This gets into the big engine small engine problem. I have a friend with a 2.5L v6 in his Sennica and see really poor MPG readings until he gets into a higher gear with his automatic at about 40mph where as in my 1.5L xB I get really high MPG readings lugging it at idle in 5th gear at 20mph like 70mpg on the level road.

what is 1200 to holy cow is 1600 or so for me

This is correct. I have a vacuum/boost gauge in my car and I can see the vacuum per throttle angle. u can get to 0 vac. without opening the throttle all the way.

on a map of maf the throttle angle isn't used for fuel enrichment unless the throttle body is open x amount in x amount of time. quick acceleration needs more fuel (in theory). if your moving steady its not even going to do anything. rpm's increasing is going to add more fuel. but like said, at 0 vacuum your at a higher efficiency.

I wanted to add you can get to 0 without going into open loop as I've seen it on my vacuum gauge. also how the tps.

a vacuum gauge is great for exactly this cause I know exactly how much throttle to use to get to 0. you can also see lugging, the needle shakes.

By faster, I meant accelleration. What you described is how I drive normally because of how the car is set up. It's very hard to drive it normally. It's lloking promising. I'll see after this tank is done.

Yup, that's what I was talking about too. I don't accelerate any faster than the traffic around me. If you can't control your acceleration enough by shifting because you have too much idle-RPM torque, it won't work for you.

Even if you can, it still may not be the most efficient way to drive your car...it sure is for mine, though.

Isn't lugging an engine damaging to the rod and main bearings?

Lugging will destroy the engine quickly. Modern engines are equipped with knock sensors and can't lug. They run or they stall. At very low RPM with wide throttle openings they may rumble or growl; the extra vibrations may prematurely wear stuff. I avoid letting that happen.

I would think 75% throttle at such a low rpm would just be hammering the bearings constantly. The engine is attempting to spin faster but cannot due to the resistance of moving the car forward so as the combustion happens it's forcing the pistons/rods down but they cannot move so the force would be put on the rod and main bearings.

That's my thoughts on it but I have nothing to back it up.

At such a low RPM, there just isn't much power being made. There's so much more torque at higher RPM that the lower RPM can't even begin to stress that stuff.

Basically, the difference between 50% throttle and 100% throttle at, say, 1000RPM, is merely the energy wasted sucking air through the throttle...it's not making much more power, pulling in much more air, or using much more fuel.

Unfortunately I can't even use that technique....oh well. Thanks for the explanation.

So what you are saying is that if I was to short shift, be driving around from 500-1500 rpm, but flooring it, it would [possibly] help MPG? Without hurting bearings or putting undue stress on anything?

This tank, I am driving (accelerating) at high RPM with low load. I'll see how it turns out. There is likely to be some noise in my figures, cause I can't stop playing (working on) my truck! (Changed from 10-40 to 10-30, changed fuel filter, so lost a bit of gas... etc.)

Next tank, I may be willing to try driving at very low RPMs, but I am worried about hurting (or putting more wear on) something driving so low.

I don't recommend going straight into the 500-1500 range. Gradually go down until you're not comfortable going any lower. For me, that happens when it growls audibly, but for others it may just be a vague feeling of fear. I recommend against making a habit of being at such low RPM that it's growling a lot.

I have seen no good logic to support the common fear that low RPM driving will hurt a modern vehicle. Your 1986 model may not quite qualify as modern...even if it has a knock sensor, its ability to compensate may not be very well developed.

Be careful and if you're not comfortable, abort.

Yeah..... No.

Modern doesn't quite cut it, you're right.

I'll keep that in mind. I may be forced to drive higher than that just because I haven't got enough power down there :) but, even if not that low, I think I will try lower RPMs next tank.

It seems there's always an exception. Who would have thought it would be a Cadillac?

I'm a big fan of GM and I love driving Cadillacs, but they do seem to have an abnormally high percentage of bad Cadillac-only engine designs. :(

That recall says the problem is with low-octane fuel and aggressive driving, which I'm sure to them means high RPM (especially since it's probably only available with an automatic).

Aggressive driving could also mean flooring it from low rpm. Regardless, the recall describes classic pre-ignition knock, which wasn't prevented by the wonders of computer programming in this particular case.

not entirely true. this CAN be used to diagnose what code is stored, giving a rough idea of what sensor or system is being affected. it cannot give a data stream of live and recorded events of all systems.

often times, for example, an O2 code will come up. of course AZ will try to sell you an O2 sensor--not always the right idea.

Ok guys, here are my results (or, half of them).

This last tank, I shifted anywhere from 2.5k to 3000 RPM plus. As you can see in my gaslog, it is a hair higher than the last 2 tanks.

I have not been very scientific about this test, because I have been changing more than one variable at a time. I went and removed the radiator fan (was mechanical, now electrical), made the alternator switchable, and did a bit of EOC. The pump I used to fill up was also doubtable (all of this outlined in the gaslog).

You can draw whatever conclusions you like from this data.

This tank, I have been driving at lower RPMs, and shifting like a diesel. I went and unplugged the WOT switch, and have been using the gas pedal like a switch until I get to cruising speed. My engine seems to do well down as low as I have taken it (idle speed). The only thing I have noticed is a LOT of vibration at the really low RPMs.

Don't accept "a LOT of vibration"...that may not be good for your engine's health and it's certainly an unpleasant way to drive. IMO it's not worth any potential savings (which it may not provide anyway).

I was expecting something like that :D

I am not sure that I have lugged this engine yet. Either I have and don't recognise it, or this engine goes really low without lugging.

With the TPS, when I go to WOT, won't it richen the mix whether or not it has vacuum? I have unplugged the WOT switch.

Edit: according to google, lugging is when the engine is under so much load, that it cannot accelerate (or drops RPM) even at WOT. Looks like I found the lower limit :thumbup:

There's a link about lugging in my sig. Technically it is when you have severe predetonation, such that you hear explosions under your hood.

With the disappearance of manual transmissions, and doubly so with the appearance of knock sensors, its meaning has shifted to be as you describe, or to be when the engine is giving you the vibrations you described, or when it's making a growling sound.

I imagine that it will go rich with the TPS measuring WOT, but then why does it have a WOT switch if it can do that?

I don't know, actually. I wish I had the documentation on my ECU, or could take it apart and tweak some settings.

I would imagine that it stays in closed loop operation (according to the speed sensor and MAP sensor, among everything else)(no, it has no MAF sensor) until the WOT switch (just a little micro switch) is closed, in which case it goes rich.


Would it be odd if I said that sometimes it seems to have more power around 60-75% throttle instead of 100%?

Just remember that those BSFC maps are good for a proper running engine. If you have bad rings poor sealing knocking faulty timing all bets are off. In the case of worn out engine heavy loading is a no no you generate more blowby increase loading on rod bearings and crank etc better to use light acceleration and rev gently if it will rev with light throttle then shift.

Re: Full Throttle at low RPM, or High RPM and low throttle
 

I was once told (30+ yrs) ago by a thermo teacher that in general the best mileage is obtained, all other things held constant, is low RPM & WOT providing you aren't detonating! This only makes sense because an engine is basically an air pump & the less friction getting the air thru the engine (blockage in the carb/air butterfly valve for injected engines) & low RPM for minimum mechanical friction in the engine helps maximize MPG. Think of other things that also help are low restriction air filt (K/N), 4 valves/cyl/bigger valves & low restriction exhaust system. High rpm is generally a destroyer of both mileage & engine life (diesels last 500000 miles because of their slow rpm) even though high rpm is where the HP is & the fun. Remember the HP equation has RPM in the top of the equation! Using these methods I have gotten as high as 71 mpg in my 95 civic VX & 20 in my old 97 powerstroke.

Re: Full Throttle at low RPM, or High RPM and low throttle
 

Hell yeah! definitely! ;)

Re: Full Throttle at low RPM, or High RPM and low throttle
 

Diesels last because they cannot incur pre-ignition(detonation). That coupled with their heavy duty build qualities makes for a very stout, long lasting engine. If they could physically rev higher there's no doubt they would from the factory but the fuel power isn't there on large diesels and the sheer piston speeds they see due to long rod stroke is why they're rpms run so low.

The main killers of engines is pre ignition and oil starvation. Lugging an engine even equipped with knock sensors is silly. Remember; knock sensors work by reading pre ignition and retarding timing to deal with it. That means the engine is still seeing knock which could still damage a bearing.