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ST5 Rear Diffuser Clarification


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In ST5 if one does not use the "BTM Aero" mod we are allowed a rear spoiler (within specifications) with no mod factor and a front splitter (4" protrusion rule) with a mod factor. It also appear that a rear diffuser is allowed if it is an OEM option. However, am I correct in assuming that an aftermarket rear diffuser is entirely illegal in ST5?

I see evidence of them being used in ST5 in 2018 but perhaps the rules have changed since then.

Or does "Cutting/Removal of the rear bumper cover/fascia..." somehow include an aftermarket rear diffuser?


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After going back through many old forum posts I think I answered my own question. My understanding is that the only non OEM aero mods allowed in ST5 would be rear wing, front splitter and vertical front air dam.

Someone correct me if I'm wrong.

However, just to be clear, if a vehicle comes from the factory either in BTM or as an OEM option with what might be considered a rear diffuser this is legal in ST5 as long as one is not taking the BTM aero mod factor, correct?

I ask because there are a lot of places the rules say you can't take the BTM aero mod factor if you have an BTM rear diffuser:

For example, ST5-6 rules 6.1.4 says "Vehicles that maintain the unmodified Base Trim Model (BTM) body lines, do not have non-BTM aerodynamic aides or modifications, and do not have a rear wing or rear spoiler or rear diffuser may assess the Modification Factor for "BTM Aero" in calculating the "Adjusted Wt/HP Ratio"."

This makes me assume the BTM rear diffuser would be ok if not taking the BTM aero mod factor.

Edited by Magnuson
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Greg G.

No, rear diffusers are not permitted in ST5. 

All ST5 and ST6 Aero rules start with what is permitted under BTM Aero (not a rear diffuser as you quoted).  Then, the additional items and modifications that are listed in the next two sections of the rules are the only other Aero mods permitted.

So, this is permitted in ST5 and ST6 (but that it in regard to the rear bumper area):
4) Cutting/removal of the rear bumper cover/fascia where it does not cover the
rear frame/bumper cross beam.

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