vaccum and effiency...

[Look Its Aaron]

When driving a stock stang, how much vacuum, should you see at idle?

I just deceided to check my car today and I'm pulling 20" at idle.

To be efficient on road travels what is a good number to stay constant at?

 

[HISSIN50]

20" hg is very good. Basically, the more vacuum you see, the more efficient you are. At WOT, for instance, you will have very little manifold vac.

When I am easy on it, I can stay above 15" hg.

 

[Look Its Aaron]

I am assuming you have a vacuum gauge connected in your car?

So when taking a road trip the more vacuum the more efficient the car is working?

I am assuming this affects MPG as well?

 

[HISSIN50]

I am assuming you have a vacuum gauge connected in your car?

So when taking a road trip the more vacuum the more efficient the car is working?

I am assuming this affects MPG as well?

You assume correctly. Since the 88 GT vert is SD, I like to monitor vac. Manifold vac is inversely proportionate to how far the throttle is open. At idle, the throttle is closed and the vac reading is high. As you step harder into the gas, you have noticed that the signal diminishes.

Yep - the more vac you show on the guage, the more efficient you should be, which translates directly into MPG.

 

[Look Its Aaron]

Thanks

 

[vristang]

JT
I hope you don't think I'm trying to start a fight, but this is contrary to what I learned back in the university.
I thought that high vac meant that the engine had to work harder to pull the air past the TB. This restriction decreases efficiency. There is a point in the throttle which triggers wot via the tps. The efficiency theories I have heard have been to open the throttle (reducing intake restrictions) to just before wot is triggered.
Assuming the same engine rpm, opening the throttle more will create less drag on the engine, and the a/f ratio should be unaffected until wot.

I have been trying to drive this way for the last month or so since installing my vac gauge, but I seem to have another problem which is causing periodic and erratic rich conditions. So I guess I am just going on theory with this one.

Hope you can clear this up for me
jason

 

[HISSIN50]

Jason, I think we are lookin at efficiency vs economy. Does that seem reasonable? You are speaking about engine efficiency, while I am speaking about fuel economy. Now when I read Aaron's post, he did mention efficiency, as you correctly noted. However, I took that to mean fuel efficiency (read: economy, to me).

The fuel metering/programming is often for leanness with light and medium throttle. And for fuel economy, I like to keep the throttle rock steady (think setting the CC).
There are a few things from this light-throttle application and leanness which counteract economy, but all in all, if one uses light throttle (keeping manifold vac high), fuel economy should be better overall than using more aggressive throttle application. Note that I did not say the motor would be more efficient though.

FWIW, Charles Probst quotes this phenomenon (use a vac gauge and keep the reading high for fuel economy) in his book, as I recall.

Whatcha' think? Jason.

 

[vristang]

Yep, Fuel economy does not equal engine efficiency.
That's where I was getting mixed up.
As I re-read now I am not sure why I didn't catch that last night.

jason

 

[TheUser]

so what I got from this is:

Light throttle (high vac)= uses less gas, but slows the velocity of the air because of the throttle body blade being mostly closed, which hinders the engine's efficiency; if the throttle were open more (low vac), it would be more efficient because of the higher velocity of air (less restriction), however, it would use more gas which hinders the fuel economy:SNSign:

 

[Look Its Aaron]

I believe so

 

[Sicarius428]

Have a feeling that mine is more efficient than economy. hehe. 14-15Hg at idle... 10 when driving nice. lol
Kevin

 

[vristang]

Have a feeling that mine is more efficient than economy. hehe. 14-15Hg at idle... 10 when driving nice. lol
Kevin

Holy Gas Mileage!

I get 14-15" at idle, but my economy is 19.5mpg when "driving nice."

What gears are you running?

 

[Sicarius428]

I get about 20mpg with stock gears(3.27? maybe) right now. I got a new set of 3.55s on the shelf but with gas prices I'm scared to drive the 40 mile round trip(mostly freeway) every day. You get 20mpg on a 408!?! Damnit!
Kevin

 

[HISSIN50]

Kevin, as you know, if you really have 3.27's in there now, not only will the change to .55's be almost inperceptable, but wont change economy much.

 

[Sicarius428]

hmm... yeah its really not a big difference. Thanks for the suggestion. I'll check this weekend to see what gears I really have. I think a new tranny is in the works before the rear end though. All in due time...
Kevin

 

[vristang]

I get about 20mpg with stock gears(3.27? maybe) right now. I got a new set of 3.55s on the shelf but with gas prices I'm scared to drive the 40 mile round trip(mostly freeway) every day. You get 20mpg on a 408!?! Damnit!
Kevin

Through them in. If it does hurt your economy it won't be by much. Also, there are many people on this site that will swear that taller gears can actually improve economy, especially in city. I have no experience with that though.

 

[vristang]

Driving home today I decide to throw a monkey wrench at this thread, just to see what happens.

How 'bout if you have a vac gauge and an a/f gauge. I am running one of the autometer flashy light stoich. gauges on the stock O2 sensor. Maybe not very accurate, but it gives me an idea of what is going on.

I have noticed that upon "tip in" (when I just start to depress the throttle), the gauge indicates rich. I assume this is the reason for avoiding quick throttle changes?

I guess the question is: If I can run less than 10" of vac, and still be indicating lean, will this yield better economy over running at 15" and lean? Is there a factor I am not considering?

Thanks for the great thread by the way!
:SNSign:

 

[TheUser]

I guess the question is: If I can run less than 10" of vac, and still be indicating lean, will this yield better economy over running at 15" and lean? Is there a factor I am not considering?

I thought we all agreed that more vac results in more economy; based on that, I'd say the 15hg of vac would net better economy

 

[HISSIN50]

James, LOL.

On topic, as we all know, that NB A/F gauge will not yield proper results. But.............. The puter supplies fuel appropriately for the amount of air entering the motor. So if you are wisking more air into the motor (throttle opened further), though still reading 'lean', you are using more fuel (than if you were very miserly with the throttle application).

This stuff with the vacuum reading is as much a relationship as an absolute. Since the vac reading is higher with less throttle opening, it is an easy way to relate how far open the throttle is open. If we had TPS gauges (showing what our relative voltage is), we would use those instead ('keep the reading under 1.5 volts for best fuel economy').

 

[vristang]

I thought we all agreed that more vac results in more economy; based on that, I'd say the 15hg of vac would net better economy

What I am seeing when driving the car is that rapid throttle changes affect the a/f ratio. A steady/slow throttle allows running at lower than rich a/f levels. The reason (as I understood it) that the higher vac readings provide better economy is that the a/f ratio increases when the vac signal drops (part of the fuel regulator compensation i assume).

My question still stands, If the a/f ratio is not in the rich zone, is it more efficient and economical to run at lower vac levels? This would allow the engine efficiency of an open throttle valve and leaner fuel metering.

Maybe some one who has read the Probst book more than once, can answer this? I haven't gotten back to it yet.
Maybe this weekend?
jason