It's for a business sim game (you produce and sell cars).

I'm interested about engines (and possible other components, but primarily engines) and their corelation to the car's "quality". I'm interested in two ascpects:
* how it works in reality
* how it can be simplified for the purpose of a game

I guess speed and fuel efficiency are the two primary stats.

Fact from the real world I found so far:
- the key for high speed is the ratio of total car's weight to the HP of the engine (especially for early cars around year 1900)
- aerodynamic forces that affect a car is the square of the speed (but I read about it in the context of fuel efficiency for heavy trucks, no clue how it relate to the max speed)

For the purpose of the game the simpliest formula could be: speed = total weight (chasis+body+engine) / engine HP. And the customers would pay depending on it. Of course I would prefer something more realistic

In addition to speed and fuel efficiency, even a basic car simulation needs to model acceleration (change of speed over time) and turning radius.

From there, you can also add different gears and gear shifting, different turning radii based on speed (can't do a tight turn going 40mph), the effects of speed on acceleration and turning (Formula One cars start accelerating really slowly, they need to build up speed so that the airflow over the car pushes it down and keep the wheels glued to the ground, then they can really hit the gas), effects of friction on driving (be able to go into a fishtail or spin out).

And that's just a tiny list.

It is not a car simulator, it is business simulation of a car manufacturer (like old Detroit, MotorCity/OldTimer). Turning radius would be an overkill, but acceleration indeed would fit. Maybe: speed, acceleration, fuel efficiency, reliability (when it will break down), safety, max cargo/passengers (and aestetic, but that's not a technical thing so ignore it in this topic).

Can it have hybrid cars and weird stuff like that? :3

Also max torque might be good so you know if it can be used to pull stuff.

Fact from the real world I found so far:
- the key for high speed is the ratio of total car's weight to the HP of the engine (especially for early cars around year 1900)
- aerodynamic forces that affect a car is the square of the speed (but I read about it in the context of fuel efficiency for heavy trucks, no clue how it relate to the max speed)

For the purpose of the game the simpliest formula could be: speed = total weight (chasis+body+engine) / engine HP. And the customers would pay depending on it.

no that is not correct. At least if for "speed" you mean "max speed"... weight has nothing to do with it. Maximum speed is mostly a factor of engine HP and aerodynamic drag... the weight only slightly influences the max speed by a phenomenon called rolling resistance, but it is a tiny factor in the equation.
In automotive terms the aerodynamic drag is identified by a value called "Cx", you can sometimes find this value for different cars.

Weight comes in when you are analysing accelerations... such as 0-100kmh times, or how fast you can go around a track, there a lighter car will be faster because it will accelerate faster and turn faster.

So a lighter car will just reach the maximum speed more quickly but it'll stay there where the HP/drag will keep it.

@turch.. Formula 1 cars DO NOT accelerate slowly.. 0-100kmh times are easily under 3 seconds, that is nowhere "slow".

no that is not correct. At least if for "speed" you mean "max speed"... weight has nothing to do with it. Maximum speed is mostly a factor of engine HP and aerodynamic drag... the weight only slightly influences the max speed by a phenomenon called rolling resistance, but it is a tiny factor in the equation.
In automotive terms the aerodynamic drag is identified by a value called "Cx", you can sometimes find this value for different cars.

Hmmm, they mentioned the weight to HP ratio was the key to winning early car races, maybe the meant acceleration not max speed

How important the aerodynamic drag is for low speed/HP cars? Like 500kg car, 1 cylinder 5HP engine (the cheapest early cars)? I guess the speed for these could be significantly increased by using a better engine since the aerodynamic forces are still rather low?

You can include aerodynamic drag in the fuel efficiency and top speed functions. The drag is the square of the speed. Based on the speed people want to drive their car, you can estimate how much power you need to overcome drag. You then figure out the efficiency of fuel to forward force and this becomes your fuel efficiency. For top speed, it's the point where drag is equal to maximum power output.

For older cars, required speed would be fairly low so drag has little impact. When you move to highway speeds, it becomes a lot more important.

Hmmm, they mentioned the weight to HP ratio was the key to winning early car races, maybe the meant acceleration not max speed

but that is correct.. races are not won on straight lines, they are won in corners.. acceleration (how fast you gain speed, brake and change direction) is the main factor. Look at F1, they have wings that produce lots lots of drag, without wings a F1 car would be way faster on a straight line. But still, they trade max speed for faster cornering speed and grip.

But in a business simulator this doesn't really apply. If weight to power ratio would make your business then Lotus would be a market leader, which is not.
Style, reliability and product placement are way more important than pure performances to sell a car..

@turch.. Formula 1 cars DO NOT accelerate slowly.. 0-100kmh times are easily under 3 seconds, that is nowhere "slow".

Well, what I mean is their acceleration curve is shaped more like an exponential function, whereas a regular car is shaped more like a linear one. They start acceleration "relatively" slowly in the first second, and then their acceleration skyrockets up in the following two as they gain downforce and increase grip.

[quote name='kunos' timestamp='1326987010' post='4904294']
@turch.. Formula 1 cars DO NOT accelerate slowly.. 0-100kmh times are easily under 3 seconds, that is nowhere "slow".

Well, what I mean is their acceleration curve is shaped more like an exponential function, whereas a regular car is shaped more like a linear one. They start acceleration "relatively" slowly in the first second, and then their acceleration skyrockets up in the following two as they gain downforce and increase grip.
[/quote]

nope.. it doesnt

with downforce comes drag, with every gear change acceleration goes down.. just like every other car.

maybe you're think about potential cornering power, then yes, it is exponentially going up.. but straight line acceleration is a curve going to an horizontal asymptote.
This is a telemetry from Fisichella's car in 2007... acceleration is nothing like exponential.