Porsche 901 4 speed vs 5 speed

I have a speedster kit with a 901 4 speed transmission. It runs great but I was wondering if there is a huge benefit in getting the 5 speed? It seems like you cant really launch with the 5 speed and its more for cruising on the freeway. Also I hear about people not liking how complicated the shifting is. Is it worth having the 5 speed or should I just roll with the 4 speed?

You would probably not benefit from the 5-speed.

Depending on where you live it makes a big difference.
I love my 5 speed.
I don't cruise.

Cruising speed does NOT exist in Terry's world!

Looking forward to seeing you next week, Jim!

Likewise, my friend!

So nice shifting a 5 speed - you get use to the "stump puller" 1st. Besides the economy needs the infusion of cash - buying and rebuilding a 5 speed with correct gearing is not cheap. Would be nice to have a 5th for quiet flat land cruising.

You ain't kidding Greg, I just got done paying for my late model 915 5 speed with aluminum case. I am now walking bow legged;(

How much is the transmission rebuild? Is there a place that has a kit?

Mat, rebuild from what? A good core is 1200 to 1500. Angela is the expert, but figure twice that for a rebuild with no problems. Run into any problems and than you can add another 500 plus.

Wolfgang---so what would be the gear ratio and R/P for such a 5th gear.

My "overdrive" 4th gear is .821 with a 4.120 R/P and 25" tall tires.
I turn 4500 RPM at 100 MPH and 88 MPH at 4,000. I have 132 ft lb of torque from 3,500 to 6,800 RPMs.(and 120 at 8,000 RPMs so it doesn';t drop off all that much.) It dynoe'd at 143 MPH at 6,500 RPMs. This is not a huge Type IV engine---just 2,143 cc with 40 Dells and race balanced.

Disabuse me if I'm wrong, but I have always read that having a 5th gear means just having to shift through one more gear than the usual four with a "final" gear like I have.

Help me understand all this!


Thanks---Jack

Uh, yeah, you have to shift "once more time" to get to the same top gear - but you will get to fifth gear FASTER! I've got a big type four also, but I can still get bored waiting for each gear to wind out.

In Theory you could justify a taller than normal fifth gear (over a four speed) if you spend time on open freeways and want to relax the engine RPMs a bit. That presumes you have the torque to haul your ass at lower engine speeds and still get sufficient cooling.

Did you ever notice that a lot of "big" V8 cars with manual transmissiions have six gears when their smaller V6 and 4 cylinders counterparts only have five speeds? Seems backwards don't it? Well the V8s have sufficient low end torque so they can install TWO overdrive gears - 5th AND 6th gears. That gives the V8 better "highway" mileage to meet the Federal CAFE standards for fleet average gas mileage.

Cool. of the miles on my Type IV, almost 100% is spent in "final" gear. I don't drag race and rarely see 4,000 RPM in 2nd or 3rd gear so there's no rush to arrive at final gear.

For my driving style I believe I'm set.

Interesting thread.

Jack, don't ever drive your car in LA or San Diego traffic. We wear out our 2nd and 3rd gear synchros regularly!

1st in a 901 5 speed is the weak link in that tranny. If you want to launch hard off the line then I'd stay way from that one.

Brian--seems right to me. That 1st gear looks to me like 7 MPH at about 5,500 RPM!

I read that Porsche made the 5 sp 901 basically out of the 4 sp by adding on a shaft that is supported only on one end for the 1st gear. (over simplified I'm sure) Under stress this shaft will deform, bend, wrecking the unit or at least disabling 1st. There are places that correct this (like the late George had done for his IM build) but I gather they are expensive. Porsche's thinking was apparently in racing 1st is only used to get the car rolling and then you use 2nd to 5th, that's why those gears are in the H pattern. I had an old 67 Alfa Duetto and ditto.

However, I have the 901 5 speed and its a lot of fun rowing throught the gears, especially after I adapted a 915 sport shifter to work with the 901 and with a basic gate kit. Just don't try and launch off the line, take pleasure in going up and down the gears instead.

Go with a Berg 5-speed.
Choose your own gearing.
Relatively simple to install.
Relatively expensive ($4-5k)
Worth every penny.

The Berg conversion is tempting. I've read on other websites that the Berg nose cone (which holds first gear?) is the "weak link" if you are into hard launches on a regular basis. That was some years ago. Maybe issue is fixed? Do your homework.

The Berg 5spd leaves you with same top gear ratio --- wouldn't you want RPMs to drop 800 RPMs when cruising at 70 MPH? You get closer ratios but no over drive gear.

Not an expert, but it seems to me that between available top gear ratios of .72 to .89 and rear axle ratios from 4.37 to 3.88, that any number of final gearing combinations can be obtained to give a "desired" freeway cruising RPM.

The trick is to have suffficient engine torque at freeway RPM so you don't have to downshift to get over a railroad overpass. For example, last Sunday I drove to a car show that was located about 30 miles east and over 4000 feet higher in elevation. The speed limit was 70 MPH. The traffic was moving 80 MPH all the way there. There were NO level stretches of road.

You should ask yourself, will my air cooled car do that without serious overheating?

What Dave said---you gotta have the torque plus good oil cooling to enjoy an overdrive type final gear. Especially the torque.

torque is what makes the world go round.

Can you convert the 4 speed into a 5 speed by rebuilding it?

The internal castings of the cases are different

As I understand it, the Berg five speed conversion requires his proprietary new nose cone casting. It is a bolt on casting, but the internals are obviously different. He places your first gears in the new nose cone and juggles all the rest of the ratios to suit the application. First gear is therefore subject to the largest torque multiplication loading.

In the past, Clyde Berg, (son of deceased Gene Berg) held true to his fathers wishes and wouldn't install a "super overdrive" 5th gear. It was Gene's theory that if you slowed the engine down too much at freeway speeds, it would overheat. I don't know if Clyde has changed his opinion.