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Aerodynamics of the Gremlin

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WesternRed View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote WesternRed Quote  Post ReplyReply Direct Link To This Post Posted: May/17/2023 at 6:42pm
I'd be thinking 0.43 is closer to the money for the pacer too.

Tom, that's really interesting on the pro-stock weight breaks, always a lot more going on than you think, being able to run less weight and also getting a body with less drag would be significant gains.

Back to the Gremlin, the Kammback body is supposed to be a reasonably efficient design in terms of aerodynamics.


I've researched enough on the internet to be dangerous now, nut I'm thinking the upturned lip in the back of the Gremlins roof is probably a design flaw in terms of reducing aerodynamic drag, but might add a little bit of downforce, generally a bit of a trade off between downforce and drag. Looking at most modern hatchback designs, where reducing drag is all about increasing fuel efficiency, the roofline generally curves gently over into the the rear or there is a spoiler the either sticks straight out from the roofline or is angled downwards. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PAGAENT Quote  Post ReplyReply Direct Link To This Post Posted: May/17/2023 at 7:32pm
Do a search for Alex Tremulis, one of the foremost aerodynamicists of his time, he passed in 1991 and was able to help my project which was hampered by rules. After all these are supposed to be stock body cars. He might be able to help you work within the rules and get your CD down, even .02 is significant. Ultimately my quest was hampered by being forced to use a too small optional spoiler whereas the Chevy Monza owners were allowed to construct their own because there was none available. The consequence of this was that above 200 MPH the car got lift and lost traction even with 680 lbs of lead for ballast.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote 73Gremlin401 Quote  Post ReplyReply Direct Link To This Post Posted: May/17/2023 at 9:06pm
I think the issues with the early Gremlin aero were not so much that the CD wasn't good (heck, no cars were good in that era other than a Citroen DS) but it was what the Gremlin body did to the air from a flow management standpoint.  Back when I hung out at the Team Highball shops in the late 70s, I was a sponge for anything that I could pick up, and the comparisons that Amos Johnson made between racing the Gremlin vs racing the Pacer always stuck with me. Yes, he said that the Pacer had a better overall aero, but it generated a lot of rear-lift at high speed, which created major stability issues that prevented the car from going any quicker effectively (ie., as a viable race car that can win races). The pre-77 Gremlins had, like the 70-72 Hornets, a forward-canted grille, which combined with the down-swept angle of the bumper and the splash pan, created a ton of lift on the nose - also way not good - but a completely opposite problem from the Pacer aero.  The 77 Gremlin redesign was a huge help with managing front-end airflow, with the primitive sheet metal behind-bumper front-splitter - something that was barely getting any notice in stock-bodied racing at the time - being a major contributor to the improved air management up front - and helped put the Gremlin back in the winners circle, along with the Spirit that replaced it for some time.  My own personal seat-of-the-pants experience on tracks like Rockingham, Charlotte and Road Atlanta with both my 73 and 77 Gremlin, with fundamentally identical drivetrains, bore this truth out. Did the 77 Gremlin have a better CD number than the 70-73 Gremlin? I don't know that any money was ever spent to find out - but was the air management better - oh definitely. Long story short - it is unlikely that the Hornet Hatch had a substantially better CD than the Gremlin - but it almost certainly managed air flowing around it better, which could have made the car quicker in the 1/4 mile.

Fun semi-related footnote - the aero package that debuted with the 78 Concord AMX, but is best known on the 79-80 Spirit AMX - really DID work. In testing for the Nurburgring race, Team Highball spent time at (IIRC) Charlotte motor speedway, running the Spirit both with and without the front airdam and fender flares.  Up to that time, they were running Spirits in IMSA competition without those parts, and were having plenty of success. When they attached the spoiler and flares to the test car, and then re-ran the test, the amount of downforce the air dam created was enough to bottom out the suspension. So yes - those parts really did work, and found their way onto the Nurburgring cars and history was made.


Edited by 73Gremlin401 - May/17/2023 at 9:12pm
73 Gremlin 401/5-spd.
77 Matador Wagon 360/727.
81 Jeep J10 LWB 360/4-spd
83 Concord DL 4-dr 258/auto

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Post Options Post Options   Thanks (0) Thanks(0)   Quote PAGAENT Quote  Post ReplyReply Direct Link To This Post Posted: May/17/2023 at 10:15pm
Yes, it's not an AMC but it did get a lot of redesign which improved the aero considerably without drawing attention to it. I'm sure all these things were considered at AMC and they had to work within a budget that probably was not very big. It's close in size to a Gremlin so many of the same problems would have a similar result.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote WesternRed Quote  Post ReplyReply Direct Link To This Post Posted: May/18/2023 at 8:07am
Looking at the later Gremlin the front end does look to be a bit more slippery, one of these Gremlin GT front spoilers might be a good thing. I notice the Pro-stock era cars seem to have been run with a lot of rake on the body and very low at the front so maybe there is something in that. 

Hard to know with race cars if they did things to reduce drag or to increase downforce, which would increase drag. How fast you are going would change your priorities too.

Simple things like putting flush covers over the headlights could make a difference, but would it be enough to be measurable.


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Post Options Post Options   Thanks (1) Thanks(1)   Quote PROSTOCKTOM Quote  Post ReplyReply Direct Link To This Post Posted: May/18/2023 at 11:06am
I think making changes as small as sealing the voids around the headlight rings to the plastic bezel doors would yield benefits. I am actually closing up all the gaps and openings on the front grill area on one of my Hornets right now. Once I get it finished I am going to make a mold off of it for a 1 piece doghouse. Then I'll have the grill, headlight, and bezels air brushed back on. 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Hurst390 Quote  Post ReplyReply Direct Link To This Post Posted: May/18/2023 at 11:41am
I sure would love to have a fiberglass dash if you get time
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PAGAENT Quote  Post ReplyReply Direct Link To This Post Posted: May/18/2023 at 12:03pm
Originally posted by PROSTOCKTOM PROSTOCKTOM wrote:

I think making changes as small as sealing the voids around the headlight rings to the plastic bezel doors would yield benefits. I am actually closing up all the gaps and openings on the front grill area on one of my Hornets right now. Once I get it finished I am going to make a mold off of it for a 1 piece doghouse. Then I'll have the grill, headlight, and bezels air brushed back on. 

Tom
Every small thing you can do to manage airflow around the front will help. I went to great effort to close up all the fender & door gaps only to find out the air flow went around the car making my efforts ineffective. This was determined by rolling the window down at 200 and sticking my elbow out and finding no airflow. The ridge of the fender and abrupt transition of the windshield to the side caused the flow to separate. In my case the air dam was probably the most improvement but it has to be within 1" of the ground to be effective. Might cause loss of traction in a drag Gremlin, you would trade off ET for MPH
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PROSTOCKTOM Quote  Post ReplyReply Direct Link To This Post Posted: May/18/2023 at 1:35pm
Originally posted by Hurst390 Hurst390 wrote:

I sure would love to have a fiberglass dash if you get time

I haven't giving up on that project. I just got burnt out working on the plug, so I took a break.

Actually I bought the materials for it a few weeks ago. Maybe sometime this year I'll get it finished.

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Post Options Post Options   Thanks (1) Thanks(1)   Quote WesternRed Quote  Post ReplyReply Direct Link To This Post Posted: May/18/2023 at 8:18pm
Here is a thought, the belly pan I made a little while back for Street Outlaws might make a comeback with a bit of a re-design. I have the Canton road race oil pan and a deep transmission oil pan installed, both square boxes sticking out the bottom of the car that are probably offering some wind resistance.


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