Windtunnel Testing the Datsun S-30 Z

[bjhines]

I for one would like to get some test data on relative pressures and flow at various points. I too was thinking that we could eventually get negative pressure under the hood... but the hood vents always flowed out not in.

 

The tunnel has a test rig that we can use for this. We will have to make a project of preparing a hood as well as other areas to install the mass of hoses that must be attached to perform the tests. We will also need to think of some ways to achieve negative pressure under the hood. perhaps wheelwell and splitter designs would help... along with a well thought out airbox in front.

[ihatejoefitz]

I've obtained most of my information from wikipedia, so take this with a grain of salt. http://en.wikipedia.org/wiki/Drag_coefficient

 

Some values provided for a 280zx http://www.mayfco.com/nissan.htm

 

Anyway, I needed some perspective.

 

The test mule baseline gave a CdA ft² value of 8.71 putting us in line with some boats! (92 Crown Vic, 91 LeSabre) and by run #18 you had it down to 7.33 (a little better than a 01 Civic).

[bjhines]

The 01 Civic is higher and larger than the S-30 Zcar. So we still don't have a slick fish.

 

We can get better with some more work fairing the windows, A-pillar, and drip rails. The wide-rounded shape of the IMSA flares could pay off in more ways than just reducing drag. The wheel wells could also use some work inside and out.

[JMortensen]

I think our test cars had positive underhood pressure, and that air is flowing out the vents because of the pressure differential. The Bernoulli effect is helping, but it's effect is secondary.

 

If I am understanding you correctly, you think the underhood pressure is negative, and air flows out the vents solely because of Bernoulli.

No, I think that COULD BE what happens. And the point with the Bernoulli effect is that air doesn't necessarily have to flow out of the vents because of pressure. But when the top area between the core support and the hood is left open, and all the holes in the core support are left open I think it's very difficult if not impossible to obtain a negative pressure under the hood.

[X64v]

Just a few questions/observations...

 

How to “read†the data being provided… A little explanation of the fields…

 

C LF front lift neg # = lift, pos# = down force on front end

C LR rear lift neg # = lift, pos # = down force on rear end

 

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?

[cygnusx1]

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.

[BillZ260]

I'm trying to keep up, but alot of this is over my head. I'll keep reading untill it starts to make sence...

 

Are there any close up pics of Roddy's hood access pannells? They look like vents.

[Bob_H]

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

[Mikelly]

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

[gnosez]

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?

[jt1]

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

[TitaniumZ]

Photos will follow,but yes the area above the nose inlet to the core support in the blue SDI car is sealed off.

[JMortensen]

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.

[240hoke]

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.

[BillZ260]

Moved to new thd.

[JMortensen]

Photos will follow,but yes the area above the nose inlet to the core support in the blue SDI car is sealed off.

Any pics? Were the holes in the core support open?

[Tony D]

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.

[Mikelly]

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

[rags]

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

[cygnusx1]

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.