Transmission Out, Changing Gears Options

[Dr Evil]

Flipped H in 5th is the tallest gear you can have in a 901. There is no more base circle left after this to shrink, and no gear options left. I tend to talk people out of a flipped H on anything not water cooled for exactly the reason of RPM = Cooling and lowering it too much risks overheating. So many seem to want an air cooled engine that runs at V8 RPM on the freeway, different beasts, different diets.

[GregAmy]

"Sorry, officer, but I had to go this fast otherwise my engine would overheat!!!"
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(A-HB-N-U-ZA with 4.83FD on the race car, stockers on the 2056 street car for that ^^^^ reason).

[ClayPerrine]

Flipped H in 5th is the tallest gear you can have in a 901. There is no more base circle left after this to shrink, and no gear options left. I tend to talk people out of a flipped H on anything not water cooled for exactly the reason of RPM = Cooling and lowering it too much risks overheating. So many seem to want an air cooled engine that runs at V8 RPM on the freeway, different beasts, different diets.

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The only reason we are running an flipped H in the 4 cylinder car has to do with the low profile tires. With the stock ZD 5th gear, the engine spun at almost 4 grand on the highway. With the HB, it is right at 3 grand.

But if you are running stock height tires, don't try to use a flipped H or HB for fifth. It will overheat because the fan is not turning fast enough.

[jfort]

That's a nice gear selection for your application. I should move to something similar for mine but the car just doesn't get driven enough to justify the effort. What tire are you running? My trans has a "Y" top gear and Toyo 245/45/16 with diameter of 24.7" is 71 mph at 3500 rpm in 5th.

front wheels 7x16, 21mm 944 spacer

rear wheels 8x16

tires: TOYO Proxes R888 225/45ZR16 front, 245/45ZR16 rear

I used a D of 24" for the spreadsheet.

[jfort]

Flipped H in 5th is the tallest gear you can have in a 901. There is no more base circle left after this to shrink, and no gear options left. I tend to talk people out of a flipped H on anything not water cooled for exactly the reason of RPM = Cooling and lowering it too much risks overheating. So many seem to want an air cooled engine that runs at V8 RPM on the freeway, different beasts, different diets.

I thought/worried about the heat, but now I have a front oil coiler. It never gets too hot.

[brant]

The fan also cools the heads
Which are not benefited by an oil cooler

[flyer86d]

I’ll purchase your old S 4th gear if you want to sell.

Charlie

[Dr Evil]

If you google "tire speed calculator" or "wheel speed calculator" you come up with bunches of really cool sites with calculators to see what wheel speed will be and you can back calculate to engine RPM, or find a calc that does that for you. I used to have a link to one that did it all, that is either no more, or I cant find it easily. The link was lost several computers ago. Here is a fun one:

http://www.agcoauto.com/content/plugins/Ti...speed/calc.html

[Dr Evil]

Another interesting phenomenon evidenced here is preference. I get asked weekly about gears and gear ratios and I usually reply with, "That would be like me trying on a pair of pants for you and then telling you that you should buy them because they fit me so well." I can make most people happy with some generic recs based on a few parameters figured out during a discussion. But, for real custom fit gearing one needs to start with a ball park set up and put their but in the seat with their wheels and tires, and engine, and see what they like/hate and go from there. Takes far longer, more expensive, but when that is what you want then nothing else will suffice.

[GregAmy]

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I'd suggest one basic question first: why do you want to change it? What is your mission? What is it about the current ratios that don't meet that mission?

It's not that the stock ratios are written-in-stone perfect; of course they're not. But it's what Porsche (or VW?) decided the mission was for the stock 4-banger cars.

Of course, changes in engine mods and/or expected vehicle use will change that equation.

My race car, for example, significantly changed that mission from stock: I needed a basic 1st gear to get me off the grid once per session (hopefully), then a close ratio 2-3-4-5 H-pattern for racing on the track. I had an expected highest and lowest speeds (roughly 40 to 115), a higher expected shift point (roughly 6300), and an adjusted (and dyno'd) torque band to keep RPMs within. I did not care about revs at cruise because I was never going to be at cruise. I did not care about fuel economy. I did not care (as much) about engine longevity.

Until you define your mission, you really can't start talking about even *if* to change ratios or not; you're certainly not ready to begin discussing any specific ratios.

GA

[VaccaRabite]

I thought/worried about the heat, but now I have a front oil coiler. It never gets too hot.

This thinking will destroy engines.
You oil cooler is doing nothing to keep your heads cool. Air going over the heads is entirely governed by the fan. If the fan is running too slow, your heads will bake even while your oil is showing normal. Baked heads drop valves. Dropped valves bend rods.
It is imperative that both oil temps and head temps stay in check.

Get a CHT gauge, and mount the sender on your hottest cylinder (#3 on a Type4). Happy head temps are ~275-375. At 400 the heads are overheated. It will happen FAST if you are driving on the freeway and the fan is not spinning fast enough to keep the heads cool. Like within minutes. You will find yourself driving at lower gears in order to keep the heads from cooking. This is the same reason why many people recommend not using 5th gear till you are going ~70mph on a stock gearset - to keep RPMs over 3000.

Zach