To understand what installing a gearbox from a diesel car on a petrol engine will lead to, it is necessary to delve into the technical nuances and find out what exactly the difference is between such units.
Gear ratios and number of gears
We covered the principles of transmission operation in detail in the article "What happens if... you engage reverse gear while moving forward." In this case, let's focus on a parameter such as "gear ratios."
The gear ratio is one of the key parameters of a gear pair, which determines the amount of torque transmitted to the wheels.
When shifting gears in a car (i.e., when engaging certain gears), both the transmitted torque and the ratio of rotational speeds of the driving and driven gears change.
Torque transmission using a two-shaft gearbox as an example:
a — forward movement; b — reverse movement; 1 — input shaft; 2, 3, 5, 6, 7 — gears; 4 — output shaft
The gear ratio is the result of dividing the number of teeth on the driven gear by the number of teeth on the driving gear.
For example, if the driven gear (3 or 7) has 60 teeth and the driving gear (2 or 5) has 20, then the gear ratio of the pair will be 3 (60:20): the output shaft rotates three times slower than the input shaft, and the torque on it will be three times higher than on the input shaft.
The engine's torque is not constant — it changes depending on the crankshaft speed, and to ensure confident movement at different speeds, a gearbox with various gear ratios is used.
We wrote about how torque affects a car's dynamics in the material "Why a car needs torque." In short:
- torque determines the car's ability to accelerate and overcome obstacles,
- engine power is responsible for maximum speed,
- engine speed adds complexity, since at different speeds, power and torque have different values.
It is worth adding that the higher the gearbox ratio, the higher the torque at the wheels.
Thus, the following pattern can be derived:
- A higher gear ratio provides better acceleration but limits maximum speed. This is a "torquey" but not fast gear.
- A lower gear ratio worsens acceleration dynamics but allows reaching higher speeds. Such a gear is "long" but not designed for active acceleration.
To improve dynamic characteristics, manufacturers use gearboxes with 6, 7, and even 8 "short" gears, where the gear ratios on the higher gears are as high as possible. And the closer the gear ratios of adjacent gears are to each other, the smoother and faster acceleration occurs during sequential shifting.
The downside is that the driver has to shift more often.
For measured and calm driving, without sharp accelerations, 4-speed gearboxes with "long" gears are still used, allowing movement over a wide range of speeds in one gear.
How a "petrol" gearbox differs from a "diesel" one
Now about the engine: the main parameters of a motor are power, torque, and the speeds at which these indicators reach peak values.
Power and torque are not constant values — they depend on engine speed, and this dependence for each motor is described by complex graphs.
Diesel engines stand out for their high torque at low speeds, while petrol engines develop maximum torque at high speeds:
The gear ratios of the transmission are selected by the manufacturer taking into account the power and torque characteristics of the engine, wheel diameter, braking system capabilities, and other factors.
Hence the main difference in gearboxes: for diesel and petrol cars, gearboxes with different sets of gear ratios are used.
"Torquey" diesel engines, which pull confidently from low revs, do not need large gear ratios on lower gears — they are already capable of moving the car from a standstill and quickly accelerating it. But a petrol engine at low speeds requires help from the transmission in the form of large gear ratios. At high speeds, the situation reverses.
It is worth noting that manufacturers produce a huge number of different gearboxes with very different characteristics, and the difference between gearboxes for diesel and petrol can be either very noticeable or minimal. Moreover, sometimes absolutely identical gearboxes are installed on cars with different engine types.
So, what will happen
If you install a gearbox from a diesel on a petrol car, pulling away will be noticeably more difficult due to the relatively small gear ratio on first gear.
On second gear, there will also be a lack of torque.
Third gear will most likely be comparable to that of a petrol gearbox, but on higher gears, the car with such a gearbox will become noticeably more dynamic.
If you do the opposite — install a gearbox from a petrol model on a diesel — the car will get huge torque at low speeds, but at high speeds acceleration will become very sluggish. Moreover, such a transmission will not last long: the loads from the diesel engine will be excessive for it.
Why install a "diesel" gearbox on a petrol car
Let us remind you that the main differences between gearboxes for different engine types are the set of gear ratios and the strength of the gearbox (diesel units are designed for higher loads).
During tuning, when engine power is increased (see the material "What happens if... you greatly increase engine power"), the gearbox is always strengthened and gear ratios are recalibrated. In some cases, it is more reasonable to install a gearbox from the diesel version.
As a result, the car gets improved dynamics at high speeds, and at low speeds, the power of the forced engine is sufficient to feel confident.
For the same purpose, a "diesel" gearbox is also installed on standard, non-tuned cars, but with the condition that the engine is powerful enough to move the car from a standstill without problems under new conditions.
Summary
- Installing a "diesel" gearbox on a petrol car improves dynamics at high speeds but makes pulling away and driving on lower gears more difficult.
- Problems on lower gears can be solved by forcing the engine.
- It is not recommended to install a gearbox from a petrol model on a diesel — it will quickly fail.
