Do holes in your rear bumper really make you faster?

you wont notice any difference in acceleration, well you shouldnt, its just the top end speed would in theory be greater due to less of a 'parachute' effect being created by the bumper as air passes under the car and travels up into the recess in front of the bumper. Drilling holes in it might be an idea but it could create other problems. If holes were cut then the air underneath the car is going to behave differently, air will be sucked up from under the car due to negative pressure created by the holes in the bumper, so it sucks the car to the ground. But this will only work if the holes are big enough and placed appropriately. On the other hand,if the holes arent big enough or spaced right, the airflow under the car will build up (yes despite the new holes),thus raising it up and your car will handle like a dog on ice. You could just 'sheet' the bottom of the car, thus making it entirely smooth, throw on a diffuser at the back which would 'clean' up the air passage underneath the car and would be a better job than drilling holes.......

I ran the same car with no extra mods other than holes in the rear bumper and ran .2seconds quicker over a 1/4mile. It does make a difference this is why you see trucks etc with the rear hatches down when empty as it improves mpg on motorways etc

ninety-one-racing said:

you wont notice any difference in acceleration, well you shouldnt, its just the top end speed would in theory be greater due to less of a 'parachute' effect being created by the bumper as air passes under the car and travels up into the recess in front of the bumper. Drilling holes in it might be an idea but it could create other problems. If holes were cut then the air underneath the car is going to behave differently, air will be sucked up from under the car due to negative pressure created by the holes in the bumper, so it sucks the car to the ground. But this will only work if the holes are big enough and placed appropriately. On the other hand,if the holes arent big enough or spaced right, the airflow under the car will build up (yes despite the new holes),thus raising it up and your car will handle like a dog on ice. You could just 'sheet' the bottom of the car, thus making it entirely smooth, throw on a diffuser at the back which would 'clean' up the air passage underneath the car and would be a better job than drilling holes.......

Click to expand...

So what you're saying is that the rear bumper creates excess air resistance. When you take away (part of) that air resistance, your car won't accelerate faster? But top speed is higher? Sounds contradictive to me.

IF acceleration does not increase, that should mean that difference in air resistance is minimal and then top speed shouldn't increase noticably either.

Alright, I'll be leaving in 30 minutes, to go to a friend of mine, dilling the holes and then to the autobahn

Back. Went to a mate to drill the holes. So I did:



That looks like at least 150 horses extra

But it started to rain, just before we drilled the holes. So thats **** cause the weather conditions changed. It was way more moist. Anyways, we still went to the Autobahn. Yesterday I got 206km/h one way and 204 the other way. Today that was 207 and 202...

So I don't really know what to make out of this. We will be trying this tommorow one more time, if it doesn't rain. And thats the last time, cause this just slurps petrol and I'm just a student. Any donations are welcome

i imagine due to holes in that design might be odd aerodynamic effects

rear diffuser, flat underneath is my prefered option but getting 1 made isnt easy for most

keep up work jt, good to see people going out and just experimenting

_JT said:

So what you're saying is that the rear bumper creates excess air resistance. When you take away (part of) that air resistance, your car won't accelerate faster? But top speed is higher? Sounds contradictive to me.

IF acceleration does not increase, that should mean that difference in air resistance is minimal and then top speed shouldn't increase noticably either.

Click to expand...

no its not contradictive, acceleration is a measure of how quickly you reach a certain velocity or point, top speed is not! top speed is the maximum velocity that can be reached, look at some of those cars that do the land speed records, their acceleration isnt anything massively impressive yet 300mph+!


air resistance at low speeds isnt that important, but when you get above 120mph, it is! so when you're accelerating from 20mph,the shape of your rear bumper is about as important as the length of your aerial, stick your head out the window at 20mph and 120mph and you'll see exactly what the nature of air the bumper is experiencing, thus if you allow this air to escape by cutting holes,then top speed should increase (if the air pressures behave as expected and allow it)

just go out and try the damn thing!!

I've already tried, read my previous post. I'll try again tonight, if it doesn't rain.

But I have never heard that air resistance is not relevant, when accelerating. I've always heard from everyone that air resistance is the one thing to overcome when accelerating on high speeds.

_JT said:

I've already tried, read my previous post. I'll try again tonight, if it doesn't rain.

But I have never heard that air resistance is not relevant, when accelerating. I've always heard from everyone that air resistance is the one thing to overcome when accelerating on high speeds.

Click to expand...

Look, if you really must argue that it will affect the acceleration, I'll agree. But it is to such an infantismally small degree that it is not worth mentioning.

Top speed might be 1-2mph different, but this could be attributed to slightly differing wind speeds, ambient temperatures etc etc.

IMO (having studied Aerodynamics at uni) on a Civic, is going to be a looks thing. And IMO it looks shite.

Just my 2 pence.

Yeah it does look shite I agree My expectation is that it will indeed make no more than 3mph difference. To overcome the differences in weather conditions, I will be changing bumper on the spot tonight (if the weather is good enough for it)

Im getting ideas on making some holes like these on my ek
but its a pre facelift and has deflectors ..
i like these holes

Those wheels look mad

That bumper doesn't look too bad either

Anyways, just did some good runs...finally! Hoep you guys appreciate the results, I sacrificed more than a tank of petrol for this and the other runs and thats a lot for a student At least 50 euro's In short the top speed results are:

Bumper with holes
From A to B: 208km/h
From B to A: 208km/h

Bumper without holes
From A to B: 205km/h
From B to A: 206km/h (just barely)

Description of experiment
We drove to a parking spot next to the autobahn. There we launched. I kept accelerating untill a certain spot (next parking spot, few km's) and then we noted that top speed we got there. We turned around at the next point where we could. Then we would go flat out again from the beginning on the autobahn, only opposite direction obviously. Untill we came across the first parking spot, we then noted the top speed again.

Description of results
With the bumper with holes we got to the top speed pretty quick. With the new bumper, we were not only slower, maybe it was even more surprising how long it took to get at the top speed, compared to the bumper with holes. For example, on the B -> A run with hole-bumper we got to our top speed relatively quick and just before we got to the parking spot we had the feeling it might even get to 209, but there was a car that got in front of us. The run without hole-bumper was pretty different. It took way longer to get at 205 and only 100 meters before the parking spot where I'd let go of the gas pedal it went to 206. I have yet to download the G-Tech results, but for my feeling the bumper with holes made some difference in higher rpm's in 4th and 5th gear.

Some pictures:

My high tech professional autobahn racer bumper


Notice the craftmanship




My high tech professional autobahn racer bumper next to my puny inferior stock bumper

Think the holes look pretty cool dude! Respect for conducting a proper scientific investigation too!

Given that a major aerodynamic package in F1 - costing a lot of money and man hours in R&D - produces gains of a fraction of a second per lap, i reckon the improvements are a pretty good result!

Well, it looks shite imho Maybe if I'd have aligned everything out and would have finished it nicely, it would look ok on a black bumper. But it's not my thing And thanks!

EPIC WIN!!!!!!! I am adding this thread to the links archive i created a while back, i need to sticky that thread so people can find these threads.

Ok, i added this to the links archive now stickied in the general section

Wow, awesome! I'll check out the G-Tech readings tommorow. If I'm right we will have accelerating number in g-forces and speed in second and third gear.

Can you do a run without the bumper?

Interesting results, I wonder if a diffuser would produce the same results? IF designed right could even reduce lift to some extent I expect!