Can diffuser increase performance, speed or acceleration?

K

ksher

Veteran
 Bedfordshire
After having my Z4 roadster over 6 months now, I am considering fitting a rear carbon fibre diffuser on it (I may have to keep my Z4 for 2 to 3 years). I got a fitted price from local BMW dealer, which is £450 including VAT, labour and part. The only outstanding issue is insurance.

My current insurer is Morethan, and the policy does not allow any modifications that increase performance, speed or acceleration. How do you interpret it? The insurance policy expires in 2 months, I can change it to other insurers later anyway. But if I have the diffuser fitted now and something happens later, they will refuse any claims.

My understanding is that rear diffuser helps increasing the downforce (performance???). I spoke to Morethan, and they said if OEM parts are to be used, the cover will still be valid and premium will not be affected. Only when aftermarket parts are used, then they will not cover. Does it make sense?

For CJ/Breaker: Do your exhaust system/diffuser affect your insurance premium? I compared my car with/without diffuser, the difference is between £100 and £150.
 
You kiddin - you should feel just how much faster the car is. Sticks down like an F1 car once I hit the power, spits gravel out at pursuing Mondeos and corners on rails.
Oh - sorry just dreamin' for a moment.....

I doubt if the diffuser adds anything at all to the performance of the car since it's not part of a coherent under body air system. It's purely cosmetic, but I don't tell the kids in Corsa's or Mondeo rep behind :roll:

No change on insurance for me, nor the strut brace, non runflats, m seats, etc. etc. They don't care unless I mess with the motor.
 
cj10jeeper said:
You kiddin - you should feel just how much faster the car is. Sticks down like an F1 car once I hit the power, spits gravel out at pursuing Mondeos and corners on rails.
Oh - sorry just dreamin' for a moment.....

I doubt if the diffuser adds anything at all to the performance of the car since it's not part of a coherent under body air system. It's purely cosmetic, but I don't tell the kids in Corsa's or Mondeo rep behind :roll:

No change on insurance for me, nor the strut brace, non runflats, m seats, etc. etc. They don't care unless I mess with the motor.
Click to expand...

I am surprised that even your quad exhaust system does not affect insurance premium. What insurer are you with?
 
Tesco on a multicar policy.
The only thing they ever got funny about was the hardtop that they thought was a permenant structural modification. Took a bit of explaining that is was removable and the car remained a roadster, not a coupe.

Fundamentally nothing I have done has affected the performance and the policy has a £30,000 flat valuation. If anything was ever damaged in theory they only refit standard.
 
Yeah, the diffuser adds about 25kph in a stright line to the 2.5si top end and adds 0.5 of a G in cornering grip on the skid pan, also takes off 20 secs of the Nurby laps time for the E85 2.5si (say reports). :wink:
 
ksher said:
After having my Z4 roadster over 6 months now, I am considering fitting a rear carbon fibre diffuser on it (I may have to keep my Z4 for 2 to 3 years). I got a fitted price from local BMW dealer, which is £450 including VAT, labour and part. The only outstanding issue is insurance.

My current insurer is Morethan, and the policy does not allow any modifications that increase performance, speed or acceleration. How do you interpret it? The insurance policy expires in 2 months, I can change it to other insurers later anyway. But if I have the diffuser fitted now and something happens later, they will refuse any claims.

My understanding is that rear diffuser helps increasing the downforce (performance???). I spoke to Morethan, and they said if OEM parts are to be used, the cover will still be valid and premium will not be affected. Only when aftermarket parts are used, then they will not cover. Does it make sense?

For CJ/Breaker: Do your exhaust system/diffuser affect your insurance premium? I compared my car with/without diffuser, the difference is between £100 and £150.
Click to expand...

Follow the link for a 'Wiki' explanation of diffusers and splitters - all to do with Bernoulis Theorem!

http://en.wikipedia.org/wiki/Diffuser_%28automotive%29
 
Mine with the diffuser leaves M's for dead!! :P (more to do with the driver mind I think!! :poke: ). :P

Being serious, I doubt if it did have any noticeable effect on performance or handling, at least not driving on the road.

I have found however that the rear doesn't get so dirty in the rain so something is going on aerodynamically.

I'm with Swift by the way and as long as it's OEM, they don't care.

Kicks ass on the looks department though!! 8) 8) :D
 
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