According to history, Wally Booth ran both the Gremlin and Hornet hatch with the exact same driveline and the Hornet was 3 mph faster, ending the career of the Gremlin in super pro stock racing (let me know if this is inaccurate).

Have to assume the difference was mostly due to aerodynamics, now the front half of the Gremlin and Hornet is pretty much the same, so presumably the difference in rear body shape is the issue.

What can be done to improve the aerodynamic performance of the basic Gremlin body and how much difference could it make? I notice a few people running roof extensions off the back of the car and Kevin has some fancy little winglets on the back of his Gremlin.

I’ll just pop this here for future reference  https://aia.springeropen.com/articles/10.1186/s42774-020-00054-7" rel="nofollow - Drag reduction by application of aerodynamic devices in a race car | Advances in Aerodynamics | Full Text (springeropen.com)

Fixed the link in my first post, some interesting data there for those who are interested.

Tom, I used to think that Wally actually swapped out the engine at the track, but makes more sense that they would compare performance between cars with the same engine output. From some online reading, the difference in ET was 0.15 seconds and 3.5 mph, quite a significant improvement. Not sure if the wheelbase difference would make much of a difference in ET and MPH, but would probably help with stability. Any other differences? A production Hornet is probably heavier than the Gremlin, but pro stock would presumably have some weight requirement that had to be met. 

Cand find any numbers for the Hornet so far but apparently the Pacer was much more efficient at 0.32 (or 0.43 depending on source), would be good to have some verifiable data.

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. 

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.

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.

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

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.

Red is high pressure, blue is low pressure.

If anybody has an accurate 3D cad drawing of a Gremlin, let me know.

My cousin used to run stock cars and although they had many class rules, they all took advantages in hopes of not getting caught and getting an edge. He would rake back the windshield 3/4" to 1" and pull side panels/fenders in were it was hard to accurately measure. Radiuses of some curves were increased and others near the bottom sharpened. Never had the software as you show, but getting an extra lap out of the tank of fuel and winning races was all the proof of concept he went by.

You see new cars like my son's Honda with the full aluminum underbelly engine pan and almost smooth bottom under the car.

The 1-1/2" air dam added to the front bottom of the motorhome added a measurable 0.4 to 0.5 MPG.

It's the only picture I can find anywhere of a Gremlin running any sort of front aero device, so I'm thinking either it didn't work, or was not allowed under the rules. Do notice that they tended to run the cars very low at the front, which might reduce the amount of air going under the car or maybe they just ran a lot of rake on the cars to try to help with downforce at higher speeds.

I suspect the front spoiler would be against the rules, they were pretty strict on maintaining stock appearance by the looks of it.?

https://www.gapponline.net/2010/01/29/the-1973-nhra-drag-rules-pro-stock-eliminator/" rel="nofollow - https://www.gapponline.net/2010/01/29/the-1973-nhra-drag-rules-pro-stock-eliminator/

Tom

Just revisiting this post, under car view without the belly pan doesn't look very aerodynamic:

this is my revised belly pan:

Will hold less oil now, but is re-engineered as an aerodynamic device. I ran this at the track last time out with some interesting results, average of best 3 runs (different events):

Without  belly  pan: 60' 1.602, 330' 4.587, 660' 7.109, 1000' 9.309, 1/4 mile 11.176 @ 120.61 D/A 222'