Vacuum = Good but Vacuum = Bad?
 

I've heard all my life that driving with as much manifold vacuum as possible is the best for mileage. I've tuned older cars and their instructions say to tune idle speed and mixture to highest indicated vacuum within an RPM range. It makes sense with a carb that more manifold vacuum (less carb throat airflow) would draw less fuel from the bowls.

BUT

Now I've started reading about pumping losses and throttle losses and I understand the benefit of EGR but it seems counter intuitive that high manifold vacuum (small throttle opening) would yield better mileage because the engine is working against the throttle.

SO

A high reading on my vacuum gauge means my engine isn't under a heavy load BUT it also means my engine is working burning gas to create that vacuum. So what's better, more manifold vacuum or less pumping loss?

A typical 4 cylinder engine can waste up to 2hp just on throttling losses.
2hp doesn't sound like much, until you realize you need less than 10 to 12 horses to maintain highway speed.

Go for less pumping losses. Get a diesel. No throttle therefor no throttling losses.
Or swap in a lawn mower engine that runs WOT to maintain speed, no throttling losses.

High vacuum, better mileage, worse efficiency.
Low Vacuum, better efficiency, worse mileage.

That's the recipe for pulse and glide, engine on or off. P&G is using your vehicles mass to store energy. 10-20 MPG on the pulse, 150-infinity mpg on the glide, infinity when the engine is off.

Compression is the secret to efficiency. When you restrict the air flow into the cylinders by creating manifold vacuum, then your real compression is based on the pressure available to the cylinders. Your engine could create 10 atmospheres of compression, but with manifold vacuum of 50% of atmospheric pressure, your compression is half as much or about the same as cars 100 years ago.

regards
mech

Vacuum readings are hypermiler feedback. In general you want it high because it means you're demanding less of the engine, burning less fuel.

The comment about the diesel is entirely correct, and of course you also get the added bonus of the higher energy content of the fuel. But that's a big job, swapping in a diesel.

This is where pulse-and-glide comes into play, asking your engine to pull in the efficient but thirsty low vacuum mode, then relying on inertia to maintain speed for a while. But that big box won't coast really well when empty, and the '89 Econoline wasn't offered with a manual, so P&G is a lot more problematic.

Another way to look at it is the difference between efficiently generating energy, and efficiently covering distance.

At low vacuum readings (close to WOT) the pumping losses are low, and you're generating a whole lot of energy--you're either going up a steep hill, or you're accelerating the car to a higher speed. You're burning a bunch of fuel to do it, but you're producing more power for each bit of fuel you burn.

At high vacuum readings (close to idle) the pumping losses are high, but you're not generating much energy--you're either decelerating or idling. You're not producing much power, so you're not burning much fuel, but on a per-power basis you're burning more than in the low-vacuum case.

As has been mentioned, pulse and glide lets you mix and match these so you can take advantage of both.

-soD

Think I get it.

If Idle is 20 then 20 is a great number to see on the highway. If wide open throttle under heavy load is 5 then 5 is a better number to see for 1 minute than to see 10 for 2 minutes. High mileage comes from high vacuum but if you're gonna work it, work it efficiently.

Yeah, actually it was. Every year of the 3rd Gen Econolines was offered with a manual trans. 75-88 or so was either the SMOD or SROD (both typical 3 speed transmissions but with an added OD 4th gear) and 87 or so-91 was the M5OD (5 speed OD transmission). I'd actually prefer the 4spd because the shifter is next to the driver's seat vs the 5spd which has its shifter exit through the engine cover, requiring shifter removal prior to accessing the rear of the engine. The 4spd also has its slave cylinder outside the case and just bolted on, whereas the 5spd has a hydraulic throwout bearing which requires transmission removal to service.

I have an automatic transmission (C6) but there have been two E-vans with manual transmissions on craigslist in my area in the last 3 months or so and both were pretty darn cheap. Thankfully (?) they've both had the older steering columns (pre-84 or so) that make a direct swap from AT to MT for my 89 less than seamless (ignition switch moved from dash to column). If a van with a MT and the correct column comes along cheaply enough I'll probably have a hard time not making the change.

Here's the one currently available in my area: 1977 ford econoline 150 van

Here's the EPA ratings for 1989 Econolines, listed with 3 speed AT (C6), 4 speed AT (AOD [E150] or E4OD [E250 & E350]), and 5 speed MT (M5OD): http://www.fueleconomy.gov/feg/bymod...conoline.shtml

If I swapped to the MT and drove normally I should expect an improvement of roughly 1 mpg in mixed driving. I have no doubt I could make that 4 mpg or so very very easily. BUT would it be worth it?

10,000 miles at 14 mpg = 714.29 gallons = $1,613.58
10,000 miles at 15 mpg = 666.67 gallons = $1,506.00
10,000 miles at 18 mpg = 555.56 gallons = $1,255.01

Fuel assumed constant at $2.259/gal. I don't think there's anyway in hell I could swap for less than $400. I'd put in a new clutch and throwout bearing, have the flywheel resurfaced, and put in new ujoints. That's most of the budget right there. I'd also need a different starter. I think buying a van with MT, swapping both trannies and reselling the donor with my C6 in it could come close but still, sketchy. I do plan on owning the van long enough to put on many more than 10,000 miles but I doubt it'll be driven much more than that amount in any given year.

That's news to me. According to Wikipedia the manual option was dropped in '88. Of course that's Wikipedia, so we know that info is 100% reliable.

And that's academic in any case, since yours has the beefy auto.

Idle is typically 14 to 15 inches of Mercury.
Driving on the highway you can see up to 21 inches of mercury.

My friend has a 4 cylinder Tacoma, when he goes up hills he leaves it in top gear as long as possible. Very low vacuum show much better mpg on the scan gauge then shifting it in a lower numbered gear, more RPMs and more manifold vacuum like you tend to get with an auto transmission.
Ideally you would have a 1L engine and something like a 7 or 8 speed manual transmission and put it in the highest gear you can maintain speed and hold the gas on the floor.

Depends on car and engine. Usually lower RPM helps with cruising MPG by reducing friction and pumping losses. However in my case I have noticed I get better mileage avoiding areas between 1000-1700 RPM. my mileage peaks at 1900 rpm or 45mph in top gear and drops off going higher or lower. So sometimes when I'm accelerating I try to avoid top gear unless it will drop me at or above 1900 rpm or unless I'm ready to cruise. For pulse and glide I also avoid engine speed lower than 1800-1900. I find some vacuum to avoid enrichment and moderate engine speed (1.9k-2.5k rpm) to be best for efficient acceleration or p&g. I just get up to speed faster due to much higher available horsepower at or above 2k rpm and I can drop to neutral and coast for much longer sooner where it really helps my trip mileage.