understood. I think enkei's goal for this thread is observation and attempting to control for one variable across a range of vehicles. I think if it's done properly it could be very interesting.

[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).

[enkeivette]

And for the record, I don't think that there is any C5 or C6 anywhere, that will get the best mileage at 90 mph in OD on a flat road.

As you said, my car is atypical. And the high rpm carbed motor is not smart enough to cruise at a low rpm.

[SeanPlunk]

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, 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).

Fair enough, and right on.

[SeanPlunk]

Originally Posted by enkeivette

And for the record, I don't think that there is any C5 or C6 anywhere, that will get the best mileage at 90 mph in OD on a flat road.

As you said, my car is atypical. And the high rpm carbed motor is not smart enough to cruise at a low rpm.

Your car is one of a kind, I can't wait to see the new paint.