Why get a vacuum gauge...

[copternadley]

Originally Posted by livewire516

Why get a vacuum gauge if I already have an mpguino? Is it beneficial or is my money better spent elsewhere?

I suggest you go for AutoMeter. It's cheap and it's durable.

[Red Lion]

I got a Sun Pro from Autozone. $26 with tax. Cheap and easy to install.

[user removed]

Actually when vacuum is lowest the engine is running most efficiently, when throttle position is less than 75%.

In those carbureted engines of yesteryear, there was a point in acceleration where the volume of fuel and the rate of delivery was matched to a certain rate of acceleration.

When Fuel Injection came along efficiency would be improved at different rates of acceleration.

While the highest vacuum reading at steady speeds would indicate best mileage, it does not indicate best efficiency. This is clearly demonstrated by Pulse and Glide strategies, which, when combined with engine off coasting, eliminate all engine operation with any significant manifold vacuum..

Try to understand that vacuum is detrimental to efficiency because it indicates a reduction in the compression of the mixture at the point of combustion. If your vacuum is 50% of atmospheric pressure, say 15 inches, it is the same as WOT at 18,000 feet altitude where atmospheric pressure is only 15 inches. Engine power is significantly lower, and in many cases in aircraft, supercharging or turbocharging is necessary to restore engine power at high altitudes.

Another way to understand it is to consider a dyno test on a 4 cylinder GM push rod, old tech engine.

With a 20 HP load at 1500 RPM the engine burned 1 unit of fuel. Increasing the load to 50 HP only resulted in a 50% increase in fuel consumption.

1 unit = 20 HP
1.5 unit = 50HP

You can see the difference easily. The greater load, which would reduce manifold vacuum and increase the in cylinder compression resulted in;

16.66 HP on .5 units of fuel
Versus
20 HP on 1 unit of fuel

When you consider the accomplishments of dedicated hyper milers like Pale Melanasian and the super high over EPA ratings mileages they achieve, they do so by eliminating any part load high vacuum engine operation. It's either accelerating at 0 vacuum, or engine off at 0 vacuum.

Most important point, although a high vacuum reading indicates better mileage, it does not indicate better engine efficiency. Those who achieve the highest mileage understand this perfectly, or they should.

regards
Mech

[puddleglum]

Is this only true, though, if you have the ability to P&G? In a car with auto that cannot EOC, wouldn't it be better to keep vacuum high for better mileage at a constant speed, since the extra fuel used on the pulse can't be made up for with a glide, or am I not understanding this concept correctly?

[kennybobby]

i'm driving a big ass 6160 lbs Ford van with AT. For years i got 13.5 mpg using cruise control and slow acceleration. i have added a vacuum gauge and use P&G, DWL, etc. and brought the mileage up to 17.

This stuff is very counter-intuitive--it takes a real mental effort to get past the commonly-accepted folk lore of driving and vacuum gauges, such as that the vacuum must always be running high (closed throttle) to get the best fuel economy. It may be true at cruise, but it's doesn't work that way all the time.

On hills and during pulse the vacuum gauge lets me keep the maximum open throttle before reaching the point that the tranny wants to kick down to a lower gear.

[D.O.G.]

I don't have an MPGuino for either of my cars (yet), but I do have vacuum gauges fitted to both and will be keeping those vacuum gauges when I do get MPGuinos. I think the two should complement each other nicely.

Neither car is OBD2, so I can't use a scangauge or similar to get engine load % readings for the pulse of P&G.
Taking full vacuum (idle) as no load and zero vacuum as full load, I can guestimate the 80% engine load that's usually recommended for pulses.
I treat pulses the same for both auto and manual, it's the glide that's different.

I also use the vacuum gauge readings when climbing hills for DWL.

[Varn]

I have been using a vacuum/boost gauge on my Turbo Diesels. Keeping the vacuum high on a gas or gas turbo engine or the boost low on the TD works for economy.