This is everything you need to know, right here.
Looking at your car at Sebring it should be good to go as it sits, should be doing easy low 30s. Once you reach that point on street tires then we can talk about more mods.

Spend the money either on a BBK or a handful of track days.

 

Thanks for the positive feedback on it, appreciated.

The bumper venting was something I did about 10 years ago and if I recall the grill I bought off an ebay seller which came in square sheets which had 3 different size holes you could choose from, mine are the smallest I believe. What I liked about it was that it was already pre powder coated black so that saved me a step unlike my diffuser and its made from aluminum of course, so its light and easy to cut/bend by hand. Probably wont be seeing any DYI's on any of my rear end diffuser work here, I did it in multiple stages over the years, all said and done it was rather expensive, time consuming and not exactly DIY friendly to execute.

The goal for me was to design a deffuser that i didn't see on the market at the time, which performed some level of function even if limited, which actually complimented and extenuated the visual appeal of the car, since its a car that is compromised with double duty like most, being a street legal DD and track lapper at my local course. Its a full interior/full weight NA car on big street tires that i have managed to get less then 1 sec off lap record at my 2.5m course in its config/equip level. I dont run a rear wing and I run top down.

 

I once ran a full session with no rear bumper at all and it made absolutely no difference in lap time or top speed down the Summit Main front straight of 132mph so I'm dubious of the benefit of bumper venting.

 

I think having no bumper at all may be better then having the stock one in place, at least non trapped air canceling out added turbulence possibly, which may have been why you saw no appreciable change. I'd expect the driver to experience some sensational difference in the seat though at that speed, either positive or detrimental, and as we all know confidence in the seat usually equates to better lap time, even if the car itself isn't in a state to producing it for the driver in equipment.

Examining the stock bumper cover, it has a large area behind the rear of the car with large openings underneath and behind the rear wheels to trap air, and without closing it off aka some form of under tray/diffuser, its an open parachute. The rear bumper cover venting is just a secondary measure beyond the deffuser to evacuate any remaining trapped air. I have to think that my particular coverage areas of the diffuser are adding something measurable, I just cant say how much. I have no before/after data to say how effective it has worked, other then pleasing my eyes and maybe adding some confidence since I dont run a wing.

 

As posted not worth the effort to gain performance(for me) but I really love what you did Junky. I like the look of the vented bumper and the how the piece is formed around the exhaust and plates to the outside. Great job! The thing is there but not ricey in your face. That fine line between ricey and track ready is a fine one but you always seem to end on the right side of it.

 

Yes, you would think "having no bumper at all may be better then having the stock one in place" and "Examining the stock bumper cover, it has a large area behind the rear of the car with large openings underneath and behind the rear wheels to trap air, and without closing it off aka some form of under tray/diffuser, its an open parachute" but that's not true. Like I said the car felt exactly the same with or without the rear bumper cover, top speed, lap time and high speed handling where aerodynamics are most important. So the bumper doesn't "act as a parachute." I'm just trying to save everyone some effort on their track car. If you like how the bumper vents look then cool, do it. And the raw aluminum diffuser posted above looks very cool too.

 

That was my intent. Thank you

 

Its a theory, and design based loosely around what's already been incorporated on these cars. You experience mentioned is somewhat surprising, but I cant argue with it. Obviously a full under tray would be the most effective, but for the marginal gains, its not realistic for most double duty S2000's. Thanks for the positive feedback on my work regardless, it was a fun and satisfying project.

 

I have an Alloy craft one I just pulled off mine here in Orlando I'll sell you. I only pulled it because TT4 rules won't allow it next year.

 

The phenomenon with the rear bumper may be similar to pickup trucks. Driving with the tailgate down may intuitively seem more aerodynamic, but it's been proven in testing to be less so than with the tailgate closed, which maintains a high pressure zone in the bed that forms an aerodynamic "roof" over the bed.