Presented by Nine Lives Racing
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9/24/22 10:30 p.m.

I've been lurking here for a while, mostly following along with the LS1 Kart build. I have my own Kart based on a C4 Corvette. I autocross and do HPDEs with the car. The car is just under 54%/46% weight front/back so downforce in the back is important. The bottom of the car is pretty flat with the exception of the drive shaft tunnel between the front and rear wheels. I'm considering building a rear diffuser to get more downforce. Considering that the bottom is flat, and doing some search on how diffusers work, my understanding is that built properly it should do something...but I'm also fully aware of how ugly the airflow around the top of car is. 

Even if the air above the diffuser is extremely turbulent, will the principals of a rear diffuser work as long as the airflow below the car is relatively clean?

VolvoHeretic HalfDork
11/17/22 1:11 a.m.

Ok, here is an aerodynamic question of a never before annalized car...

This is my Volvo 1800. When it ran well, I could reach its 6000 rpm redline in 4th gear with a 3.31 rear gearing and 26.8" tires which was also my top speed in 4th+ O.D. at about 5000rpm. (calculated 143.14 mph) MOTORISTO: Vehicle speed calculator

In the below top speed video below, it would not go higher than 5000 in 4th as it was missing as indicated by the widely fluctuating tack, at a calculated speed of 119.52 mph. Surely caused by bad points, plugs, and gas.

Before the video, I warmed it up with a flying mile pass and the always unhooked front hinged hood raised about 6", enough that I had to stop because I couldn't see. After duct taping it down, I made the video. Should I just build an aluminum louvered hood like they used to sell for every hatchback car rear window ever sold in the late '70s to help evacuate under hood air? Anybody care to casually take a look at this things aero?

It went even faster before I put the wheels, tires, flares, and air dam on. It was as if I had thrown out a home made parachute.  I have a thread around here somewhere talking about thinking about a giant rear wing to slow it down even more.

Back before online speed calculators, I did the long hand math on a 2x4. There was probably something wrong with some math... smiley 

When I first built the thing, I didn't realize that a 6 cylinder block wouldn't bolt up to a 4 cylinder bell housing so I had to bolt up the B30's block and 3 speed automatic to the 1800's driveshaft and 4.3 rear axle. eventually I found the correct trans and bell housing bolted to the 4 bangers driveshaft and axle and immediately sheared off the driveshaft u joint yoke. I than installed the 164's 3.31 rear axle and shortened driveshaft. I now want a 3.73 Windstar 8.8" rearend.

With the 185/70-15 tires and 4.3 rear axle, the Michelin XWX tires didn't last 10,000 miles but it was entertaining. I had a 2 page driving record including 3 exhibition driving citations. More is better, right? World's Scariest Volvo

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