The Mustang Forum for Track & Racing Enthusiasts

Taking your Mustang to an open track/HPDE event for the first time? Do you race competitively? This forum is for you! Log in to remove most ads.

  • Welcome to the Ford Mustang forum built for owners of the Mustang GT350, BOSS 302, GT500, and all other S550, S197, SN95, Fox Body and older Mustangs set up for open track days, road racing, and/or autocross. Join our forum, interact with others, share your build, and help us strengthen this community!

Ask AJ Hartman - Aero Questions

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Fabman

Dances with Racecars
6,696
8,483
Exp. Type
W2W Racing
Exp. Level
20+ Years
Pleasanton: 1/2 way between Sonoma and Laguna Seca
Excited to hear your feedback once you get it out on track.
Completely eliminated any high speed understeer. Turns I used to have to hold my breath on are now easy as pie. Definitely a major improvement. I just wish I could get even more tunnels in there.
 
I honestly think that would be the best way to really cool these things.

AJ: Have you tested the different versions of the nose?
Unfortunately I have not. And since tunnel time is very expensive, any back to back tests on such small variables as front bumper shape is unlikely for me to do. Theres just to many variables to make quick changes in the tunnel. Main example is if you duct your rad to a GT350 bumper, but then swap to a different bumper and that ducting doesn't line up right, the numbers are gonna be quite off as its not an apples to apples comparison.
 
353
359
Exp. Type
Autocross
Exp. Level
20+ Years
So Cal
I really looking at the Mach1 front bumper.
I just dont like the 2 round things in the front grille.
But i feel is a great.front shape. Much more than 350 or 500 which seem odd

Maybe pull dedicated caliper cooling off it?
 
562
715
Exp. Type
Time Attack
Exp. Level
3-5 Years
SoCal
Unrelated to mustangs, but I was reading about the new 911 GT3 RS and all the attention they put into the aero.
I noticed (correct me if I'm wrong) they claimed the rear wing made 70% of the car's total downforce (so a 30/70 split F/R).
I get that manufacturers want to make cars safe for your "average driver" which is why so many cars understeer from the factory, but this aero imbalance seems extreme, especially for a car that is supposed to be the best handling 911 ever.

I didn't link any sites since none of them seem to agree with each other. Some say the wing makes 900 lbs at 124mph and others say the car makes 900 lbs @ 124
 
353
359
Exp. Type
Autocross
Exp. Level
20+ Years
So Cal
Unrelated to mustangs, but I was reading about the new 911 GT3 RS and all the attention they put into the aero.
I noticed (correct me if I'm wrong) they claimed the rear wing made 70% of the car's total downforce (so a 30/70 split F/R).
I get that manufacturers want to make cars safe for your "average driver" which is why so many cars understeer from the factory, but this aero imbalance seems extreme, especially for a car that is supposed to be the best handling 911 ever.

I didn't link any sites since none of them seem to agree with each other. Some say the wing makes 900 lbs at 124mph and others say the car makes 900 lbs @ 124
My take...

You're unlikely to get anything real on the aero numbers. The wing might make both 900# and 70% of the total downforce, but that does not mean the car has a 30% front aero. We don't know what the body is doing.

Having said this, you never see much of a splitter on a Porsche. Why is that? It's because a Porsche, specifically the 911 derivants, all have a high percentage of weight on the rear of the car. Because of that, they have to have an even higher percentage of rear downforce (to induce a little aero understeer, which is good). They don't need to make front downforce for the same reason they don't run large front tires. They can't use the grip.
 
562
715
Exp. Type
Time Attack
Exp. Level
3-5 Years
SoCal
My take...

You're unlikely to get anything real on the aero numbers. The wing might make both 900# and 70% of the total downforce, but that does not mean the car has a 30% front aero. We don't know what the body is doing.

Having said this, you never see much of a splitter on a Porsche. Why is that? It's because a Porsche, specifically the 911 derivants, all have a high percentage of weight on the rear of the car. Because of that, they have to have an even higher percentage of rear downforce (to induce a little aero understeer, which is good). They don't need to make front downforce for the same reason they don't run large front tires. They can't use the grip.
Thanks for the reply.
So the aero balance is directly related to the weight distribution - makes sense.

I was always curious why Porsches have good sized wings but non-existent splitters. Thanks
 
210
322
Exp. Type
Time Attack
Exp. Level
10-20 Years
75024
Thanks for the reply.
So the aero balance is directly related to the weight distribution - makes sense.

I was always curious why Porsches have good sized wings but non-existent splitters. Thanks
Also most porsches since the 2000's have flat, paneled floors and noses with brake cooling ramps/tunnels from the nose. That and the sloping, low hood are probably making good usable downforce to balance the wings. And the GT** cars had radiator venting directed into the front brakes and sometimes over the hood (center radiator). Lots of freedom for that with no engine up front.

I chuckled to see the new gt3rs has a DRS button to flatten the rear wing flap for 'passing'. Just the thing for point by's.
 
353
359
Exp. Type
Autocross
Exp. Level
20+ Years
So Cal
Thanks for the reply.
So the aero balance is directly related to the weight distribution - makes sense.

I was always curious why Porsches have good sized wings but non-existent splitters. Thanks

Yep, exactly. The Porsche doesn't need a ton of front aero, but you still have to balance it. The Cup cars have a little plastic air dam piece on the front of the nose. It's sacrificial, so if you go off, it's gone. When it's gone, there's a _big_ aero understeer.


20220915_204516.jpg
 
6,674
8,773
Yep, exactly. The Porsche doesn't need a ton of front aero, but you still have to balance it. The Cup cars have a little plastic air dam piece on the front of the nose. It's sacrificial, so if you go off, it's gone. When it's gone, there's a _big_ aero understeer.


View attachment 79087
Porsches are probably a bad example, one of the things that are readily apparent is their tendency to totally dump the rear end once it goes past a specific yaw angle, it was clearly apparent in Pcup, and in some of the SCCA runoffs where the point of no return is immediate. I'm assuming some of this is the pendulem (engine) at the extreme rear of the car, and the trade off is better braking ability on all 4 wheels... in a straight line anyway. Since every sanctioning body disallows aerodynamic devices that are moveable under power, I think they just live with the barn door wing, we used to drop the rear of the car, after exhausting all the wing adjustments to try and get all the straight line speed we could. Anyway, I have seen more Porsches than I ever wanted to, except maybe in a container ship heading to the ocean floor, but that's my take on the Porsche wing.
 
Last edited:
6,674
8,773
@AJ Hartman
My 2007 Gt500 keeps pulling timing and occaisionally showing intake air temp codes, I've done everything possible to divorce the cold air intake from the engine compartment heat, but it still gets plenty hot. Since I don't want to carve up a Gt500 hood, would you happen to make, or could suggest a modification, to the existing louvres in the hood that seem marginal at best with regards to removing underhood air. It is a street only car.

20221013_125802.jpg
 
Last edited:
353
359
Exp. Type
Autocross
Exp. Level
20+ Years
So Cal
Porsches are probably a bad example, one of the things that are readily apparent is their tendency to totally dump the rear end once it goes past a specific yaw angle, it was clearly apparent in Pcup, and in some of the SCCA runoffs where the point of no return is immediate. I'm assuming some of this is the pendulem (engine) at the extreme rear of the car, and the trade off is better braking ability on all 4 wheels... in a straight line anyway. Since every sanctioning body disallows aerodynamic devices that are moveable under power, I think they just live with the barn door wing, we used to drop the rear of the car, after exhausting all the wing adjustments to try and get all the straight line speed we could. Anyway, I have seen more Porsches than I ever wanted to, except maybe in a container ship heading to the ocean floor, but that's my take on the Porsche wing.

Honestly, I haven't really had that experience. I've heard all the Porsche handling horror stories, but I've never experienced them myself. (That's a lie. The first Cayman GT4 was mess because they'd coil-bind and Jorg set them up crazy loose. New springs & rates were a big help.) Yes, they're unusual with the rear engine and 4 struts (fewer, now), but I don't find them difficult to make go around the track at a pretty good clip. Anything is tricky when scrounging for the last tenth, but I find modern Porsche's have a larger window of operation than most of their competition.

The older stuff, no question, there were all sorts of funky geometry problems and those look like nightmares. I'm really talking about anything 996 or later (the last 20 years).
 
6,674
8,773
Honestly, I haven't really had that experience. I've heard all the Porsche handling horror stories, but I've never experienced them myself. (That's a lie. The first Cayman GT4 was mess because they'd coil-bind and Jorg set them up crazy loose. New springs & rates were a big help.) Yes, they're unusual with the rear engine and 4 struts (fewer, now), but I don't find them difficult to make go around the track at a pretty good clip. Anything is tricky when scrounging for the last tenth, but I find modern Porsche's have a larger window of operation than most of their competition.

The older stuff, no question, there were all sorts of funky geometry problems and those look like nightmares. I'm really talking about anything 996 or later (the last 20 years).
We were down to trying to eak out a MPH or 2 down the straights, pile that on to the minimum air pressure requirement at the start of the race, which meant a 5 plus psi difference front o rear until they warmed up, minimal bars and springs.. what a PITA Pcup was for me.
 
@AJ Hartman
My 2007 Gt500 keeps pulling timing and occaisionally showing intake air temp codes, I've done everything possible to divorce the cold air intake from the engine compartment heat, but it still gets plenty hot. Since I don't want to carve up a Gt500 hood, would you happen to make, or could suggest a modification, to the existing louvres in the hood that seem marginal at best with regards to removing underhood air. It is a street only car.

View attachment 80255
Only thing without cutting up the hood would be some gurneys or similar in the front of them. I've seen them help quite a bit on cars with larger openings, all the way to almost nothing on my GT350 stock hood, which has an extremely small stock opening. Yours is larger than that so it should help some but wouldn't be what I'd consider a big gain.
 
@AJ Hartman
My 2007 Gt500 keeps pulling timing and occaisionally showing intake air temp codes, I've done everything possible to divorce the cold air intake from the engine compartment heat, but it still gets plenty hot. Since I don't want to carve up a Gt500 hood, would you happen to make, or could suggest a modification, to the existing louvres in the hood that seem marginal at best with regards to removing underhood air. It is a street only car.

View attachment 80255
The problem is the extremely tight vents combined with the fact that a portion of the vents are covered by the inner sheet metal. No one that I know of makes custom hood vents anymore so you're in a bit of a bind without either cutting up the hood or finding someone willing to fabricate custom vents for you.

35213530223_1fc4ea41b8_o.jpg
 
The problem is the extremely tight vents combined with the fact that a portion of the vents are covered by the inner sheet metal. No one that I know of makes custom hood vents anymore so you're in a bit of a bind without either cutting up the hood or finding someone willing to fabricate custom vents for you.

View attachment 80268
Yeah, kinda a crap setup no matter what. But since Blacksheep asked for any options without cutting the hood, a gurney or ramp on the leading edge is about your only option.
 
6,674
8,773
Let me guess, I prolly need an AJ Hartman custom carbon fiber hood to solve my problem.
And I'd buy one too, the cowl panel you made for my SN95 was still on the car when I sold it after 10 years.
 

TMO Supporting Vendors

Top