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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > Tracking, Autocrossing, Dragstrip, Driving Techniques > How much drag can a spoiler add?



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      01-15-2019, 12:04 PM   #1
Biginboca
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How much drag can a spoiler add?

I recently swapped my trunk spoiler from the m3 style trunk lip to the performance looking high kick.

This is the old trunk lip spoiler:




This is the new spoiler on the car:



I did a 1/4 mile run in April on the left with old spoiler. Since then I added the bigger spoiler and some power mods which showed the faster run on the right:




I expected it to be quicker than April’s, but honestly more like 1-2 tenths quicker, not like 1/2 tenth quicker. Then I looked at the 1/8 mile times and speeds. Today was 2.5mph faster at the 1/8 but only about .5mph faster at the end of the quarter. Somehow I lost the 2mph advantage at the 1/8 in the speeds that followed from 80-102mph over the second half of the quarter mile run.

Can the difference in the drag from that larger spoiler be significant enough to cost me like 2mph over the second half of the run?

Last edited by Biginboca; 01-15-2019 at 12:11 PM..
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      01-15-2019, 12:19 PM   #2
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Spoilers do not generally add drag. They create a pocket of high pressure which allows air flowing over the roof to pass over the rear of the car more easily. This generally reduces drag and rear end lift. A wing uses that airflow to generate down-force which is where added drag can become significant.

Dragy is a decent app but you're talking about comparing different conditions, different dates, different mods, different roads, different slopes, etc. Way too many variables. Spoiler surely isn't one of them though LOL.

Last edited by bbnks2; 01-15-2019 at 12:32 PM..
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      01-15-2019, 12:39 PM   #3
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Quote:
Originally Posted by bbnks2 View Post
Spoilers do not generally add drag. They create a pocket of high pressure which allows air flowing over the roof to pass over the rear of the car more easily. This generally reduces drag and rear end lift. A wing uses that airflow to generate down-force which is where added drag can become significant.

Dragy is a decent app but you're talking about comparing different conditions, different dates, different mods, different roads, different slopes, etc. Way too many variables. Spoiler surely isn't one of them though LOL.
Well it’s the same exact stretch of road and temp was within 2 degrees on those runs. Also no wind on those runs either both runs at 4:00am on a cool perfectly still night. Same driver too... The only variables are the changes to the car really.
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      02-12-2019, 09:11 AM   #4
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I checked out your build thread and think your dissatisfaction might be due to at least one of these 2 points:

1. You increased your throttle and diameter of the airbox-throttle pipe. Considering your car is NA, the diameter might be too large and might work against anything else you've done for power, even if you added a second airbox inlet. As I understand it, increasing the throttle body size has absolutely zero benefit for the N52, but in combination with larger piping, you've probably altered the way air flows. You'd figure that larger piping means "more available air," but if the suction of the engine isn't matched to the pre-intake piping, larger pipe diameter will decrease airflow in that section. Additionally, your piping is made out of silicone, and unless it's internally reinforced, the shape can deform as well (not saying that it is, but that it might).

2. Swapping your front brakes and adding rear spacers have added rotational mass at the wheels, so part of your acceleration and even torque performance increase is being negatively offset.

And as bbnks2 mentioned earlier, it could additionally be due to a combination of a whole array of other small conditions. Your alignment could be off. The tune may not be optimized for your current setup (if you bought from BPC's product webpage). I know BPC's website doesn't list some of the mods as tuning options you've made to the pre-engine piping. Having added additional oil piping may be messing with oil pressure. Ambient temperature and humidity are probably different, and drivers, which are the most significant variable, probably don't drive perfectly or consistently in harmony with the car even if it's their job to do so. I think that's the reason autocross seems to be so much more popular on this board--knowing and driving your N51/N52 car better may simply have greater reasonable access to better numbers than modifying the engine.
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      02-14-2019, 06:59 PM   #5
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Quote:
Originally Posted by Biginboca View Post
Well it’s the same exact stretch of road and temp was within 2 degrees on those runs. Also no wind on those runs either both runs at 4:00am on a cool perfectly still night. Same driver too... The only variables are the changes to the car really.
How do you like dragy? Same stretch of road why are the slopes different ?
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      02-14-2019, 07:02 PM   #6
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Quote:
Originally Posted by Sgop335 View Post
How do you like dragy? Same stretch of road why are the slopes different in the stamps?
I like the dragy a lot! It has allowed me to test mods easily and save hundreds on dyno fees. It’s a really great tool for anyone who’s chasing power mods.

I don’t have an answer why the slopes are different but obviously it’s some error in Dragy. Both those runs were started on the same line on the same road going the same direction.
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      02-17-2019, 08:14 PM   #7
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      10-02-2019, 03:05 AM   #8
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the spoiler is insignificant at those speeds. the biggest variable is the launch.
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      10-03-2019, 04:05 PM   #9
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also a few extra gallons of gas can make a small difference as well.

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