88IROCZJG

This has long been a topic of arguement among me and classmates. My car has the stock 700-R4. From a dead stop I put my tranny in OD and just go. It is what I have seen my parents do for as long as I can remember and it is just what I learned. I just assumed that the whole point of an auto is that your car knows when to shift and over-drive is just another gear. Others at my school with automatics with over-drive say that it is bad to start in OD and you should only use it on a major highway were you wont be doing alot of stop and go driving. We all know that none of us really know what we are talking about but can somebody clear this up for me? Is it bad to start in OD or doesnt it really matter?

SC88

It generally does not matter. If you are towing statues you may want to keep it in Drive. If your driving in stop and go, you can put it in drive just so the tranny doesn't have to shift as much. Many people will argue. Just do what you like, it's yours.

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1988 Camaro Sport Coupe: Orignally: 305 TBI 700r4
Now: 1974 355CI, 2bolt, Badger Flat Head Pistons w/ valve relief, Melling high performance street cam, GM Cast Vortec Heads, Edelbrock RPM Air Gap Intake, Edelbrock 600cfm w/ electric choke, GM HEI w/ vacuum advance, Weiand Low Profile Air Cleaner, Mallory 5 port Fuel Pressure Regulator, Mr. Gasket Throttle Cable Bracket, Corvette Shift Servo, 16" IROC Wheels, Every thing else is STOCK
Best Times:60ft 2.26 1/8th 9.68 @ 73.63 Soon: 3.73 or 3.42, Gears, Hoosier Q/T

Tom84L69

trannies are designed to be left in OD for normal driving, use other gears for big hills or for slowing down.

transfixleo

Generally speaking you should be fine with it in O/D. That being said there is one advantage to leaving it in D until you need O/D. That is the Sprag won't have to reengage after everytime you let off the gas but to be honest that is not a big deal since you are at low torque. WHat kills them (and other parts) is when the sprag is worn and you kick it down from 4th to 3rd or 2nd. Under these circumstances the sprag is applying at high RPM and load and if it does a slip-grab it can literally bend parts inside the tranny. I milked my '88 along using the method of manually downshifting to D when I wanted to pass someone until I decided to O/H the tranny since I knew that the V-6, having lower gears would have more Sprag wear and I didn't want to be left at the side of the road. When you see 700's with stripped out Forward clutch drive teeth, it is because of this problem.

That being said, what do I do now that I O/H'ed it and have the best design sprag in there? Leave it in O/D!

[This message has been edited by transfixleo (edited November 12, 2001).]

irocaz28

there is no reason for you not to be in overdrive all the time unless u r in traffic. It will shift into fourth gear if u ever go fast enough by itself. regular drive jus keeps u in 3rd but in overdrive u get the added gas mileage if that matters even though its marginal.

AJ_92RS

So is the sprag only engaged in 4th gear? Is that what you meant by saying, "That is the Sprag won't have to reengage after everytime you let off the gas but to be honest that is not a big deal since you are at low torque."?

I was told at work, by a guy that used to rebuild trannies, that the Sprag is mainly disengaged at idle. Like at a stop, or coasting, or whatever.

The reason we were talking about it; We were talking about how Dodge Caravans don't use one and how many of those trannies he rebuilt because of the way Dodge has that tranny set up.

If you can briefly explain what a Sprag gear does, it would help. I guess the main reason I'd want to know is so I can tell if it's going bad. So I guess the real question is, what are the symptoms?

Thanks,
AJ

[This message has been edited by AJ_92RS (edited November 14, 2001).]

ChevyLuva3

The reasons for D and OD:

D, is for driving. If kept in D rather than OD, you will have much better pickup. The car will respond better.

OD, is for driving, also but it was made to save mileage. If kept in OD you will have less pickup, but save on gas. It will also shift earlier. The car won't respond as well as D.

So in conclusion, if you're just driving around, save some gas and keep it in OD. But if you're racing against someone or just want better pickup, put it in D.

Remeber D, isn't your only option in racing, you can always shift 1,2,D. Then if you need those last extra mph, put it in OD. Hope that helps.

stingerssx

Yeah, that's right. I was waiting for someone to mention the fact that in o/d the tranny upshifts each gear earlier. In drive, it holds out a little longer. It doesn't really matter unless your the type that has to get where he's going quick. But to race, manually shift.

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'82 Firebird, dead stock, 9 bolt disc rear, over 200,000 miles and still going strong, more to come...
http://www.spinfrenzy.com/stingerssx...easures.html#4

transfixleo

The sprag is an overrunning clutch, like a ratchet only without the backspin. It holds when you are accelerating, in 1st, 2nd and 3rd, and it overruns in 4th and whenever you are coasting. When you put the car in D there is another clutch pack that comes on that keeps the sprag from being able to overrun, that is why you get 'engine braking' in those gears and not in O/D.

What are the symptoms? Well that is the hard part. When it starts to go bad it is usually under whats called dynamic load (acceleration) that it will slip while under static load (idle) it may slip/grab causing a delay-bump on engagement but that is harmless. When you are accelerating and you downshift from 4th into 3rd or 2nd (when it has to reengage under max load) if it slips then grabs it can evenyually do allot of damgae. On the mild end it will wear the teeth out on the forward frictions but I have seen many times where the lugs on the Sun Shell and Reverse Drum are mysteriously flared out. This is because the sprag slips and the car over revs then BANG it grabs and the centrifical force makes these parts distort.

By driving a higher mileage 700 in the D position you thus keep the sprag if not technically engaged, in the same position it needs to be in so that as I said coast to accel won't make it go on and off.

Z28DJP1987

Driving in OD reduces the RPM's on the engine
only after you reach 40 MPH. You can use D or OD anytime with no problems, however; OD
is best suited for highway driving. Forget the BS about wear on the Trans parts.

AJ_92RS

Thanks Leo, (Is that your name? or am I assuming too much?)

That's a lot more than what I expected you to say, but that's a good thing. That also makes a lot of sense now. I've seen pics of them and wondered what in the heck they did.

Now I know, 'The rest of the story'!!! Thank you.

So from what you're saying, since I have 175K miles on my car, engine, and trans, then it wouldn't be a bad idea to leave it in 'D' while I'm in town, right?

I've heard it's just a good idea anyway, that way the trans isn't always shifting back-and-forth from 3rd and 4th when your riding at like 37-42 mph (that's what mine does).

Thanks again,
AJ

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92 RS w/t-tops 305 TBI Auto.
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transfixleo

Yes, especially if it is the original sprag- you are definately on borrowed time!