Solved

Income from motor customers

Hello,

The amount of profit you get from selling your engine doesn't follow a formula as simple as cost x number of customers. 

To quote Jack Basford from the iGP Manager News Article, "A manufacturer’s profit and loss value per race is determined by subtracting the manufacturer’s cost per race for its operations (750k) from the profit per engine it sells. The profit margin is 20% per customer engine. The resulting formula looks something like this: (customerCost * 0.20) * totalCustomers - 750k."

Quotation taken from iGP News Article #157.

I hope this clears up why you haven't had a £4m profit each race from your engine. 

Hello,

It is worth noting that, as you level up and get unlock more customer slots for your engine, your potential profits per race increase.

For example, at Level 30, the limit of 10 engine customers will give you a profit of between £250,000 and £1,250,000 depending on the cost of your engine. 
This means that it takes a minimum of 100* races at Level 30 to make back the original £100m investment. 
* This does not include any profits made from being a manufacturer between Level 20 and 29

Additionally, the benefits of being an engine manufacturer are not just financial. 
Being an engine manufacturer allows you to control the spending of Engine Points which can mean up to an additional 30DP per track into an attribute of your choosing. This is fine control that engine customers do not get.

EDIT: There is a guide pinned to the top of the Help and Support subforum which gives additional information in relation to Engine Manufacturing - Engine Manufacturer Guide

EDIT 2: This post has been edited to correct a previous error. Thank you Antonio for the information. 

Hello,

"The positive effect on the races is actually limited to the first 14 to 15 races, because then i have the maxed out my acceleration and handling and because of the deduction for brakes and downforce, you have to choose the combo fuel efficiency/ tire economy."

In relation to this, the benefits you see from being able to add engine points to certain attributes is dependent on the number of races your league has per season. 
For example, a league which consistently runs 15 race seasons has a much lower starting design that a league running 22 race seasons. This means that teams in a 15 race league are less able to max their car attributes and therefore less likely to focus their engines away from Acceleration and Handling at the end of a season.

Being able to change the weakness of an engine at Level 30, as you say, will probably be a huge benefit to engine manufacturers and their customers but the true effect won't be known until people start to hit the new level cap.