cutting holes in your rear bumper
 

Lately I've come across some drag cars that have cut holes in their rear bumpers. They justify the action by saying that it allows air to escape from the rear bumper and helps prevent the parachute effect. I have seen claims of people dropping anywhere from .1-.4 seconds off of their 1/4 mile. I personally drive over 300 miles a week on the interstate, and if this could help me at all, Id like to give it a try.

Has anyone done any testing to prove this an effective technique?
Does anyone have the knowledge to explain to me why it wouldn't be effective?

worst case scenario, I will find an extra bumper to experiment with myself.

here is what I am talking about...
http://i11.tinypic.com/67sqhiq.jpg

Hello,

I would be concerned that your insurance company would use it as an excuse not to pay a claim, saying you damaged safety equipment.

Why not just enclose the underside?

The holes wouldn't actually be in the bumper. just in the thin plastic cover
it wouldnt make a difference in the structural integrity of the car

mostly just because this would be easier for the time being. Im lazy :p

It really wouldn't be any easier than cutting a piece of cardboard for testing purposes. The claims about "Speed holes" in rearward body cladding probably aren't all they're cracked up to be, either. Imagine how little air is actually succumbing to the parachute effect, and how little air is actually under there to begin with. You're almost universally better off smoothing the flow path.

Those holes create much more turbulence, as well.

It is better for air to go around than through a grille. I vote for a partial belly pan ahead of the "parachute." If I were modifying a race car, I'b be tempted to say that what didn't help did, and vice-versa, trying not to help the competition.

Hmmm I am more inclined to think this INCREASES drag which for a racing car MIGHT result is more rubber on the ground in the rear which might explain the faster times.

IE worse for mpg concerns :-) Just a guess.

Now you have me curious: How does more rubber on the ground in the rear make this car quicker??? :confused:

lol yep
civics are front wheel drive

if anything the downforce on the rear would hurt

Why not try just removing the rear bumper cover? That would be an easy way to see if it helps.

thats easy frank. First MOST "racing" cars are rear wheel drive for exactly that reason Second the more force you have pushing DOWN (drag helps push down or at least STOPS some "lifting up" results the same.

the more you push down the more POWER you can "push" into your rubber without losing grip.

its one of the reasons I can DUST pretty much any sports car in the hole shot in my Cherokee. I have an ass ton of torque and enough rubber that NO amount of throttle causes me to lose traction even in the rain. SO when I hit go "I GO" while the 911 chirps his tires and loses the hole. (and then 3 seconds later blows by me like a missile of course but I have already humiliated him at that point) ie most people who have those cars have no clue how to actually handle them :-) same goes for 99% of people driving SUV's

so the more you can push down or avoid "lifting up" the rear tires the more Horse Power you can push to the wheels.

The more Horsepower you can push to the wheels the FASTER you can go in theory.

this of course assumes you have excess horsepower at your disposal and are limited by your tires ability to translate it.

It was just a guess as to why some who claimed to get better times by doing this.

mmmmm. Grand Prix's have parachutes for rear bumbers. My SE has about 8-9" of just plastic hanging out, that can catch air off the rear subframe. The GT and GTP have over a foot of plastic that hangs down. There is nothing to stop air from trapping, and those that do race their GTP's have shown a few tenths or more dropped in the 1/4 by sealing it up.

I wouldn't cut holes, I'd seal it. Ya spent all that money on the car, why butcher it?

Do a search here for belly pan. I agree it will give much better results, just watch the muffler.
The balance is that the underside of a car is very dirty aero wise. The gains are small but every bit adds up.

I'd say those holes are just to get people talking about your car instead of just another reasonably fast car. I'd seal it from underneath.

Why butcher it? Because it "looks cool"... like body kits.

dont do it man. youre better off with a partial pan back there. plus im sure you dont want every honda ricer/tuners out there trying to race you at every light (hate that). for some reason some of them come to think that the grill block has something to do with racing(it actually might help).

Maybe some fat tires would block off all the air that's filling that bumper???:confused:

If you really want to cut part of your bumper then go lower, those holes are to high in my mind, trim the lower edge that hangs down and catches the air, then add some stream lining ahead of it.

Yes, the holes work. *no sources, but i've seen it happen*

Guy gained about .2 on his 1/4, with a faster MPH.

Sorry, I'm not buying it. I have a slow car, and just a little wheel spin can be a .2 difference. You just can't use a dragstrip to measure aero changes. I could probably list about one hundred things that could happen between two back to back runs that could alter the ET. Have this dude tape up them holes then do some coast down testing, A-B-A, then you can make a believer out of me.

yeah I agree but that's a way to do it and have noticed like someone else said.

the holes will help and get you attention
a belly pan will help more and not get you attention.



no one will blame you if you'd like to have speed holes.
"we" will respect sealing the sir out more than we will the holes.
the rest of the world might appreciate the "holes" more.

the theory is sound.
if the bumper is filling with air then holes will allow the air to escape.

the better idea is to not let the air get there to start with.


and then there is the few who like it cause it looks cool.

I wouldn't know if the theory was sound, I just know that the testing method is silly :) I drag raced for many years and saw how much missunderstanding there was about going fast and how to test mods to see if they improved performance or not.

sorrow six. While my knowledge of this is limited and I don't want to be combative My theory logic is the opposite. The bumper will only cause increased drag until it is FILLED with pressurized air. once that happens (micro seconds) it will from then on behave like a solid object. ie think air dam. when you add the holes you are letting this "bubble" escape and now the DRAG of air through the bumper will now persist and not convert to acting like an air dam which my butt sense says is lower drag than the holes.

for an example of this (I have NO tested this and I could be dead wrong on this but my gut says its so) take a CUP aim the open end into an airstream measure the drag.

take the same cup and CAP the end of the cup and do the same thing.

I think you will find the "drag" to be very similar is not slightly lower than the cup without the cap.

now take the open cup and drill holes in the bottom and add STUFF inside the cup (to represent the bolts frame brackets etc.. in the bumper.

now measure the drag. I think you will find the stuff cup with closed bottom will have far less drag than the stuff cup with closed bottom. Because of the closed bottom the stuff in the cup becomes irrelevant to drag. once you allow the passage of air it becomes relevant.

at least thats my gut check take on it.

How about cutting 1 hole in your bumper to fill your inflatable boattail??;)

http://ecomodder.com/forum/showthrea...ttail-896.html

Someone please do a test for me:

Turn on a faucet at 6 gallons per hour.

Put a funnel under it that can only flow around 3 gallons per hour.

Measure the "wasted" water that doesn't make it through the funnel.

That will prove that those holes aren't what they're cracked up to be. Keep the air from getting in there to begin with, you'll be better off for it. If you look at those holes, they represent less than half the surface area of the bumper. If the bumper in theory is filled with air, those holes are only relieving some of the pressure build up, causing (probably) more harm than good.

Imagine what happens when you force a fluid through a hole at pressure that isn't tapered, as well. The coanda effect says that the fluid (air) will attempt to adhere to the object it's being forced through, which means that it will expand very quickly as it leaves the hole, possibly causing more drag.

So first, you're pressurizing it, then, you're allowing it to expand. Sounds like a huge waste of energy to me. Why not save your grief, build a small belly pan, or just cut the bumper and add vertical stabilizers?

Now that would probably be a good use of existing drag to shape the larger section of flow.

"...No, madam, that is NOT an automotive condom, it's an aerodynamic codpiece!"

Why not cut the bumper bottom edge higher, and combine that with a pan? It would probably create downforce while also reducing drag. How is it different from the boattails everyone loves, unless your vehicle is very low? You guys need to put down the haterade and get off the ecomodder hatewagon.

http://img258.imageshack.us/img258/7742/assend2xz5.jpg

http://i5.photobucket.com/albums/y16...fbe21a741b.jpg

1. There might be confusion as to the purpose of a boattail.
2. What flavors does haterade come in?

You forgot:

3. Hatewagons are more aerodynamic usually than hatepickuptrucks, but less aerodynamic than hatehatchback-ecoboxes.

Inb4 all this BS again that happened with Hermie.

That does it. I'm gettin' on the hatewagon.

How much are tickets?

please quit killing my IQ points... I NEED those! :D

Yet another thread I'm about to jet from. Pce out, guise.

oh Christ.... been dieing to say that.

1) Isn't the purpose of a boattail to cause cross section to decline rearward, IE create inward-slanting surfaces? That's what we are talking about here, cutting holes in the bumper fascia, or trimming it like in my examples.

2) It comes in "Front Aero Doesn't Matter Fruit Punch", "You Don't Need Downforce At Hypermiler Speeds Strawberry Surprise", "Just Add Wheel Skirts Juice Blend", and "Engines Don't Need Cooling Kiwi".

Edit: I forgot "That's For Racecars Where Only Downforce Matters Rasberry" which is the haterade being drunk in this thread.

The Aerodynamic Performance of Automotive Underbody Diffusers

So you're saying that those holes produce negative lift?

I'm relatively sure they don't produce anything, actually, rather negating positive lift that would normally be occurring due to the parachute effect. The notion still stands that the parachute effect wouldn't be a problem if the offending area were covered by body work, hence the obvious advantage of a rear belly pan, or the option to just cut the offending section of the rear bumper, rather than add the ineptly named "speed holes" to it.

If that wasn't your complaint, I'm not sure where the relevance was.

What you are basically saying is that a hatchback isn't more aerodynamic than a wagon. The plain rear belly pan would be like the wagon, and cutting the lower edge or making holes (big enough holes) would be like the hatchback or adding a boattail. Air doesn't care whether it's flowing past the top or the bottom of a car.