Miata Drag Reduction

Since I now have some comparative data on Miata aero, I wanted to share it.  I've already posted in my 'build' thread, but thought more would see it here.  The aero in question is your basic homebrew Supermiata S2 setup:  birch splitter, plastic airdam and lexan trunk spoiler.   I also added Left Lane Design fender vents, and removed the lower portion of the rear bumper.  I think everyone understands why this setup reduces drag, but I haven't seen 'back to back' numbers posted anywhere.

The car:

1992 Miata, NB VVT/6speed swapped, 4.3 torsen, hardtop.  135rwhp, 2100lbs without driver.  No changes to anything between the data sets other than the aero mods.

Pic before:

Pic after:

Results:

Both data sets were taken at Arizona Motorsports Park, 2.2 mile road course in Phoenix, AZ.  Before numbers are from March 2020 (in black), and After numbers are from December 2020 (in red).  As noted, no changes other than the aero mods, although March was probably ~12-15deg warmer over the day.  Data was gathered via Race Chrono cell phone app.  I've found this app to be pretty good for straightline speeds, but the sampling rate doesn't provide good enough data to evaluate corner speeds.

Quantitatively, the car has gained 2-3mph on every straight.  Some of that improvement may come from increased drive off the corner, but for most of these straights, at 135hp, I'm at full throttle from the apex and was previously.  The exception is the straight coming off the carousel complex, where the improvement was 3mph on a relatively short straight... and this was the one corner I can definitively say the aero made the car more stable and I was able to get on the throttle harder and earlier.

Qualitatively, the car is more stable in high speed corners.  Previously the car was a little touchy and quick to rotate on trail braking, the aero has settled it down, and made it more predictable and easier to drive.  I need a real data system in order to provide a quantitative look at corner performance.

Is there a particular rule book you're building to?

Straight speed can also be affected by corner exit, so if you're able to corner harder it would show up as a higher peak speed on the straight.

If you wanted to check drag, a coast-down test would probably be a good repeatable one. 

Any indication of improvements in brake cooling? I'd love to see hard data from the fender vents. 

In reply to sleepyhead the buffalo :

No.  HPDE car + local TT classes that are primarily hp/weight based.

In reply to Keith Tanner :

I'm not seeing a clear increase in corner speeds in the available data, but as I noted my cell phone app lacks the resolution to pull corner data that I feel is fully reliable.  The carousel complex (between the 100mph and 94mph straights) is the only corner I can definitively say I'm getting a better drive out of and that shows in the 3mph delta on a relatively short straight.  The following straight is about the same length, but only shows a 1mph delta coming off a much slower corner.  I'm comfortable in crediting the improvement on the two longest straights primarily to drag reduction.  I've edited my initial post to remove the impression I'm crediting ALL of the improvement to drag reduction. 

Our roads have become busy enough that a coast down test in a barely "street legal" car is a bit of a challenge, but I'll give that some consideration.

I can't really comment on the effectiveness of the fender vents, as they were done at the same time as the rest of the work.  I can say the ducting to the Singular backing plates is moving a lot more air over the brakes.  I've noticed a huge increase in brake dust on the wheels over even one session.  I haven't noticed a change in braking performance, but this car already has way more brakes than it needs for the current power level (Dynapro radial mount on 11.75" rotors).

The reason I was wondering about trying to measure the drag more directly is because when I put the wing on my car, my peak speed on our track went up fairly considerably. Obviously not due to lower drag, but because I was able to accelerate more on our curved back "straight". It wouldn't have shown up on a lower power car as they usually don't hit traction limits before the braking zone. Not saying this is what's happening here - that's a very specific behavior - but it pointed out to me how I couldn't use this particular metric on my own track and got me thinking about alternate ways to look at it.

That's an interesting side effect of the increased brake dust, and I'd take it as a very positive sign that the fender vents are working as intended - dropping wheel well pressures so the ducts are moving more air. Nice!

Keith Tanner said:

The reason I was wondering about trying to measure the drag more directly is because when I put the wing on my car, my peak speed on our track went up fairly considerably. Obviously not due to lower drag, but because I was able to accelerate more on our curved back "straight". It wouldn't have shown up on a lower power car as they usually don't hit traction limits before the braking zone. Not saying this is what's happening here - that's a very specific behavior - but it pointed out to me how I couldn't use this particular metric on my own track and got me thinking about alternate ways to look at it.

This is a really good point about how to attribute an improvement to a particular change in vehicle dynamics.  I'm assuming you're talking about Grand Junction as opposed to High Plains?  As you said, at GJMS, I probably would have seen very little improvement because of the low speeds.  IIRC, I was only seeing high 60s on the back straight there, and traction under acceleration was not an issue.   For my car at AMP above, I'm at full throttle from the apex leading onto every straight except the one I noted above, so it's almost the opposite case from your car at GJMS.  Better drive was not a factor, and from what data I have, the corner speeds were pretty much a wash in the corners in question, so I'm comfortable attributing the majority of the improvement to drag reduction.

Maybe I should have titled the thread "Aero Improvments"?  

Yeah, GJMS. The straight at HPR would be much more affected by pure drag, but it can be windy so that's some noise in the data.

The limit I hit was not really the limit to accelerate, it was cornering speed on the sweeper that we call a straight. Most cars can't hit that limit so they accelerate through the entire thing. My car hits the cornering limit so it can't accelerate any more - and it gets dodgy trying to put power down when you're already nudging the edge of the traction circle. The wing increased the size of that traction circle enough that my maximum speed through the sweeper went up to 75 mph from 70 or so - I could accelerate again. The front wheels didn't gain much.

Aero is more interesting to me than engine power and so much better suited to fun experimentation.

I'd be curious about tufting around those fender vents.  I reckon the upper one is working fine, but I'm suspicious about the lower actually pulling air back into the fender well... based on some other flow visualizations I've helped with.  But, that'd require a willing second car with some kind of camera device.  Although, you should be able to get pretty good data on any road marked above 55mph.

It looks like you're radiator opening in the front airdam is close to the same size as the radiator?  I know it's Arizona, and it gets hot there, but I think you'd be able to close down the opening if you make a good sealing duct with "gentle-ish" curves to the face of the radiator.  Although, I know that getting the top around the bumper bar structure can be a bit difficult.

The opening is about 2/3ds the area of the radiator.  In the near-ish future, there will be an intercooler in there as well, so I was willing to err on the side of more airflow.