Aero improvements for my old race car; whizzing up a rope?

So being the inate over thinker that I am I've started looking the aero on my Datsun 1200:

I've still got rather big flat street car mirrors on the car. Installing small aero style mirrors (I have a set on the F500 that i autocross that aren't needed on that car) or I could mount convex mirrors on the cage.

The mirrors represent .3 reduction on the frontal area.

I have no air dam on the car; my home track Spring Mountain Motorsports is medium speed and the car over steers so I don't need it for front grip. I exit the key corners at 80mph and top speed is 111. I'm looking to cut drag. My thought is mount the air dam further back and focus on keeping as much air out from under the car.

There are no inner fenders, the firewall essentially forms the backside of the fender well but stops 3" shy of the outer fender so it forms a big air scoop. The alternatives are block this off or channel the airflow out the seam where the leading edge of the door meets the trailing edge of the fender.

The rules stipulate that I need to keep the grill but nothing stopping me from channeling the air into the radiator versus letting it tumble through the grill and slamming into the flat surface that is the core support.

The real over think is things like the vents at the base of the windshield. The from these vents directly inside the car since all of the climate control is removed. So what effect does this have on the aero? Is it possible that this pressurized the interior to the point of keeping air from filling in the side windows? Or is it better to seal the vents in an effort to get the air moving up and over the windsheild.

Drip rails aren't helping but that seem is part of the structural integrety, so make some little fairing pieces or just fill them with clear silicone?

The rear quarter windows are at least 1" inboard of the fender and roof, do I space them out so they are flush?

And here is the rub, I'm sure some or all of,this,would help but here is the reality; when I get placed I the small bore group I run in the top 5 but when I get thrown in with the B-sedan cars (510s & 2002s) my 1676lb 105whp car is dead meat down the straights against 2000lb 180-190whp cars.

The real problem is I'm to cheap and lazy to run a proper race engine so instead I think up creative ways to eek out the equivalent of 7 horsepower.

I'd love to talk more but I have to out to the garage and see if I can mill 6.2 lbs of various brackets on the car because that's worth .28 horsepower.

Tom

Real Flat bottom and diffuser would do worlds of good, I agree with the mirrors. Shaving the drip rails is a bridge to far.

IMO power is easier and cheaper to find then aero improvements.

Not factoring in drag, aero improvements or gear ratios, you need to find another 20-50hp and drop 50-300lbs to be in the same league power to weight wise.

By all means experiment with aero, but I still think regardless of what you do, you'll need to find another 10-15hp minimum somewhere and probably drop some weight to save yourself from being butchered by those more powerful cars once you hit the straight

Block off as much of the grille as possible. It is surprising how little opening you need at track speed.

How does the under hood air get out ?

Rig a true cold air intake with a slight ram effect.

I think aero is even more important the less power you have. Remember, drag goes up exponentially with speed.

Run_Away wrote: I think aero is even more important the less power you have. Remember, drag goes up with the square of speed.

Exponential would be even worse.

This approach is basically what chapman did with the lotus 11's. But, those were smaller open top car.

Would be helpful to have a few pictures to look at.

.

iceracer wrote:

Block off as much of the grille as possible. It is surprising how little opening you need at track speed.

How does the under hood air get out ?

Rig a true cold air intake with a slight ram effect.

This seems like a good start. Make sure all the air coming in towards the radiator isn't spilling away from it, and that the inlet and outlet are sized correctly.

I dig the cold air intake idea. Do you have to run with the headlights? If not, removing one and putting a pickup point for the intake where it was is "the easy button" for this.

Would a small spoiler on the back help any? Curbing some of the oversteer would help your corner speed and as you know, exiting a corner 5 mph faster translates to that much more speed through the entire next straight. (I know, I know, you're smarter about this than I)

My point being, not all aero work needs to be in making it slippery, some of it can be to aid grip.

Speaking from a position of complete ignorance, I'm wondering about those windshield vents. Even if the air flow isn't treating them, and the interior of the car, as a giant air brake, it's at least got to be really turbulent and messy around there. I would definitely see about sealing them up and re-evaluating what's going on. You might even get better, cleaner flow heading up and over to KyAllroad's rear spoiler.

Did anybody race these cars back in the day? That is, are there any old photos, team notes, magazine features, etc. somewhere that you could get aero ideas from?

Is this what we're in?

Undertray for engine compartment. Don't forget a way out for air. Seal radiator with shroud. I like the cold air intake off a headlight. Air dam/small splitter. Then find another 30 hp

I like an air dam as far forward as possible, then with all of the air being forced around or over the car you can limit the amount that enters the engine compartment via grill restrictions behind the grill cover.

Not the same thing but I had a Ford van that was a big brick on wheels. To improve aero (LOL!) I blocked off 50% of the radiator openings which still left a radiator size "hole" for air to enter. The van never overheated! I also added an air dam below the bumper which is the most forward part of the van. What this all did was improve "MPG's" by a small bit but really helped keep the van stable at speed and crosswinds so it was doing something.

sleepyhead wrote:

Run_Away wrote: I think aero is even more important the less power you have. Remember, drag goes up with the square of speed.

Exponential would be even worse.

(pedant mode)

Squared IS an exponent.

In reply to Knurled:

but with the, and the...

awww, E36 M3. Busted.

wearymicrobe wrote: Real Flat bottom and diffuser would do worlds of good, I agree with the mirrors. Shaving the drip rails is a bridge to far. IMO power is easier and cheaper to find then aero improvements.

I don't know about power being easier/cheaper,I'd work aero 1st as it'll improve corner grip and straightline speed unless massive wings are involved.

Getting the majority of the air to go around and over the car smoother while using a flat bottom and diffuser for the air that has to go under the car really costs mostly effort and minimal cash outlay.

Once the aero is sorted and the car able to use more power then add power,makes a bunch more sense to me then adding only power for speed on the straight bits.

On a more GRM (and likely rules friendly) note, the 1200 seems to have a relatively flat floor, so I'd add a couple 1-2" tall airdams just forward of the leaf spring mounts to help move air around / under the rear suspension:

There's lots of good advice happening in here so far I think. Hopefully I can add. You can do some of your own aero testing and research by sticking a bunch of (in aviation) yaw strings to the car and see where you can make changes to smooth things out, where positive and negative air is etc.

I ran an air dam on my production MG car with less power than you have. I ran aluminum down to within a couple of inches of the ground and then a piece of rubberized canvas conveyor belting material down to about a half inch off the ground (under braking it would contact the pavement and be very slowly ground away). Picked up 400-500 rpm on a long straight that way.

JG did a FB broadcast a while back testing aero on their Mustang build w/ Magnehelic HVAC pressure differential gauges, quite informative. Gauges are $60-70 online. Broadcast archive should be on FB page.

Ok first since I'm bad at posting pictures here is the link

http://datsun1200.com/uploads/photos/26131.jpg

Second @Kyallroad the car is purposely set up so that once it starts rotating it wants to keep gently rotating so that you never have to lift off. Once the exit trajectory is dialed the gas pedal is pinned to the floor. If the car had 40 more horsepower it would need a different set up but even with and extra 15-20 it won't be an issue. Note also the corner speed is higher than most of cars I run with.

1200s have 17.5 ft of frontal area and a CD of .42

The rules are such that any under tray cannot extend back past the leading edge of the front fender opening.

As for exhausting the air out of the engine compartment that's a tough one because the coach work has to basically remain standard other than air dam & bolt on fender flares.

The gearing is fairly low; 4.38 with 20.6 tall Hoosier bias ply 185/60-13. As well as the rolling resistance being lower with a 185 width tire, they also only way 11lbs a piece.

Tom

wspohn wrote: I ran an air dam on my production MG car with less power than you have. I ran aluminum down to within a couple of inches of the ground and then a piece of rubberized canvas conveyor belting material down to about a half inch off the ground (under braking it would contact the pavement and be very slowly ground away). Picked up 400-500 rpm on a long straight that way.

I am not processing this right. You sayin' that putting an air dam on the car REDUCED aero drag? I would have thought that making a big low pressure area under the car would increase it.

Knurled wrote:

wspohn wrote: I ran an air dam on my production MG car with less power than you have. I ran aluminum down to within a couple of inches of the ground and then a piece of rubberized canvas conveyor belting material down to about a half inch off the ground (under braking it would contact the pavement and be very slowly ground away). Picked up 400-500 rpm on a long straight that way.

I am not processing this right. You sayin' that putting an air dam on the car REDUCED aero drag? I would have thought that making a big low pressure area under the car would increase it.

Reading that, it sounds to me like wspohn made a front airdam that was in contact with the ground. That wouldn't necessarily create a low pressure area under the car (afaik, air from the engine bay would get pulled into that area at whatever speed the sides and back of the car could suck it out (after having been slowed down by going through the radiator and engine, etc. So it'd be pretty close to atmospheric under the car, I'd expect.)

And if it ended up being slightly lower pressure under the car compared to "unity", then if anything it'd be good 'cause that low pressure would be helping to suck the car down.

.

Tom1200 wrote:

1200s have 17.5 ft of frontal area and a CD of .42

The rules are such that any under tray cannot extend back past the leading edge of the front fender opening.

As for exhausting the air out of the engine compartment that's a tough one because the coach work has to basically remain standard other than air dam & bolt on fender flares.

The gearing is fairly low; 4.38 with 20.6 tall Hoosier bias ply 185/60-13. As well as the rolling resistance being lower with a 185 width tire, they also only way 11lbs a piece.

can you list your gear ratios?

when you're at 111mph, what rpm are you at (estimated) compared to the rpm you're making 105hp at?

Those are some pretty restrictive rules... can you post a link in case I get some time to meander through them?

Clearly making a flat bottom isn't legal, so making a defuser is out too. Making the biggest air dam you can, that covers all the way out to the tires, and helps turn the flow so it goes outboard of the tires is probably your best best.

There's an outside chance that putting a spoiler on the back will help too... because the converse curve of the back window/trunk is most likely seeing "separated flow". Adding a spoiler could help give that flow something to attach onto. Tufts on the back window will look "limp" if the flow isn't attached, and will follow a "front to back" line if it's attached.

If you build a spoiler for back there, make sure to build one that you can try different heights, there'll probably be a point where you'll just be adding drag (I think you'd probably top out around 6" max).

You might look at the V12 MG build on here, the last couple pages have some flow visualizations that might help you see what's going on.

Air dams work because they keep air from going under the car but around and over it. The "outside" of a car is smooth and more aero then the "bottom". Also any air that goes under the car is turbulent air and that hurts aero and creates lift. An air dam also creates a low pressure area just behind it that helps pull air through the radiator and out of the engine compartment. This air doesn't have to fight it's way out like it would if you had no air dam. That's why you can reduce the grill area and still keep the engine as cool if not cooler.

Would some type of side skirts be an option? They'd pair nicely with the airdam to keep air from tumbling around under the car.

When installing an air dam be sure to allow some air flow to cool the brakes.

I have a friend that didn't.

Some years ago, Car and Driver mag. showed that an air dam will improve fuel mileage. Less drag.