Critique my Vac gauge driving

[spacemanspif]

I'd like to summarize the way I understand vacuum gauge driving and open myself up for critique as well so that I don't start to build habits that are not helping my economy.

According to what I have read, its best to accelerate at ~5"Hg. Does this only apply when the goal is a high rate of speed? I'm wondering if it applies to red light driving...for instance, should I accelerate through 1st and 2nd (up to ~30mph) with low vacuum and then start coasting up the next light? Or is it better to accelerate slowly with high vacuum when I know I'm doomed to stop again quickly? My current driving style is high vacuum acceleration

What I am assuming with 5"Hg acceleration is that once the clutch is engaged you increase gas pedal pressure in accordance with keeping the gauge at 5"Hg, correct? Then shift up to the next gear at no higher than ~3000rpm?

When on highways (not doing P&G) its best cruise at the highest vacuum possible. My car is showing idle at 20"Hg, 60mph (2000rpm) at ~16"Hg. What i am surprised at is the 8" drop in vac when climbing even a small hill. Is this behavior when using cruise control normal?

I always thought Delaware was a flat state until I installed my vacuum gauge yesterday. With cruise set to 60mph it seemed like the needle never stopped moving due to all the hills and overpasses. I think I am going to try foot driving in the near future where I accelerate on the downs and bleed off speed on the ups in order to try and keep vacuum at a constant high-ish reading. What would be better...steady 15"Hg or constant swing between 10"Hg and 20"Hg??

TIA,

[Occasionally6]

Understanding what you are trying to do is more important than any specific method. Find a BSFC contour plot - any will do, they're all much the same basic shape - and look for the parts of the island with the highest thermal efficiencies. You are trying to operate the engine, when you have to, in those areas for as much of the time as is possible and not all or very little (= coast) when you can't be in those islands.

If you look at the axes that reference the islands, the rpm range and load range that you want to be in should be clear. You might have to convert the axes from a unit of absolute pressure into inches of Hg if that makes more sense to you.

I do prefer to use kPa on a vacuum gauge though because "0" vacuum is close enough to 100 kPa that I can view the "desired" manifold pressure as a percentage i.e. in the range of 60-80% (~kPa) of maximum.

Throttle position doesn't correspond directly with inlet manifold vacuum. The other variables are the load on the engine and rpm. You have noted that you can hold the throttle and rpm constant while climbing a hill and see the manifold vacuum fall.

That's good because the load is high(er) and towards being in the high efficiency part of the island. If the load gets too high - manifold vacuum gets too low - and you start to fall off the other side of the thermal efficiency peak, back the throttle off (allow your speed to reduce) or change down a gear.

Trying to hold a particular vacuum reading i.e always at the peak of the efficiency island, is not possible without varying your speed so much you will impede other traffic. There's not all that much to be gained over being just close to the peak anyway.

Always (neutral, or better is engine off) coast up to a red, unless you have to brake, in which case, stay in gear and motor the engine. Try to avoid the braking part though.

Even on a highway, constant light load is worse than some variation of pulse and glide. The P & G can be subtle.

[spacemanspif]

Here is the Saturn DOHC map. I just have no idea how to read it , any insights towards understanding it are much appreciated.



Thank you for the advice, Occasionally6. You have given me much to think about...and much more once I figure out what the above pic is telling me

[PaleMelanesian]

Read it like this: at 2500 rpm, best BSFC is from about 60 to 80% of max power. Then roughly translate that to vacuum level for best acceleration. Despite what these charts show, I've found better results keeping to the lower end of the rpm scale, more like 1500-2200 and no higher.

My Scangauge reads vacuum in psi. I like ~12-13 psi MAP for accelerating. 14.x is ambient, so it's nearly but not quite open throttle. Am I right in saying that your gauge reads 30 in Hg = perfect vacuum (below idle) and 0 = ambient = wide open throttle? Assuming so, you'll want to accelerate somewhere in the 5-10 range, and cruise as high as possible (most negative).

You are correct that for cruising you want the most vacuum you can get away with = most closed throttle.

[user removed]

Your bullseye is dead center in the 250 range. As Pale stated stay in the lower half of that range for your acceleration. You are probably close at 2 inches acceleration, or somewhere slightly higher. 80% load should give you around 5 inches (assuming your ambient is 30.

regards
Mech

[spacemanspif]

Thanks gents! I sort of understand it now. You are correct that my gauge reads from 0 - 30"...I wanted a full face gauge because the full swing is more sensitive than the small section that is used on a boost/vac gauge.

So basically the best driving technique is accelerating at 5" (or a little lower) and up-shifting at no more than 2500rpm? Obviously tweaking for nuances of the car/diver as I track MPG over tanks of gas...

Thanks for all the insights so far.

[cbaber]

What I've found is that it's better to accelerate at 70-80% load and then focus on DWL, than try to accelerate gingerly. Since your on a vacuum gauge you have to translate my numbers to the appropriate vacuum equivalent, which Pale already mentioned was ~5-10". Also keep in mind that for any given speed, the higher gear (lower RPM) is more fuel efficient than the lower gear (higher RPM) assuming your staying within the 70-80% load range we discussed. It may seem natural to downshift when your RPM's are around 1500 and your vacuum gauge is really low, but its still more fuel efficient to drive at high load and low RPM than lower load at higher RPM at a given speed. Basically, unless you are having to go WOT, stay in the highest gear possible and keep your shift points low.

If I am city driving I do by best to time lights. If I am accelerating from a red light I need to know my situation up ahead. If I need to stop again, or slow down, I will accelerate 70-80% load and then neutral coast or EOC sometimes. You still want to follow the rule of driving without brakes (DWB), and when you need to accelerate do so at your engines highest efficiency.

On the highway the goal is to use the least amount of power possible to maintain speed. This is load driving. For my car and my commute I plan out the engine load for each hill so that my speed tapers off slowly. If I were to hold my speed it would require a higher load, using more gas. If I taper my speed off from 60 mph to 55 mph in the duration of the hill, I use less gas that way. On the other side of the hill I can make it up using gravity as an aide.

I don't like to slow down too much, though. I've found that if I take off too much speed up a hill, I am having to use more load down the hill to regain speed. Instead I only allow 5 mph to taper off, and then I can quickly get back to up 60 mph and use very little load to go down the hill (or sometimes EOC).

I see massive swings of engine load (or vacuum in your case) when driving over different terrain. Up the steepest grades in my area I have to use 70-70% load just to maintain speed. On downhills I only use 25-35%. Below 25% is idle basically. Flats are 35-45% load. It's so nice to have a Scangauge that can tell me engine load, as well as the MPG I am getting at that load.

[Occasionally6]

Originally Posted by spacemanspif

Here is the Saturn DOHC map. I just have no idea how to read it , any insights towards understanding it are much appreciated.

I assume you realize that it is a 3D map, like a terrain contour map. With the units of BSFC it is a valley/hole, and you want to be near the bottom of the hole at the times when you are operating the engine.

The units used for the BSFC plots may be different to this one. If you converted the BSFC figures into thermal efficiency you would, instead of the hole, have a "mountain" that represented how efficiently the energy contained in the fuel is being converted into mechanical energy. It may be easier to comprehend in that form. You still want to be in the same place on the plot, just near what becomes a peak when plotted with those units.

It is a map of efficiency, not of the quantity of energy being converted, so you do want to operate the engine for as little time as possible, period. It's just that, when you are operating the engine, it should, ideally, be near the peak/hole.

That plot doesn't include lines that represent power output (maybe find another one that does for insight). Those lines would represent the rate of energy conversion. In order to access the higher efficiency load/rpm points you will have to be releasing (and finding a use for) higher power outputs.

That is what you are doing when you accelerate, pulse and glide or climb a hill and coast; you are storing and releasing that energy. Those are quantities of energy. You want to store energy in those forms, energy obtained from the engine while it is operating at high efficiency, and extract it or use it, with minimal loss.

So that's the three concepts required: efficiency, quantity and rate (of energy flows).

Also, that plot uses torque as an indication of load. Engine torque, at a particular rpm, is a reasonably close proxy of manifold pressure but needs some explanation.

If you take the maximum torque value at a particular rpm, and assume that occurs with 0 manifold vacuum, you can use the plot to determine where you want to be in terms of engine load, and hence manifold pressure.

If you feel it would help, maybe replot the data shown with the torque converted to a percentage of the maximum at each rpm, that as an approximation of the percentage of maximum manifold pressure.

Originally Posted by PaleMelanesian

You are correct that for cruising you want the most vacuum you can get away with = most closed throttle.

You still, ideally, want the engine to operate at near peak efficiency. The problem with cruising is there's less opportunity to manage the flows of kinetic (constant speed) and potential energies (on a flat road). That is unless you artificially create the flows with pulse and glide. You don't want to travel too fast either, or you'll lose what you gain in engine efficiency through aero drag.

If you don't want to P&G, then ignore manifold pressure and pick a precise speed-gear-throttle combination based on the lowest instantaneous consumption rate somewhere near the speed you are comfortable travelling at.