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

[r_towle]

Yes

[r_towle]

G50

[r_towle]

See gear ratio charts at carquip

[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

[mlindner]

Nice explanation Dave, Thanks, Mark

[thelogo]

See gear ratio charts at carquip

Looked but said 901 data missing

Id like to know what rpm equals 80mph

I am using stock tires size

[Steve]

Here's a gear chart

[Steve]

ZD is the stock 5th gear.
Here is a page with more info.
http://www.914world.com/specs/regearing_901.php

[Matt Romanowski]

In my experience, it takes a lot of power to get a 914 over about 140 mph.

A 915 is much better than a 901 and shifts much better. A G50 would probably be amazing as well.

[GeorgeRud]

Most reliable way is to use a GPS speedometer app on your cellphone to see what rpm equates to 80 mph. The VDO speedometers are sometimes rather optimistic in my experience and tire diameters also affect the actual speed you're traveling.

[bernbomb914]

my trans has c ,1st. 904 2nd, std. 3,, v 4th, ha 5th. the engine is a 78x104 which should have lots of torque.

[thelogo]

I'll attempt to confirm 83mph at 3500 rpm with gps

But something tells me that a bit on the high side .

I think its more like 75 mph (IMG:style_emoticons/default/bye1.gif)

[PlaysWithCars]

I'll attempt to confirm 83mph at 3500 rpm with gps

But something tells me that a bit on the high side .

I think its more like 75 mph (IMG:style_emoticons/default/bye1.gif)

You could have a non-standard 5th gear, but its doubtful. More than likely you have tires that are less than 25" in diameter. (note that piece of info at the top of the gear chart posted by Steve.) Speed at rpm can be calculated by referencing the gear chart and multiplying by the ratio of (your tire diameter)/25". You can find your tire diameter, or a very close approximation, by looking up your tire size on TireRack and looking at the spec sheet.

[r_towle]

In my experience, it takes a lot of power to get a 914 over about 140 mph.

A 915 is much better than a 901 and shifts much better. A G50 would probably be amazing as well.

I think matt tried to go over 140 for a few years. $$$$$