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3.42 gears or 3.73 Posi

  
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3.42 gears or 3.73 Posi

 
78TransAm400 78TransAm400
New User | Posts: 25 | Joined: 12/13
Posted: 03/19/14
05:10 PM

Hey everyone,

I have a 1978 Pontiac Trans Am with a W-72 400 and 4 Speed Borg & Warner. I'm currently running the factory 3.42 gears in the rear end, but I'm looking for a little more..."ophm" out of the car. The 78' W-72 engine only makes 220hp from the factory, and what I have is mostly an original engine. Correct me if I'm wrong, but the main issue with that motor is the lowered compression....

But without changing the crank to raise compression, do you think changing from a 3.42 gear to a 3.73 Posi rear end would really make a difference?

How hard is that to change into a Posi system?

And highway driving....with my 3.42 gears I'm at 3,000 RPM at 70 MPH. If I change to that 3.73 posi, how bad would it be on the highway?

As always thanks everyone!  

shyrgfuh3 shyrgfuh3
Enthusiast | Posts: 446 | Joined: 11/13
Posted: 03/19/14
11:16 PM

around 3400 rpm at 70 mph with 26.5 tall tires, 3.73 gears and 3 speed auto or 4 speed manual trans.

gear ratio calculator

http://www.wallaceracing.com/gear-speed.php  

ROWSLEY ROWSLEY
Guru | Posts: 1015 | Joined: 07/11
Posted: 03/19/14
11:30 PM

changing the gears will make a difference and may be the cheapest way to go. or you can keep the gears and go for a 5 speed. i love my tko and 3.42s, 2000 at 70!

you will have the carrier out to change gears thats the best time to swap in a pozi unit. eaton clutch type. most trans am already have pozi rear ends!

about 3500 at 70 with 3.73s.  
76 455/4spd TRANS AM
69 GRAND PRIX 455/5SPD

barneyformula barneyformula
Addict | Posts: 3306 | Joined: 01/07
Posted: 03/21/14
07:24 PM

You won't notice enough of an improvement going from 3.42 to 3.73 to make it worth the effort or expense. Changing the compression ratio doesn't require changing the crankshaft at all. There are two ways to do it, either smaller combustion chamber heads or domed pistons. Earlier heads will raise the compression ratio but be careful, you don't want too much, about 9:1-9.5:1 is the max for the street. Your W72 should have 8.5:1 stock. Your TA should also have a posi already.
You didn't mention what, if any mods have been made but if you still have the restrictive single catalytic converter and small dual resonators, then that's where I would start. Get a nice 2.5" dual system with low restriction mufflers, maybe headers (I like Hedman 3-tube headers because they fit well and are easy to install).
Next would be an upgrade to a better cam, lifters and timing set especially if it's still stock. Lots of room for improvement there! You don't need something extremely lumpy but there are much better grinds available today that will outperform the original and don't even need higher CR or different intake/carb.
Hope this helps to steer you in the right direction.  
A little help... 'cause we don't all have to learn the hard way!

ROWSLEY ROWSLEY
Guru | Posts: 1015 | Joined: 07/11
Posted: 03/21/14
09:52 PM

good point barn. their is a lot a person can do to a car that is all stock to boost hp. you can also tweak the carb and timing curve.

if you don't like headers look at ram air manifolds.

a 455 stroke crank will raise compression but will also lead to a complete rebuild!  
76 455/4spd TRANS AM
69 GRAND PRIX 455/5SPD

78TransAm400 78TransAm400
New User | Posts: 25 | Joined: 12/13
Posted: 05/04/14
04:03 PM

Hey guys,

Sorry for the late response.

When I bought that car it had been sitting for about 17 years under a cover in some guys garage, maybe driven a maximum of 3k miles from 1993 to 2010. Needless to say, I inherited numerous little issues with the car. Within the first two months of owning it I replaced the water pump, clutch, radiator, mufflers and a few hoses. The only real upgrades to the engine was an Accel distributor coil/ECM and a K&N air cleaner. The car only has 61k original miles on it, so I don't want to replace anything noticeable (intake, exhaust manifolds etc..)but I do try to gain some extra ponies anywhere I can sneak them in.

I did cut off the giant catalytic converter, when I took it off it was so heavily clogged you could shine a flash light through one end and it wouldn't come out the other. Besides from that, its as stock as you can get. It is a more rare T/A than most thing, for one it has the 4 speed on the floor and it's a no factory A/C car with W72 and WS6 packages which I've found to be a rare combination.

I was however thinking about advancing the stock timing of 18 degrees before TDC to maybe 14 or even 10 before. What are your thoughts on this?


Other than that and a few replacement parts over the years the engine is original with only 61k miles on it, so tearing it apart is something I'm shying away from. I don't want to hurt the originality of the car too much as it still shines with factory paint and interior, but I'm always looking for slight tricks to up some power without modifying too far.

The distributor ECM did go out about 6 months ago, so instead of just a simple OEM replacement I bought an Accel Unit with a bigger coil which was worked well for it.  

78TransAm400 78TransAm400
New User | Posts: 25 | Joined: 12/13
Posted: 05/04/14
04:04 PM

Hey guys,

Sorry for the late response.

When I bought that car it had been sitting for about 17 years under a cover in some guys garage, maybe driven a maximum of 3k miles from 1993 to 2010. Needless to say, I inherited numerous little issues with the car. Within the first two months of owning it I replaced the water pump, clutch, radiator, mufflers and a few hoses. The only real upgrades to the engine was an Accel distributor coil/ECM and a K&N air cleaner. The car only has 61k original miles on it, so I don't want to replace anything noticeable (intake, exhaust manifolds etc..)but I do try to gain some extra ponies anywhere I can sneak them in.

I did cut off the giant catalytic converter, when I took it off it was so heavily clogged you could shine a flash light through one end and it wouldn't come out the other. Besides from that, its as stock as you can get. It is a more rare T/A than most thing, for one it has the 4 speed on the floor and it's a no factory A/C car with W72 and WS6 packages which I've found to be a rare combination.

I was however thinking about advancing the stock timing of 18 degrees before TDC to maybe 14 or even 10 before. What are your thoughts on this?


Other than that and a few replacement parts over the years the engine is original with only 61k miles on it, so tearing it apart is something I'm shying away from. I don't want to hurt the originality of the car too much as it still shines with factory paint and interior, but I'm always looking for slight tricks to up some power without modifying too far.

The distributor ECM did go out about 6 months ago, so instead of just a simple OEM replacement I bought an Accel Unit with a bigger coil which was worked well for it.  

78TransAm400 78TransAm400
New User | Posts: 25 | Joined: 12/13
Posted: 05/04/14
04:07 PM

Also, did they come with a stock POSI setup? I thought they where open rear ends from the factory.  

barneyformula barneyformula
Addict | Posts: 3306 | Joined: 01/07
Posted: 05/04/14
06:53 PM

Posi was standard on Trans Ams unless ordered without it. The old pellet style converter was heavy and inefficient, but you never could see through them due to their internal design. If you are required to pass an emissions test or you care about the air, today's universal cats are light years ahead of the pellet cats, both in flow and efficiency, I would not hesitate to use one on your T/A since it is basically stock. In fact, you might want to have the stock crossover pipe removed and along with it, the y-pipe leading into the mufflers and have 2' or 2.25" true dual pipes installed back to the factory muffler and use dual universal cats (optional of course).
If it sat as long as you say, replace all the rubber lines (rad and heater hoses, fuel lines, vacuum lines) to avoid problems.
You mention "advancing" the timing from 18 to 14 or even 10 degrees. You have it wrong, the higher the number, the more advanced the timing is. I'm not familiar with the actual timing specs from 1978 but I would start with 12 base (with the vacuum advance hose disconnected and plugged) at idle and then have a helper slowly rev the engine up while you check the mechanical advance with a dialback timing light, you want to see how far the mechanical advance advances the timing before it reaches it's maximum advance and you also want to see what rpm it reaches max advance at. Ideally you want full advance of 32-36 degrees all in by 2500rpm (still with the vacuum disconnected and plugged), you may need to buy an advance curve kit and try different springs to get it where you want it.
The distributor "ECM" that you refer to is called the ignition module and it is not really an ecm, it just replaced the function of the old breaker points that were used up until 1974.
Aside from the exhaust and ignition mods we've discussed, the only other thing you can do to maintain the originality with better performance is to have the Q-jet carb rebuilt by one of the professional carb shops that specialize in setting up Q-jets. Beyond that you are looking at internal mods such as higher compression pistons or a head swap (but the head swap can be detected by any savvy Pontiac enthusiast as the external markings will be a dead give away) and a cam change. There are much better cam grinds available today that will increase power and driveability than the 30+ year old W72 cam, the timing set should also be replaced due to wear in the nylon gears that were used originally. If you choose to upgrade the cam, don't go too wild on the specs. Consult with the manufacturer to get one best matched to your combination, don't just pick the ne with the biggest lumps, you'll wish you hadn't!
Steve  
A little help... 'cause we don't all have to learn the hard way!

shyrgfuh3 shyrgfuh3
Enthusiast | Posts: 446 | Joined: 11/13
Posted: 05/05/14
12:48 AM

hello

3.43 to 3.73 is a change of 8.75% i have done this several times and it is definitely noticeable but not a drastic nite and day OMG difference.

if you run smaller rear tires it will do the same thing.

you can still run an the fwy with 3.73 and 26.5" tires.

going to 3.91 will be very noticeable but sill reduce your fwy cruising speed to around 60 - 63 unless you want to run at high rpm.  

78TransAm400 78TransAm400
New User | Posts: 25 | Joined: 12/13
Posted: 05/09/14
10:04 AM

Yeah i already have to cruise at 70, even at 70 im at 3k RPM with 3.42 gears. The rear tires are factory size,can't recall the exact number,but factory recommendation size.  

barneyformula barneyformula
Addict | Posts: 3306 | Joined: 01/07
Posted: 05/09/14
08:17 PM

Factory size should have been 225/70R15.  
A little help... 'cause we don't all have to learn the hard way!