Little Red Devil

[kaanage]

Yeah heaps apparently Clay. A hatchback coupled with a rear torsion beam suspension set up is no friend to efficient aerodynamics.

I don't see the rear torsion beam as being an issue - in fact I think it is better than anything else for clearing the space at the rear. Rear wishbones or a multi-link rear end would place more components in the air path.

Yes I don't really think a perfectly functional rear diffuser is an option with this car.

I think you could make it work if you were to remove the spare wheel well but then you would need to reroute the exhaust to the side directly (use a big resonator instead of a rear muffler?) to make it full width.

[VWindahouse]

I don't see the rear torsion beam as being an issue - in fact I think it is better than anything else for clearing the space at the rear. Rear wishbones or a multi-link rear end would place more components in the air path.


I think you could make it work if you were to remove the spare wheel well but then you would need to reroute the exhaust to the side directly (use a big resonator instead of a rear muffler?) to make it full width.

Greg, sorry I meant that you can't attach a flat diffuser to any part of the beam or over it because of the suspension movement.

I would love to delete the well, but in both categories I'm looking at competing in you can't. Plus I might look at adding a fuel cell for the E85 in it eventually.

[kaanage]

I don't see the air path being long enough with the spare well in the way to prevent early separation in a rear diffuser - there is just just too little overhang in the bodywork, even if you were to try to re-energise the airflow with an exhaust slot at the top.

[team_v]

Are you able to raise the spare wheel well so that it sits higher up in the bodywork (essentially having the top of the spare sit up into the boot floor area)?

[VWindahouse]

You're right, that's why most functional diffusers stick out a fair way past the rear bodywork. But again not allowed in one of the two categories. Plus in both I can only move the exhaust 100mm from the original placement. All leading me to the decision to just reduce drag at the rear.

And I must say even at highway on ramp speeds I can feel a positive difference.

[VWindahouse]

Are you able to raise the spare wheel well so that it sits higher up in the bodywork (essentially having the top of the spare sit up into the boot floor area)?

Ummm I'll check the ruling but I don't think any modification of the chassis is allowed.

[kaanage]

You're right, that's why most functional diffusers stick out a fair way past the rear bodywork. But again not allowed in one of the two categories. Plus in both I can only move the exhaust 100mm from the original placement. All leading me to the decision to just reduce drag at the rear.

And I must say even at highway on ramp speeds I can feel a positive difference.

The other issue with the minimal rear overhang is the lack of cantilever effect which gives any downforce generated by a diffuser greater effect (though that then needs to be balanced at the front with a splitter).

Could you run the exhaust outlets high up through the valance? Say in the middle? That would further help reduce the wake drag over the openings you have made already (which is where I think you are getting the real drag reduction benefit).

[VWindahouse]

Sorry do you mean cut another hole in the remaining top bit of the bumper for the outlet?

[kaanage]

Exactly - then the hot (expanding) exhaust gasses will increase the pressure as close to the rear stagnation point as possible which should reduce the wake drag by the maximum amount (thought experiment going on here ).

[VWindahouse]

Not a bad idea actually. Two things I'll have to measure up. The first is the no greater than 100mm from the original outlet. And the second and the one most likely to kill the notion is having to cut so far up into the chassis and R-bar



So if I can't my back up was always calculated. The plan was to shorten the outlet (now single) and raise the muffler up in the tunnel to be higher than the bottom of the wheel well and torsion bar. That's why I went with a 6" round muffler away from the Milltech which is 8.5" wide 5" tall oval with long 10" dual outlets. So no part of the exhaust will be in the path of airflow at the rear.

Anyhoo I measure up and get back to you