vaccum road results
 

this is my first post.(had this post in instrumentation but didn't get muich of a response) . don't know why i am interested in this stuff. i had a
toyota previa; it got the same mileage whether i dogged it or flogged it: 21 mpg on the highway.

My current Car is a 95 infiniti g20 with automatic trans. obd1 so i can't use one of the fancy gauges. i ran the tube for the gauge
with the hood release cable, then under the plastic wheel well liner then down and up into the eng. compartment.

the gauge reads 20-0 in driving. i figure that is about right as i am at
5000' and one is to lose an in. vacuum for every 1000' above sea level.

i am getting about 10" at idle in gear (unfortunately i have an automatic
trans.). that increase to about 14" in N.

usually for optimal economy one is to shift up at about 2k rpm. that sure doesn't appear to be the case with this car.

while cruising at 50 mph with the cruise control on flat ground i pull about
10" in overdrive. when i shift the button to go out of overdrive the vacuum
level increases to about 14". This is also true at 65 mph and to a lesser
extent even at 75 mph!

I am informed that the gear is not locked mechanically until one is in overdrive, and that one will always have hydraulic losses in any other gear.
so it seems that overdrive would be the way to go.

so it appears that these engines have less of a load on them when revving
high. I do not want to cruise in 3rd on acct. of engine noise and wear, but
it sure looks to me that in order to save gas one should almost avoid overdrive unless going very fast. I am very surprised.

comments?

My olds only reads 20" at idle. It will drop to 2" on a steep hill and then down shift. It reads 20-21" while coasting and reads about 15" on cold start. If I hit the gas it goes right to 0 and stays there until I let off. I have touched 40 mpg and it has plenty of power, so I assume my readings are normal, so yours should be ok.

I think you have to abandon the idea that more vacuum means better efficiency, it's only true for some special cases, and those have very little to do with actual driving.

I suggest reading the threads about BSFC-charts as a start.

You mentioned that you can't use a Scanguage because of the OBD1, perhaps you could use an MPGuino? They are designed to work with any fuel injected car, including yours.

Do a search for engine efficiency curves, see if you can find one for your vehicle. Low load/high RPM means you are using more gasoline (efficiency is lower). Since you use the same power to travel at the same speed, you want to be toward the center of the efficiency curve. Higher load is ok, since for most cars ~70% load at 2,000 rpm is where peak engine efficiency is. This is why pulse and glide works.
I echo KY's suggestion for using an MPGuino. They work great. Mine isn't spot on, but it gives a real good idea of what is actually happening.
For me, at 45 mph in 5th I get about 10 mpg better than in 4th. You have a different car, so you will have different results, but they should trend the same.

While vacuum might not be true indicator for highest mpg , 14" at idle in N seems way low . It should be in high teens with slightly higher idle speed (something like 17-21"@1500-2000 ).
Those numbers seem low to me for condition you indicate .
Might have retarded timing or vacuum leaks .