engine and drivetrain 4-speed, 5-speed

[JFT]

Just a general question about the need to swap out the 4-speed for a 5-speed. An important change, or not?

I ask because I’ve noticed, having owned a number of ‘70s-era German cars, that their engines all seem to be tuned to run at medium to high revs at freeway speeds. My ‘71 M-Bz 250 was like that, my ‘69 Porsche was like that, and of course 1602s and 2002s are like that. Right?

The rule of thumb, back before the fancy tunable cams and engine-control electronics that we have now, was that an engine’s best efficiency was found in the area where the horsepower curve intersected the torque curve. If that’s the case, if you add a 5-speed trans to an ’02, for instance, especially if that trans uses 5th gear as an overdrive, do you not risk screwing up that formula?

Just curious. Any input will be – well, interesting.

-JFT

[ricortes]

More options become available like lower ratio rear end so you can still match RPM to your speed comfort level on the freeway. You can keep your engine RPMs closer to optimal performance at all other speed too.

Rick

[JFT]

So you're saying, Rick, that you swap the diff ratio at the same time you swap to a 5-speed to keep the overall final-drive ratio as stock, in effect just giving the car an additional intermediate ratio, right?

Thanks - Jon

[Pierre]

Replacing a 4-speed with a 5-speed adds one gear. All the other ratios are the same. 5th is an overdrive which drops you RPMs during cruising. At the very least, you get a lower noise level, lower vibration and longer engine life if you do a lot of freeway cruising. You may also see improved mileage.

Most people who instal 5-speeds like it. A poor installation can cause your shift rod to be out of its natural location. The tranny does weigh a bit more but that weight is low and at the center of the car and won't affect the handling characteristics.

Highly recommended IMHO.

[FunkyLaneO]

The national speed limits have also increased proportionally, when our little cars were build 55mph was the max speed limit everywhere, now it is 65-75mph adding a fifth gear at those speeds keeps us at the same rpms as when we were supposed to be driving 55.

[DavidLMM]

The double nickel was not enacted nationaly until 1974, intended to reduce fuel consumption by 2% or so. Some states had already reduced to 55, prior to the Fed holding hostage all the highway monies for compliance. The rest begrudgingly followed, but enforcement was not always strict.

02s were built for the autobahn, not US highway speeds. Of course, over there, close ratio trannies and different diff ratios were more common.

David

[FunElan]

That's what I like about the 02s motor, even at lower rpms I still have power to pass cars on the fwy. And like Pierre said my whole shift pattern has moved backwards about an inch. Wasn't really happy about that.

[ArdenK]

you swap the diff ratio at the same time you swap to a 5-speed to keep the overall final-drive ratio as stock

You can do this so you can get off the line faster, spin up quicker and get to top speed quicker. If you keep the rear ratio stock then it takes a bit longer to spin up but you get a higher over all top speed.

I wish there was a 6 speed we could use :-/