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

[thecheese429]

Originally Posted by theholycow

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.

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.

[theholycow]

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.

[thecheese429]

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.

[FrugalFloyd]

Originally Posted by theholycow

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.

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

[theholycow]

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).

[FrugalFloyd]

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.

[bowtieguy]

Originally Posted by SentraSE-R

Take it to Autozone and they'll hook up a diagnostic scanner to the OBD-II port for free. That will tell you what the cel codes are, and you can sort out what to repair/replace from that information.

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.

[thecheese429]

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.

[theholycow]

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).

[thecheese429]

I was expecting something like that

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