Transmission gearing

Which drivetrain to choose, so many choices, 4-cyl? 6-cyl? NA? Turbo?
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Transmission gearing

Post by Midlana1 » 06 Feb 2018, 12:27

Short story: if you're building a car just for cruising you can skip this. If you're planning on doing autocross or especially trackday events, read on.

Here are some things to keep in mind when researching a drivetrain:

1. Midlana requires roughly 300 whp to reach 150 mph, and 364 to reach 160mph.
1a. Decide if you care - if so:

2. Determine your redline
3. Determine the gear ratios in your donor drivetrain, including the final drive.
4. Part of the "geartrain" is the diameter of the driven wheels; for Midlana it'll be around 25".
4. If you're going forced-induction, both turbo and supercharging increase torque. The downside is that the gears will have to deal with that, but the upside is that the engine probably doesn't have to be spun as fast. Unless the engine already uses forced-induction, it's likely that the gearbox won't last.
5. Conversely, if the engine is kept normally-aspirated, torque on the gears will be less. If more power is wanted though, the engine typically has to be spun faster.
6. Calculate shift points - most street cars typically have first gears that are intended to get the car moving on a steep hill and aren't good for much more than heavy traffic. For a lightweight car, a numerically lower first gear makes it much more useful - or even skipping it altogether and starting off in second.

Decide how big a deal the donor gear ratios are, whether they'll be okay* for your application; if so, great, you dodged a bullet. If not, look into the availability - and expense - of aftermarket gears. The point of all this is avoiding getting beaten badly at an event due to poor gear ratios, then being shocked at what it costs to fix that. Knowing what you're getting into before buying the drivetrain can save money and potential disappointment.

*Given that building a car is a luxury, what "okay" means is highly subjective. Getting beat by a Z-06 but saving $4000 on gears can make one pause.

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