Rpm

[frosty2]

Hi everyone just looking for an opinion. My 72T is coupled with a 911/01 transmission
And this past weekend was out for a fall trip with some friends. We put approx. 400 miles on our cares with some nice fall weathers.
Anyway on our way back home we went about an hour on the 401 highway at 60 to 65
Miles an hour. I know that isn’t very fast but at 60 miles an hour I’m doing 3200 rpm.
My question is, is that safe to maintain that rpm for that long in an old engine.

Thanks
Mike

[kent olsen]

I have a 72T also. I'm a performance nut so I'm on my 3rd engine, now a twin plug 3.0L. I have 246,000 miles on mine. My experience has been, driving it is better than not and these engines are so reliable you won't hurt them driving up to the redline frequently. I took the 2.4L out after 6 months when the valve guides started to leak and a friend wanted my engine for his VW bus. He traded me his 2.7L which I drove 105 miles to/from work all year long in Oregon. No problems.

[GH85Carrera]

Yep, no problem. It does use more gas than ideal, but so what. The engine was designed to run at 6,400 RPM continuously if necessary.

For my 85 911 I changed 5th gear to a "taller" ratio. I drive lots of multi thousand mile road trips. I used to run a 3,400 RPM a lot for 12 hour days.

When I had my transmission open for other work, I ended up swapping in the 5th gear from a 1973 915 that is in my opinion perfect for highway use. Now at 65-75, the legal speed limit in most areas I am running 2,5000 to 2,800. Pleanty of power, and my gas mileage did go up.

It is a 29,21 5th gear.

[frosty2]

Thanks for the replies guys, Glen I was thinking of the taller 5th gear for highway
driving but wasn’t sure what that would do to me when driving at 50 miles an hour.
Right now at that speed I think I run around 26 to 2800 which seems to be a nice rpm for the engine but was afraid that might change to around 2000 which would be
sluggish for 5th but 50 mph would be a little high for 4th.
I do need to do something with the transmission as 2nd gear is becoming an issue,
I have to baby it when shifting from 1st up and when down shifting from 3rd down
Or it will grind.
Thanks
Mike

[Trackrash]

The 914s had a taller 5th. It is designated ZD. Your trans, if it has original 911 gears, has a ZA. I made that change to my old 911 trans, and is was worthwhile.

[frosty2]

Thanks Gordon.

[r lane]

I bought a 1958 cabriolet the summer before my last year in college. The redline was 4500 rpm. On the interstate between Pensacola Fla and Auburn Ala and back, that is where the tach needle sat and depending on the wind, 90 mph give or take. Never occurred to me that the engine couldn't handle it. For the 3 years that I owned it, it never seemed to mind, in fact I think it liked it.

[GH85Carrera]

Originally Posted by frosty2

Thanks for the replies guys, Glen I was thinking of the taller 5th gear for highway
driving but wasn’t sure what that would do to me when driving at 50 miles an hour.
Right now at that speed I think I run around 26 to 2800 which seems to be a nice rpm for the engine but was afraid that might change to around 2000 which would be
sluggish for 5th but 50 mph would be a little high for 4th.
I do need to do something with the transmission as 2nd gear is becoming an issue,
I have to baby it when shifting from 1st up and when down shifting from 3rd down
Or it will grind.
Thanks
Mike

That is the issue. At 50 MPH, 5th is too tall except on dead flat roads. 4th is perfectly fine to use and I can get to 105 MPH in 4th (only on the track at Charlotte superspeedway) and driving along at 2,800 to 3,000 in 4th is not an issue in 4th. Fortunately the speed limit is usually 65 up to 75 around here on the highway.

On Highway 1 in Florida going down to Key West the speed limit and traffic make it a LONG 3rd gear drive. Likely the furthest I ever drove in third.

If you indeed are driving a lot at 50 MPH, the gear swap may not be for you. It just is not an issue for me. Simple test, next time you are in traffic at 50, try it in 4th gear. It is not a big deal.

I just love the lower RPMs and fuel savings of the tall 5th gear for highway driving. I would love to know what my real world top speed is now, but it is unlikely I will ever find out. On the track at Charlotte I was at 5,200+ RPM in 5th with the stock gear. That was plenty fast enough and the engine screaming, and the wind noise is LOUD. Then seeing the huge banked curve ahead and the instructor saying "whatever you do don't hit the brakes" is a real adrenaline pumping moment. Everyone should try it.