Can running at 65% or less hurt your engine?

[Capt. Geoffrey Thorpe]

So one question I've had on this that maybe you know more about. It seems that many GM transmissions are programmed to run at really low engine speeds (1000-1500 RPM) with load on them and have been back to the 700R4. Not being much of a lube system guy, I've wondered what they've done towards that not qualifying as lugging.

I haven't done transmission calibration - but in general, low engine speed high manifold pressure is where you find the fuel economy (as you well know). The limits for this thing typically are a result of performance / perceptible roughness - if you push the pedal down you want it to go without too much waiting for a shift - and customers will not tolerate any kind of low frequency input from the engine into the pedals / seat / etc. Before you get to engine damaging loads, the transmission would have already gone to a lower gear.

Computer ignition timing gives you more room up in that corner than the old mechanical / vacuum advance by allowing you to retard all the way back to MBT even at speed / load points you shouldn't oughta be running.

According to the aircraft engine POH it does.... which specifically prohibits Manifold pressures greater then 25" at RPM's of 2000 or below on the IO-540-C4B5. Your modern car has many automatic safety features to prevent you from damaging the engine... your aircraft not so much.

More likely they are concerned about knock triggered by the fixed ignition timing at low speed / high pressure than mechanical damage due to high cylinder pressures caused by the flame advancing fast w.r.t. the increase in volume during expansion.

 

[Ted]

I haven't done transmission calibration - but in general, low engine speed high manifold pressure is where you find the fuel economy (as you well know). The limits for this thing typically are a result of performance / perceptible roughness - if you push the pedal down you want it to go without too much waiting for a shift - and customers will not tolerate any kind of low frequency input from the engine into the pedals / seat / etc. Before you get to engine damaging loads, the transmission would have already gone to a lower gear.

Computer ignition timing gives you more room up in that corner than the old mechanical / vacuum advance by allowing you to retard all the way back to MBT even at speed / load points you shouldn't oughta be running.

Makes sense. For me I use the roughness at very low RPM/high MP as something of my personal "stay out of" zone. I remember when I was first learning about cars, the OWT going around was to never load an engine heavily below 2,000 RPM. Meanwhile, even 700R4s would routinely do so in overdrive. As it's gone on, it's seemed that GM transmissions get lower and lower RPM.

In my 400k or so of driving miles, the only engine damage I've had that was noticeable was my Jag V12 with no rings (happens when you use the gas as an on/off switch) and my 6.5 diesel that threw a rod (just not a good engine design). I could never attribute any engine issues to how I drove, normally shifting manual vehicles in the 2k-2.5k range unless trying to go fast. So I guess I'm doing it right.

Of course, my VR4 is hard on the mains, and I'm thinking it's time to pull the oil pan and change them out. Oil pressure is lower than it should be, but it's not been good since I bought it.

 

[Henning]

Makes sense. For me I use the roughness at very low RPM/high MP as something of my personal "stay out of" zone. I remember when I was first learning about cars, the OWT going around was to never load an engine heavily below 2,000 RPM. Meanwhile, even 700R4s would routinely do so in overdrive. As it's gone on, it's seemed that GM transmissions get lower and lower RPM.

In my 400k or so of driving miles, the only engine damage I've had that was noticeable was my Jag V12 with no rings (happens when you use the gas as an on/off switch) and my 6.5 diesel that threw a rod (just not a good engine design). I could never attribute any engine issues to how I drove, normally shifting manual vehicles in the 2k-2.5k range unless trying to go fast. So I guess I'm doing it right.

Of course, my VR4 is hard on the mains, and I'm thinking it's time to pull the oil pan and change them out. Oil pressure is lower than it should be, but it's not been good since I bought it.

I used to wonder about the old Datsun Z cars with the Mercedes 230 six cylinder based engines that had "0" oil pressure at idle. Seems that was normal and they were all that way from new....

 

[Ted]

I used to wonder about the old Datsun Z cars with the Mercedes 230 six cylinder based engines that had "0" oil pressure at idle. Seems that was normal and they were all that way from new....

At idle you need very little oil. I demonstrated this with my '69 Cadillac Fleetwood (472) that ran out of oil (oil light) at 3 AM some winter near Effingham, IL. Drove at idle to the next gas station, added oil, was fine.

I think the issue is that most pumps have a performance curve where you'll have some correlation between idle oil flow and performance oil flow. On the 6G72 (my Mitsubishi engine) there are often low flow/pressure issues that people don't correct and spin a bearing. Of course, the possibility for that increases with extra power produced, which is part of why I like my power level. High enough to be fun while still comfortable for the engine. But I do need to address the bearings.

 

[bluerooster]

The truth of the matter is that yes....a good test taker can easily pass the written part of the test with no problems even if he doesn't know much in real life. In fact, if the examiner is of the type to pass a student for other than realistic reasons the practical could be done too....

I think most examiners and takers of A&P tests are honest and have some reasonable ability or knowledge of mechanics. I've met quite a few A&P's from schools who should probably be working at walmart but that is likely true of a lot of car mechanics...doctors, teachers etc.....etc... or in fact any job.


I think the consensus is right though, the guy is an idiot. Cars rarely use very much of the available power of their engines and normally don't run at any real power output of more than about 20%. Having worked on race cars for a long time I can say that car engines worked hard wear out quicker than those run on the road by the average joe. Run them easy like we do going back and forth to work and they last forever. Run them like they do at the indy 500 and they last about 650 miles. (we ran them in during pre race practice and sometimes did a few miles prior to the race....)

Frank

Run 'em like the AA fuel guys do, and they last about 1/4 mile.