How much does a Diffuser really help?

[aliab]

I run an 89 Swift SE-3, I recently bought it and went through the sfr drivers school. I took it off for drivers school because I didnt want to break it being a novice. I just attended srf regional at thunderhill on may 17-18, and I was only putting out an average of 2 minutes flat with a rare 1:58, while all the other FC's were doing close to 1:50's, a whole 10 seconds faster than me. after race 1 I started asking around and noticed that they were all runnning a diffuser. In the next morning we went out for qualifying and i was trying to stay behind another FC's draft, while being behind him I noticed extream turbulance. and even on the straights he'd pull away from me like i was only in a 1600!! i was only barely able to keep up with a 1600 in the straights and was only able to catch up through the turns. Neil Porter took a look at my carburator because my dad (crew cheif, and ex-FA,FC,FF driver) notice it would load up and ran really rich causeing a misfire at the beginning of my race. so i was also using a poorly built carb as well .

Would adding a diffuser remove alot of the turbulance from drafting behind another FC? does anyone know the laptime i should be doing? and do i need to to change front and rear wing settings to complement the diffuser?

thanks for reading and hope to hear your thoughts on this subject!

Peter Gregor

[Rick Kirchner]

The diffuser will allow you to run more downforce with less drag (if you run the narrow chord low-drag wings vs the pre 1990 wide-chord things), but those 10 seconds of difference are mostly about experience.

If you are worried about going off and tearing it off, leave it off until you get more comfortable, and then you'll go even faster.

For relief from the turbulence, get lower in the car and look at helmet aero aids.

[Bob Wright]

I'll leave the aero specifics to the aerodynamic engineers...short answer is they work. I owned a SE3 and cars in that era did not have highly developed diffusers (DB3, DB6s didn't have them at all), so yours was probably added later.

The diffuser works in conjunction with the wings and you balance the cars with all three

At Pocono years ago (and probably now as well) we took wings and diffusers off the cars. The removal of the diffuser alone was worth 4 mph at the end of the straight, so....I think your speed problem is probably more likely in the engine bay.

[EYERACE]

remember these expressions: 1.speed will come [rephrased]...there's no substitute for seat time. 2.exit speed is king [rephrased] if you can go through the corner faster than your competition, on exit you will hit your top speed sooner thus pulling further away from those slower in the corner

[nulrich]

A diffuser can actually reduce drag while increasing downforce. The efficiency will depend on the overall aerodynamic design of the car, of course. A diffuser accelerates flow between the ground and the undertray, ideally reducing pressure on the entire underside of the car.

At high speed tracks with no aero corners (like Pocono) if the aero design is good you're probably still better off leaving the diffuser on while taking the wings off.

In terms of efficiency (how many pounds of drag for a pound of downforce) a diffuser is the best, the front wings are the second best (ground effect in clean air) and the upper rear wings are the worst (dirty air, no ground effect).

Nathan

[Brett Lane]

Run the car with the diffuser. 1) It was designed that way. 2)You will have to learn to drive the car with the additional downforce and be able to figure out how to balance the car and it's aero package by adjusting the wings( front wing flaps, upper rear wing angle, etc.). 3) seat time, seat time, seat time. 4). If you want to drive a car without aero downforce, drive a FF for a while- a great developmental driving experience.

To give you an idea of just how much downforce a diffuser makes: I have a VanDiemen FB conversion that has run at least three different diffusers. For a while, I ran a FC diffuser, then went to one of Mike Devin's bigger FB specific larger diffuser( a work of art, BTW). To keep the thing from bending and bottoming out, I had to use twice the bracing. Then I had a problem with the whole rear of the car bottoming out and dragging on the track. Eventually, I had to go to heavier springs over the normal FC springs to resolve this. Heavier springs on a conversion that is almost 200lb lighter- think about it...

[andyllc]

I don't think your car was originally designed with a diffuser. Our old Swift DB-3 wasn't. We added one and it did not make a massive difference on that car. On a modern car (ie mid 90's and newer) I would say differently.

[aliab]

Thanks everyone for you responds!
I was planning on running the diffuser anyway so I can get used to it, I was told a lot of different things about diffusers at the track but I think I got the jist of it; less drag, more downforce, better straight line speed!

[Purple Frog]

By design the DB-3 is a very narrow bodied car with it tapering even narrower at the floor. That said, designing a diffuser that will actually work requires some thought. The best that I have seen extend the flat plane of the diffuser forward along the sides of the cockpit for as much as three feet, and as wide as almost 90 cm.
Also, ride height for an efficient diffuser is critical. Some older cars like the DB-3 ran fairly high with soft springs making it hard to keep a diffuser working. Stiffening it up to control chassis roll will help the diffuser work. Just adding a diffuser to an older car, while looking cool, may not make it faster.

IMHO, it's best to copy the development done with the advent of the DB-6 FC car. A great example is the car Cole Morgan took to the 05 Runoffs. If fitted with today's HP numbers (~154) that car may still be very competitive. It's front wing was much like the current Citations. It also had a great rear upper element and diffuser all teamed up with a tiny frontal area. (It also didn't hurt to have James Lee turning the wrenches.)

It is fun to try to improve on some of the older cars, not that you will get as fast as a modern car, but that you will learn a ton about what really works. In the learning process you will learn why the modern cars are faster.

OBTW, a DB-3 is a hoot at a track like Daytona, especially in a draft behind a Atlantic.
Story is a DB-6 turned over 160 at Daytona in the late 90s...when 143 was a lot of HP.

[ccoffin]

So Frog, are you saying that my Dad should take his R.K. Smith DB3 that has been updated to 89 specs, throw in the pinto with today's HP numbers, and plan on making the trip to Daytona next fall in an attempt to beat the land swift FC speed record? Cause I've been telling him that for months now.

Where did Cole and company get their aero stuff from?

Our DB3 hit 146 at the end of the straight at Spokane a few years back on tow from an atlantic, but consistently is about 3 mph ahead of our Mygale at the end of the straight, in current trim, with not nearly as good a pinto pushing it. No diffusor on ours, in regards to the OP. 1.5 seconds per lap slower than the Mygale. I doubt a diffusor would change that by much, if at all.

[gcoffin]

Actually the DB-3 will do 146 mph all on it's own.....towed by the Atlantic it was in the 150 range......it has a strong regional motor with an up rated cam and flywheel. I considered a diffuser, but decided against it because like Frog says, soft springs and ride height......it is typically 3-5 mph faster than the Mygale on a long straight......a really long straight

[SV@RHC]

Peter, You did just fine for being on the full course for the first time. You just cant beat seat time.

I know that a DB6 driven by a very competent driver in 2012 turned a few 1.48's without a diffuser. The Majors just a few weeks ago had a few cars dip into the 1.45's and from what i remember your car has run in the vicinity of a 1.52+-.

The best thing you can do is gain seat time and keep asking questions. it was great being out there with you. Hope to see you out there again, soon!

Scott

[LAJ]

A properly designed total AERO Package (and adjusted) will work better than no diffuser at all !!
The diffuser creates more downforce with very little drag (most of witch is induced ) letting you reduce the higher drag upper wing element to balance things out (creating less overall drag and more total downforce). FF2000 cornering G's .1 more at Mid-Ohio and 1.6 seconds faster a lap.Our corner exit speeds were up and typical 3 to 4 mph faster peak speeds at the end of straights.With the diffuser we could do turn 1 flatout in Qualifying and turn 13 flat with a touch of trail braking. I have also had the same experience setting up Sports 2000s in 2005 SCCA Runoffs with Bart Wolf at Mid-Ohio Qual. #2 @1:26.9 finished #2 on 3 and 4 heat cycle Avons.
PS. remember that controlling the air on top of the diffuser is just as if not more important than the air under!! I do love the smell of methanol in the morning even if it does make my eyes water !!
Lee A. Jordan
My first Indy 500 1977
me on far right