So, this here is my starting car which i'll be using for this guide, and these are its not-so-amazing, stock power figures:

So, lets remember 114ish horsepower (HP) as our stock untuned, unmodified number. Here are some of the ways to increase that FOR FREE!!!

Making Your Car Run LEANER or RICHER :
Ok so the way to do this is to first of all, use the tuning tool, to the right of the drill (wrench in MWM and LE) icon. Now that you are on the tuning tool, click on the feul rail, which is the long thing inbetween the intake manifold and the cylinder head. You will then see a number, this number will vary between cars, but try to remember your unmodified number along with your current HP number. Keep in mind, sometimes your car might already be on the optimal value.

BEFORE:

AFTER:


Now lets look at my HP increase:



An increase of about... 9HP!

Adjusting CAM TIMING :
For this, we need to be on the tuning tool again and this time, click on the cam gears (SLRR puts these WITH the cam shafts, so they're not separate), these are the circular gears you see on the end of the engine block with the belts on them. Note that, some engine types (i6, v12, etc) have them covered, in that case you'll have to take the cylinder head cover(s) off to see them. Again, keep in mind, sometimes your car might already be on the optimal value.
In my case, the intake side cam gear was already on the optimal value, however, the exhaust side was adjusted. (I changed it to -8 degrees)
I wont bother posting before and after pics, the increase in power was only 2-4ish HP for me :P (this wont always apply to you though)

Removing Exhaust Headers
(only on N/A cars, AKA a car with no turbo, or supercharger)
This is pretty simple, no tuning, just remove the exhaust headers, which are the 4+ pipes which lead up to the actual straightish looking pipes.
BEFORE:

AFTER:


Increasing 'Boost Pressure'
(ONLY on turbo charged cars!)
Pretty simple too, using the tuning tool, just click on the turbocharger and pull the slider all the way to the right for some extra MAAAD turbo action!
No pics this time because my starting car which i used for the purpose of this guide was N/A, so no turbo nor supercharger.

This is pretty simple, all you gotta do is remove all the bits on the exterior that you can, and yes it actually save quite a bit of weight.
BEFORE:


AFTER:

I may have forgotten to take a photo of the car stripped down, but it looks like crap anyway, so thank me later :P But that's not the point, look at the whopping decrease in weight!!

PLUS i got SO much money from selling all the stripped parts:


NOTE: I HIGHLY RECOMMEND YOU KEEP AT LEAST ONE BRAKE LIGHT, AND ONE HEADLIGHT!!!