5th Gear question?

[bigrubberjeep]

On my 915 my stock 5th is ZD/U ( he said it's off the chart tall )

My wrench is sugesting go with SN, what do you think?

[RoninLB]

Originally posted by bigrubberjeep
On my 915 my stock 5th is ZD/U ( he said it's off the chart tall )

My wrench is sugesting go with SN, what do you think?

is it the same R&P as mine ?

[bigrubberjeep]

I have no clue, all I know is the R/P is stock

[rdane]

A stock 915 is 28:23 and is about 145mph at 6400 rpm using a 8:31 r/p.
I have a 26:25 in my short box which is 6 down from stock and about 127mph at 6400 rpm and I think a bit short for most uses. But not many cars will pull me in 5th gear from freeway speeds to my top end either

Take a look at the www.carquip.com gear calculator and see what suits your needs for a 5th gear ratio.

http://www.carquip.com/parts/911/trans/gears/index.asp

[RoninLB]

Originally posted by rdane

A stock 915 is 28:23 and is about 145mph at 6400 rpm using a 8:31 r/p.

that's what i thought.. I love the 8:31 in 5th on the hy. It's rpm perfect at 60mph speed limits in 4th. In 5th on interstates, 90mph is real nice for engine EGT&CHT, and decent fuel milage. Track is another story.

I think the G-50 is noticably taller overall. I wouldn't want taller or lower than the 8:31 for my act.

[Bill Verburg]

'85 US 911 used a 915/73
11:35 3.18
18:32 1.77
23:29 1.26
26:26 1.00
38:30 0.789

w/ an 8:31 3.875 r/p

for street use leave it alone, for track you would probably(depends on home track though) make 3,4, 5 lower and closed together

[bigrubberjeep]

Originally posted by bigrubberjeep
On my 915 my stock 5th is ZD/U ( he said it's off the chart tall )

My wrench is sugesting go with SN, what do you think?

What do the above equal in numbers?

Anyone?

[wevoid]

SN = 28/23 = 1.217 overdrive
ZD = 38/30 = 1.266 overdrive

This is about a 250 rpm difference in these two gears at 6000 rpm, so for street driving the difference will be almost invisible.
The SN will pull slightly harder in 5th albeit to a lower top speed - if you can make the redline in top gear ......unlikely.

Perhaps the factory switched to the 38/30, as the higher tooth count is possibly a little more quiet, as well as taking advantage of the additional torque of the 3.2

SN was used as top gear from the 2.7 liter cars onwards.

Regards

Hayden

[bigrubberjeep]

Originally posted by wevoid
SN = 28/23 = 1.217 overdrive
ZD = 38/30 = 1.266 overdrive

This is about a 250 rpm difference in these two gears at 6000 rpm, so for street driving the difference will be almost invisible.
The SN will pull slightly harder in 5th albeit to a lower top speed - if you can make the redline in top gear ......unlikely.

Perhaps the factory switched to the 38/30, as the higher tooth count is possibly a little more quiet, as well as taking advantage of the additional torque of the 3.2

SN was used as top gear from the 2.7 liter cars onwards.

Regards

Hayden

How many of you concur?

[rdane]

The carquip website I listed has a break down of gear numbers matched to the letter format. How correct it is I don't know.

[wevoid]

This is further granularity to this, but it's probably beyond the original request for information.

The 38/30, is listed as ZD on the Carquip site. The gear sets I have actually had in my hands with 38/30 teeth, from 915 equipped 3.2 liter cars, have not had an alpha designtation stamped in them, just a year marking.

ZD/U is not something I recognize, or an alpha ID that fits the system for Porsche 915 gears.

SN was common from 1974 through until the 3.2 carrera engine was installed.

SM 28/22, 1.272 was used on transmissions that were not USA spec, so you might find one of those in a gray market import, or imported transmission.

Your wrench is giving you good advice, an SN 5th gear is fine for a 3.2 liter car.

Regards

Hayden

[Carrera3.5L]

Originally posted by bigrubberjeep
What do the above equal in numbers?

Anyone?

George,

That is real easy to determine. Simply count the number of teeth on the input and output gears to determine the ratio. If you don't have the gear pair in front of you, have your mechanic do it. Takes all of about 30 seconds.

The tallest factory ratio that I have seen in a 915 is 30:21. Don't know what transaxle it may have come in or if it is even still available.

According to my old notes, 28:23 and 28:22 are the 2 ratios used in production 915's for 5th. The 28:22 is NLA.

Ralph

P.S. - Just for a comparison, my '88 G50/01 has the following ratios (stock):

3.500 (42:12)
2.059 (35:17)
1.409 (31:22)
1.125 (36:32)
0.892 (32:36)
3.444 R&P (9:31)

With a 245/45/16 rear tire (24.68"), 6,500 rpm in 5th is a theoretical 155.36 mph.

I have an excel spreadsheet made up for ratios, I'll gladly make one up for you if you provide the following info: ratios, r&p & tire diameter (size would be fine).

[klaucke]

I was under the impression that the 1974 cars used the tallest 5th gear ever in a 915, taller than the 75- onward. It used a 7:31 r&p so the 2.7 could still pull such a tall gear.

EDIT: I just checked, and 5th for 1974 is a .724 according to "Original Porsche 911" by Peter Morgan.

[bigrubberjeep]

Hayden and Rlaph, with the information you both have provided, would you say both the SN and the ZD are about the same? Any "REAL" difference?

Only thing I noticed (don't laugh) is what I can relate to my R/C racing days, is the pitch of the gears, much finer pitch on the ZD vs. the SN this usually means quieter.

[wevoid]

They are "about" the same.
In real terms, the 28/23 will turn your motor at 3330 rpm to make 80 mph. The 38/30 gear will turn the motor 3200 rpm to make 80 mph. Both numbers for a 225/55/16 tire.

Will you notice the 130 rpm difference at 80 mph?

If you think you will, have your mechanic find you another 38/30 and put it in the transmission, you will be stock for a 3.2 Carrera. The 38/30 is not that rare.

Klaucke - is that for a US delivered 1974?

Regards

Hayden

[bigrubberjeep]

Originally posted by wevoid
They are "about" the same.
In real terms, the 28/23 will turn your motor at 3330 rpm to make 80 mph. The 38/30 gear will turn the motor 3200 rpm to make 80 mph. Both numbers for a 225/55/16 tire.

Will you notice the 130 rpm difference at 80 mph?

If you think you will, have your mechanic find you another 38/30 and put it in the transmission, you will be stock for a 3.2 Carrera. The 38/30 is not that rare.

Klaucke - is that for a US delivered 1974?

Regards

Hayden

There is nothing WRONG with my 5th gear, It does not need to be replaced, he's just sugesting it for the sake of "PULLING" in 5th gear vs. not, he claims what I have is "WAY TOO TALL" for a 3.2

At a cost of course.

thank's again Hayden

[bigrubberjeep]

Originally posted by wevoid
SN = 28/23 = 1.217 overdrive
ZD = 38/30 = 1.266 overdrive


Hayden

My wrench said it was a 38/35, I'm starting to think he's having way too many German beers during lunch.

[wevoid]

As Ralph suggested - have your mechanic count the teeth on the 5th gear pair.
If it's a 38/30 it's probably not worth chanigng to an SN gear.
If ZD/U is a weird super tall ratio, count anyway and post the number, we will all be wiser.
If it is taller than 38/30 a change to 38/30 or SN will be benefitial for pulling in 5th gear, but beware, if 5th is non-stock tall, 4th and 3rd might also be non-standard....

Regards

Hayden

[wevoid]

Well.... a 38/35 is a very nice short top gear, it will pull way harder in 5th that an SN or the 38/30.

Seems like you have some miscommuncation with the mechanic.

Most people wanting to modify a 915 to shorter ratios would be eager to bag a 38/35 for short 5th gear. That would be a very rare 5th gear ratio if it's O.E.

If that is truly your 5th gear ratio, my comment about 3rd and 4th still stands... but to the shorter rather than long side.

Regards

Hayden

[rdane]

PQ 25/26 1.04
QQ 26/26 1.00
QP 26/25 0.96
RP 27/25 0.93
RO 27/24 0.89
SO 28/24 0.86
SN 28/23 0.82 Stock
ZD 38/30 0.79
NM 28/22 0.79
TM 29/22 0.76 Stock '72-'73
ZT 38/29 0.76
TL 29/21 0.72 Stock 915/08
N 40/28 0.70
28/19 0.679
42/25 0.594