901 gearbox questions and, How fast am I really going? Options

[thelogo]

Any backround info on this trans appreciated

But more specifically

Q. Does gearing or gear ratio determine the cars
Speed, combined with rpm no matter what engine
Power you have

Q. With a more poweerful then stock engine how does that
Affect my cruise rpm

Q. What is stock 5th gear good up to 90mph ?

Q. Never down shift to 2nd gear right
Do not abuse 1st

Q. If money was no object what trans would be ideal for a
914

Shout out to the cheap sob

[Dave_Darling]

From these questions, I'm not sure you understand how a transmission works... So here goes:

Most engines don't have broad enough power ranges to be directly connected to the wheels. They need some sort of gearing in between.

Each gear set will take the input RPMs and multiply that by some fixed numbers (fixed for each gear set, that is), which then get multiplied by the ring and pinion ratio to get the RPMs that the output shafts spin the wheels. So a given gear, like 3rd gear in a regular 914 box, will always give you about 45 MPH at 3500 RPM (if you have stockish-sized tires). It doesn't matter how you get those rotations, so the engine doesn't matter a hoot.

Now, with a different engine, you may want to cruise at a different RPM than you had before. For example, a small high-winding motor may be happier at higher revs when cruising--or an air-cooled motor may be happier when pushing more air through the cooling system. Or a large engine that makes tons of torque and doesn't have any cooling challenges may be happier at lower RPM.

Stock 5th gear with stock-sized tires can potentially be pushed to an absurdly high speed--something like 150 MPH, or possibly more, depending on the engine's redline. But the power to push the air out of the way of the car goes up hugely as the speed gets higher, so very vew 914s will be able to move that fast regardless of gearing.

Your choice of transmission is greatly affected by your choice of engine. If you have a Type IV engine, the stock type transmission is pretty much going to be the best option, even with a high budget. Even small 911 engines will be very happy with the stock trans. Bigger 911 engines would work well with a 915 gearbox, while big torquey V8s would suit the 930 four-speed or the G50. Since the G50 has been bolted to just about every kind of motor in various track-day cars or kit cars or low-budget supercars, that probably would be the cost-no-object option for the biggest and most powerful engines.

Unless you want to take many millions of dollars and get Quaife or ZF or someone to build a custom box for you....

--DD