Q's about Stillen diffuser kit

[t32b]

I've been considering swapping the brittle, expensive stock rear diffusers with the ones from Stillen. I feel like I'm living on borrowed time knowing that sooner or later I'll break those things so I'd like to put them aside along with a few other spares. So, a couple of questions:
There were comments some time back that the 'large' diffusers would imbalance the car. Since then, Stillen has come out with OEM-sized versions. That said, the on-line description states that the large ones have had lots of track miles and are the optimum size. Any consensus as to which ones are best?
Re the replacement, do they use the same pan holes as the originals (not sure from the .pdf whether the drilling is in the diffuser or the pan)? I assume the holes are re-used but thought I'd check. No way I'm drilling new holes in the pans.
Any other recommendations or referrals on these beasties? I actually think the large ones look rather cool (i.e. if you're going to have a rear bumper, why not have these too!) but, despite the fact that the car rarely sees high speeds, I'd like to get the optimum design.
Thanks,

 

[shelbyelite]

I have had both here at our shop. I personally prefer them in the stock size. Yes, they will use the stock holes. You will have to drill out the rivets that hold on the stock diffusers. Other than that, it is not that big of a deal. Oh, you will have to remove the belly pan for installation.

 

[kumar]

From what I gathered, the larger sized one will create more downforce but also might hurt you in top speed runs because of the additional downforce.

 

[kmillen]

As Shelby said the only drilling required is to remove the rivets holding in the factory diffusers. We supply new rivets to re-attach the new urethane replacement diffuser's. The entire process (from pan removal to re-installation) usually takes around two to three hours from start to finish depending on your skill and comfort level.

In regards to the diffuser's performance at high speeds. We have not heard of any problems with the longer diffuser's causing an imbalance in the aero stability. We have reached fairly high speeds (around 150-160) in our car and did not feel that the diffuser's were causing any turbulence or imbalance.

Here are the installation instructions:

http://www.stillen.com/product_files/FGT102000~inst.pdf

 

[roketman]

I have a pair for sale if you are interested. They only exit air ,no effect on down force .The only reason the stock ones are shorter is because the factory guys thought they looked ugly.

 

[Indy GT]

They only exit air ,no effect on down force .The only reason the stock ones are shorter is because the factory guys thought they looked ugly.

I am not quite sure what this statement is trying to say....

Sure diffusers exit air, that's their whole purpose. However they DO SO in a very specific fashion; to lower under body pressure AND THUS create down force.
Look closely at any F1 or Indy car specifically under the car or side pods if you can. The aerodynamic wizardy the aero engineers use on these cars is very specific, taylored for a specific car and track and provides huge amounts of downforce to enable faster and faster cornering speeds. It works or the teams would not spend millions of dollars testing and refining their use.

I just have to disagree with any statement to the contrary especially if it is just an opinion and not supported with actual loadcell measurements which I wager no owner has or will conduct. Unlike the Ford team which DID measure these loads and did copious of wind tunnel testing to arrive at the correct pressure balance knowing all the system variables. Again I disagree. The Ford design team hardly made the diffuser geometric selection based on asthetics.

I hate to keep preaching, but those who think they understand these aerodynamic attributes should again review the SAE engineering paper Number 2004-01-1254, "2005 Ford GT - Vehicle Aerodynamics - Updating a Legend". Please don't take my word for this, read the paper for yourself and appreciate what the designers did to arrive at the production configuration.

I have no axe to grind with Stillen. I as others very much appreciate their sponsorship of our Forum and having us over to their facilities at Rally II. I am sure they make some great products which are undoubedly taylored for their highly modified GT for a specific racing circuit. Will they work on an unmodified production car in street driving conditions? Maybe. Will addition of their long runner diffuser change any of our car's handling characteristics? Maybe. Don't know. Havn't seen any data other than someone says the car feels the same at the one condition he drove to. However, the OE Ford selected geometry diffuser was obviously carefully examined, tested and certified as part of our car's aerodynamic interactive "system".

 

[roketman]

My words come from talking with Tom Reichenbach.

 

[t32b]

Not trying to trigger a family feud here, but it is an interesting topic. I don't have easy access to the SAE paper, but will continue to try to get a read. But it's still not clear which way to go. I totally respect that the GT team did incredibly sophisticated work with modern tools to calculate the pressure, balance and flow functions that radically improved the aerodynamics versus the original. OTOH, engineering is all about compromises. It will be interesting to know if they tried and rejected the larger parts because of function, or perhaps appearance, or something else. With my cursory knowledge of aero/hydro, I have some opinions about the net function and tradeoffs of these parts and the impact of the size change. But I'd like to hear from someone on the design team to relate some first- hand knowledge. Again, all very interesting.

 

[Kingman]

....not stock

 

[JimF]

Diffuser

The simple solution is to put the Stillen front splitter on as we..:biggrin

That is what I did. I have run the car at 200 + in the Silver State Classic, and not for a short burst. I was at 165 for the better part of 90 miles. The balance was great. I have run the event with and without the Stillen mods. The car was more stable with the larger pieces at high speeds.

I would have no concern about adding both of these.

Sorry Kyle, but the hardware the Stillen includes for the front is a joke. I tossed it and bought black, button head allen's so they don't show up as bright heads on the black.

Jim

 

[Indy GT]

I have attempted to contact Tom Reichenbach as well as he is one of the co-authors of the SAE paper and could undoubedly give us some background on the subject.

If he happens to read the Forum posts and would give us some insight on the front splitter and rear diffuser geometry, I am happy to accept his guidance. Because his has the data to support his opinions.

So Tom if you are out there can you help us better understand this reoccuring topic?

 

[Ed Sims]

Ford GT Team

The Ford GT Team did the best they could but 2 major factors they had were time contraints & cost.

Ed

PS Short rear diffusers & blocked front ducts are some we've been discussing lately.

 

[t32b]

I'd love to. But the last thing I need is less ground clearance. I don't mind scuffing the snout of my 997 (lowered, GT3 bumper/splitter), but I just can't bring myself to do the same with the GT. But it's the same reason that I want to take the original parts off the rear.

The simple solution is to put the Stillen front splitter on as we..:biggrin

That is what I did. I have run the car at 200 + in the Silver State Classic, and not for a short burst. I was at 165 for the better part of 90 miles. The balance was great. I have run the event with and without the Stillen mods. The car was more stable with the larger pieces at high speeds.

I would have no concern about adding both of these.

Sorry Kyle, but the hardware the Stillen includes for the front is a joke. I tossed it and bought black, button head allen's so they don't show up as bright heads on the black.

Jim

 

[kmillen]

The simple solution is to put the Stillen front splitter on as we..:biggrin

That is what I did. I have run the car at 200 + in the Silver State Classic, and not for a short burst. I was at 165 for the better part of 90 miles. The balance was great. I have run the event with and without the Stillen mods. The car was more stable with the larger pieces at high speeds.

I would have no concern about adding both of these.

Sorry Kyle, but the hardware the Stillen includes for the front is a joke. I tossed it and bought black, button head allen's so they don't show up as bright heads on the black.

Jim

I don't disagree with you in regards to the hardware Jim. I tried talking our engineer's and Steve into using black hardware but I was overruled.

How are those front bumper supports working out for you?

 

[Tiny]

anyone have pics of the 4" longer Stillen diffusers on the car?

 

[Tiny]

Let me clarify, pics with the 4" longer diffuser installed on a bumper delete car thanks

anyone have pics of the 4" longer Stillen diffusers on the car?

 

[dpdive]

I'm lowered 1" in the front and 5/8" in the back. I'm using Tecnocraft's front splitter, side diffusers, and and rear diffuser and all are slightly bigger then stock. Looks great, no issues at speed other then more downforce equals slightly less speed, and although I have to watch some driveway/parking lot entrances I've had no issues with clearance.

 

[cobra498]

I have talked at length with both Tom and Mark McGowan about the stock GT diffuser, they both agreed that the stock diffuser as well as the Stillen version were ineffective because of their height. Mark said the car was far more sensitive to changes in the height of the rear spoiler lip than to changes in the diffuser. The Stillen front splitter is very flexible because of the material used, not a good trait for a splitter, I have one if you are interested.

 

[Indy GT]

I'm lowered 1" in the front and 5/8" in the back. I'm using Tecnocraft's front splitter, side diffusers, and and rear diffuser and all are slightly bigger then stock. Looks great, no issues at speed other then more downforce equals slightly less speed, and although I have to watch some driveway/parking lot entrances I've had no issues with clearance.

dpdive- when you say "no issues at speed" and "more downforce" are these opinions or have you conducted telemetry tests?

What "speed" have you tested to and how much more downforce did you measure?

For our 200+ top speed FGT, the high speed aerodynamics are all integrated (front splitter, rear diffuser, wickerbil, under body NACA channels, etc.) and were developed through much wind tunnel and closed track testing. These are all very important numbers to know and have supporting data to substantiate.