What new 5th gear would be ideal?

[GH85Carrera]

I want to swap my 5th gear to drop my RPM for long trips. Every year I make a road trip and this year it will be back to California.

I have a stock 915 in my 1985 Carrera. Right now with the 15 inch tires I am running 3,500 RPM to maintain 75 or so MPH. I would like to drop it to 2,880 or 2,900 range.

The stock 5th gear now is a (38:30) or .798 ratio. I am think a .724 (29:21) would be about right.

What do you think?

I am not looking for a land speed record just a little lower RPM for the long drives.

[CCM911]

Talk to Joel Reiser in Rochester, NY. He is also one of the tech guys in the PCA. He did my buddies trans, and really knows what he is talking about.

[GH85Carrera]

After some Google time and talking to some experts I am thinking I want a 29.21 which came in the 73-74 911 it is the TL gear. Now I just need to find one for sale.

Anybody have a lead to find one?

[KTL]

My gear chart/spreadsheet says that gear change would gain you approx 430 RPM.

Give Porsche84 (Jerry) a PM and see what he's got in stock. He's got a lot of 915 used gear sets in good condition.

Porsche 915 Transmission Parts - Austin Motorsports LLC

Guard makes a 28:19 (0.679) if you want to drop the rpms to ~2,800 RPM.

915 Gears | Guard Transmission

[uwanna]

Go to Gbox, 915 TRANSAXLE SPECIFICATIONS-GBOX Transmission and in the info box, download
the gear ratio chart. It's an Excel spreadsheet, and you just enter your tire size, gear teeth numbers, RPM, etc and it will create a custom chart. You can play with all kinds of variables. If you have Microsoft Office, you should have Excel spreadsheet.
It's quite entertaining!
I just bought a TL gear set 29:21 (.72), to put behind my 3.6 transplant. With the TL set
3000rpm=80mph.
I previously tried a custom 32:20 (.63) 5th gear set, but it was a little too high geared, 70mph=2250rpm 92mph=3000rpm

Play with the spreadsheet and have fun!
The TL 29:21 gearset is also the 5th gear in the 923(915 type) trans used in the late '70s 912. A TL set is hard to
find, just keep looking and eventually you'll find one. Took me about 6mos.

[darrin]

another thought -- what about simply swapping on slightly taller rear tires before the trip? This could get your 5th gear rpms down to closer to where you want them and would likely be a LOT cheaper than regearing your tranny (and would be easier to reverse)

[KTL]

Tires have to get really big. If I change the tire diameter by two inches in my gear chart, the 80mph RPM only drops by 300. Glen is looking to lose 600 RPM

[GH85Carrera]

The trip to California is for Porsche Parade in Monterrey California. I have not heard the official announcement but I bet they will have a DE at Laguna Seca. I want to drive the track and do the corkscrew and have fun. I don't even care about track times since I am on all season 15 inch tires. I just wanna go play and enjoy the track.

My car is not a track whore at all, but I have been to several DEs. Changing the tires and wheels is not an viable option.

This will be my 12th road trip to Parade. I drive my 911 every year.

Thanks for the links and possible sources. I will call them ASAP.

[pitsqrd]

I had to address the same question recently. I have a 3.6 conversion in my 78 SC and found the stock 5th gear ratio with my 17 inch Kinesis was great for acceleration out of 4th and up to 130+mph, but a little short for cruising on the FWY. I think the stock 28:23 5th for the stock 3Ltr motor in the 78SC is fine, but with a more powerful 3.6, a streetable 3.6 conversion can use a taller 5th gear.

At about 70mph my tach was reading at about 3K and just seemed wanting of another gear, "overdrive." It similar to "forgetting" I have a 6th gear in my 993 and leaving it in 5th while driving on the FWY, 3K+ RPM exhaust note is great for short burst, but slightly annoying for long drives.

My 915 trans is being rebuilt with a 28:22 (rebuilt was required to refresh the 30+ year old trans anyway), which I believe is the stock 6th gear ratio for the 1997 993. Since the 3.6 conversion in my 78SC is very similar to the 3.6 in my 993, I think the 28:22 in the 78 will be reasonably well geared for FWY driving, but since the 78SC is lighter than the 993, I should still get very decent acceleration with the 28:22 5th gear ratio.

For what it's worth, that was the logic in changing to the 28:22 for 5th in my 3.6 conversion.

Thanks
GP

[KTL]

GP,

One thing you need to keep in mind is the final drive (ring & pinion) ratio for the 915 is 8:31 and the G50 993 is 9:31. So the same gear used in either trans is going to have a different road speed for a given rpm. Point being is your SC is going to cruise at a slightly higher rpm than the 993 due to the higher numeric ratio of the 915 final drive.

[Jack Stands]

Though the Parade will be in Monterey, I doubt they will be using Laguna Seca for their driving event. Ever since Chicago Region had a Parade in Appleton, WI many years ago, and ran the autocross was at Road America, they haven't been using race tracks. There may be a regional DE event run around the Parade, but I don't think it will be a Parade activity. Liability concerns always abound.

Is Laguna a 5th gear track? If not, a taller 5th gear would certainly help keep cruising interstate revs down.

[GH85Carrera]

Originally Posted by Jack Stands

Though the Parade will be in Monterey, I doubt they will be using Laguna Seca for their driving event. Ever since Chicago Region had a Parade in Appleton, WI many years ago, and ran the autocross was at Road America, they haven't been using race tracks. There may be a regional DE event run around the Parade, but I don't think it will be a Parade activity. Liability concerns always abound.

Is Laguna a 5th gear track? If not, a taller 5th gear would certainly help keep cruising interstate revs down.

Yea, the DE is always separate from Parade and run on the last day of Parade. I ran at Robeling Road & Charlotte and others after Parade.

[uwanna]

Originally Posted by KTL

GP,

One thing you need to keep in mind is the final drive (ring & pinion) ratio for the 915 is 8:31 and the G50 993 is 9:31. So the same gear used in either trans is going to have a different road speed for a given rpm. Point being is your SC is going to cruise at a slightly higher rpm than the 993 due to the higher numeric ratio of the 915 final drive.

What he said! Going from a 28:23 to a 28:22 in a 915 8:31,will only change your speed at 3000RPM from roughly 70mph to 73mph. Mainly negligable.
A G50 with 9:31 final drive and a 28:22 6th gear will give approx 82mph at 3000rpm.
A 29:21 (.72) 5th for your 915 would be a much better choice. Approx 80mph at 3000rpm.

Get this Excel gear ratio download from Gbox and play with the ratios!
915 TRANSAXLE SPECIFICATIONS-GBOX Transmission

[GH85Carrera]

Just an update.

I am REAL happy with the new 5th gear ratio. The TL is just what I wanted. It drops the RPMs right at 500 RPM. Now at 75 MPH I am at 3,000. For the long trips it is going to be great. If I get stuck in a string of 50 to 55 MPH trafic I will be in 4th but I don't get into that situation very often. Most of the interstate cruising is 70 to 80 and this gear will be great.

A 6 speed transmission is certainly not practial so I think it is the perfect gear for a street 911.

The 5th gear on a 915 is right on the end. It can be changed without opening up the main transmission. Since my transmission was out and just sitting there during my engine rebuild it was the perfect time to swap the gear. It is not a big job at all "while you are in there"

If anyone wants a Stock 1985 Carrera 5th gear I have one for sale!




.

[uwanna]

On your old 5th gear for sale thread, you said this:
"gear change was done while the transmission was on the bench due to a engine rebuild. The engine should be back in the car and all buttoned up soon. Then I will know. I just hope the new gear is not real noisy. The old gear has the little groves to reduce noise and increase oiling. The new gear does not have them since it is from a 1973 transmission."

Just wondering if the new/used gear you put in was noisy or not. Hopefully it's nice and quiet! I have the same gear on my workbench (actually from a 912/915box), ready to install, but waiting for a lot warmer weather. Glad to hear you are pleased with the gear ratio, sure I will be also.

[GH85Carrera]

I have not had a chance to drive it a lot in 5th gear. From what I can tell it is pretty quiet, about like before. While breaking in the engine and seating the rings I try hard to NOT just drive at a steady speed.

I just can't wait to get it broken in and run it red-line a few times