Years ago a friend of mine and I finally lined up. We made almost identical power and had the same chassis, once he cut out for bumper holes he picked up 2MPH in the 1/4 and would inch me out on the big end every single time! We would literally be door to door until 4th gear and he would start pulling ahead slowly and prior to the holes it was dead even running identical MPH and times. It certainly made me a believer because this wasn't even on high HP cars, I'm talking cars trapping low 100's he picked up 2MPH. I'm sure on cars trapping over 120MPH the difference is more than 2MPH based on what I had personally experienced.

 

 

just so some people can learn some things; a rear splitter with flat bottom would have an equal effect. It's all about aerodynamics. If you look under your car, the rear bumper actually works like a small parachute at higher speeds. Cut holes in the "parachute", or add a flat plate so the air totally bypasses the "parachute", and you've drastically reduced the wind drag. Obviously adding a plate adds weight and cutting holes reduces weight. So the choice is pretty obvious.
Ever stick your hand out the window at 120mph and cup the air? It's a considerable amount of drag. Now imagine the surface are of your hand compared to an entire rear bumper. Well, half the bumper...

 

 

look up the cars that run at World Cup Finals and check out their rear setups. That will let u know whats good. I saw some pretty crazy rear diffuser designs this year.

 

You can make your own rear diffuser out of ABS, this is how mine turned out:

 

Great concept, but I’ve seen soooo many parts fly off cars over 100mph that I’d rather just cut than take the risk

 

 

Thank you! I definitely attempted to make mine functional, from the research I've done, a rear diffuser angle of around 7-10 degrees is optimal. I still need to do work on a full length flat undertray to really increase its functionality.