Tall vs short 5th gear: Why does 911 cruise at such high RPMs ?

[sugarwood]

In 5th gear, my 911 only does 65mph at about 3000rpm
As such, it feels like the engine really needs to work hard just for basic highway cruising.

In contrast, my daily driver does 65mph at only 2100rpm.
I need to go 88mph to get to 3000rpm.
Let's say this represents the typical modern car.

Why does the 911 require such high rpms for normal highway cruising?
Does this mean it's geared to have a lower top speed?
Does the 911 have a "short" 5th gear?
Is this why people install a taller 5th gear?

What are the pros and cons of a tall vs short 5th gear?
For racing, do people prefer staying more responsive at high rpms ?

[Jack Stands]

I always assumed it was designed that way to assure proper cooling air flow over the engine and to keep the RPM drop reasonable when shifting from 4th. However, they seem to run just fine at that (and higher) RPMs all day long without any problems.

[pavegeno]

Seems to be a sweet spot as far as still having some push if you floor it but still economical.

When I lived in the US I thought the same thing...5th at 60-80 mph seemed like it was working hard. Since I got to Germany last summer I have spent quite a bit of time cruising in 5th between 5000 and 6000 rpm and it feels smoother at those RPMs and quite happy to be there all day. No sense that it is laboring and the throttle response is great. I've only pushed it to the bottom of the red arc at about 6200 RPM and it just purrs.

Now when I hit the 120 and 100 kph speed limits and she settles back down below 4000 RPM I don't even notice it.

[dipso]

Originally Posted by sugarwood

In 5th gear, my 911 only does 65mph at about 3000rpm
As such, it feels like the engine really needs to work hard just for basic highway cruising.

Are you sure you are not in 3rd? That doesn't sound right to me.
Here's 5th.
120 mph at 5000rpm.

[Oh Haha]

My Euro SC runs at about 3100 RPM at 70 mph in 5th gear.

I don't know what gear ratio is in the gearbox but is stock as far as I know.

This thread will help>How fast will your SC go???

[Bill Verburg]

Originally Posted by sugarwood

In 5th gear, my 911 only does 65mph at about 3000rpm
As such, it feels like the engine really needs to work hard just for basic highway cruising.

In contrast, my daily driver does 65mph at only 2100rpm.
I need to go 88mph to get to 3000rpm.
Let's say this represents the typical modern car.

Why does the 911 require such high rpms for normal highway cruising?
Does this mean it's geared to have a lower top speed?
Does the 911 have a "short" 5th gear?
Is this why people install a taller 5th gear?

What are the pros and cons of a tall vs short 5th gear?
For racing, do people prefer staying more responsive at high rpms ?

Consider the 911s heritage, it's roots are '60s performance and racing cars

Until the US govt started mandating higher fuel economy standards in the late '80s the 911 continued to use the performance oriented gearing that had made it's reputation

here is a summary of the predicted performance w/ a specific tire, different tire will predict different speeds in gear



In the late '80s and then into the 90s the gearing got taller on the street cars giving more relaxed highway cruise rpm, US cars generally had taller gearing than RoW until the later water cooled cars rationalized the specs to be much closer worldwide than previously

[javadog]

Porsche picked the 5th gear ratios to give a high top speed. They were usually pretty well chosen for that. Your daily driver top gear was chosen for other reasons, usually fuel mileage being the most important of those. Few normal sedans can get anywhere close to their redline in top gear and some can't do it in the gear below that, either.

You may actually be traveling a little faster than 65mph at 3000 rpm. There's a gear chart in your owners manual taht would tell you. Your tach and speedo may have a little error and the rear tire size you run can affect the speed as well.

JR

[Matt Monson]

Racers very rarely make their 5th gear taller. Porsche matched top gear very well to the power and aero of the car. As posted in a picture the car can pull very close to redline in 5th. Ask you Camry or Accord to do that.

Also keep in mind we had a 55mph speed limit when you car was built, not the 65-75mph interstate speeds of today.

[javadog]

And, you could still drive some places in Europe as fast as you wanted, not to mention the publicity Porsche got from magazine tests, so matching the gearing to the available power and aero drag was more than an academic exercise to them.

JR

[G50911]

what size tires are you running? on my 68 running my fuchs set up (low profile) i too am winding out. when i switched to my steelie set up (65 series tires and almost 2" taller) my cruising speed is in the pocket.

[safe]

Originally Posted by sugarwood

In 5th gear, my 911 only does 65mph at about 3000rpm

I think that is pretty normal for most modern manual cars. It compares to most daily drivers I've had.

[uwanna]

After I did a 964/3.6 transplant into my '80SC with a 915/70, I got tired of the 3000 RPM at 70mph due to the noise/buzz and thought with the higher HP/Torque I could push a higher gear ratio in 5th. Found a 5th gear set with a tooth count of 32/20 or .63.
I installed the set to see if a .63 5th was usable or ridiculously high. After install, I could cruise 70mph at 2200RPM, 3000 rpm was approx 87mph. The 3.6 seemed to push the car AOK and all was well except the gear set, being after market fabbed, was way too noisy!
I put the original 38/30 .7895 5th gearset back in, and set about to find a stock OEM Porsche TL 29/21 .7241 gearset. I have since found the TL gearset, but have not found the time to install it yet. Several folks on PP have installed the TL gears with 8:31
R/P and seemed quite pleased. With the Gbox Excel gear ratio calculator, the TL gears would seem to give 70mph @ 2600rpm, and 80mph @ 3000rpm. A much more "sane"
upgrade. I think the .63 5th gears were meant for the small block V8 911 transplants!

[Bill Verburg]

Here's a survey of most of the air cooled transmissions w/ oe tire fitments from '72 thru '98, some factory race transmissions and sport transmissions are included


and here are g50/ 5 speeds


and the g50/ 6 speeds


compare the speeds @3k over the years

[jjeffries]

Tall top gears and final drives are one of the nice things about the new cars we're all used to, and are (of course) related to the CAFE standards all vehicle MFG's must pursue. Whether that's achieved via 6, 7 or 8 speed automatics or CVT's, the "new normal" is being able to canter down the highway with the rest of traffic at 80 mph while pulling only 1900rpm , yet with easy access to enough power to pass without massive revs, and not having to suffer endless down/up shifts to maintain cruise speeds over slight inclines and declines. This is, I feel, a beautiful thing, but certainly makes a ride in the old sportscar something of a shock, having to readjust to the relatively high revs at even moderate highway speeds. But after a little while at the wheel, we readjust our senses and expectations. I changed the rear end in my Alfa from the stock 4:55 (!) to a 4:10 from a later year. This may have brought 75 mph down from something like 4000 RPM to 3600. It was still welcome, although it blunted the sprinter nature of car's character slightly. John in CT

[wwest]

Originally Posted by sugarwood

In 5th gear, my 911 only does 65mph at about 3000rpm
As such, it feels like the engine really needs to work hard just for basic highway cruising.

In contrast, my daily driver does 65mph at only 2100rpm.
I need to go 88mph to get to 3000rpm.
Let's say this represents the typical modern car.

Why does the 911 require such high rpms for normal highway cruising?
Does this mean it's geared to have a lower top speed?
Does the 911 have a "short" 5th gear?
Is this why people install a taller 5th gear?

What are the pros and cons of a tall vs short 5th gear?
For racing, do people prefer staying more responsive at high rpms ?

Look at the HP/torque curve of your DD vs the 911....

The 911 needs ~3000 RPM to generate enough HP for that road speed.

My V6 RX300 only needs 2300 RPM.

[Cherry Bomb]

AIRCOOLED.

The VW Beetle cruised 65mph @ 3250RPM for proper cooling fan CFM at that speed. Any faster and the fan cavitates, resulting in less cooling to the engine. On the flip side, Aircooled cars will suffer with less fan speed when pushed to max "cruising" speed. (as designed) Of course the Porsche is more refined mechanically, but it does happen. Lowering the RPM on the highway, like your Honduh DD, will surely allow the heat to creep up on your precious H6 engine. Gearing will also affect your transaxle cooling at sustained HW speeds.

Just my $.02

[javadog]

Originally Posted by wwest

The 911 needs ~3000 RPM to generate enough HP for that road speed.

No, not even close.

JR

[911pcars]

Originally Posted by Cherry Bomb

AIRCOOLED.

The VW Beetle cruised 65mph @ 3250RPM for proper cooling fan CFM at that speed. Any faster and the fan cavitates, resulting in less cooling to the engine. On the flip side, Aircooled cars will suffer with less fan speed when pushed to max "cruising" speed. (as designed) Of course the Porsche is more refined mechanically, but it does happen. Lowering the RPM on the highway, like your Honduh DD, will surely allow the heat to creep up on your precious H6 engine. Gearing will also affect your transaxle cooling at sustained HW speeds.

Just my $.02

Engine load doesn't necessarily correlate with engine speed. There are other factors that enter into the load picture including altitude, throttle opening, ambient temp., vehicle speed, etc.

Engine temps. can be confirmed by monitoring the oil temperature gauge (and/or cylinder head temp.). IMHO, it would be nice if the engine fan could provide optimum air flow via an automatic temp. or speed-sensitive drive (or fan blades).

Sherwood

[autobonrun]

If you have your owner's manual, at the back, about three pages from the end is a horsepower/torque versus rpm chart. At least for my 1979, 68mph is about 3000 rpm so you're not far off.

The 911 SC horsepower curve is very steep. At three thousand rpm, it's only putting out about 95 horsepower on it's way to 172 hp. I've always found the speed vs rpm figures on this chart very accurate to what I've experienced in the car. Given the accuracy of the instruments I think you're performing as designed.

Looking at the horsepower and torque curves, there's a lot of acceleration left after you hit 3000 rpm. I typically try to stay in this sweet spot anyway.

[wwest]

Originally Posted by Cherry Bomb

AIRCOOLED.

The VW Beetle cruised 65mph @ 3250RPM for proper cooling fan CFM at that speed. Any faster and the fan cavitates, resulting in less cooling to the engine. On the flip side, Aircooled cars will suffer with less fan speed when pushed to max "cruising" speed. (as designed) Of course the Porsche is more refined mechanically, but it does happen. Lowering the RPM on the highway, like your Honduh DD, will surely allow the heat to creep up on your precious H6 engine. Gearing will also affect your transaxle cooling at sustained HW speeds.

Just my $.02

Cavitates, no, stalled "airfoil" maybe, but not likely.