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Confirmed V6 Nut
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Discussion Starter #1
Right now I have 3.27s with a TracLok, a mildly JModded 4R70W, underdrive pullies, no air silencer, and a K&N panel filter.

I normally don't pull more than 2500 RPM. In town, I'm generally about 1500RPM and 2000 or less on the highway. With the the stock Singleport V6 cam, what should be my optimum RPM range for mileage.

I got the sick feeling I'm running too low in the RPM band for the engine.

What ways can I force the engine to run in the optimum band?
 

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Right now I have 3.27s with a TracLok, a mildly JModded 4R70W, underdrive pullies, no air silencer, and a K&N panel filter.

I normally don't pull more than 2500 RPM. In town, I'm generally about 1500RPM and 2000 or less on the highway. With the the stock Singleport V6 cam, what should be my optimum RPM range for mileage.

I got the sick feeling I'm running too low in the RPM band for the engine.

What ways can I force the engine to run in the optimum band?
Get a simple vacuum gauge and watch what your vacuum does vs rpm vs speed.

You'll want the highest vacuum reading you can get while maintaining your speed.

Shoot, some of the old sytle vacuum gauges even had yellow, green, and red sections painted on them to indicate optimal fuel economy.

 

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The only way you will shift where the car runs in it's optimal range of power output is to change the cam (which will alter the actual RPMs the power is best at) or the rear end (which will change the RPMs the engine is turning for a particular speed).

For reference, when I had my 3.8L I had 3.27 gears and turned about 2200 RPM at 75 MPH, and I got anywhere from 26-32 MPG, usually right around 30 with no city whatsoever, A/C on.
 

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Confirmed V6 Nut
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Discussion Starter #5 (Edited)
Chris,

Your observation is what I'm getting at. If I drive highway at less than 75, usually 65 to 70, I'm only doing at most 2K RPM, but not getting the 26 to 32 MPG. I've gone faster and gotten the 27 to 30 MPG.

With stock programming, the car is usually in the 1500 to 1700 RPM range. Evidently there is a economy band up around 2200 RPM. Looks like I'll either need to speed or get a chip, some gauges, and an 8.8" pumpkin for some 3.55s I've got sitting in the garage.

Chris, got a forcast for the weather in SoCal 14-15th of August? I'll be flying in and driving my daughter out then.
 

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Chris, got a forcast for the weather in SoCal 14-15th of August? I'll be flying in and driving my daughter out then.
Should be hot and dry. Dress for temps in the 90s or 100s, depending on where you'll be.

About MPG, I noticed that too. My mileage did not improve and in most cases got worse under 70. It may be the torque characteristics of the 3.8L below 2200ish RPM.
 

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Confirmed V6 Nut
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Discussion Starter #9
Thanks for the weather reminders. I haven't been in a SoCal summer for over a decade.

Back to my original note, maybe I ought to get a 8.8 out of a Super Coupe and put in 3.55s to run about 2200 at highway speed limit.
 

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Get a simple vacuum gauge and watch what your vacuum does vs rpm vs speed.

You'll want the highest vacuum reading you can get while maintaining your speed.

Shoot, some of the old sytle vacuum gauges even had yellow, green, and red sections painted on them to indicate optimal fuel economy.

Hah, couldn't be farther from the truth. Vacuum = pumping loss

This is why diesels get better economy, they don't have this pumping loss. Ideally, you'd want 0 vacuum at all times. You can't get there with a gasoline engine, but things like variable cam timing let you get a lot closer to it by massively retarding the cams during cruise.
 

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A vacuum gauge was used by the factory to indicate MPG in some old Cadillacs years ago before there were computer controls. It wasn't really very accurate but it was there all the same. It can be used as a rough tool to indicate load as 94 Daily said the higher the load the more the vacuum will drop. You really can't compare this to a diesel engine where the accelerator controls the amount of fuel injected into the engine and has nothing to do with airflow. The thing that contributes the most to modern diesel engines power and economy is the ability to control not only the amount of fuel injected but also when it is injected directly into the combustion chamber.
 

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Uh no, the thing that massively contributes to ANY diesel engine's ability to get decent economy is the total lack of vacuum in the intake tract.
 

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The energy lost due to vacuum in the intake is not the only consideration. There are multiple pumping losses in an engine diesel or gasoline. Pumping loss from the compression stroke. (It takes energy to compress the air [for a diesel] or air/fuel [for gasoline] before combustion happens.) Pumping loss from the exhaust stroke( It takes energy to pump the burnt gasses out of the engine.) A diesel engine actually has more pumping loss during the compression stroke due to the much higher compression ratios of those engines. It makes up for these losses due to the higher compression and the fact that diesel fuel has more stored energy than gasoline. If I remember correctly its in the range of 25% to 35% more energy. I'm going off memory here on the energy % correct me if you will.
All I'm saying is there is a lot more to a diesel engines efficiency than your statement would indicate.
On a gasoline engine vacuum is an indicator of lost power, it does take energy to create the vacuum. I am agreeing with you. It is also a indicator of restricted airflow. In a gasoline engine, if there is less airflow there is less fuelflow. Less fuelflow per mile moved equals more mpg.
 

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Well, I've been playing with variable cam timing on my 3V truck, retarding it quite a lot. I have it as near 0 vacuum as much as I possibly can. This means the throttle plate is wide open, even though my foot is barely on the pedal at cruise. Guess what, it's picked up about 2MPG. Considering it got 12-13 mpg before, that is a ~16% improvement simply by eliminating the manifold vacuum whenever possible.
 
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