Jim Huseby
Senior Member
Does anyone have figures on the comparison of coefficient of drag on both a '30 town sedan and a '31 with slant windshield (and no wind-catching visor)?
aerodynamics
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Jim Huseby
Senior Member
Does anyone have figures or calculations indicating the HP required to push say a '30 model A coupe at 35 mph to 70mph in mph increments of 5? Assume conditions at sea level, 70 deg F., 70% RH with 35 psi in 475-500x19's and 600 wt gear lube at operating temp, no drag in brakes, etc.
Rock Hornbuckle
Well-known member
Almost everything that any engineer ever thought of, or worked on will have charts, graphs and tables with results/measurements/calcs, .etc. I would try googling aerodynamics. You might be surprised, someone probably tested automobiles of various shapes and recorded the results. Let us know what you find. 600w is not wt. it's a part number. It's might be difficult to ascertain bearing drag/friction. You might also try to locate a wind tunnel that would be willing to test your car. I think many here on the barn might be interested in how wind drag on our A's fare compared to a modern car. Good luck
1931 flamingo
Well-known member
WHY ??
Paul in CT
Paul in CT
Rock Hornbuckle
Well-known member
It may be related to his driving a speedster. just a hunch
Tom Wesenberg
Well-known member
Years ago I saw an article where someone rigged their Model A to drive backwards, claiming it had less wind resistence that way.
Rock Hornbuckle
Well-known member
that's really getting inovative!
mass A man
Well-known member
Jim may have a valid point. How many of us have tried to open up the windshield while tooling along at only 35 MPH? It's almost impossible, at least with one hand! I was very surprised how much wind force is against that windshield. The slant sedans, sans visor and canted w/s frame may provide some streamlining, but I still have to think only a bit, maybe 5%, taking the whole front end in consideration.
Jim Huseby
Senior Member
Even though I build speedsters pretty much full time, I don't have one for myself right now, as I was behind on delivery and sold my demonstrator to a waiting customer. Also the deer are thick here so I feel safer in my CCPU with a fairly stout wind shield and posts. Chances of surviving a deer collision with no windshield are slim. But anyway, it takes noticeably more HP to push the truck at 60 to 65 than the speedster. The speedster, with less HP, would just keep accelerating right up to the point (about 75 mph) when I remembered I was winding up a nearly stock lower end and would back down to about 55. But it didn't seem to need a lot of HP to cruise above 60 or so. In my ccpu, you can feel the "barrier" at about 63 mph and its no fun to mash the gas just to prove how fast the truck can go. With a "97" on it, it wasn't noticeable, but I'm back to the sweet running single B carb and I think more sensitivity to strain on the engine.
So I'm thinking now about fuel mileage, or actually fuel wasted and unnecessary strain on the engine to push a frontal profile that has been compared to the side of a barn through the wind at 45-60 mph. I'd like to have precise measurements rather than my own rampant speculation, but here is some speculation anyway. I can't push the windshield out at 20 or so mph which makes me wonder how much HP it uses up at various speeds. Of course, it is only a small portion of the overall drag situation I did some aerodynamic study and experimenting, with measurements, when we were racing sports cars in the 70's (and also made some horrible and embarrasing blunders in this area, that I could tell you about but not right now). So I have some ability to almost see what air is doing as it moves past various shapes. And I can almost see the air slamming against the windshield, then deflecting up to the visor where it curls around forward and around and hits the WS again. I punched the visor full of louvers (slats, for you pilots, as the idea was to relieve trapped pressure and promote laminar flow over the apex), knowing it wouldn't make any measureable difference, but just couldn't stand the idea of trapped air and extreme turbulence there. You would be horrified to know that I've made a couple of '29 slant windshields with no header and no visor out of tudors just to see if it makes a difference. One, I slanted so far back that the windshield had to be opened to get clearance between the glass and the steering wheel. Well, now that you're thoroughly disgusted with me, tell me what you've done to significantly improve your A mileage and what mpg you're getting.
So I'm thinking now about fuel mileage, or actually fuel wasted and unnecessary strain on the engine to push a frontal profile that has been compared to the side of a barn through the wind at 45-60 mph. I'd like to have precise measurements rather than my own rampant speculation, but here is some speculation anyway. I can't push the windshield out at 20 or so mph which makes me wonder how much HP it uses up at various speeds. Of course, it is only a small portion of the overall drag situation I did some aerodynamic study and experimenting, with measurements, when we were racing sports cars in the 70's (and also made some horrible and embarrasing blunders in this area, that I could tell you about but not right now). So I have some ability to almost see what air is doing as it moves past various shapes. And I can almost see the air slamming against the windshield, then deflecting up to the visor where it curls around forward and around and hits the WS again. I punched the visor full of louvers (slats, for you pilots, as the idea was to relieve trapped pressure and promote laminar flow over the apex), knowing it wouldn't make any measureable difference, but just couldn't stand the idea of trapped air and extreme turbulence there. You would be horrified to know that I've made a couple of '29 slant windshields with no header and no visor out of tudors just to see if it makes a difference. One, I slanted so far back that the windshield had to be opened to get clearance between the glass and the steering wheel. Well, now that you're thoroughly disgusted with me, tell me what you've done to significantly improve your A mileage and what mpg you're getting.
SeaSlugs
Well-known member
yea they look about as aerodynamic as a jeep wrangler (maybe find cd stats on them? about as similar as your going to get) they too have a mostly flat windshield roughly the same size, huge flat front and grille, front fenders that catch air, high ground clearance like an A, and about as boxy as the car bodies. the 4.0 straight 6 they had gets about 18mpg...
i think with the A efficiency is going to depend on tire PSI, how efficient the engine is running (like advance/reatrd exactly spot on for peak power output), driver style, and elevation and how exact the fuel air mix is to a given elevation and temperature.
i feel aerodynamics for an A are about as good as a garden shed and not a whole lot you can do to it short of making your own body like a dome/jellybean (think geo metro or small hatchback shape)
Allanw
Senior Member
If you looking at reducing drag on a Model A, reducing the frontal area is what you want to do! Those posts above, talking about the windscreen etc are spot on.
You not going to be able to reduce the Cd much, but if you can reduce the frontal area, the HP you have will be more effective at moving the vehicle. For economy, you'd have to go slower
The increase in HP required to increase speed is very exponential.
A slight tangent: I find it hilarious how car makers spout off about the Cd of the cars they make. The thing is, without knowing the frontal area, the Cd is just a number. You can't use it as an absolute comparison car to car. The only way to compare is to know the frontal area, and the Cd:
Example:
1990 Range Rover Wagon Cd= 0.45 Frontal Area=29.1 sq feet Cdxsq ft=13.1
1990 BMW 325i Convertible Cd= 0.45 Frontal Area=20.9 sq feet Cdxsq ft=9.41
The BMW is still a LOT easier to push through the air!
You not going to be able to reduce the Cd much, but if you can reduce the frontal area, the HP you have will be more effective at moving the vehicle. For economy, you'd have to go slower
The increase in HP required to increase speed is very exponential.A slight tangent: I find it hilarious how car makers spout off about the Cd of the cars they make. The thing is, without knowing the frontal area, the Cd is just a number. You can't use it as an absolute comparison car to car. The only way to compare is to know the frontal area, and the Cd:
Example:
1990 Range Rover Wagon Cd= 0.45 Frontal Area=29.1 sq feet Cdxsq ft=13.1
1990 BMW 325i Convertible Cd= 0.45 Frontal Area=20.9 sq feet Cdxsq ft=9.41
The BMW is still a LOT easier to push through the air!
Jim Huseby
Senior Member
I'm liking these responses. Any practical measures or mods that were doable by 1931 or at least no later than 1945 would be interesting to me. I would like to know what fuel mileage some of you are getting and how you're doing it. I can tell you (you already know) that an A engine with larger valves, ported, port-matched, 7:1, etc. is very sensitive to ign. timing and very sensitive to any variation in timing cyl-cyl (slightly inaccurate dist cam or worn dist)
Dave in Boise
Senior Member
Craig and Eddie!!
Easy enough to do for anyone who has accidentally put in their Ring and pinion in backwards during a rebuild.. (lol)
- Dave
Pete
BANNED
By doing a search for Cd and frontal area I found data for many cars but nothing for old cars. By doing some seat of the pants interpolation it looks like it takes about 450 hp at the rear wheels to go 150 mph in a model A sedan.
We know from talking to many people that a model A with a slightly modified engine (so called touring) will go approx. 75 mph. These engines usually develop around 60 to 70 hp. With a couple of data points you can guestimate a curve.
I can tell you that the 450 for 150 is fairly close because the SBC in one of my A's (stock body 2 door) pushed a 38 Chev coupe 154 at Bonneville and it will go 140 by GPS on the stretch of road I have to test on.(still accelerating)
The wind noise and buffeting at 140 is quite loud even through a helmet and ear plugs.
I think the easiest way to get the data you want is to get one of your cars on a chassis dyno and get hp at a given mph. Then you can figure in the frontal area/Cd. This will get you practical data but a wind tunnel would be the last word.
