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Old 10-16-2010, 10:13 AM   #1 (permalink)
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DWL at 3k+ rpm = higher vacuum?

Just installed my first vacuum gauge and am so amazed after my test drive, I simply must post a question. If I try to stay above 15 pounds of vacuum while DWL (), my truck does a better job maintaining speed or even accelerating in 3rd or 4th gear at 3000 to 4000 rpm rather than 5th gear at a lower RPM. Really??? Ten or more continuous seconds of 3rd gear-4000rpm on the highway is getting me better MPG than 2000 rpm of DWL in 5th?

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Old 10-16-2010, 01:31 PM   #2 (permalink)
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Actually it's inches of vacuum. Sounds like your 5th gear is tall enough for good cruising, but 4th and 3rd are better for acceleration.

Logically speaking 5th will be your most efficient gear.

There are two opposing but valid arguments about accelerating in 5th or the lower gears.

One rationale is get there in much less time using the lower gears then get into your high vacuum 5th gear cruise for max economy.

The second rationale is even though it takes longer to get there in the highest gear you should be using less fuel because even though the consumption is greater the number of revolutions of th engine is lower.

Best rate of acceleration by your vacuum gauge reading should be at about 2 inches. If you gearing is so tall that the acceleration is insufficient, then use the lower gear at 2 inches reading.

My VX had very tall gearing with 3rd gear red line at over 110 MPH. That is about the same as a lot of the performance Hondas in 5th gear, same as 3rd in the VX!

Think like the truckers who have many gears to choose from. If you get below 2 inches of vacuum, you are near WOT enrichment, so the lower gears will most likely get you to your desired speed with less overall fuel consumed.

Bottom line it really depends on the overall ratios of your gearing. Ideally (my opinion) most cars would benefit for gearing that allowed these speeds at max engine RPM.

1st--------30 MPH
2nd--------60 MPH
3rd--------90 MPH
4th--------120 MPH
5th--------150 MPH or more

In my 76 280 Z that was the ideal combination of power and economy. Fast 0-60 times with only one shift, as well as 30+ MPG at speeds averaging 55-60 MPH.

While different size and performance engines would probably necessitate a change in the top speeds in each gear, the relationship should remain the same.

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Old 10-16-2010, 02:03 PM   #3 (permalink)
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Thanks Old Mechanic. I did some errands this morning and find that no matter what gear I am in, if I plan on accelerating and the engine is less than 2200 rpm, I should down shift one or two gears and let the engine scream up to 4000k.

For example, I have always assumed it was most efficient to accelerate with light throttle and shift to maintain a 1200 to 2000 rpm range. When doing this however, the vacuum immediately falls as low as 1 inch. The only way to accelerate and keep it above 1.5 inches is to wind first gear to 3700 rpm then skip to 3rd or 4th gear and maintain speed.

This is the opposite of the old dummy upshift arrow lights; seems completely counter-intuitive. Is this the definition of Power and Glide?
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Old 10-16-2010, 04:03 PM   #4 (permalink)
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No matter what gear you are in the best engine efficiency will be at about 2 inches of vacuum. (1.5 is fine as long as you do not floor the accelerator)

Adjust your rate of acceleration using the gears while maintaining 2 inches of vacuum.

When at a constant speed use the highest gear practical and try to keep the vacuum as low as practical.

Think of it this way. If you want the most power for the least fuel 2 inches of vacuum is a good point to get it. Your engines best fuel consumption for each horsepower produced is probably in the 1500-3000 RPM range. 4000 is probably too high.

The vacuum reading you see on the gauge is the percentage of atmospheric pressure that is going into the cylinders. If it is half of atmospheric pressure (30 atmospheric 15 reading on the gauge) then you are only achieving half of the engines designed compression ratio, or about what a model T Ford got a full throttle.

While this may seem to be counter intuitive, let me give you and example.

A 4 cylinder engine on a dyno producing 20 HP at 1500 RPM uses 1 unit of fuel. Increase the load on that same engine at the same speed to 50 HP and you only use .5 more units of fuel.

The first 20 HP cost you 1 unit. The second 30 of 50 total cost you half as much fuel as the first 20. This uses more fuel per unit of time but less fuel per unit of hp.

1=20 or 20 to 1
1.5=50 or 13.33 per half unit or 26.66 per unit.

That is why you should accelerate briskly to your desired speed. It cost less fuel.
Full throttle is to much. Higher RPMs are to much, and accelerating very gradually does not get you to peak efficiency. You need to find the "just right" balance.

This "balance" is going to be the rate of acceleration of normal traffic moving away from a light (your acceleration environment) that you can obtain using the highest possible gear. If your gear is to high then you cant maintain the rate.

Accomplish this with 2 inches of vacuum when you accelerate and you should get to speed with the least possible amount of fuel.

When you are travelling at a constant speed try to keep the vacuum reading as high as possible and as steady as possible. This will get you the best mileage as long as you are not using pulse a glide to improve your mileage further.

Pulse and glide employs an "all or nothing" principle. Accelerating at peak efficiency, then coasting with the engine either idling or turned off.

Lets say your engine used .25 gallons of fuel per hour idling and you are going 50 MPH in a coast. You are getting 200 MPG. With the engine off you are getting infinite MPG.

These coasting periods at astronomically high MPG can really increase your overall MPG if you have the patience and dedication to employ the tactic.

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Mech
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Old 10-16-2010, 08:48 PM   #5 (permalink)
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Quote:
Originally Posted by 4ringcircus View Post
Thanks Old Mechanic. I did some errands this morning and find that no matter what gear I am in, if I plan on accelerating and the engine is less than 2200 rpm, I should down shift one or two gears and let the engine scream up to 4000k.

For example, I have always assumed it was most efficient to accelerate with light throttle and shift to maintain a 1200 to 2000 rpm range. When doing this however, the vacuum immediately falls as low as 1 inch. The only way to accelerate and keep it above 1.5 inches is to wind first gear to 3700 rpm then skip to 3rd or 4th gear and maintain speed.

This is the opposite of the old dummy upshift arrow lights; seems completely counter-intuitive. Is this the definition of Power and Glide?
I'm assuming you've got the I4? My father has the S-10 in my profile, and it will pull from idle in top gear. Not very fast, but it'll pull it. It seems to stop lugging at any throttle position around 1800 RPM, so that's about where we floor it if we're getting on the highway. Letting the revs fly up is the only way to get better acceleration out of the engine, true, but that is going to hurt fuel economy, no two ways about it.

I am of the opinion that you just do not use a vacuum gauge for acceleration. That it is for steady-state cruise to show you how steady your right foot is.

Pulse and glide involves brief periods of maximum acceleration at the lowest RPM's that won't lug the engine, followed by coasting, often with the engine off throughout the coast.
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Old 10-16-2010, 08:55 PM   #6 (permalink)
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Quote:
Originally Posted by Old Mechanic View Post
When at a constant speed use the highest gear practical and try to keep the vacuum as low as practical.
For constant speed, wouldn't he want to keep it as high as possible?

Quote:
Originally Posted by Old Mechanic View Post
When you are travelling at a constant speed try to keep the vacuum reading as high as possible and as steady as possible. This will get you the best mileage as long as you are not using pulse a glide to improve your mileage further.
Must've been a typo. 4ring, higher vacuum is better, but only in top gear at steady speeds. Steady vacuum is even better than higher vacuum.
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Old 10-16-2010, 10:01 PM   #7 (permalink)
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Right AA it should have been as high as possible at constant speeds. There is some evidence that it may actually be slightly better to do mini P&Gs when there are slight hills.

I experienced that with my CVT insight and it seems to hold true for my CVT Altima. Slight grades seem to make little to no difference in fuel mileage (read from instantaneous gauge) while the same downhill sections will see mileage jump to 60 Max gauge reading).

I did a 40 mile stretch of road today at 55 MPh and got 41 MPG in the Altima.

Definitely try to keep the readings as steady as possible if the terrain is flat, but even in hills of very shallow grade you might try to mini P&G and actually achieve better mileage than on flat ground.

Again very shallow grades probably less than 2%.

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Old 10-17-2010, 09:49 AM   #8 (permalink)
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Thanks everyone. I've reread your comments a few times, and it seems to make sense. I'll drive this tank doing my best to maintain 2.0 on the gauge and post back the results.
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Old 10-22-2010, 09:47 PM   #9 (permalink)
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We'll I am 3/4 of the way into the tank and here is what I discovered: With moderate throttle and higher RMP's the vacuum goes up to 1.5 and higher. However, this really sucked down the gas, and I went through the first quarter tank way too fast.

Doing some more research I found threads on BSFC ~ break specific fuel consumption. If accelerating, it is better to put it in the tallest gear possible and give it no more than 80% throttle (avoid fuel enrichment) and quickly get up to speed. This of course causes the vacuum gauge to plummet to nearly 0 vacuum. Despite the low vacuum, the engine is extremly efficient at converting fuel into speed. And as accelerating is not an efficient manuever to begin with, the best way to maximize FE is to work within the engines highest efficiency. Search BSFC threads in the hypermilling section for more info.

Once up to speed, I begin using the vacuum gauge. I try to keep it between 1.5 and 2.0. Best practice while cruising is to ease off the throttle to let the vacuum nudge up but not so much as to start loosing speed. The gauge makes it very easy to do this, and has really helped me drive more efficiently. My truck is good for around 45 to 55 mph when driving with this amount of vacuum.

Using these techniques, I have earned back all the fuel economy I wasted on the first quarter tank and I might be heading for yet another new mpg record! I'll post my results in my garage.

Thanks everyone.
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Old 10-23-2010, 12:01 PM   #10 (permalink)
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Have you considered buying an Ultragauge? Since your truck is OBDII, you can get complete impg and trip and tank mpg, load, etc. displays, for $60. A vacuum gauge is a poor second choice, by comparison.

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