does diffuser actually improve mpg

romero1990 · 03-06-2021, 03:16 PM

just wanted to share/check an anecdote. after I installed an Hstyle diffuser, the mpg went from 18.2 to like 19.6 so far. always highway drive/commute tho.

roastbeef · 03-06-2021, 04:17 PM

Not significant.

Redd · 03-06-2021, 06:21 PM

I have a full Varis lower diffuser. No it does not.

InnerBlueSkies · 03-06-2021, 06:21 PM

Quote:

Originally Posted by romero1990

just wanted to share/check an anecdote. after I installed an Hstyle diffuser, the mpg went from 18.2 to like 19.6 so far. always highway drive/commute tho.

When it comes to things like this I'm reminded if BMW could achieve an extra 1.4 MPG wouldn't the engineers have just integrated a diffuser into the existing bumper?

EDWYER · 03-07-2021, 02:42 PM

No.

6ixSpd · 03-07-2021, 07:55 PM

Lol.

Navman56 · 03-08-2021, 09:08 PM

Can it? Absolutely. Does it? No.

A well designed diffuser acts to slowly reintroduce the underbody flow into the wake of the car. For a diffuser to work well, you want the floor ahead of it to be as smooth and continuous as possible. See this CFD shot of a Tesla Model S's underbody:

A discontinuous floor often has the flow separate well ahead of the diffuser. Separated flow doesn't 'see' the diffuser, and at that point you just have ballast hanging off the back of your car.

The back half of an M3s floor is anything but smooth and continuous. As most diffusers are just designed for aesthetics, I'd wager 100% of them do absolutely nothing.

Source: Am aerodynamicist

03-06-2021, 03:16 PM	#1
romero1990 Lieutenant 176 Rep 586 Posts Drives: E92 M3, F25 X3 Join Date: Sep 2018 Location: Bay Area iTrader: (0) Garage List 2017 X3 [0.00] 2009 M3 [0.00]	does diffuser actually improve mpg just wanted to share/check an anecdote. after I installed an Hstyle diffuser, the mpg went from 18.2 to like 19.6 so far. always highway drive/commute tho.
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03-06-2021, 04:17 PM	#2
roastbeef Lieutenant General 11585 Rep 12,725 Posts Drives: E92 M3 Join Date: Nov 2010 Location: Orange County, CA iTrader: (4)	Not significant. __________________ Instagram; @roastbeefmike
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03-06-2021, 06:21 PM	#3
Redd Brigadier General 3897 Rep 4,160 Posts Drives: 2010 BMW E92 M3 Dakar Edition Join Date: Jul 2015 Location: Malaysia iTrader: (0)	I have a full Varis lower diffuser. No it does not.
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03-07-2021, 02:42 PM	#5
EDWYER Yoda of mods 1673 Rep 3,004 Posts Drives: X5 RWD Join Date: Feb 2018 Location: NC iTrader: (40) Garage List 2015 X5 RWD [0.00] 2016 BMW M2 [10.00]	No.
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03-07-2021, 07:55 PM	#6
6ixSpd Save the manuals! 6015 Rep 6,745 Posts Drives: '16 M3, '23 718 Spyder Join Date: Feb 2016 Location: 416 iTrader: (4)	Lol. __________________ '16 M3 \| '23 718 Spyder Past: E92 M3, F87 M2, E39 M5, etc
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03-08-2021, 09:08 PM	#7
Navman56 New Member 54 Rep 23 Posts Drives: 2008 E92 M3 Join Date: Feb 2020 Location: North Carolina iTrader: (0)	Can it? Absolutely. Does it? No. A well designed diffuser acts to slowly reintroduce the underbody flow into the wake of the car. For a diffuser to work well, you want the floor ahead of it to be as smooth and continuous as possible. See this CFD shot of a Tesla Model S's underbody: A discontinuous floor often has the flow separate well ahead of the diffuser. Separated flow doesn't 'see' the diffuser, and at that point you just have ballast hanging off the back of your car. The back half of an M3s floor is anything but smooth and continuous. As most diffusers are just designed for aesthetics, I'd wager 100% of them do absolutely nothing. Source: Am aerodynamicist
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