Is it ok to shift from OD to D while on the highway ?

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1988 Sport Coupe 305 TBI
K&N
Flowmaster exhaust

 

Yup, so long as when you shift into Drive (1:1 ratio) the rpms don't exceed 4500 rpms. That way you have room to accelerate.

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George P. Lara
1984 Z28
2001 SS Coming Soon
SCCA, SCFB, SC3GFB

[This message has been edited by MRZ28HO (edited July 06, 2001).]

 

going OD to D will not hurt anything. Just bump up the RPM's a little.

 

Actually, I have done some thinking on this and IMHO, at high speeds it probaly isn't a good idea unless you step up the RPM at the same time. The reason is that the overrun clutches are just two little frictions that are not really meant to hold much torque but at high speeds (let's say, 100 MPH for example) there has to be a load placed on those that is higher than what they are meant for (essentially, coasting).

 

You wouldn't be shifting into D at 100 MPH, unless you were looking to do some replacing.

But, to humor you, you're just saying that (provided that you are at an exceptable speed) if you're going to downshift, just give it a little gas?



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1985 Camaro SC - 2.8L, auto.

http://www.xenodrgn.f2s.com/Frontright.jpg

 

No humor needed. There are two reasons for going to D, one is two accelerate (which won't affect the O/R clutches since the sprag is going to take over, theoretically)but the other reason is to slow down, which from 100 MPH is a real possibility. I was trying to point out that this would likely put too much strain on them.

As a tranny builder I have seen plenty of 700's with burned O/R frictions but no real reason as to why. The reason I say this is they are only supposed to 'work' on decell, as during acceleration the sprag holds, the O/R's KEEP the sprag from overrunning and therefore you get engine braking when otherwise the RPM's would drop on coast. Try it yourself. Go @50 in O/D, coast then put it into D. You will feel nothing until you go to D then it will slow down as the engine speed rises. One theory I have has for O/R slipping is that the sprag slipped (common problem) on accel in D, or lower gear position (they aren't on in OD) and all of a sudden those little buggers had to hold, which they can't, so they slip and burn. This new theory I have could also explain it very well too. So to get back to the question, yes by increasing RPM's you are not loading the O/R clutches as they wouldn't be working. Bottom line: Don't use lower gear positions from higher speeds like that to slow the car down.

 

slowing down.....isn't that why they put BRAKES on a car!? you should have your head examined if your using the trans gears to slow down!!

 

Uh... that is why the overrun clutches are there. If you don't want to use them, fine. There is a technique where you can drive in slippery conditions in a lower gear without having to be on and off the brakes and is very effective. Also, in a lower speed panic stop, dropping the tranny into Low gear is VERY effective.
One other note, in man 2 and 1 there is no TV boost as it is already boosted to max pressure. Ever notice that the shift feels firmer in Man 2 than auto? The reason for this is to keep the tranny from slipping when you DO put it into those gears to slow down.

[This message has been edited by transfixleo (edited July 09, 2001).]

 

I always wondered why it seemed to be a little stiffer shift hmmmmm.....I like it. I have heard about giving the car a bit of gas before a down shift. Line pressure is low and then you put additional torque into the tranny. Without the extra line pressure over time doing this you could burn evrything up.

 

just floor it on the highway and it will go to third on its own. and the tranny does shift harder in 2 than D, a friend of mine that works on trannys told me something like it uses more clutches or something like that.