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Mikelly

Windtunnel Testing the Datsun S-30 Z

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One thing that needs to be understood. Think of a "ball" of compressed air as part of the car. With the windows down, the air fills up the car interior with a mass of air. Once that mass reaches it's static point, it won't allow any more air in. It will force the approaching air to go around. If you allowed that air to escape through the tailgage, now you have a flow problem where the air has to turn, enter the window, turn again and squeeze out the hatch. This may worsen drag. The same principle applies to the nose of the car. You want to stall a big (smalller is better) ball of air against the nose of the car to force the oncoming air around. Preferable over or around the car and not under. Think of it as an air-dam made out of compressed air.

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Exactly. The book Tune to Win has some more detailed info on flow and radiators, how to shape them so that you get good flow across the entire radiator, etc. if you're interested Bob.

 

I assume that was for me? Do you mean that book has info on how to shape the ducting? What are the details on the book? author/etc..?

-Bob

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Guys, I'd recommend we start another thread on the items that you'd think WOULD be tested in a follow-on session. Let's keep this one on track with what WAS tested.

 

What I'm concerned about is mixing hypothesis and "ideas" to test with the outcome of the actual testing we did.

 

AND I'd like to start ANOTHER thread on BOOKS and links that might help us all understand what's going on in this thing we call aerodynamics. The reason I came on so strong w/ JonM. in reference to the ducting for the radiator/BOX was because at this point I personally don't even want to "think" I understand this stuff. There were a lot of assumptions we made that didn't pan out during the testing, which makes me no longer assume a specific mod will "work".

Mike

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Cygnusx1 - lesson one, never post when tired. Your reply re: trapped air keeping any more air from entering was so simple I slapped my own head this morning. Modding the "A" pillar is simple and straight forward. As are the winglets in front of the tires. The rocker aero skirts look like simple pieces as well but, in our case, would need to be removable, as getting on/off our trailer would rip them off unless we modded it.

 

All these would be easy to do to a track car but adding them on to a street car and keeping the overall appearance in mind will be harder IMHO.

 

Did I miss something, was a "cowl" hood tested?

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posted by x64v

Or did it miss the second latch and pop a few feet up or something?

 

That was an eye opener. The hood raised up about 7 -8" and stabilized. The mule didn't have a safety latch, neither did the gnose car, and both did about the same thing when the hood flew open.

 

posted by cygnusx1

You want to stall a big (smalller is better) ball of air against the nose of the car to force the oncoming air around. Preferable over or around the car and not under.

 

Exactly what Bob Smith explained to us. If you know where the center of the ball of air is, the "stagnation point", that's the best place to put openings for cooling air in the front of the car.

 

posted by BillZ260

They look like vents.

 

They are vented with perforated screen, just another one of the many cool details on Roddy's car.

 

John

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I assume that was for me? Do you mean that book has info on how to shape the ducting? What are the details on the book? author/etc..?

-Bob

http://www.amazon.com/Tune-Win-Carroll-Smith/dp/0879380713/ref=pd_bbs_sr_1/103-6497924-6233420?ie=UTF8&s=books&qid=1179156406&sr=8-1

I haven't read it in a while, but it has do's and dont's for duct shapes, and I think it talks about pressures inside ducts as well.

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I had a discussion with bob on the inlet size on the front of the car. He said that for a track car that saw mostly high speeds you need an opening with an area roughly 1/4 the size of the radiator. He also said that for a street car 1/3 would be much more appropriate dude to sitting in traffic etc.

 

This goes along exactly with what corky has mentioned in his turbo book he recomends on intercoolers that they only need 1/4 the area if properly ducted.

 

On my personal car i plan on blocking off the entire area above the bumper and then use adapqute ducting to transfer the air through the i/c and radiator. I will probably split the air in between the two as well.

 

Another thing to think about of how the air is ducted a duct shaped like this (radiator on the bottom shown as the solid line):

 

___/

 

will provide ALOT more turbulence and actually hinder airflow as opposed to something like this:

 

/___

 

Hopefully in the next few weeks I can materialize some of these ideas and get some pictures. Alot of what Bob was telling us was almost comon sense when you sit down and think about how the air will flow and what can be done to alter and take advantage of it. I think alot of people almost over analize aerodynamics when It comes to street cars. We have a LONG way to go before we can even get close to correctly employing alot of whats used in professional race cars since the entire body is designed around producing down force. This windtunnel data is defintly getting us ALOT closer and I really hope we can collectively decide on a package to test in round 2.

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From subjective viewpoint at El Mirage and Bonneville, when we run in ALT class instead of PRO, the car seems to be FAR better planted up front. The basic difference between the PRO class and ALT is that we totally block the radiator inlet on the G-Nose with sheetmetal, flush to the sides and bumper. no air whatsoever is allowed into the radiator during the run. Yeah, it gets hot, but something is cooling it....

 

GCC class has us running a full underbelly pan, and it doesn't seem to make mumch difference between the ALT configuration at all. You can see in the Record Books that the difference between PRO and ALT configurations is about 10mph (163 in PRO, 173 in ALT) at Bonneville. GCC was in the same area as ALT, so that kind of lends credence to what the Aerodyn operator was seeing.

 

The test data also reveals why we had less wheelspin when adding 200# of ballast in the spare tire well...and went faster.

 

I do know from a video we recently did, that in ALT configuration--even with all the radiator opening blocked off, the safety latch on the hood will keep the hood latched if the idiot strapping in the driver forgets to 'beat it' for good luck just before making the pass---so there is STILL pressure under that hood in a stock 280Z, even with the radiator opening of the G-Nose blocked off---anyone care to venture a guess how THAT is happening? We run no splash pan, either BTW, as it makes for a difficult installation of the bellypan. The only time the splash pan area is covered is when we run the GCC class, full belly pan, and covered radiator inlet on the G-Nose.

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My guess is it's exactly what I mentioned above, and what Bob Smith told all of us in the control room. You're gonna get LOTS of air stacked in at that firewall. Deal with it the same way you deal with having the windows down. Let it pack in and fill up, and the rest will spill out around and find the path of least resistance. I think a radiator box will do a lot to make the minimal opening more efficient for cooling, but don't know how much it will aid in fixing the other issue. You still have all that air trapped at the firewall.

 

Mike

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Tony,

 

Where do you think the air is coming from? The only place it could come from is under the car but why is it doing a 90 degree turn to pressurize to bottom of the hood instead of just flowing under the car? Do you have a picture of the belly pan?

 

Joe

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Heres a crazy idea. Look at the SDS side skirts. They turn in at the front of the rocker and stop at the TC buckets. What if you continued them so they meet up to a point at the bottom center of the lower radiator support? That should pull some air out of the engine bay and possibly decrease underhood pressure.

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I still think you guys are wrong about the air under the hood. If NASCAR can get neg pressure under the hood, then we can. NASCAR cars are very basic in terms of shape, previous to last month's release of the COT they didn't have splitters or anything else. No undertrays, no nothing. All they do is get the nose of the car ON THE GROUND, and use the shape of the car to extract air from under the hood.

 

I see no fundamental reason why we can't do the same thing.

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I have no idea where the cooling airflow comes from when it's all plugged and panned. Even without a pan, with the radiator blocked, it doesn't seem to really affect it. The temperature we saw at Bonneville can probably be directly related to holding the car at 8000+ rpms for five miles straight....

 

I mean, I never took a static pressure reading in ALT configuration, but now you guys got me wondering if we indeed have some sort of 'negative' under there, and the radiator is basically 'ground breathing' for circulation. Our Bonneville car is pretty low, without anybody in it, I can't fit my wrist sideways under the T/C section of the frame at the firewall---I have to use a can picker-upper to stuff the plugs in the header collectors. Though now since we didn't see any loss by running the twin 3" exhausts, we can plug them at the rear of the car instead of trying to plug them from underneath.

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I won't speculate further. What I know is what a guy who does it every day for a living recommended to the 9 of us in the control room. Without testing the setup, we can mentally spar until we're tired.

 

We can certainly add it to the list for next time, and do some myth busting.

 

Mike

 

I still think you guys are wrong about the air under the hood. If NASCAR can get neg pressure under the hood, then we can. NASCAR cars are very basic in terms of shape, previous to last month's release of the COT they didn't have splitters or anything else. No undertrays, no nothing. All they do is get the nose of the car ON THE GROUND, and use the shape of the car to extract air from under the hood.

 

I see no fundamental reason why we can't do the same thing.

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So how about a reinforced home depot lip? Held in place by some spring steel...

On my car at full down on the suspension my home depot lip (hard plastic lawn edging, not the soft rubber stuff) is about 1" from the ground. I usually run about 2.5" higher than that though to get over normal street bumps and road transitions. I am going to be mounting my belly pan soon to help reinforce the air dam and block off the radiator-airdam gap.

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One source of high pressure air is just in front of the windshield. You can see the yarn is some of the photos drops into the windshild wiper grill. I know there's one post about overheating at speed caused by a rear hood cowl. High pressure air in front of the windsheild slows flow through the radiator enough to raise engine temps significantly. The problem was solved by moving the vents closer to the stock outboard locations. Maybe this answers the low 15mph radiator exhaust flow.

 

With a couple of the test cars having hood latch problems and that there are small passages from the wiper area to under the hood, I think future tests could make sure the rear of the engine compartment is sealed as well as the front.

 

I know it's being debated whether or not negative engine compartment pressures can be acheived, but it looks to me that efforts to reduce pressure will not only help with down force, but with drag by increasing the pressure differential between the front and rear of the radiator, thus allowing for a smaller mouth opening.

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I still think you guys are wrong about the air under the hood. If NASCAR can get neg pressure under the hood, then we can. NASCAR cars are very basic in terms of shape, previous to last month's release of the COT they didn't have splitters or anything else. No undertrays, no nothing. All they do is get the nose of the car ON THE GROUND, and use the shape of the car to extract air from under the hood.

 

 

Yeah I think the important thing to think about is if your cars a street car or a race car. Im sure it would help to have the airdam scraping the ground but then you cant go over any bumps etc. Just thought Id mention that.

 

Anyways like on the car of tomorrow the front air dam routes air out the sides in front of the tires. If you were to make a panel that extends out from under the air dam and then goes around the side of the car to the front of the tires where some kind of flare routes the air out sideways? Would that help. Ill work on some kind of picture its hard to explain.

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Just a few questions/observations...

 

 

 

Isn't this backwards? If you look at #9-#10 (change to the whale tail), the rear lift numbers went from positive (lift) to negative (downforce), not negative to positive, as that says it would.

 

 

Also, looking at total front lift during #5: the difference between the hood popped test and the retest is 120lbs of lift! Correct me if I'm mistaken, but doesn't that mean that an easy way to reduce front lift for those of us with stock/near stock bodied s30's is to just pop the hood to the second latch when we near those speeds? Or did it miss the second latch and pop a few feet up or something?

 

This is what we were starting to conclude in the whaletail thread.

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