Aerodynamic upgrades?

[Map63Vette]

My commute might be changing in the near future, so I've been thinking about trying to optimize my 67 Dart to get a little better gas mileage as it might be turning into my main daily commuter. My current setup is a modern 5.7 Hemi with a T56 and 3.55 rear gears, so I've got a fairly low cruise rpm and have got it up to around 20-25 depending on the conditions (~20 is my average, 25 only on real flat road). I think I might be able to squeeze some more out with some tune optimization, but ultimately I know I'm pushing a brick through the air, so I'm curious if anyone has done anything specifically to try to address that. Off the top of my head I'm thinking maybe something like closing off the engine bay better and maybe something like a front splitter or other sort of front aero to clean up the airflow, but I'm curious if anyone has any examples of those kind of mods as I'm guessing they aren't exactly off the shelf parts. Would be curious to see any pictures or designs people have put together over the years. Seems like a lot of the stuff out there is for the later model 70+ cars, but may always be able to adapt those ideals to earlier cars.

 

[harrisonm]

Hello from Topeka. I thought about this for a bit, and I can't think of anything you could do to change the aerodynamics of your car without changing its looks or costing a lot. A beautiful 67 dart that was physically altered to improve aerodynamics would no longer look like a 67 Dart to me. I can think of two fairly easy things to do. First would be to change your rear tires to something a bit taller if possible. Or switch to a 3.23 rear gear.

 

[Yunick13]

Having a flat bottom would help. It'll speed up the air under the car, reducing drag. Lowering the car will help, which reduces the amount of air going under the car. Haven't really heard if vortex generators make any difference.

Running skinnier tires reduces frictional drag on the road surface.

 

[72bluNblu]

My commute might be changing in the near future, so I've been thinking about trying to optimize my 67 Dart to get a little better gas mileage as it might be turning into my main daily commuter. My current setup is a modern 5.7 Hemi with a T56 and 3.55 rear gears, so I've got a fairly low cruise rpm and have got it up to around 20-25 depending on the conditions (~20 is my average, 25 only on real flat road). I think I might be able to squeeze some more out with some tune optimization, but ultimately I know I'm pushing a brick through the air, so I'm curious if anyone has done anything specifically to try to address that. Off the top of my head I'm thinking maybe something like closing off the engine bay better and maybe something like a front splitter or other sort of front aero to clean up the airflow, but I'm curious if anyone has any examples of those kind of mods as I'm guessing they aren't exactly off the shelf parts. Would be curious to see any pictures or designs people have put together over the years. Seems like a lot of the stuff out there is for the later model 70+ cars, but may always be able to adapt those ideals to earlier cars.

Lower the lid and tilt the windshield back? 'Bout the only thing that's going to help lower your frontal area...

One of biggest things hurting you is probably the deep inset grille, the hood catches air and creates lift which not only makes the front end feel light at really high speed but also lowers the efficiency. Pretty much an issue with most muscle car era designs. Unfortunately, there isn't going to be much you can do about that. Adding a front spoiler helps with the lift, but that doesn't help with the drag that you want to reduce for better mileage.

Look at what Dodge did for the Charger 500- flat front grille, lost the recessed rear glass. Both massive aero improvements. More on that stuff here
https://www.roadandtrack.com/car-culture/classic-cars/a29285/dodge-charger-500-nascar-history/

Getting ride of the deep inset grille was one of the reasons the '72+ Challenger got the "frowny" grille, it was set much further forward and was a significant aero improvement vs the '70 grille

The other stuff is little, tucking in your bumpers helps some especially at higher speeds, that was another NASCAR trick- they'd fully smooth them into the bodywork. And even on the backside they covered up the inset bodywork. Obviously this probably isn't something you want to do to a street car, but if you wanted to see if it would help you could just tape those gaps on your car and see if there's any noticeable improvement.

Hello from Topeka. I thought about this for a bit, and I can't think of anything you could do to change the aerodynamics of your car without changing its looks or costing a lot. A beautiful 67 dart that was physically altered to improve aerodynamics would no longer look like a 67 Dart to me. I can think of two fairly easy things to do. First would be to change your rear tires to something a bit taller if possible. Or switch to a 3.23 rear gear.

You obviously missed the part where he said he has a T56. As in, a 6 speed transmission that has not one but TWO overdrive gears. 6th gear is either .63 or .50 on that transmission, so the 3.55's aren't hurting him at all. He can probably cruise at 70mph at 2k rpm or less. Hell I run 4.30's with my T56 and I'm doing 75mph in 6th at 2,500 rpm.

 

[junkyardhero]

You obviously missed the part where he said he has a T56. As in, a 6 speed transmission that has not one but TWO overdrive gears. 6th gear is either .63 or .50 on that transmission, so the 3.55's aren't hurting him at all. He can probably cruise at 70mph at 2k rpm or less. Hell I run 4.30's with my T56 and I'm doing 75mph in 6th at 2,500 rpm.

do you suppose that part of the problem could be that with the double OD and 3.55's the final ratio might actually be too low and the tune can't put enough timing in to take advantage of the lower RPM and be efficient?

 

[MV8]

I would not change anything. Driving technique is a major factor. Very slow throttle changes to prevent added enrichment over what's called for at that point in the map. Parasitic loss is lowest at 1:1, so running a numerically lower axle ratio to work with 4th gear at cruise speed is more efficient. Numerically lower ring and pinion usually have thicker teeth so a lighter duty axle can be used for the same strength. Removing the side mirror(s) and wipers, flush fuel filler and trim, reduce the grill opening, add an air dam under the bumper to reduce the amount of air flow under the car and matte black skirts under the rockers (to hide) to do the same, drums instead of discs for less rolling drag, lengthen the rear fenders and trunk lid with flat extensions (a longer shape is generally more aero), higher tire pressure, narrower wheels to allow rear fender skirts, tuck the rear of the front fenders to get the high pressure out of the wheel well, etc.

 

[go-fish]

Things I would do:

Air dam, grille to radiator support close-out panel, block off the grille except for what is absolutely necessary for radiator airflow, flat panels underside where feasible, and a Gurney flap.

 

[TrailBeast]

All this for maybe 1-2 mpg?
Just drive it, or get a Prius.

 

[A56]

Dropping weight always helps as well as a slick wax job.

 

[Locomotion]

A slightly nose-down stance, about 5* I believe, tends to help aerodynamics and a front spoiler will help minimize airflow getting caught up in the "rough" underside. Similarly, tractor trailers have air dams under the box shaped in a "V" to direct airflow around the trailer's rear tires.

There was some controversy about spoilers on late '60's Camaros in Stock & Super Stock drag racing a while back. Some racers were using the front spoiler without the ducktail rear spoiler that was part of an option package. The reason was that while the front spoiler helped aerodynamics, the rear spoiler actually created drag! I believe NHRA stated that either both spoilers are used or neither.

Readjust fenders & doors to minimize the gaps.

Synthetic oils - a little thinner, if possible, in the engine, trans & rear.

Cost vs return does need to be considered. Most benefits will only be obvious at highway speeds.

 

[RAT ROD AL]

Back in the day we would jack em up . Carpooled with my dad to work. He would say
" bet you get good mileage going down hill all the time !"

 

[Locomotion]

View attachment 1716297006

Besides the rear spoiler, rear wheels & passenger mirror - looks like a twin to my first car, a 340!

 

[Bodyperson]

Ya shutting the grill must be a big deal, a lot of the manufactures are using an "Active Shutter System" mostly for aerodynamics though it does help with emissions too with a faster warm-up. Mercedes has been cleaning up the bottom of the cars for decades. No pinch welds on the rockers, things like that.

The most aerodynamic shape is a whale shape. Look at those solar powered competition cars. Maybe some Titanium K-member bolts would help.

 

[72bluNblu]

do you suppose that part of the problem could be that with the double OD and 3.55's the final ratio might actually be too low and the tune can't put enough timing in to take advantage of the lower RPM and be efficient?

Possible I suppose but I’d think that’s unlikely. I don’t know what cam or tune he has, but the OE gearing for a 5.7L Challenger would set a 70 mph cruise at about 1,600 rpm. That’s with a .50 6th, tires that are almost 29” tall (20’s) and a 3.9 rear gear as equipped from the factory. And of course that’s a nearly 5k lb car.

 

[AJ/FormS]

With computerized timing controls, and EFI, yeah you might be able to cruize efficiently at 65=1600, but I would think such an endeavor would take a special cam.
But getting my 367 to burn efficient at that low an rpm was not easy.
I installed a dash-mounted, dial-back, timing module with a range of up to 15 degrees.
I only ran that 65=1600 for one summer, until that cam lost lobes. With the smaller cam (223/230/110), as to mpgs, I found that running at 1600 in 2ndOD versus 2050 in firstOD@2050 was just not that much better.
My new cam, 230/237/110 does not get mileage anywhere near the former, and so I ditched the A833od in favor of the commando box, (still with the .78GV behind it) and economy at 65=2240 is what it is .The New cam combo runs at 65=2240.
My current timing set-up, runs ~14initial+10Mechanical@2240+22VA=48 plus whatever I dial in. I run a minimum of 54*@2240, and sometimes up to 60, and one time at 63*. You cannot run that with just a distributor, unless you manually crank it over, and promise not to floor it.
As to aero/drag , I think my stock-body Barracuda must be pretty good. When I stuff it into neutral at 65mph, it takes forever to coast to a stop, even with the low tire pressures that I run.

 

[harrisonm]

Lower the lid and tilt the windshield back? 'Bout the only thing that's going to help lower your frontal area...
View attachment 1716296938

One of biggest things hurting you is probably the deep inset grille, the hood catches air and creates lift which not only makes the front end feel light at really high speed but also lowers the efficiency. Pretty much an issue with most muscle car era designs. Unfortunately, there isn't going to be much you can do about that. Adding a front spoiler helps with the lift, but that doesn't help with the drag that you want to reduce for better mileage.

Look at what Dodge did for the Charger 500- flat front grille, lost the recessed rear glass. Both massive aero improvements. More on that stuff here
https://www.roadandtrack.com/car-culture/classic-cars/a29285/dodge-charger-500-nascar-history/

Getting ride of the deep inset grille was one of the reasons the '72+ Challenger got the "frowny" grille, it was set much further forward and was a significant aero improvement vs the '70 grille
View attachment 1716296939

View attachment 1716296940

The other stuff is little, tucking in your bumpers helps some especially at higher speeds, that was another NASCAR trick- they'd fully smooth them into the bodywork. And even on the backside they covered up the inset bodywork. Obviously this probably isn't something you want to do to a street car, but if you wanted to see if it would help you could just tape those gaps on your car and see if there's any noticeable improvement.
View attachment 1716296941




You obviously missed the part where he said he has a T56. As in, a 6 speed transmission that has not one but TWO overdrive gears. 6th gear is either .63 or .50 on that transmission, so the 3.55's aren't hurting him at all. He can probably cruise at 70mph at 2k rpm or less. Hell I run 4.30's with my T56 and I'm doing 75mph in 6th at 2,500 rpm.

No, I didn't miss it. I realize the T-56 would nicely offset a low gear ratio. I was just thinking that on top of what he had already done. Changing to a 3.23 would definitely give a mathematical improvement to RPM vs Speed, and I thought that might help gas mileage a bit. It isn't much of a drop in gear ratio, but I thought that every little bit might help.
Gee, I wish I was at 2500 at 75. I have an 833 with 3.55s. I am at about 3,000 and 65. I have thought about an overdrive of some kind, but I don't drive my car on the highway enough to make it worthwhile.

 

[Map63Vette]

Wow, thanks for all the great ideas and discussion so far! For a little more background, the cam is an SRT Max Plus from Inertia Motorsports with a slightly different LSA than the catalog. I've only got one dyno chart for it and it wasn't great as I later found the car had a cracked pressure plate fork and the tune in general was very conservative (I did it myself and didn't want to push any numbers due to lack of experience). The car cruises 70 mph at around 1600-1700 rpm with the current transmission, rear gear, and tires, though I expect that number to go up a little bit when I put on my next set of tires as I currently have some ET street radials on the car that will be changed to a more highway friendly tire and I'll probably go to a common size on all 4 corners to make spares or moving stuff around easier. The EFI setup is a Megasquirt MS3X, so I've got full control of pretty much anything I want in the tune now, and I recently finally got a knock module installed in it, so I'm willing to push things a little further with the added information I can get from them. I'm planning to lean out my highway cruise a little more (currently running the typical 14.7, thinking I'll try to target more like 15:1 to start with) and will need to add in timing to compensate.

Lots of interesting ideas on the body to try to improve things. I know the recessed grille is basically the Achilles heel that I'll never really be able to fix, but some simple mods like closing out the core support to the K frame and maybe some Gurney flaps seem like easy enough things to try that could make a noticeable difference. I'm not really trying or expecting this car to ever be anything like a hyper-miler, more just seeing if I can get from an average mpg in the low 20s to the high 20s as I know that's what the modern cars with the Hemis can do. I know they are going to most likely have better aero, but not sure how much better. If I could get 5 mpg from a better tune and some limited aero mods, that would be pretty great for long term. I think a good 40+ miles of my 70 mile (round trip) daily commute are all highway, so that was the main reason I was looking at aero updates. Might have to see about some baffling behind the grille to better force air into the radiator opening instead of just randomly around the structure up there, but I know the gap is pretty limited, so not sure anything I could make would be a huge difference. Will have to just go stare at it one of these days and maybe play with some cardboard. I've got a 3D scanner, so might be tempted to see if I could get some CAD data going to play with as well, but that's probably overkill.

Regarding the Charger 500 rear window and Gurney flaps, would it be any benefit to put a flap at the back of the roof just before the rear window? Or does it only really make a difference at the very rear of the car where the air totally leaves the body? I think I could get some small aluminum angle bracket and attach it with some double sided trim tape easily enough as a test, but wasn't sure where to put it.

 

[go-fish]

Regarding the Charger 500 rear window and Gurney flaps, would it be any benefit to put a flap at the back of the roof just before the rear window? Or does it only really make a difference at the very rear of the car where the air totally leaves the body? I think I could get some small aluminum angle bracket and attach it with some double sided trim tape easily enough as a test, but wasn't sure where to put it.

I don’t see why not. A gurney flap is good for getting the boiling air from a flat surface of a tail panel going upwards. The magic happened on its back side.
Darts rear window probably has the same drag characteristics.