JC in NY

Of course you want to cruise at 65 in sixth gear for the best fuel economy.

Suwipin

I'm pretty sure that I'm at ~3000 rpm on 6th gear when cruising around 70mph. I'll double check

5000-6000rpm are my shift points. If it were my sustained run speeds, I'd definitely get a much much worse fuel economy (which we're getting on the track ).

Dr. No

There is a graph in Paul Frere's Porsche 911 Story showing (to my non-engineer's read) that the 993 engine runs most economically in a several hundred rpm range centering about 3000 rpm. I used to get ridiculously good gas mileage in my 914 cruising at 3000-3400 rpms, even in 5th (which I recall was about 20+ mph over Jimmy Carter's speed limit).

tsjoel

I personally don't care about fuel economy (not politically correct) because I would go buy a hybrid if I did. That being said I am mainly interested in enjoying the fun that my car offers without damaging it. I am hoping that driving for short periods of time (10-20 minutes) in the 4-5k range accomplishes both my goals.

blackcat

I made a critical mistake driving back from visting the in-laws over the holidays:

I shifted into 6th and left it there - engine was about 2800-2900 RPM.... after about 200 miles the CEL came on (!)

I know there's no good guidelines for predicting the onstet of the dreaded CEL syndrome, but I'm convinced leaving the car in 6th for so long at low RPM's definitely accelerated the onset of the CEL... that and the PO put too much oil in shortly before my picking it up.

Pete
96 C2
41K at time of CEL onset... she was in her prime <sob>

jdoc7

I'm just curious, below what RPM do you consider 'lugging the engine?' I typically drive my 95' around town at 3000 RPM's.

Edward

A "lugging" engine is somewhat akin to "stall speed" in aircraft. That is, there is a definite minimum speed a plane can fly at, depending on its design, before it can't maintain lift and just falls out of the sky. Similarly, given whatever engine, there is a minimum RPM an engine can turn, given the load it's currently under, under which the engine will begin to shudder in protest. This is why we have transmissions. Given a downhill at say 40MPH in 6th, the engine is not experiencing enough load to tax it, and thus will cruise happily down the hill. Given a good uphill at 40 in 6th, the engine's load increases dramatically, and RPMs will fall as the engine's torque will not be able to cope with the load. When the RPMs fall to the point that the engine shudders, this is "lugging." I've noticed that when I ride in others' cars, typically point-and-steer econoboxes, many drive this way as they slow for a intersection, turn right, step on the gas, and never drop a gear or two...they THEN realize they should have downshifted when the car shakes (and doesn't go anywhere) as they gas it, THEN they downshift. While we clearly want to avoid this kind of driving, I also personally feel it's prudent to keep the RPMs up high for acceleration, which helps minimize carbon deposits; yet steady crusing on the freeway at 3000 RPM +/- is certainly no problem...as long as one clears the cobwebs on occasion (!) Perhaps an expert can corroborate or correct what I've said here.

Edward

914und993

I think Edward characterized it well. When cruising at constant speed on the freeway, running as low as 2000 rpm doesn't bother me a bit. Obviously, if I'm anticipating the need for some brisk acceleration, I'll have it winding at higher rpm in a lower gear before I hit the go pedal.

When I was flying a Cessna 182 - which had a 6 cylinder air-cooled boxer engine, I was instructed not to run the engine "oversquare". What that meant was at 20 inches of manifold pressure, have on at least 2000 rpm. At 30 inches (wide open throttle normally aspirated engine at sea level), have a least 3000 rpm, etc.

While most of us don't have manifold air pressure gauges in our 993, a similar principle should apply. For the sake of the engine, the wider the throttle opening the more revs you should have on. But while cruising down the highway at part throttle, running at a lower rpm saves wear and tear on your engine and does not contribute to "carbon buildup". I take that only to a point, though. I never run the rpm less than 2000 if there is going to be any open throttle at all, even small amounts of part throttle.

Chip

CP

Pin,

Now I remember, you have a shorter rear diff.

My RUF has a taller rear diff. than OEM to begin with, so 80 MPH in 6th is like 2,600 RPM. Fortunately, the motor has so much torque (435 ft-lbs at 5,000 RPM), so I never lug the engine under any circumstances.

'Problem' with the RUF is that it's so fast, WOT runs to high revs makes me too obnoxious/conspicuous to fellow drivers, and or worse, cops. So I rarely could open her up. I need roads with less co-occupants.

CP

914und993

Pete, I don't think that driving 200 miles in 6th at 2800 rpm caused the conditions that your CEL is complaining about. More likely during your drive certain engine parameters were constant enough that the DME was able to "believe" that a detected abnormality represented a true fault and not just a transient condition.

Edit: And one other thing. I think there is a good chance the car was setting the CEL before you picked it up. The PO might have just had it reset before seling it. It takes a while before the DME can run its tests and set the CEL again.

Chip

Tom W

CP: Pin has a gear change (I think to the /21 gearbox), not just a ring & pinion change.

I typically drive at about 4k rpm in 4th gear (I have different ratios from stock - shorter, so it's about 70 mph) so if I need the power, it's there. When cruising on the highway in light traffic I'll keep it closer to 3k rpm (my 6th is the stock 5th). I shift before 5k rpm until the engine is warm and then shift wherever I need to (typically about 6 when accelerating). At the track I'm usually shifting at 6.5k to 6.8k rpm.

blackcat

Hi Chip

Porsche Uber Alles!

Yeah, I was kind of waiting for the CEL to go off ever since I purchased my C2. The PO had been getting caught in rush hour traffic in the Bay Area - he'd been commuting from East of Sacramento to Cupertino (I can hear the Bay Area folks cringing now ). He wanted a tiptronic for some reason...

Good point about the abnormality - IIRC I had the cruise control set and since I-5 is pretty flat after the grapevine I doubt there was a lot of variation in RPMs... I just purchased a OBD II Diag tool (laptop based) - as soon as I can get the damn com port on my laptop working I'll see what the problem behind the CEL is...

Thanks for the insight - hopefully it's nothing $eriou$...

Pete

CP

Now this raises another question - cold engine shift point. I've read threads talking about 4,500 RPM max, and others talking about 3,500 RPM. I'm playing it safe shifting at 3,500 RPM max. when cold. Am I too conservative?

CP

mooty

cp, pin has custom gear may be even shorter than g50/21 thus his rpm in 6th gear can't be used as ref. i have G50/20 usa spec. it's pretty hard to cruise in 6th, i feel rpm too low and engine too quiet. cruising at 5-6k rpm, you really can't b/c you will be very close to 100mph in 4th or 5th gear. lower gears, i shift around 4500 or 5000rpm. cold engine shift point, my foot is faster than my hand. i can't shift before hitting 4k... i am not so sure about shifting at 3500 rpm. you won't feel much power at all.

CP

John,

I agree with you about the paltry RPM in 6th. The only savior is the BIG torque in my motors (both the RUF and the M5). That makes low RPM cruising doable. I tried that in a Z06, and it felt weak. You should try the RUF and see the difference in torque for yourself.

I do agree with you cold shift at 3,500 RPM is definitely not satisfying. I'll try 4,000 next time.

CP