mrl

I must apologize if this question has been answered before. I did a search for C4 manual transmission, but could not find anything relevant.
I fail to understand the logic of the 4+3 transmission. I can understand the benefit of having an overdrive for the top gear (4th in this case) but why would you want to have an overdrive option for 2nd and 3rd gear?

Randy T

As I understand, purely for fuel economy. The 4+3 was developed when GMs corporate average fuel economy was under pressure to rise (that part really hasn't changed much, CAFE standards look to be under ever greater pressure to rise significantly in the near future).

A 4+3 standard trans equipped Vette offered greater fuel economy.

Also, please fill out your profile.

mrl

ok for the profile.

I forgot to ask, what is the final drive ratio of the 4+3 in 4th with overdrive?

What is the final drive ratio in 6th for the ZF?

The reason I am asking, is that I will be buying a C4 in the near future and was wondering if it is worth spending the extra $$$ for an 1989 and up with a ZF trans. I read that the 4+3 were problematic.

tequilaboy

4+3s were available in two varieties:

MH5: 0.62 (more common)
MK2: 0.68 (Seems to be linked with Z51)

By comparison, the ZF6 has 0.75 5th gear and 0.50 6th gear 0/D ratios.

I think the 4+3 has a better overall (wider) ratio spread in the first 4 gears with a low 2.88 first gear and more reasonable O/D ratios.

The ZF6 2.68 first gear is rather tall resulting in a narrower ratio spread.

I'm in the minority, but I like the 4+3.

They're both costly to repair or replace.

c4cruiser

With regular maintenance, the 4+3 will last and perform well. You need to do a fluid change every 12 to 15K miles (two quarts of Dexron II or III and replace the filter (same filter as the Ford FMX tranny and costs about $6 to $8).

The OD does not really like to be beat on in terms of shifting into or out of OD at full/throttle and high RPMs. The OD will kickdown similar to an auto tranny like you would do in passing a car. It will handle around 360-375HP and 450+ ft-lbs of torque.

One nice feature about the OD is that the gear splits in the OD with 2nd and 3rd gear is that you can cruise at arterial/boulevard speeds while lowering the RPM. I frequently use 2rd and OD for speeds between 30-40 and 3rd and OD for speeds up to 50. 3rd/OD is about 400 RPM lower than 4th/no OD at the same MPH so you can get some fuel savings.

Even on the freeways around here where the speed limit is 60, I will get into 4th/OD and just shift down into 3rd (leaving the OD engaged) if traffic slows some or I want to pass a car without using the kickdown feature.

Not all years of the 4+3 operate the same. Check this link to see how they work.

5speedC4

Stay away from the 4+3.

Mine came with a 4+3.

Now it has a Keisler Engineering TKO 500, 5 speed manual.

That cost me $3,500 with me doing all the work myself.

Stay away from the 4+3.

You have been warned.

Now listen to everyone else tell you how great they are.

I bet I really irritate them.

Fact is, the 4+3 is a fun tranny, and reliable if taken proper care of.

But when it breaks...........

You will remember my words.

Stay away from the 4+3 transmission.

corvettedan22

I love my 4+3 and if thats what u can afford dont let theese tight wads steer you wrong.

In an effort to keep it lasting.

I use it

Clutch
1
Clutch
2
Clutch
3
Clutch
4
Clutch
(over drive as 5th)

if you take care of it it will last.

Car Guy 88

GM used the Doug Nash 4+3 in order to meet new federal fuel economy standards. it consisted of a 'Super' T-10 trans (a little stronger than the normal T10) with an overdrive unit behind it. I just looked at my original sales booklet from 88 and GM advertised it a making city driving easier as the reason for the option of overdrive in 2,3 and 4 also the seem to indicate for example that 2 OD is not the same ratio as 3 no OD. so I guess it gives you more gear ratio choices, I own a 4+3 car and here's my opinion on it: it Totally died like 2 weeks after I bought the car, I had it rebuilt with roller bearings, any other H.D components I could upgrade to, a centra force clutch, short shift kit, and chip that allows OD shifting in all 4 gears, worked well for a bit then the O.D stated shifting wildly on it's own, had that fixed but it still to this day always comes on the first time I shift into second after I turn the car on. So I got a manual overide switch to use for racing( or for getting out into traffic) so it can't come on or so I don't hit it by accident, also it got real hot during racing so I had a big sperate trans cooler put in. Now it works great, I don't ever have to shift in traffic since I can do 62mph in 1 OD, So If you like a 4 speed a don't mind playing with it go for it, if you want max performace just buy a 6 speed, I personally like the nostalic part of having a 4 speed Corvette, anyway thats my 2 bits on the Doug Nash 4+3.
By the way the tran shop said my 'beefed-up' trans was good for 500hp/ 500ft-tq, and they guaranteed that, so it is possible for them to take some power.

jhammons01

If you wind up with one, understand that there are a lot of input sensors that need to be paid attention to. One sensor fails and the OD stops engaging. Bad mechanics (gasp that would never happen) could take advantage of you and tell you the OD is out, and charge you to replace it.

If you are analytical and you can do some of your own troubleshooting, you'll be fine with one. If you find yourself constantly paying high repair bills seemingly as if it only happens to you.......you are not the type for a 4x3.

As far as use....shift normal all the way up to 4th. Once there, push in the clutch and hit the OD button for highway speeds. Slowing down.....push in the clutch and hit the button again...shift normal.

So basically it is a 5th gear.

PLRX

Your profile still empty...

mrl

What more do you want me to add?

FOURSPEEDVETTE

redfast

i hate my 4+3 it works though, but id rather just have extra gears than pushing a button for an extra overdrive gear, im swaping it out for the zf-6

nassau66427

Don't forget to test your 4+3 to make sure that the automatic 4th gear overdrive engagement function is working properly.

johnkos

I f you have to drop the trans don't let your mechanic rip the boot the linkage runs through. You will have a fun time finding a replacement. Going through it now.

Bob CTS

that is how I drove mine, I had no problems with mine.

Randy T

Blood type....favorite movie...

Kidding. Looks like you filled out your profile.

I don't have a 4+3 trans - mine are both auto. I wouldn't be afraid of one, though, IF I could determine that it had been properly maintained. If that could not be determined...then ya got a crapshoot.

Leftoverture

You could add your name! I have the 4+3 trans and I like it just fine. You will not have to spend much more to get a six speed car if that's what you want. In fact, you could find some 89-91 cars for less than 84-88 cars, depending on condition & mileage. And even if you pay a bit extra for the six-speed car, it will be cheaper than swapping trannies later. Drive both then make your decision.

mrl

Oh yeah...my name! I left it blank because I thought it would show up during posts. I did not see anyone elses name so I thought everyone left it blank. Sorry, noob mistake. It's filled out now.

Anyways, good idea about trying them both. Thanks.

redwing76

Good advice...........