I'll try to help you out
.
First of all, you have downloaded the non-insim tweak, so you do not need to bother with insim. The only reasons you would want to use the insim version are ease of use, you wont have to run LFS in a window, or alt-tab all the time, AND, the insim version lets you alter the redline value.
I think neither the insim or normal version work on the test patches by the way.
The tweaking itself .. is just what it says. Tweaking
you are not able to put in power or torque values, you tweak some values and look at the results, tweak again etc.
So you want to make something like a fiesta RS turbo engine .. I say 'like' because sometimes it's just impossible to get the exact values, but you can make something like it. (read on to find out I couldn't get the RS turbo right)
First you will have to know what all the values stand for. On the left side of the tweak window you have the Cartype editor, and on the right side the single car editor. The cartype editor can be used with all cars (although it is quite limited compared to single car), and the single car editor only with the demo cars.
That means that with the demo cars you can make use of both sides.
Right, start LFS and LFStweak, select the XFG in LFS and press refresh in tweak if all fields are empty.
I'll go over all available fields and try to make a RS turbo engine;
Cartype editor
Weight: You can alter the weight of the car here, negative values allowed, watch your weight balance if you tweak this (and tweak suspension accordingly)
Min/Max gears: What it says, all gears including neutral and reverse. If you want a 6spd, make both values 8 (it will force the minimum amount of forward gears to 8)
Drive: Change drivetrain
Min/Max displacement: displacement 'boundaries'. If you want to make an engine bigger than the max value, or smaller than the min. value, adjust these accordingly.
Torque: This is kind of like you powerband width. If you increase this value, you will have a bigger powerband, in higher RPM. It will have more power and less torque. I use values between 1 and 2,5 (those are quite extreme).
Power: Like Power in single car editor, but in cartype editor (so available for all cars).
Idle: Adjust idle.
Single car editor
Gearbox: Available gearboxes.
Drive: Available drivetrains.
Engine: Inline, Flat, or V.
Cylinders: How many cylinders your engine has, remember to adjust min/max in cartype editor.
Displacement: The displacement of your engine, remember to adjust min/max in cartype editor.
Torque: Not the same as the torque field in cartype editor. If you lower this value, you will have more torque and less power, both at lower RPM. (note: turbo's don't spool up well with values under 0,5) If you increase it, more power, less torque, higher RPM.
Power: Adjust general power .. if you increase this, more power and torque at slightly higher RPM, but the power/torque ratio (for lack of a better term) stays more or less the same.
In general, if you decrease displacement or increase the number of cylinders (so you would get cylinders with less displacement) you will generally have more power at a higher RPM, and less torque.
I will leave the wheels tab for what it is, I only ever use the Tyre width field. For XFG it is 0,185 , so I'd imagine those tyres would be 185 millimeters wide.
In the Other tab you will find the cartype and turbo fields, to be honest I do not really know what Cartype does. You can check the box for a turbo, and enter pressure in bar.
Sooo .. to make the RS Turbo engine. I do not know the turbo pressure of this car, I will go with 7 psi which is 0,48 bar. For power and torque I will use 133hp@5500 and 183Nm@2400. Displacement will be 1597cc.
I enter 1597 in the single car editor displacement field, check turbo and change pressure to 0,48 and click apply. Look in the garage for the effects, re-enter if you were in garage already. (you will have to re-enter after every applied tweak.)
Way too much power at too high RPM, and a relatively small powerband. First I will raise the Torque in Cartype editor so the powerband will get bigger. This will also raise RPM, so I will have to lower torque in single car editor. I used 2 for cartype editor torque, 0,5 for single car torque, and 0,562 for single car power.
This gives me 133hp, but at 6844RPM, and 159Nm@4844. To get it right I could tweak Cartype editor torque (more) which would in turn force me to lower single car editor torque, sooo .. this particular engine will be impossible to get right, because you will have to use torque values below 0,5 , and as I said earlier this will make the turbo spool up way too slow.
So I'm sorry I couldn't make the RS turbo engine for you (but you can definitely make other good engines which resemble real ones!), but I think I may have given you some advice on tweaking ...
One more thing, the presets in LFSTweak only save cartype editor data so that would only work with non-demo cars, which you cannot use the single car editor for. I myself take screenshots if I want to save engines.
Oh, thanks for reading, I hope my english is understandable, and please correct me when I'm wrong.
Have fun!