rosco28

Something i find amazing in the 915 transmission is that it can be driven in a high gear even while coming to a roll stop, or speed-bump, and the clutch need never be pressed.

How is it that these cars can be driven in high gears at such a low speed and never even shudder, threatening to stall?

what is the effect on the engine?


Thanks!!

rosco28

old man neri

I thought it was something like '10mph/gear' anything less and you should downshift. At least for the 915. I have heard of a few people having problems getting it out of gear at low speeds.

-matt

Amber Gramps

Just because you can, doesn't mean you should. I try to always stay in as low a gear as I can. Makes the car really fun to drive. I'd rather wake up the next morning to a stiff neck than an empty wallet.

rosco28

matt- i have no problems getting it out of gear at low speeds, in fact even if i'm in 4th and come to a speed bump, i don't push in the clutch and there is still plenty of power to go from 5mph-45mph in no time, leaving her in 4th the whole time. Mind you, i don't do this often, i am just amazed that I can. is it really bad for the car, and why?

Douglas - Yes, driving in the lowest gears possible is the most fun, but it definitely burns much more fuel than coasting in a higher gear where the REVs are lower. In your scenario you'd wake with with an empty wallet too.

KC911

I always keep my RPMs above 3k...it's a sports car!

ps: I can get 27 mpg (at approx. 3.5K rpms) in 5th going 80-90 mph, so imo, if you're that concerned about mpg...ride a bicycle

Keith
'88 CE coupe

Brads911sc

I agree with Keith. If you want gas mileage, buy a civic or corolla... People that worry about whether they can run cheap gas, or drive in a way to get 1-2 mpg better... probably shouldn't be driving a sports car where an engine rebuild is 10k+. Not driving it the way it was intended... will strain the engine and transmission parts.. whether you can feel it or not. For me.. I buy only Chevron Premium... buy only 1/2 tank at a time... and drive the car how it was meant to be driven... my daily driver is an 08 Accord... which actually prefers cheap gas... and still gets 26-28 mpg... Drive your car and don't damage your engine and trans trying to save $2 at the gas pump.

dshepp806

I'm a "3K' kinda' guy, too!!

Best,

Doyle

Mike Murphy

rosco28, to answer your question, the engine can pull from 1000 rpms or even lower mainly due to the sophistication of the engine design to allow for such flexibility. It's not actually the transmission but the engine's flexibility that can allow for this. Assuming you have a 3.2 Carrera, the Digital Motronic computer has the responsibility of trying to keep combustion just right. So as long as you aren't pinging or have too low oil pressure, you should be able to pull at low RPMs without any issues. I'm sure Porsche designed the engine this way on purpose because they wanted a car with the utmost flexibility.

And while most people would agree that revs should be kept much higher, there are plenty of automatic transmission Porsches that are daily driven with relatively low revs due to the standard shift points associated with typical non-spirited street driving. And those haven't been prone to failure any differently than the manually-shifted cars.

Amber Gramps

Rosco, how do I put this gently....The car gets upwards of 25 miles per gallon. My ford gets about 10, my chevy about 15. I could spend $12.00 per gallon on the Porsche and come out ahead of the cost of driving the ford.

drive the kids to san diego in the ford is 200 miles at 10 mpg at $4.50 per gallon or $90.00.

drive the kids to san diego in the Porsche is 200 miles at 25 mpg at (1/3 100 octane @ $8.25 per gallon and 2/3 91 octane at $4.80 per gallon) $5.95 per gallon or $47.60.

It's half price taking the 911 even blending in 100 octane for the fun of it. Rev the crap out of the 911 and it is still half price. Even the insurance is less.

Mike Murphy

I only got 25mpg on my car when I drove 70mph or less on the highway. I was getting no better than 17 after I had the car for a little while.

irobertson

To answer your original question as to why the car wil deal with these low speeds in highergears... I expect it is because of the electronic management of the fuel air mixture.
I wouldn't think a carburetted engine would be so smooth.

On the gearing side of things, actually, I find its just the opposite.
I have to make sure I downshift or my tranny (G50) complains.
It doesn't like to receive torque under 1500 RPM.
When doing a slow right turn on the street I'm just at the edge of having to downshift into 1st.

Is it bad for the engine?
Notice your oil pressure the next time you are heated up and turning those low revs.
It is easy for the engine to spin up from idle and get pressure if it is free wheeling.
It's another matter to be expecting it to move a car with low oil pressure in your bearings.

Another note, fuel economy is not directly related to RPM.
It is also affected by the efficiency of the engine.
For example, my diesel Jetta achieves peak efficiency at 2700 RPM so that is where it is geared to run when driving at highway speeds.
Which burns more fuel?
50% throttle at 1500 RPM or 20% throttle at 3000 RPM?

rosco28

Great feedback!!

First of all, Douglas, Keith, and Brad, I was never concerned about fuel consumption, i was just stating that driving in high revs must cost more money on fuel...

Murphey - i have a '79sc.

In posting this I was more trying to get an idea if driving in a higher gear with lower revs is good or bad or indifferent to the engine.

rosco28

KC911

IMO, it's lugging the engine (even though my car has the low end torque to do it), and I don't. These are not big American V8s and are meant to rev imo

Peter Zimmermann

It's not the engine as much as the transmission that you're hurting. At engine revs above 2500 the power is passing smoothly through the transmission, at speeds the whole system was designed to operate at. If you "lug" the car by driving it at low revs (below 2500) you are subjecting the entire system, especially the transmission, to forces that it's not designed to cope with. To try and accelerate from 5 mph in 4th gear is loading the car so heavily, so far outside the engine's designed power range, that it will definitely cause premature wear, by straining every component of the drive train, and related heavy expenditures. Your car is optimal (for daily driving/commuting) between 2500 and 4500 rpm, and you should never drive it, with the intent to accelerate, below 2500.

flatsixnut

I am just amazed that you can drive over a speedbump without stalling out the car.
I have to come to a complete stop and drive over them one wheel at a time on a diagnal.

When I am slowing down I always match revs downshifting. Even 2nd gear will lug the engine at -10 mph.