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Old 12-12-2012, 11:02 PM   #1
Reino
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Tell me more about the 4+3...

It looks like the combination of my means and desires may have me driving a 4+3 in the next few years. I understand almost everything about it, except: how exactly do you shift it?

Do you go 1-2-3-4-OD2-OD3-OD4 or 1-2-OD2-3-OD3-4-OD4, or are they entirely separated between street and highway driving?

I understand that having a simple button right under your thumb can lead to many combinations of spirited shifting. Does it truly have SEVEN speeds?

What's your method?


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Old 12-13-2012, 06:27 AM   #2
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If it were mine I would only use it when I was in 4th. In theory you do have 7 speeds but with all the issues that overdrive has I wouldn't use it that much just to preserve it. Otherwise it is basically a super T-10 with an overdrive bolted to it so the trans really is a pretty good unit.
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Old 12-13-2012, 06:32 AM   #3
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Subscribed...I'll be back with more input.
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Old 12-14-2012, 11:41 PM   #4
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Quote:
Originally Posted by Reino View Post
how exactly do you shift it?
1-2-OD2-3-OD3-4-OD4,
Actually 1 ,2 ,3 , 2-Od ,4 , 3 -O/d , 4-O/d

Ratios
1st . 2.88
2nd. 1.91
3rd . 1.33
2-O/d. 1.18
4th. 1.00
3-O/d. 0.83
4-O/d. 0.62

but as 2nd O/d is close to 3rd and 3rd O/d close to 4th there is little point.
Most only engage O/d in 4th when on the open road to get the fuel economy from the reduced revs

You need to be aware,by default ,O/d is always on ,so once the engine reaches the correct temp the computer will automatically engage the O/d ;so from a stop you will get 1st ,then straight into O/d in 2nd , 3rd and 4th gears.

"The reasoning behind the 4+3 is similar to the skip-shift on the 6-speed manuals - to meet EPA
mileage requirements in both the city and highway cycles by getting to top gear as quickly as possible.
The EPA has specific rules that do not allow short-shifting or skip shifting by the driver, so in both cases the car
is set up to skip-shift on its own.
The early C-4 solution was to put an overdrive unit on the T-10 4-speed and have the car automatically
go into overdrive when you shift to second."


Pushing the button before driving off gives you full manual control of the O/d ;
not touching the button means the computer will engage the O/d as programmed

But I found 3rd O/d handy in traffic because with the auto kickdown function , if you hit the gas hard enough it will drop into 3rd direct for passing and then shift back up into O/d when you let off the gas ;similar to a 4-3 downshift without moving the gear lever.

Different years have different functions ; read this to compare operation
http://www.5speeds.com/43table.htm

If car is new to you and the service history unknown , drop the pan on the O/d , change the filter and
replace the fluid (Dextron III )
You won't find the O/d filter at your local dealership but the filter from a FMX trans is the same
http://forums.corvetteforum.com/1571645223-post6.html

One of the reasons the 4+3 got a bad name was partly due to the O/d not being serviced at the factory
specified 12K intervals ; some owners treated it like a reg auto trans with bad results

Last edited by vetteoz; 12-15-2012 at 12:15 AM.
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Old 12-15-2012, 07:46 PM   #5
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Thank you! That is exactly what I am looking for. It sounds like a pretty fun transmission for all types of driving.


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Old 12-16-2012, 02:43 AM   #6
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There is a one way Roller Clutch that sometimes fails in the Doug Nash 4+3 Overdrive unit.
Sometimes also called a Sprag but incorrect because an automatic trans sprag has dog bone shaped rollers that locks up in one direction & freewheel in opposite rotation. Such as used on the input drum & turbine shaft assembly of a 700R4 & 4l60E.

I sometimes see those NOS GM Doug Nash 4+3 Roller Clutches show up on ebay. Might be wise to purchase one just in case.

BR
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Old 12-16-2012, 06:21 AM   #7
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Vetteoz is spot on. If ther are ever any issues, contact SK Speed in New York. They've been rebuilding them since they were new.

Service is important, so keep up with it. I'd say its not worth keeping in the car once it craps out. I'll be swapping over to a 5 or 6 speed in the future when needed.
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Old 12-17-2012, 01:51 AM   #8
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Just to add my own experience with the Doug Nash unit in my 1988, I can tell you that it is a terrific transmission. It always takes me by surprise when the overdrive kicks on, but otherwise I enjoy the transmission very much. I can use OD on the freeway and get great mileage (25mpg), or I can turn it off when I really want to feel the power of that 5.7 V8. Change the oil regularly and you'll be fine.
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