Double overdrive Good or bad idea??

OK Running around with my little 93 Van I noted how much power is lost going into OD.

It is more than you would think a 4th gear would lose, I mean it really drops the power: any load and pop it has to drop out of OD.

I was thinking of adding a seond OD Behind the auto and in some cases run both, but if I feel such a drop with the stock OD I fear it will bearly be able to move in double OD.

So I am not thinking a Under Drive and a very tall rear end gear...but that would end up being something like 2:40 or less...

So how well does a under drive work??

Any one used one?

Rich

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Double overdrive Good or bad idea??

paulorr1's picture

In another post I stated that I will be working on an secondary overdrive for a manual transmission. This project is a bit over the top since I have, so far, concentrated on mods the average guy can duplicate.

the basic problem of a "SUPERDRIVE" is one of acceleration. On flat smooth roads at steady cruise you only need your theoretical 20hp to maintain speed. If you make that at a higher intake manifold pressure and at lower RPM you'll get increases MPG. You will definitely feel cheated when you want to speed up and will have to drop one or two gears. Too bad, but that's life.

I had said I was picking up a trans for development work this last weekend but ended up installing my exhaust system. Last night I stopped on the way home at my local friendly trans shop to see about a freebee core for the development. Will check back today for availability. This is definetely not for the home handy man. You tend to get obsessed with this MPG thing. It's the challenge. It's the why didn't they do this??

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Double overdrive Good or bad idea??

mpgmike's picture

Rich, what ever happened to the 383 idea? That would give you the low end to thwart the OD blues.

Mike

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Double overdrive Good or bad idea??

Pinhead's picture

Ever hear of a "Browning" transmission? It's a 3-speed manual transmission that goes behind your regular tranny. I know a guy that has one in an old 6-cyl Chevy truck that can cruise down the highway at 70mph at about 2200 rpm. On the "low" side (he calls it "double one") it could out-pull a barge. I wonder if they still make something similar??

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Double overdrive Good or bad idea??

racprops's picture

Thanks, I am still doing the 383 motor as soon and my funding comes in.

But was wondering which way to go. All true brownie trannies I have found are way to big and have PTOs on them as well.

But I too liked the idea of a under drive AND a second overdrive system, 12 forward gears gives me a lot of gears to use.

As for the Brownie I have a pair of old Borg Warner 3 speed overdrive trannies, and have one part ready to do that with, I took out the first gear/reverse gear slider and left only 2nd and 3rd.

This set up is more like three trannies, the automatic 4 speed a 2 speed center and then a in/out over drive third tranny, as all three can be used at the same time?

And plan on running the R11 overdrive both manually and electrically.

My only problem has been supporting the front input shaft or machining it back and adding a bearing and yoke for a ?U? joint to the front.

Anyone have a machine shop and can do this conversion??

And I do know about having a U joint off center to run?figured to set the tranny off side a touch to get that.

But I was wondering if I would go all that trouble and end up with a worthless set up?that the double overdrive would not work at all or not well.

The other good thing I can see is the BW OD can be used with any gear so I can gear split the hell out of my auto?s 4 gears and with the 2/3 gear then split 8 gears?thus have 12 gears.

So feed back?? Good or bad or dumb??

Rich

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Double overdrive Good or bad idea??

E=mc2's picture

Might try emailing...

http://www.metrompg.com/

or http://www.gassavers.org/forum.php

...he put a different trans in a Metro...changing his overall gearing to a low ratio...saw some mpg gain but not as much as expected.

With a van...you have even more relative wind resistance?

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Double overdrive Good or bad idea??

racprops's picture

Well I think reading that mileage testing he is doing on that little 3 cylinder car proves something I have thought for a long time.

Small engines are good for low speed mileage, but drop off fast as speed goes up.
Note he gets fantastic 82 MPG at 34 MPH, but who wants to drive cross country at 34MPH??

But is getting 44.45 at 65.2 MPH and it was still going down as speed when up. So in this case double the speed meant ? the mileage?

Large engines start with low MPG but have a less dynamic drop off, my van runs around 14 to 20 MPG so to go fast takes a large motor, or at lease a certain power to weight/drag ratio.

Well for what I have to have in hill climbing power, and being able to cruse at 80 MPH I think I am on the best track for the best possible mileage on pump gas.

Then it is on to Hydro?

Rich

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Double overdrive Good or bad idea??

cafn8's picture

This reminds me of an early Dodge Colt transmission that once read about. I vaguely recall that it had a 3 or 4 speed manual transmission. It also had a second shifter which changed the range of the transmission, much like a bicycle transmission. It had 2 ranges, which I presume would be suited for around-town speeds or highway speeds. Sorry for my foggy recollection. I'll try to dig up more details when I get a chance.

As a guy who enjoys a nice mountain bike ride I have a first hand appreciation for the benefits of lots of gears spaced closely together over a wide range. I wonder, though, if lining up 2 or more transmissions will work against you in the form of drivetrain friction. I'll admit that I don't know specifically how much friction a transmission has, but I do know that when a car is tested on a chassis dyno they generally use a correction factor of around 15% between power at the crankshaft and power at the wheels. Obviously, there are a lot of other parts grinding away between the crank and the dyno (including the tires on the rollers), but it might be something to consider. It does sound like a cool idea though. I look forward to hearing about the results. As in the case of the Colt, it might even be something better suited to a small engine to avoid the choice between good town mileage or highway mileage.

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Double overdrive Good or bad idea??

fltcoils's picture

My lesson, from guys who started driving in the '30s, was that 60 hp is plenty to run a car (ford flathead). To maximize mpg, even those "underpowered" cars had overdrives. Thus they lowered engine speed and vacuum and reaped the benefits of better engine fuel economy.

There are a couple of factors fighting each other in the overdrive discussion.

transmission energy losses
throttle enrichment as throttle plates open up
cam timing/engine breathing optimized for high rpm operation

The principle of lowest engine speed, lowest vacuum is valid, but the factors above fight you implementing it.

The biggest battle is living with a "wimpy" engine. A 200 hp engine (rated at 4400 rpm) may make only 40 hp at 1000 rpm. Thus a wide open throttle while running in overdrive may seem to give no increase in power. But the power is there, you just need a lower gear.

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Double overdrive Good or bad idea??

mpgmike's picture

cafn8, you summed it up pretty well. My grandma had one of those, and while I worked at the Dodge dealer I got to rebuild one of those trannies.

Mike

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Double overdrive Good or bad idea??

racprops's picture

fltcoils wrote:

My lesson, from guys who started driving in the '30s, was that 60 hp is plenty to run a car (ford flathead). To maximize mpg, even those "underpowered" cars had overdrives. Thus they lowered engine speed and vacuum and reaped the benefits of better engine fuel economy.

There are a couple of factors fighting each other in the overdrive discussion.

transmission energy losses

Well this is aolder BW 3 speed so it will only have some loses due to bearing drag...

throttle enrichment as throttle plates open up

One reason for useing a MAF TPI Intake, low RPm power and system made for low RPM,s these intakes and motors were made to run under 4500 MAX RPM.

cam timing/engine breathing optimized for high rpm operation

And again my hopes to take a stock cam and lower it's Power range down to 1000RPm and with rooads lifers get full power at 2800 RPM as well.

The principle of lowest engine speed, lowest vacuum is valid, but the factors above fight you implementing it.

This why I am building a motor that likes 1500RPM.

The biggest battle is living with a "wimpy" engine. A 200 hp engine (rated at 4400 rpm) may make only 40 hp at 1000 rpm. Thus a wide open throttle while running in overdrive may seem to give no increase in power. But the power is there, you just need a lower gear.

Rich

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Double overdrive Good or bad idea??

E=mc2's picture

racprops wrote:

Well I think reading that mileage testing he is doing on that little 3 cylinder car proves something I have thought for a long time.

Small engines are good for low speed mileage, but drop off fast as speed goes up.

Note he gets fantastic 82 MPG at 34 MPH, but who wants to drive cross country at 34MPH??

He doesn't post all his testing on his website...many details are scattered in that forum. He has actually managed over 130 mpg using various mods and driving techniques....I think.

My take on it is that with most vehicles...3% larger tires will give you an easy 3% gain in mpg...but going with say a 15% lower diff ratio won't gain you 15% in mpg...not a linear relationship there.

And with a van vs a streamlined car...the faster you go...the greater the relative HP needed?

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Double overdrive Good or bad idea??

fltcoils's picture

1966 was a good year, the '66 Tbird was out, probably one of the best styled cars IMHO. But it got 14.5 mpg like most of that era.

That same year all the magazines reviewed a GM product designed to improve mpg by reducing engine rpm, lowering vacuum and setting the cam for low torque, The cutlass cruiser. So for those of you who were out of high school in 1966 and thinking of buying a car, perhaps you read this:

http://home.c2i.net/terjekl/carlife.htm
http://home.c2i.net/terjekl/motor.htm
http://home.c2i.net/terjekl/1967olds.htm

"after driving both cars over the same stretch of highway for a distance of 213 miles at approximately 70 mph. The 4-4-2 took 16 gallons to fill while the TC car required only 13." motor trend

"...They say the performance is brisk enough to suit anybody - and gas mileage is said to be over 20 miles per gallon at 70 miles per hour! " pop hotrodding

Articles typical of the times showing an improvement from 14-15 mpg to 19-20 mpg by using a lower rear axle ratio, low torque optimized cam etc.

results.

problem is, people aren't willing to accept the apparent power loss a low rpm at cruise gives you.

So drop that rpm down as far as you can and still maintain no more than 10psia mfd pressure at cruise conditions. And get a cam set for LOW torque.

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Double overdrive Good or bad idea??

racprops's picture

Well that is just about what I was planing, a 383 with a low cam torque curve to normaly peak at 200 PRM running with over drive at 2000 at 70 MPH and with the 4 gear tranny not lose all the hill climeing and passing power.

Thanks I confirms I am on the right track.

Now those rooads lifter will let me run a 2800 prm cam and with the lifters drop it back down to 2000 RPM amd when ran up to 2800 STILL Make power...

Best of all worlds.

Rich

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