Do I want a 5 speed?

c0metdot · c0metdot's Garage

3800 rpm @ 65 mph. Wow, what a noise fest. What would a fifth gear give me?

tb912 · 08-09-2009, 03:32 PM

As far as I'm told, 4th on a 4spd is the same ratio as 5th on a 5spd. I don't actually think it would make a difference.

I may be wrong..

ossiblue · 05:57 PM

tb912 is correct for standard 4 and 5 speeds in early cars like yours--the top gear in both is the same (7:31). What size tires are you running?

edit: Sorry, I mean the ring and pinion is the same (7:31). There are some slight ratio differences in the 4th and 5th gears of some early cars but without knowing what tranny you have, anything is a guess. Try this site:
http://www.roadglue.com/wiki/index.php?title=Main_Page
scroll down to the sections on transmissions and click on the link for your trans. Very helpful. Bottom line, unless you swap out to a later trans as mentioned by others, you likely will not see a noticable difference in an early, stock, 901 or 915 final drive.

Flat6pac · 08-09-2009, 04:45 PM

If your running an SC you would be in 3rd gear at 17 miles/1000

dw1 · 04:11 AM

for the 1984 3.2 (915):
1st - 5.8 mph/1K rpm (3.18)
2nd - 10.4 (1.78)
3rd - 14.7 (1.25)
4th - 18.5 (1.00)
5th - 23.6 (0.79)
with a final drive ratio of 3.88:1

For a 3.2 G50, the ratios are:
1st - 6.1 mph/1K rpm (3.50)
2nd - 10.4 (2.06)
3rd - 15.2 (1.41)
4th - 19.0 (1.13)
5th - 24.0 (0.89)
with a final drive ratio of 3.44:1

So a later-model 5 speed will make a significant difference, as with either of these 65 mph = 2700 - 2760 rpm. The problem is keeping it at 65.

rwest · rwest's Garage

My 77S 2.7 5 speed was at 3600 rpm's at 75 miles per hour, I think 65mph was around 3200 rpm's

ted · ted's Garage

You could just install a long 4th gear if all you do is freeway.
You can gear a 5 speed to do anything you want.
In your case you might want to optimize 1st thru 4th for acceleration/performance.
Keep the little engine in the power band and add a euro 5th for the freeway bogging.

tcar · 08-10-2009, 09:00 AM

Come on guys, he doesn't have an SC or a Carrera.

He has a very early car.

5 speed was an option, 4 speed standard, but I believe top gear were the same or very close to the same ratio on both.

See ted's post.

berettafan · 08-10-2009, 09:31 AM

my 2.4 is exactly 3,000 rpms at 60mph in 5th (speedo is off so i use the tach).

zippy_gg · zippy_gg's Garage

Quote:

Originally Posted by c0metdot

3800 rpm @ 65 mph. Wow, what a noise fest. What would a fifth gear give me?

I also looks like you need new front hood and engine lid struts.

javadog · 08-10-2009, 11:58 AM

You don't mention what year your car is. Since the picture is small and there have been a few changes made to the car, I'm not going to guess. Typically though, you could expect around 300-400 revs, for a comparison of stock transmissions.

The thing is, you can make it whatever you want, if you are going to change or rebuild the transmission. You can put whatever top gear you want in there...

JR

kenikh · 08-10-2009, 01:18 PM

An early car without a 5 speed is a tragedy. My favorite setup: AFKQX

McLovin · 08-10-2009, 01:31 PM

I thought on all stock, street 911s, the 4th gear was never an "overdrive" gear (i.e., numerically less than 1:1). Only 5th gear is an overdrive. But I'm surprised no one has given the definitive answer yet.

kenikh · 08-10-2009, 01:40 PM

Quote:

Originally Posted by McLovin

I thought on all stock, street 911s, the 4th gear was never an "overdrive" gear (i.e., numerically less than 1:1). Only 5th gear is an overdrive. But I'm surprised no one has given the definitive answer yet.

The 4-speed is a 5-speed 901 case, with no (5-speed) 1st gear shaft. The "1st" gear on the 4-speed sits on the same shaft as a 5-speed 2nd, thus the conventional 'H' pattern.

The ratios of the 4-speed are spaced equivalently to the 5 speed; the final drive gear on the 4-speed is roughly equivalent in ratio to the final drive on a 5 speed.

javadog · 08-10-2009, 04:58 PM

Quote:

Originally Posted by McLovin

I thought on all stock, street 911s, the 4th gear was never an "overdrive" gear (i.e., numerically less than 1:1). Only 5th gear is an overdrive.

No, not true. 4th in a 4 speed box is usually just a few precentage points lower than 5th in a five speed box. Of course, the exact gearing depends on the year...

Quote:

Originally Posted by McLovin

But I'm surprised no one has given the definitive answer yet.

That's sort of hard to do if the original poster hasn't told us which of the umpteen gearboxes, that Porsche built for a 911, is fitted to his car...

JR

c0metdot · c0metdot's Garage

This is the culprit. The car is a 67, BTW.

javadog · 08-11-2009, 07:21 AM

Okay,

Now we are getting somewhere. If stock, this transmission has a 28:24 top gear.

Below are a few other gearboxes from that era, their top gear, the ratio of the top gear and the approximate revs at 65mph, if your 3,800 figure above is accurate. Compared to your top gear ratio of .857, only the first two groups would do you any good, so I didn't bother to calulate the others:

901/0, 901/50, 901/53 28:23 .821 3640
901/02, 901/03 29.23 .793 3516
901/52 25:26 1.04 N/A
901/54, 902/50 27.25 .925 N/A

The alternative would be to have a gearbox built with a taller fifth gear. I believe Porsche offered ratios as high as 31:22 (.710) and there may be other gears available from aftermarket suppliers.

Here's a link to a series of gear charts for Porsche gearboxes:

http://www.pelicanparts.com/techarticles/mult_tranny_ratios/mult_tranny_ratios.htm

Good luck,
JR

fbarrett · fbarrett's Garage

No need to mess with the transmission gearing. By far the easiest, fastest, least expensive, and most flexible way to "gear" your car taller is to increase tire diameter.

Frank

adam912 · 08:19 AM

A lot of us 912ers have upgraded to Type 4 motors as 912 engine parts get expensive and rare. With real torque and HP now available, it makes a lot of sense to change out the 4th and 5th gearsets to match the performance curve of the motor.

Get yourself a 914 box and transfer the 4th and 5th gearsets from it into your box. (and 1st if you have a 4 speed!) Someone in your neighborhood has a 914 box they are just dying to give to a good cause.

It took me an easy weekend to do. My 2.2 now runs nice and mellow on the freeway (3100 RPM@70 MPH), yet has plenty of pull. My original green-numbers 120 MPH speedo is something of a joke now (grin)

Check out Tony Bray's website to see the grisly details on the upgrade:
http://mysite.verizon.net/res890fb/

Have fun!

Adam912.Out.

Grady Clay · 08-11-2009, 08:43 AM

BTW, Welcome to the Forum

You will find a lot of help here.

Whoa guys.

His 306914 is a normal 1967 911 originally delivered with a 902/0 4-speed. This is geared A-H-Q-X with a 7:31 R&P. The period equivalent 5-speed is a 902/1 geared A-F-M-S-X and the same 7:31 R&P. Note that the 1st gear and top (4th) gear ratios of the 4-speed are the same as the 1st gear and top (5th) gear of the 5-speed. The difference is the 5-speed has three gears between ‘1st and top gears.

The top gear highway speed vs. RPM is the same with the 4-speed and 5-speed. They have the same overdrive ‘X’ top gear 28:24 ratio.

YES, you want a 5-speed.

The best solution is to find a good used example, rebuild it and replace your 4-speed. Keep your 4-speed with the car. This is an easy DIY project.

Finding a good 5-speed can be a challenge. Buy one subject to a professional inspection. A seller should be willing to send the transmission to an independent shop of your choice. You pay UPS shipping (both ways if necessary) and the inspection.

There are quite a number of transmission types that will work. Naturally some are more desirable than others and command higher prices. Even a 914 5-speed can be converted but not the best choice. It is possible to force-fit a 915 but difficult and expensive.

Does your 911 still have the 901/05 engine? This is a VERY desirable engine. Please keep it in good condition.

How is the chassis? Rust? Prior crash repair?

Best,
Grady