Some contemplations on overdrive

I have long wondered why the auto manufacturers use overdrive transmissions. Wouldn't a high gear that is direct 1:1 be a more efficient transfer of power? So what I see is that a 1:1 top gear with a taller differential would be more efficient that achieving the same overall top gear ratio with overdrive and a lower differential? For instance a .80 ratio overdrive with a 3.55 gearset compared to 1:1 top gear and a 2.84 gear to achieve the same final ratio of 2.84. It would seem the 1:1 with a 2.84 cog would be more efficient because the 2.84 might have less drag and the 1:1 definitely has a smoother power path than the .80 overdrive does.
Comments? Did Detroit just sell us a buzz word here?

If we all drove vehicles that were the same physical size that never varied in weight, we could probably go with the simple math approach. As it is, vehicles have been (and continue to be) manufactured with everything from a two speed Powerglide to 6-speed sticks. Trucks have long had multiple options for transmissions and rearend ratio with the goal being optimization of engine performance (a relatively small range) over a wide variety of driving conditions (a vast range). An engine runs at its best within a certain RPM range. By using a transmission, the engine’s power is delivered to the drive wheels while keeping the engine within that range. As such, various gear combinations/rations are required.

While OD may seem superfluous, it allows great fuel economy when driving conditions allow long periods of constant driving at a lower RPM. Corvette 6 speed ratios are .74 for fifth and .50 for sixth. Granted, the Corvette has a lot of power being transmitted, but the principle applies from a Smart Car to a 80,000 pound semi.

Regards,
Glenn

I can only imagine my V10, 7400# F350 without OD on the open road. At 71mph, the tach is at 1900-2100 rpm. Kick it out and into direct, and it's running 27-2900 rpm. It takes more fuel to maintain that level of engine speed and ground speed. Will it pull as much of a load in OD? No, which is why that particular trans has a load/haul switch.

You could run a 1:1 ratio in high gear if you have a five or six speed transmission with very low first gear. I think Richmond sets up their transmissions like that.

I went through this exercise after I bought a 8" Ford rear with 2.73 ratio and realized that the 1:1 high gear would give good mileage BUT (!) low gear would be horrible with a TH350. The point is that with a high (low ratio gear) ratio 1:1 in the transmission the low gear is likely to not have enough mechanical advantage to get the car moving well in low gear. I used:

http://home.tampabay.rr.com/1bking/c...calculator.htm

and sold my TH350, replacing it with an 700R4 and changed the rear ratio to 3.55:1. My design goal was to have a lower low and a higher high gear than my memories of a '47 Ford flathead with a 3.78 rear, although with a SBC 350 and this is what I came up with:

Table I Present Gear Ratios with 3.55:1 Rear Gear

Gear Transmission Overall 1947 Ford (3.78 rear)

First 3.06 x 3.55 = 10.86 2.82 x 3.78 = 10.66

Second 1.63 x 3.55 = 5.79 1.604 x 3.78 = 6.20

Third 1.00 x 3.55 = 3.55 1.000 x 3.78 = 3.78

Fourth 0.70 x 3.55 = 2.49 -


Maybe my low gear is still not low enough and maybe my OD is too stiff but with about 400 ft. lb. at 2500 rpm the SBC 350 should haul it at least as good as my old flathead with the 3.78 rear and only a three speed. The second and third gears of the 700R4 are stiffer than the flathead three speed but as I recall it took a very good flathead to make even 300 ft. lb. of torque at 2500 rpm so I should be OK with good mpg in OD and still have a good low gear. Maybe I should have gone with a 3.73 rear ratio but I favored mpg. You can use the site given above to design your own drive line. Another factor is the weight of the vehicle but I estimated a light roadster will still have good acceleration with a 3.55 rear gear; you may like a lower low gear?

Don Shillady
Retired Scientist/teen rodder

I think some of you are missing the point I am trying to make. Stay with me here, this is hypothetical. Ok lets say that instead of a 5 speed that has a .080 overdrive and a 3.55 rear end, you had a 5 speed with a lower first gear and a 1:1 5th with a 2.84 rear end. For the sake of the whole argument let's say that the overall ratio is the same in each of the 5 gears but the top gear is 1:1. The engine rpm would be the same for any given speed in any given gear still but instead of the power path going through a .80 overdrive it is a direct powerpath through the gearbox in 5th. There would have to be less parasitic drag in the transmission and also I think less drag in a taller rear end gear too.

Quote:

---

Originally Posted by rumrumm

You could run a 1:1 ratio in high gear if you have a five or six speed transmission with very low first gear. I think Richmond sets up their transmissions like that.

---

Yes, I posed this on a couple other forums and that is the best answer I got. Another post on a different forum has the link to Richmond gear and they are saying exactly what I thought.


http://www.richmondgear.com/07pdfs/RG23.pdf
What has amazed me is the range of responses and the fact that very few people understood what I was postulating. Maybe I just didn't explain it right.
thanks rumm

Willowbilly---The best thing about OD is when you pull a trailer or other load you leave it in high and don't cause excessive wear on the gears. You are right about a direct drive top gear being a slight amount more efficient at highway speeds, but face it, most vehicles these days never leave city limits so the OD hardly gets used. To have the same breakaway gearing with a tall rear gear you'd need some steep gearing in Low.

I see where you're going. Move all the transmission ratios downward and the differential ratio upward to maintain the same overall drive ratio in all gears.

No reason it wouldn't work, I guess.

Well, probably would be better, but you would also have to totally redesign the differential. to be a 1:1 ratio the ring and pinion would have the same number of teeth, that's not going to work (mechanically speaking) in the rear end design we have now... I don't have any idea what the amount of drag is when in overdrive, but the new 5 and 6speed transmissions are well designed and I'm sure it's minimal loss.

Best case scenario would be a constantly changing drive ratio and no gears similar in theory to the way a snowmobile drive works. Think Chrysler tried this in the 50's. Perhaps replace the transmission as we know it with a hydraulic pump and drive assembly that would have a constantly changing drive ratio based on the power demanded to accelerate to or maintain a given speed??????

Dave, I didn't mean a 1:1 differential, I meant a 1:1 top gear in the transmission instead of overdrive.
The constant variable transmission like a snowmobile is a great idea and I have that rattling around in my brain too. I read about a guy who used a 17 horse Kubota in a 1200 pound car like an old triumph or something. He kept the 5 speed and clutch somehow. Around town he just left it in 3rd and got 67 mpg and 100 mpg highway. The whole theory there is it takes a much smaller engine to maintain cruise than it does to accelerate. Plus a constant engine speed is much better too.
Subaru did have it in the Justy too but I don't think there is anything like that for a pickup or larger vehicle.

Oh, ok... gotcha now. Sure, could leave the top gear 1:1 and lower (numerically) gears in the back... But with the od in the trans, I get to run my 4.11:1 rear gears and with a .62 overdrive I can still cruise the highway with it... I hate compromises on cars, but in the case of an od tranny I can come close to a dual use car. Besides, if it's for a Hot Rod, it doesn't have to make sense or be logical!!!!!!!

It's one of those things that would be interesting to see the real world numbers comparison on a dyno. Maybe the difference would be minute.
According to my calculations your 4.11 with a .62 overdrive would produce the same total ratio in top gear as a 1:1 tranny top gear and a 2.54 rear end.

I have had a Few GM's with the OD Transmissions . What I can Say I did not Like is . When your at Low Speeds 35-40 and it goes into OD . The Car will Lug and the RPM's go to a real Dead spot . Now on car's with 3.23 or Larger Gear gears Like the 1988 F-Body 3.27 Gear and 1987 442 3.73 Gear . They would go into OD at 30-32 MPH and what it felt Like to me was that the Rear end was trying to push the car . Making it even slightly harder to stop fast at that speed . Now this would be more of a Problem on a Car Like Mine that was driven more in the City then the Highway . My Friends that run these OD Trans Build's use a Switch under the dash to Lock out the OD When they are Driven in the City . Yes you could Shift down into 3rd But on my Pick up with the 700R-4 the Lock up Converter jumps in and out at 35 MPR . I said the heck with that and Un-Pluged that Trans Plug . It works Great Around Town now . Yes when your over 50 MPH They are Great . And you can get unreal Power off the Line and Top end . That 442 with the OD and 3.73 Gear Felt Like it never ran out of the Right gear .

Quote:

---

Originally Posted by willowbilly3

It's one of those things that would be interesting to see the real world numbers comparison on a dyno. Maybe the difference would be minute.
According to my calculations your 4.11 with a .62 overdrive would produce the same total ratio in top gear as a 1:1 tranny top gear and a 2.54 rear end.

---

That would be correct, but with a 2.97:1 low gear and a 4.11:1 rear gear, it launches quite well!!!! The lowest ratio low gear I know of in a stock box 4 speed would be a 2.20:1..... When you figure the ratio in low gear, then it becomes evident which rear gear will give the hardest launch and quickest 60 ft. time......a four speed with a 2.20 low gear, 1:1 top gear and a 2.54 rear gear will not launch near as hard as a 5 speed with a 2.97:1 low gear, 1:1 fourth gear, .62 overdrive 5th gear, and a 4.11:1 rear gear....(and we won't even talk about how much fun the five speed is with a 6.00:1 rear gear!!!!:LOL::LOL::LOL:)

If it Dead Hooks that trans Could be Dust :LOL:

Quote:

---

Originally Posted by tango

If it Dead Hooks that trans Could be Dust :LOL:

---

Not yet!!!! But with just 100 more ft. lbs. torque it would have to be a G-Force 5 speed instead of a Tremec with hardened shafts and gears!!!!:LOL::LOL::LOL: (It's all in the clutch! If I were to put the old McCleod dual disc in it doubt it would live for half a dozen launches!!!!!)

Ya I have Taken Car's Of the Line with M22 4-Speeds and 4.10 Gears at 5 Grand . Man you Should have seen the Folks Running to get out of the Street :LOL: :LOL:

The new dual friction clutches help a lot to take out some of that initial driveline shock, especially in a well prepped chassis with slicks. Drag race only, then I use the McCleod Soft Lok....

PS--used to launch my old mini motor Maverick at 6500 and shift at 7200 with a 10.5" wide Hoosier slick!!!!! Most fun you could possibly have with your clothes on!!!!!

I ran that Borg and Beck 3 Finger Clutch set up . Yes it was hard to push Down and when you Dumped the Clutch it was very Violent all the way to the back tires . My 1965 Mustang has some Hays Clutch I can Hardly Push it Down . I know I am getting Old I will have to install a Newer Type in there .

when i get enough cash im definaly looking into a g-force 4 speed!!!!!! dave showed me them boys from g-force and now i want one !!!!:whacked:

Hey Scooter I knew you could not stay Clear of any 4-Speed beat on your Car Threads **) :LOL:

Scooter You want to Launch your Nova Like This ? :eek:

thats a bad chassis set up if i ever saw one haha. no i want to keep it wheels down just shit n get :D, you know me if its a stick my nose is in it learning!

Quote:

---

Originally Posted by Dave Severson

That would be correct, but with a 2.97:1 low gear and a 4.11:1 rear gear, it launches quite well!!!! The lowest ratio low gear I know of in a stock box 4 speed would be a 2.20:1..... When you figure the ratio in low gear, then it becomes evident which rear gear will give the hardest launch and quickest 60 ft. time......a four speed with a 2.20 low gear, 1:1 top gear and a 2.54 rear gear will not launch near as hard as a 5 speed with a 2.97:1 low gear, 1:1 fourth gear, .62 overdrive 5th gear, and a 4.11:1 rear gear....(and we won't even talk about how much fun the five speed is with a 6.00:1 rear gear!!!!:LOL::LOL::LOL:)

---

Well, my whole idea was whether a 1:1 top gear had less drag than an overdrive, or would transmit power more efficiently. To me it just seems like it would. I made an effort to try and explain it was a theoretical postulation but many people have responded with how the lower gears would now be all wrong. If you read the original thread I had made my best attempt to negate that by stating that in my theoretical transmission every gear would have the same overall ratio as it's overdrive counterpart, that is the same engine rpm to tire rpm in every comparable gear, so the launch would be the same. Maybe I need to take one of those Art Carney courses so people can understand me better. I don't know any other way to explain it any better.

a transmission is more efficient when running direct in the top gear.... but what ratio would the transmission gear sets be? I'm not saying you're wrong, just gave the example of why I like the 5 and 6 speed transmissions....

Quote:

---

Originally Posted by Dave Severson

a transmission is more efficient when running direct in the top gear.... but what ratio would the transmission gear sets be? I'm not saying you're wrong, just gave the example of why I like the 5 and 6 speed transmissions....

---

The ratios would be whatever is optimal for the desired application. A drag car would be different that tow truck.
Actually it looks like the Richmond transmission attempts to do just that, allow you to tailor the ratios you want in conjunction to a 1:1 5th.

Ok, but then to equal the same overall ratio that my 5 speed has with a 4.11 rear gear, wouldn't the low tranny gear have to be so high numerically that the car would only move a few feet before shifting??

My point is, without the overdrive gear (or gears with the 6 speed) how could I still enjoy my car in the "best of both world" environment that my 4.11 rear end gear and .62 overdrive trans gear????

[QUOTE=Dave Severson;339271]Ok, but then to equal the same overall ratio that my 5 speed has with a 4.11 rear gear, wouldn't the low tranny gear have to be so high numerically that the car would only move a few feet before shifting??

My point is, without the overdrive gear (or gears with the 6 speed) how could I still enjoy my car in the "best of both world" environment that my 4.11 rear end gear and .62 overdrive trans gear????[/QUOTE
I just wrote about 2 paragraphs with examples and deleted the whole thing, if you don't understand that the total (transmission ratio and rear end ratio added together) ratio in any given gear would not change then I give up trying to explain it. It's giving me a head ache

An example of what Willowbilly is saying , My cousin has a Kenworth with a 425 Cat and 15 over trans., 4.11 gears. My Eagle has a 425 Cat and 15 direct, 3.55 gears. RPM at 60 mph is 1750 in both. Speeds in all gears is the same. The OD trans is a bit noisier in top gear, and the direct geared one seems to pull a hill better in top gear. On a long uphill grind, the Kenworth can go down to Direct and pull like crazy, quieter than the IHC in 14th (reduced gear). Once you shift them both down below that, there's no difference. Neither gets more than 5-6 MPG grossing 120,000 lbs, so I can't say if the reduced friction makes any difference in fuel consumption. Logic would say it should, but a few percent at that high rate would be awful hard to detect.

Well, guess I'm not much into theory...but I do know the 5 speed with the 4.11's works great for me, super acceleration around town and playing with the boys, and a very nice 2200 rpm cruise down the highway when travelling... If there's a transmission and gear ratio combination that will work better doing both things, I'd probably buy it....

Quote:

---

Originally Posted by R Pope

An example of what Willowbilly is saying , My cousin has a Kenworth with a 425 Cat and 15 over trans., 4.11 gears. My Eagle has a 425 Cat and 15 direct, 3.55 gears. RPM at 60 mph is 1750 in both. Speeds in all gears is the same. The OD trans is a bit noisier in top gear, and the direct geared one seems to pull a hill better in top gear. On a long uphill grind, the Kenworth can go down to Direct and pull like crazy, quieter than the IHC in 14th (reduced gear). Once you shift them both down below that, there's no difference. Neither gets more than 5-6 MPG grossing 120,000 lbs, so I can't say if the reduced friction makes any difference in fuel consumption. Logic would say it should, but a few percent at that high rate would be awful hard to detect.

---

If there were a milage difference in real life I too doubt it would be measurable on a big rig.
Thanks for the real life comparison. That is the kind of information I was hoping to hear more, pro or con.

Here is some info on the 700R-4 The good and Bad . http://www.smokemup.com/tech/700r4.php

I would love to design a car with a hydraulic drive someday. Unfortunately I do not have the requisite knowledge of hydraulics to engineer it myself.

My theory is to make the ultimate 'dancer'. have a nice fat hydraulic pump direct mounted to the engine. have hydraulic motors on the rear wheels, and hydraulics on all four corners.

When they make 'dancers' today, it requires a TON of lead in the trunk to run the electric pumps. My idea would eliminate the extra weight of the batteries, and ( should ) allow the vechicle to do more.

I do know enough about hydraulics to see some obvious design problems: first, the hydraulic drive would be a parasitic power loss. The fluid would heat ( power loss ) and any time you transfer power from one type to another ( mechanical - fluid - mechanical ) you suffer power loss. It takes power to convert power types.

BUT.. is seems to me that if you started with a variable flow pump, match your hydraulic motors correctly, you would have one SMOOOOTH acceleration.

I think there has been some big moves toward a hydrostatic car and more efficient transfer. One big advantage is keeping the engine at a constant rpm, which would allow you to just set right at the peak of the torque curve. Snowmobiles do this quite well.
The big mine trucks use electricity. The diesel runs a generator and the wheels have electric motors that are also retarders.

You guys think about all the angles! Since a fluid does not compress, why not build a drag car with the engine set to run at max torque (after warm up) and put the power through to the drive wheels hydrostatically, using a higher volume pump than the motor, and use a positive control valve controlling bypass, linkaged to the pedal on the right, you could theoretically turn the rear tires 300 + mph as soon as you got the go light! can you imagine the flying parts? Seriously though, I wondered about the logic of gear ratios as a youngster when I discovered that sprocket ratio on a bicycle was affected by the throw length (stroke) on the pedal cranks. Am I getting it?

Quote:

---

Originally Posted by JoelGrimes

You guys think about all the angles! Since a fluid does not compress, why not build a drag car with the engine set to run at max torque (after warm up) and put the power through to the drive wheels hydrostatically, using a higher volume pump than the motor, and use a positive control valve controlling bypass, linkaged to the pedal on the right, you could theoretically turn the rear tires 300 + mph as soon as you got the go light! can you imagine the flying parts? Seriously though, I wondered about the logic of gear ratios as a youngster when I discovered that sprocket ratio on a bicycle was affected by the throw length (stroke) on the pedal cranks. Am I getting it?

---

I think there has been an issue of parasitic loss in the hydrostatic drives. But in theory if you could put the engine at the peak of the torque curve and vary the multiplication to keep it at peak that would be a good thing.
You bicycle analogy is a good simplification of crankshaft stroke.

FWIW Ford does have a constant variable transmission in a few of it's smaller cars. ZF builds it.

A poster on another board offered this. This is the kind of stuff I was looking for.

You knew this was going to happen. I had to put some numbers behind this discussion.

A typical helical gearset is 98-99% efficient. In a direct drive top gear there are none, in an OD top gear there are two. For the sake of discussion, assume 99% for the input shaft gearset and 98% for the OD gearset. These assumptions are supportable given that the input set is typically quite robust and has a relatively low reduction whereas the OD gearset is typically rather weak and is being used inefficiently as a speed increaser.

A hypoid bevel gearset is 92-96% efficient; efficiency decreases as ratio increases. For the sake of discussion assume 95% for a 3.08 gearset and 94% for a 3.55. These are supportable assumptions given the range of 1.5:1 to 6:1 normally encountered.

A Ford 300 at 2000 RPM produces approx. 130 HP under full load and WOT at a BSFC of approx. 0.5 lbs/HP-hour. At cruise under some unspecified but significant load assume for arguments sake that it produces half the HP (65HP) and that the BSFC is 10% worse or 0.55 lbs/HP-hour. Assume further that the gearing is such that this occurs exactly at 60 MPH.

If this engine were to be coupled to the direct drive transmission with the numerically lower rear end, it would deliver:

65 x 0.95 = 61.75 HP to the rear wheels.

It would consume:

65 x 0.55 = 35.75 lbs of fuel per hour which is also 35.75 lbs of fuel per 60 miles

At 6.25 lbs per gallon this would be:

5.72 gallons per 60 miles or 10.49 MPG, arguably what one could expect with a heavy load.

If this same engine were to be coupled to an OD tranny with a numerically higher rear end, to produce the same 61.75 HP at the rear wheels, it would need to produce:

61.75 HP / (.94 x .98 x .99) = 67.7HP at the flywheel

That would result in a consumption of:

67.7 x 0.55 = 37.23 lbs of fuel per hour which is also 37.23 lbs of fuel per 60 miles

At 6.25 lbs per gallon this would be:

5.96 gallons per 60 miles or 10.06 MPG.

So running at highway speeds under a significant load, the difference would be 0.43 MPG. At lighter loads it would be proportionally less, meaning about half as much difference at cruise running empty or about .20-25 MPG.

This treats only the effect of transmission losses and ignores all the other sources of inefficiencies such as bearing losses, seal drag aero and rolling resistance, etc. assuming that they are equal in both cases.

Obviously YMMV both literally and figuratively.