Low RPM, lugging, newer engines ?

Deschodt · 04-06-2016, 06:32 AM

I've been driving a lot of older naturally aspirated cars, carbs, early injection etc... Most of those vehicles hate being under 3000 RPMs (try a 911S at 2500 rpm!!), and over the years I've developed a driving style that rarely if ever sees me under say 2500-2800 in normal operation (unless I'm cruising in 6th - then again most oldies don't have a 6th gear)

In the last few years my exposure to modern automatics increased, first an Audi S4 with the twin clutch DSG and recently a loaner Mini cooper auto (3 cyl but not a bad car!)... Both those things go to top gear and very low RPM any chance they get. BUT they also seem to stay there and ride their turbo (or supercharger) from that very low RPM instead of dropping several gears... I'm talking 1500 RPM here!!! Seems odd to get boost under 2000 and goes against my mechanical sympathy. The mini especially, it sounds like a diesel tractor until the brains tell it to drop 3 gears and go (which it'll only do if you nail it, other wise it pulls on the turbo from 1500 rpm - ugh, I feel like I'm lugging the car)...

I imagine the manufacturers and the gearbox computer know what's best and would downshift if it deemed necessary, which often it does not... Generally speaking I was wondering if modern engines - especially turbos - are somehow more impervious to this low RPM pull stuff (before the tranny decides to down shift 3 gears) I understand we're having carbon buildups and other issues on modern cars that are not revved high enough...

Por_sha911 · Por_sha911's Garage

Programmed to meet CAFE requirements and not for what's best for the motor. Heck, manufacturers don't mind if the turbo goes bad after 50k its a repair for them to sell. If motor blows at 75k, that's just another new car sale waiting to happen.

berettafan · 04-06-2016, 07:16 AM

CAFE standards/EPA **** all over the drivability of my Suburban. damn thing will hang around 1100 rpm on the road. that is bull****. will be getting a program once i'm done with warranty.

Otter74 · 04-06-2016, 07:58 AM

The point about CAFE compliance is a valid one, but modern engine-management systems just work a lot better than old injections setups, never mind carbs. My newest non-automatic car is from 1993, but it will happily pull from about 1000rpm in 2nd gear, or 1500 in 3rd. Even my L-Jet Fiat is really tractable at low rpm, though the turbo surely helps that.

Deschodt · 09:05 AM

That's what I mean... Seems like the car is always going for min RPM and reduced fuel consumption but in doing so is putting a strain on the engine or turbo.... That how it "feels" to me (lugging or straining while it decides whether to boost or downshift), but maybe my perception is wrong and outdated and it's perfectly safe with modern cars ? I know if I drove my 911 that way I'd kill it in a couple days ! The modern auto-box cars' I've had seem OK for whatever miles I've put on them (S4 was sold, the mini was just a loaner, I'm all stickshift) - but then again I'm hearing of issues that come from cars that were not driven hard enough (coking up, mostly DFI cars) and I'm wondering if that's not a direct result of the above...

wdfifteen · wdfifteen's Garage

I know what you mean about going against your mechanical instincts. It just doesn't seem right. But I'm guessing they've designed the whole engine to be operated this way and it's alright. I wonder what bearing loads, oil pressure, and oil volume are a 1000 rpm and full throttle. I'm not close to the industry but I haven't heard of a lot of bottom ends burning out from lugging newer cars.
OTOH my 1947 Dodge sounds like the pistons are going to come right out through the head at 3000 rpm. It will pull from 1000 RPM in 3rd gear, no problem, but the oil pressure in negligible at that speed. My old school sensibilities tell me to downshift.

rwest · rwest's Garage

If you want higher rpms, just push in the farthest pedal on the left and move the lever between the seats into a lower gear.

berettafan · 04-06-2016, 01:45 PM

my opinion is the programming to run at low rpm/shift points is a band aid intended to delay the costs associated with actually designing a more efficient engine.

DanielDudley · 04-06-2016, 01:51 PM

My GMC pushes about 40 PSI oil pressure at 1500 RPMs. I don't think it is much below 38 at 1100.

oldE · 04-06-2016, 02:05 PM

I see a couple of notes of concern for turbos when the engine is run at low revs. Why in the world would a turbo be bothered by lower RPMs?
If the oil supply is good, one of the benefits of new fuel injection systems is little or no wasted fuel going out the exhaust port.
If you've got a roller crank or an S2000, you will indeed have to spin it up, but it is easy to make engines last at low RPM.
I realize we're talking apples and oranges here, but my TDi almost never is run over 3000RPM. Torque peak is 1900. A lot of newer direct injection gasoline engines run the same way.

Best
Les

Por_sha911 · Por_sha911's Garage

Are turbos durable enough to be running 50-75% of the time and still be working properly at 100k?

masraum · 04-06-2016, 05:31 PM

Automotive engines are usually designed to run reasonably well through a given RPM range. For most cars, I think that's 1500-4000, some higher, some, a bit lower. A lot of that is based on various compromises and intended use. For higher performance engines, you get bigger valves, bigger cams, higher compression, and more RPMs for more pedestrian stuff, it's all smaller and lower.

Back in the day with carbs and distributors, you kind of had a limited range of what you could do, especially with the ignition timing. Now with computers, they can control the ignition timing not in a fixed way based solely on RPM, but also based on load and slews of other factors. I think that's a big reason why modern motors can perform better in a wider range of conditions and RPMs. Computers can now dial in any timing they want (within the programming) to maximize power and combustion for conditions. Computers and fuel injection have also been able to work their magic in the same way. Then take into account that many modern motors have some sort of variable valve timing and ways to make the intake tracts longer or shorter or any number of other changes that basically give you two motors in one. One set of parameters can allow the motor to be a low RPM performer and then another set of parameters can maximize the motor to run at higher RPMs.

Also, think about how a lot of modern turbo cars have small, quick to spool, efficient turbos bolted to motors with relatively high compression engines compared to back in the day when you had huge slow to spool turbos bolted to very low compression engines.

That makes a big difference too.

And if the car isn't a turbo, but is supercharged, well, those start making power right away.

wdfifteen · wdfifteen's Garage

Quote:

Originally Posted by Por_sha911

Are turbos durable enough to be running 50-75% of the time and still be working properly at 100k?

Can you think of a reason why not? I can't.

john70t · 07:45 PM

Quote:

Originally Posted by Por_sha911

Are turbos durable enough to be running 50-75% of the time and still be working properly at 100k?

Bump up the oil supply.
Add a fan.
And a cool-collar.

(But seriously, though, why isn't this being done already? It is temporary localized heat concentration on thin components. The Achilles heel.)

Bill Douglas · 04-06-2016, 06:53 PM

Modern engines VVT, variable valve timing, allows the car to run like a slow reving tractor when it wants to. Then advances the cam for high revs. So you can get (supposed) OK power and fuel economy at low revs and go flat out with high revs. I'm guessing the oil pumps are made to work through the various rev ranges.