Now that we are re-routing the exhaust to the side of the car our low point in the rear becomes the differential. As I understand it, the further forward we can start the diffuser ramp, the further forward that our low pressure zone will move. Our flat floor would allow me to start a ramp just behind the drivers seat if I can get more clearance in the differential. It seems no one has posted about this before.

For every inch of clearance I can gain about 8" of forward ramp.

It seems there are limited options:
1 - Cut the PPF and re-weld the diff portion eliminating the bottom spacer on the diff. This should add about an inch of clearance.
2 - Replace the diff bushings, eliminating the upper bushing portion, and mount the diff metal to metal on the sub-frame. This seems like a recipe for reliability issues. This would get about 3/4"
3 - Cut and re-weld the subframe diff mount ~0.5" higher. This is the exact clearance to the fuel tank.
4 - Cut and weld a substantial portion of the diff housing so that the pumpkin moves 1.5-2" higher with the same mounting.

From a geometry perspective this will drop the front of the engine slightly, move the transmission up slightly, and should bring the half-shafts closer to parallel.

Has anyone done this before?

Alternatively I could create more organic shapes within our flat-floor and diffuser to bring the expansion area forward but without wind-tunnel testing I don't have confidence in this doing less harm than good.

 

Mill the upper side of the carrier housing mounts to allow it to go higher?

 

to keep suspension geometry wouldn't it be better to shave the chassis rails & 'raise' the rear cradle up into the body (like a body-drop?)


lot of work but simple enough

Rich.

 

The alignment between back of transmission and front of diff can be very sensitive to changes. Not saying don't make changes, but keep in mind test test test; you don't want drive train vibration that destroys components.

 

Without having modeled it, I'm not sure your gains are going to be significant enough (if any) to offset any issues you may encounter with a) drivetrain angles (possibly including length of the rear axles), or b) moving weight higher in the car. What diffuser angle are you using?

 

A small teardrop shaped drop section around the diff seams easier to implement. The fastest Time Attack car in the world from only a few years ago did it that way.

 

I've tried to keep the angle sub 10 degrees, but I think I'm going to increase that to 15. It seems that a higher ride height supports a steeper angle.

Moving 60lbs up an inch would likely be worth it to get 100lbs 8" further forward. If I can generate enough df with the diffuser we can dump the wing and a bigger diffuser could make that possible.

At a minimum I think I'll mod the bottom of the ppf since it doesn't change anything but move bolt heads higher.

 

Just to gather some more information.
Is this a diffuser with tunnels?
Is the diffuser ramped?
What is your wing setup above the proposed diffuser?

It strikes me that going from 10° to 15°as a straight diffuser is just asking for flow separation.
The expansion of the area is desired but you still want it to be driven as hard as possible.
Are you considering a ramped transition at least to help reduce flow separation?
Pictures would be helpful too.

 

Here are some pictures. We will ramp it upward but the floor blends nicely already.

I've been reading this article: https://www.racetechmag.com/2017/08/...ort-diffusers/

The article makes it seem like a much higher angle can be supported.

Our wing is higher, but we will be testing some other positions.

 

Its hard to tell in the photos but we moved the low point higher by about 1.75" and I can move the diffuser throat 14" further forward.

Now I have to get some bolts and remove the vibration dampener on the diff. Depending on performance I may consider moving it higher still.

 

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Well, that didn't work.

 

Is there a story to go along with this?

 

It broke after 3 hours of racing. Not much of a story other than the weld wasn't strong enough. Went back to normal PPF and took first overall on Sunday though.

Moving forward we will just keep using the normal PPF and will make a streamlined cover for where the diff sticks out into the diffuser.