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PostPosted: Mon May 11, 2020 7:15 am 
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Joined: Mon Feb 29, 2016 9:12 am
Posts: 140
Location: Boston, Ma
How apples to apples is it really?

Same tires, wheels, track, air dam, flares, tune, cam, overall drive ratio?

Seems like optimizing for the NASCAR portion of the circuit at Daytona is a big deal.

I've heard stories of reduced or no air dam giving large straight line speed advantages there, and it makes sense.

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PostPosted: Mon May 11, 2020 8:23 am 
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Joined: Fri Dec 23, 2011 6:59 am
Posts: 1367
Location: Spring Hill, FL.
My Chump car teams swear having the headlights up on the Miata cost 3+ MPH. They removed all of the aero stuff for the Daytona races.

Air is really hard to predict. My slant nose Truck with a 3.5in chopped top bounced the air off of the rear window up to about 88MPH :) Then it straightened out and the rear mounted rad saw the air from front to rear. Under 80 ish the air went back wards through the rad . It was very noticeable in the rain . Every time it hit around 80 the rooster tail went 254ft. skyward from the huge wing . I added a roof extension to help the air determine which way to go.
Spent hrs tufting and videoin g the thing .

Prior to that ; Back when Chumpcar was really 500$ cars.. My topless Turbo Jetta had enough toque for an easy 145MPH but hit the wall at 125. I got so frustrated that I registered it and did extensive coast down and videos. The passenger seat head rest was a stupid amount of drag.
For most accurate results , look into "coast down testing" Keep the test parameters tight and it is really worthwhile .

https://www.youtube.com/watch?v=-fm8k_un_ps TJ

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PostPosted: Tue May 12, 2020 7:52 am 
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Joined: Thu Feb 28, 2019 5:57 am
Posts: 108
bam914ep wrote:
Check this link. There is more on 914world.com. http://www.cassidy-online.com/porsche91 ... index.html

TIL that the 914 has less drag with the LE front air dam removed...and I'm guessing it might help with cooling having more air flowing under there (esp with the leading edge flappers underneath)...fascinating. I'll have to see if I like the look, b/c that's almost as important.

The closest example to a Prod-level open top on the 914 is this analysis' "racing style" but it does not include the drag from the targa top and rollcage. It is a significant improvement. The next-closest is their "no glass on car" which may better represent the presence of a rollcage and targa; its aero is just as significantly worsened versus stock.

I was not surpsied that "top off" was worse, but I was surprised at the low level of significance.

So then I have to go back to our discussion of the Miata, which is possibly our most-numerous Prod car run in either open- or closed-config (lack of drag from the targa top notwithstanding). I'm curious how preparers are choosing which configuration to go with? It is a gut feel, config preference, or has anyone done any significant analysis?


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PostPosted: Tue May 12, 2020 8:25 am 
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Joined: Fri Dec 23, 2011 6:59 am
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Location: Spring Hill, FL.
The above modeled front spoiler created a wedge of air in the nose . The SCCA style air dam ,(vertical from bumper) dragging the ground , more closely mimics the car set on the ground . ( for a gain,IMHO) Again "square inches of involvement" is less when the under car sees slower and less air .

No splitter was modeled .
Great write up . MM

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PostPosted: Tue May 12, 2020 8:52 am 
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Joined: Mon Oct 14, 2002 6:08 pm
Posts: 691
Location: Raleigh, NC
FWIW, in a previous post (somewhere) on the topic of open versus closed Miata's Ken Kannard commented that he kept the hard top car for a reason (based on back-to-back testing).

Rick

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PostPosted: Tue May 12, 2020 9:24 am 
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Joined: Thu Feb 04, 2010 5:38 pm
Posts: 170
I guess the combinations are infinite but the one thing that dominated the open top builds on the 914 was the
"Ginther" windshield, which is not much different than the ones used on the open top Miata.

This was the one mod I wish the 914 data analysis would have included.

Re-directing some of the air slightly up and back through the glass-less targa bar could be significant.

Tom B.


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PostPosted: Tue May 12, 2020 10:28 am 
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Joined: Sun Nov 08, 2009 12:58 pm
Posts: 480
Location: Denver CO
From Daytona 2015:

Eric Prill
Q1 144.637
Q2 139.454
Q3 137.808
Q4 138.508
Race 144.127
Average 140.9068

Charlie Campbell
Q1 145.409
Q2 144.894
Q3 141.14
Q4 146.19
Race 146.978
Average 144.9222

Ken Kennard
Q1 142.122
Q2 144.637
Q3 141.14
Q4 142.618
Race 142.867
Average 142.6768

Of course. Ken only represents one data point. I averaged the data to try to account for drafting / session-to-session variances, but of course, it's still only one capture from each session.

What this doesn't account for is differences in driver and their ability to work the draft (Charlie, with his Spec Miata experience, seems to be better at it), gearing, etc. I know Charlie and Eric's cars are pretty similar and power between all three should be within a few % points.

I think anything short of a wind tunnel or other tightly controlled testing is just working off intuition, and everyone knows how that can go...
Image
Image

I think at this point it comes down to personal preference. In Prod, where everything; flares, airdam, etc. are all likely shaped by what the builder thinks will work, as opposed to strict testing, it's all probably just a guess. Winning 914's in vintage come in both flavors, hard top and open top. The driver and the rest of the combination matters more. I think the Miata data points to that as well.

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