Presented by Nine Lives Racing
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jj HalfDork
1/3/18 6:32 p.m.

My local club has relatively fast courses with several long 50mph sweepers.  Here is our last event showing the sweepers. (not my car or me driving)

The past season I finished second, but I am 1-2 seconds off the pace.  Could some basic aero get me that 1-2 seconds?  How much would I need?  How would a basic splitter and spoiler do?  Can Spoilers and diffusers be used together?

The car is a 1994 mustang.  The classes are open except treadware.  

This winter I have if Im lucky $400 in my project car budget.  Google provides a lot of guesses and opinions.  Does anyone have experience with the more budget aero and possible gains?


GameboyRMH MegaDork
1/3/18 7:52 p.m.

I'd guess you could get a full second, maybe two on that course with a really good aero setup. Diffusers and spoilers work better together than alone!  I'd say you should try a diffuser, spoiler and front flat bottom with no splitter to start with.

Driven5 SuperDork
1/3/18 8:23 p.m.

How well prepped for the class is your car currently?  Trying to use aero as a band-aid for other shortcomings is generally not the most effective solution. A 'really good' aero setup starts with a really good setup... And a really good/consistent driver that knows how to utilize it. Without both of those, I doubt you'll be gaining seconds... But I'm willing to be proven wrong too, so don't let my skepticism hold you back either.

KyAllroad (Jeremy)
KyAllroad (Jeremy) PowerDork
1/3/18 9:12 p.m.

How balanced is your car?  If it's understeering then you need more front grip, if oversteering then a street prepared style spoiler might be the way to go.  

Is your alignment optimized?  Often a really aggressive alignment is the biggest factor and then shaving tenths with aero.

Tom1200 HalfDork
1/4/18 12:22 a.m.

So if like the Cayman in the video you're on & off the throttle a lot in said sweepers and the reason you're on and off the throttle is the car starts to progressive wash wide then yes I thing aero could help.

The first step to figuring out where and why you're losing time; is it in said sweepers or is it during transitions etc?

If transitions are the problem I would start with the dampers. 

On a piece of paper down all the places where you know "you" can go faster; then break those sections down into entry / steady state cornering / exit.  Break those three sections into understeer /  neutral / oversteer. After all that note the things that could possibly improve.  Damper settings, alignment, tire pressures etc.

After all that then move on to aero; if you can put the power down on the exit but you get that slow building push then an effective splitter could cure that. If you can't ge the power down or oversteer increase with speed then a rear wing or barn door spoiler would likely help that.  

I think you could expect at least .5 and likely even a second. I think two is a big ask. Those coupe of really tight spots there are monster, anybody who can really get the power down there is going to make up as big a chunk as they will in the sweepers. Now if you can get the car to make rapid transitions in those tight sections and then get high speed stability with aero for those sweepers then yes you'll definelty make up those 2 seconds. 

STM317 Dork
1/4/18 5:10 a.m.

I'm not sure there is such a thing as "too much grip" when it comes to autocross. For your budget, I'd guess that it's the best option available to increase grip, and may decrease drag for higher speed stuff as well. Coroplast is fairly cheap and easy to modify if you're looking to experiment.

Apexcarver PowerDork
1/4/18 5:20 a.m.

I know the platform pretty well (run my 97 for 13 years now), whats the setup on the car.   There may be MUCH lower hanging fruit.  (especially in the suspension, some cheap options which can reward big)


I dont see you gaining beyond a tenth or two with aero.  These are heavy cars and you would have to generate big loads to make much difference, which is hard to do at 50mph.


So, what parts are on it and what are the alignment settings, spring rates, etc?

jj HalfDork
1/4/18 9:11 a.m.

My setup is a fairly common one.

Suspension: Koni yellows, HR super race springs, all bushings, Rear lower control arms, MM panhard bar, MM camber plates.

Chassis: MM SFCs, stock strut tower brace, solid steering rack mounts.

Brakes: stock with better pads and fluid, also stainless steel lines.

Alignment: IIRC -2.5 camber, 0 toe, caster ~4.9 right before bump oversteer.

Weight is 3100 without me.

Tires: Azenis 615 K+ 245's.  Im in the 200tw class.

Balance: The balance is basically where I want it to be, or at least as good as I can get it.  It's perfectly balanced in the 50mph sweepers.  It pushes a little in the very slow turns.  The down side is in high speed transitions or braking for decreasing radius turns, the tail likes to step out.

Driving:  I find my biggest problem is concentration.  It seems really hard for me to keep my concentration to get everything right.  Especially with the mustang since it is not a precise car to begin with.  Aside from that I would probably have to ask an instructor to sit with me, I can't think of any big areas to change.  I may be braking too early during decreasing radius turns because Im worried about the rear stepping out, but sometimes I can get the car to drift slightly while turning in which is helpful.

Power:  with only 200whp or so, putting the power down is only a problem coming out of slow right hand turns.


jstein77 UltraDork
1/4/18 9:16 a.m.

"Tires: Azenis 615 K+ 245's"

A switch to Goodrich Rival S in the widest size that will fit would get you those 2 seconds.


KevinLG New Reader
1/4/18 9:30 a.m.

First thing on the list would be bigger tires for that car.

jj HalfDork
1/4/18 9:39 a.m.

Ultimately I want RE71s in 275 width.  But the tires I have are in good shape still. 

My question isn't necessarily how to get better times by any means, but I am trying to figure out if DIY aero is worth it or not since that is all I can probably do this winter.

jstein77 UltraDork
1/4/18 9:48 a.m.

I have a similar situation with my RS.  I originally bought OEM-sized RE71's, then found out that others are running 2 or 3 sizes wider.  At first I was competitive, but as more heat cycles ensued, I'm finding the wins becoming more difficult.  I think we both need to bite the bullet and spring for wider tires before the old ones are completely used up.

But to answer your question, yes, aero will help some but probably not that full two seconds you're looking for by itself.

jj HalfDork
1/4/18 9:57 a.m.

jstein I admit I would like to, but I can't see throwing that much money away.  even after 60 or 70 autocross runs, these tires still have 50% of the tread.  And if I can figure some simple aero that works, that will be even better when I have better tires.

z31maniac MegaDork
1/4/18 10:14 a.m.

If that's all you can afford over winter, then go ahead and go for it. I can't see how it's going to hurt, unless you just end up making some poor decisions in design/execution and upset the balance of the car. 

MrJoshua UltimaDork
1/4/18 10:16 a.m.

If the rear is stepping out on high speed sweepers it seems like you would be the perfect candidate for added rear downforce. It would help with rear grip as speeds increase without you having to make suspension changes that exacerbate your push in slow corners.

pjbgravely Reader
1/4/18 10:25 a.m.

Simple aero for the Mustang would be a grill block and a front belly pan covering to the back of the engine. With the pan you can add a splitter later.  The SN-95's grill doesn't help cooling, I believe it is only for idling in traffic with the AC on. My tests showed cooling was much improved with the grill blocked.

I had a belly pan for gas mileage but I had to remove it for autocross. I made it out of  an fiberglass sheet and aluminum.

If you don't need back downforce, removing the wing/spoiler will make you a little faster. I had to put mine back on for stock.

jstein77 UltraDork
1/4/18 10:39 a.m.
pjbgravely said:

My testes showed cooling was much improved....


Fishnet pants?

pjbgravely Reader
1/4/18 11:11 a.m.
jstein77 said:
pjbgravely said:

My testes showed cooling was much improved....


Fishnet pants?

Thanks for the heads up.

Driven5 SuperDork
1/4/18 11:12 a.m.

In reply to jj :

Can you clarify what you mean by "all bushings" in your suspension.  All rubber bushings?  All polyurethane bushings?  Some of each depending on location?

jj HalfDork
1/4/18 11:14 a.m.

This article has some good info I think.

Sounds like a belly pan, air dam (even without a splitter), and a spoiler might be worth a try.

jj HalfDork
1/4/18 11:15 a.m.

Oh, all poly bushings, solid strut top mounts, rear LCA are poly/ spherical. PHB are sperical.

Driven5 SuperDork
1/4/18 11:24 a.m.

What about rear UCA's?  With your panhard, those are still rubber, right?

jj HalfDork
1/4/18 11:38 a.m.

Yes, uppers are 23 year old rubber. I left both in place also.

Trackmouse UltraDork
1/4/18 11:41 a.m.

If you are consistently off by a 1-2secs from the same guy, then focus on just beating him. What car is he running? What tire manufacturer? Does he have aero?

RevRico UltraDork
1/4/18 11:50 a.m.
Trackmouse said:

If you are consistently off by a 1-2secs from the same guy, then focus on just beating him. What car is he running? What tire manufacturer? Does he have aero?

I would even add to this and have that person drive your car. Or as was mentioned further up the page, act as a co driver for a few runs. 

Maybe there are just some little things you could adjust or improve on in your driving style or corner entry/exit that could make up some of the difference. 

I know where the video is but I have a gif I'll go look for, it's a split screen video of me and a local hot shoe driving my car, the gif is specifically where I lost 3 seconds on my run from a bad transition. 

He was acting like a rally co driver, telling me where to turn/what to do. But just going through this 180 a little off cost me 3 while seconds on the run. 

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