[Mark42]

Posted this earlier, but I think it got lost in the server move.

Was going to post in the drive train forum, but this is really a performance improvement, so I posted here.

My 98 Firebird V6 automatic coupe came with 3.08 gears and an open differential. This has proved to be very economical on gas, but not very performance oriented. Always feel like the car is lugging when the gas is depressed to go, and always having to downshift to pass.

So.... Had a local trans shop install the 3.73 Summit gears and Eaton Truetrac LSD. The Eaton Truetrac is an all gear LSD, no clutches involved, so it will not wear out with use. Install took longer than planned becuause after taking the car for a test drive, the shop pulled the pinion gear and reshimed to get a better wear pattern and less noise. I supplied all the parts: gears, LSD, gaskets, pinion install kit, and new Reluctor Ring (for 3 series carrier). Was quoted $300 for labor plus extra for broken parts and problems. Sounded fair to me so the shop got my business. The re-shim labor cost a bit and the total came to $385. Not bad for taking an entire day by an experienced old time hot rod builder.

Got past the 500 mile break in period and the gears have quited down a lot to the point of not noticing the slight noise they make. First thing I noticed was the car accelerates quicly with just a little throttle, and pulls very hard as the throttle is rolled on. Slam down the pedal and the Truetrac locks up and tires will just spin.

The truetrac is really great. Lets me pull up my incline driveway in the snow with no problem, just a little tire spin and tail wag near the top of the hill. Couldn't even get half way with the old open rear. With the passenger side tires off the road in the snow and other other tires on pavement, a little gas will cause one tire to spin for a second until the TT locks and the car drives off nice and easy, no throwing snow/mud all over the car like my 96 Camaro would have with the clutch style LSD it had.

The Truetrac is also very quiet, and under normal driving does not seem to be causing drag on the inside tire. Spirited acceleration from a stop either left or right will cause the Truetrac to lock up and the inside tire to "scuff" the pavement. But it does it without out making any noise; no chatter or clunking noises from the rear.

The engine RPM's are 21% higher in 4th with the 3.73's than with the 3.08 and that really wakes the car up on the highway. Passing is a breeze as the car responds quickly to a little throttle, and downshifting makes it take off in a hurry.

All in all, the 3.73 gears make the car much more spirited to drive. Easy as pie to spin the tires, greatly improved off the line and passing acceleration and all around better feel. The Truetrac keeps the tires planted well, especially in the rain where single tire spin was a real issue.

The whole job, including a PCM update to correct for the speedo, parts and labor came to about $1000. I truely feel this is the single biggest performance upgrade you can make for the money. A cold air intake and stainless steel exhaust w/install will run about $1000 and will not give nearly the performance improvement the gears and LSD make.

I have not got the new PCM in the mail yet, probably in a few days. PCMforless.com claims about a 10-14 hp increase for the 3800 and 1-3 mpg improvement. Will wait and see how it pans out. Right now, accountning for the 21% increase in miles registering, the MPG is coming in at 25.5, compared to up to 27 for the 3.08 gears. Not a big economy hit, and hopefully the new PCM tune will make up for it.

Will keep you posted.

Mark.

 

[Canhos]

Thanks for letting us know how it worked out.
I can't count high enough to account for the people who want to turn their V6 into a performance machine, and are looking for a bolt in mod. Sounds like the shop you went to did an good job installing it, and did it for a fair price.

Now... have you factored in the accelerated tire wear???

 

[Mark42]

Thanks for letting us know how it worked out.
I can't count high enough to account for the people who want to turn their V6 into a performance machine, and are looking for a bolt in mod. Sounds like the shop you went to did an good job installing it, and did it for a fair price.

Now... have you factored in the accelerated tire wear???

Yes, I'm counting on it so I'll have an excuse to buy 17" rims and tires

This mod turned out much better than expected. Took a bit to get used to the higher RPM's. And the car still shifts funny because it hasn't been programed for the new gears. For example, it will not go into 4th until the speedo reaches 48 at light throttle, like its still on the 3.08's, but will shift at the max RPM under heavy throttle regardless of speedo. Hopefully it will get all sorted out with the new PCM.

This does not make the V6 feel like a V8, but it does feel like an extra 50 hp was found. Its nice to be making the max of the available power and not having to modify the motor or exhaust itself.

As soon as the temps warm up to the 50's in will go a trans cooler and engine oil filter and cooler. Cheap insurance that will help keep this ride running a long time, and offset some of the wear caused by cruising at higher RPM.

 

[Injuneer]

All the V6 owners should read this. Might be a good idea to see if we can "sticky" a copy of it in the V6 forum.

Excellent information. Thanks for posting it!!!

 

[CF_TransAm]

Awesome post!!

All the V6 owners should read this. Might be a good idea to see if we can "sticky" a copy of it in the V6 forum.

and Good Idea!!

 

[99ws6wannabe]

I agree excellent post!!! I also can verify the post being that I did the same mod a week ago and have had my pcm tuned allready and it is amazing!!

 

[Pete W]

Congrats Mark and agreed it is good money spent!

 

[Injuneer]

Mark:

Please copy and paste to the "All things V6" forum, and I will "sticky" the post for future reference. It deserves to remain in this forum as well.

 

[Mark42]

Latest update:

Got the PCM from pcmforless.com and installed it. Because it is not the original PCM from my car, a procedure is needed so the PCM learns and can adjust for manufacturing tolerances in the crank trigger. Officially called a "Crankshaft Position System Variation Learning Procedure ", it has to be done with a DTC Scaner. Found a dealer that knew what i was talking about and they did it for a 15 minute labor charge. So even though many people claim they got it done for free, I was happy to pay to ensure it was done right.

I also verified the odometer against the mile marker posts on RT80. After 7 miles, it was right on. So its safe to assume the speedometer is registering the speeds accurately. BTW, 70 mph is about 2400-2500 rpm.

All in all things are well. The car runs well, and with the updated PCM the shift points have changed accordingly. Ie trans would shift to 4th under light throttle at 47-48 mph, and now with the pcm update, it shifts to 4th at 40 mph. All shifting feels right, both upshift and down shift, under light and heavy throttle. So that is another plus.

One last change, the PCM was updated with lower fan speeds to match a 180* thermostat. Picked up a stainless steel 180 thermostat by Stant and gasket. Will do that tomorrow morning. Might even dump the rear and fill with fresh gear lube as per the gear break in instructions.

Now that its all come together, I can just lean back and enjoy the ride.

 

[Mark42]

Another update......

The car now has 800+ miles on the new setup, and I have been monitoring the fuel economy closely. My driving is very constant from week to week, driving about 70/30 highway/city. Highway speeds are usually 60-65 for my normal daily commute driving.

With the 3.08's the car was getting a best of 26-27 mpg, but averaged around 26. After the 3.73's and the PCM being tuned by pcmforless.com, the fuel economy is coming out at 25.5. I had expected a bigger drop in economy, but possibly the tune has helped. PCMforless stated a 1-3 mpg increase was possible depending on the mods to the car. Seeing as my car only has a cold air intake, and is still running the stock single outlet exhaust, I didn't count on much increase. The bottom line is the car didn't take as big a hit in the fuel economy as I expected. But I also don't lead-foot the gas much (well, OK maybe sometimes)

I should also note that the PCM was tuned to run on mid-grade (89 octane) gas. Before the gear change the car was running on regular, but now it is using 89 octane. That could also be contributing to better economy due to the pcm being able to run more timing advance.

I plan on installing a Magnaflow stainless cat-back exhaust system come spring. That may also contribute a 0.5 to 1.5 mpg increase, so the car may actually end up with the same or better fuel economy than stock!

Its nice to know you can have performance and economy too.

Regards,

Mark

 

[joshjones84]

that's great that it didn't effect you MPG too much!

 

[andrew-r]

I decided to put in some new gears this spring, turned out to be pretty much the most cost-effective upgrade this car could possibly get.
Its been on the want list for a while, but I didn't have tons of extra cash sitting around while paying for college here. Then...campus security drove into the side of my SUV. Bummer, nice little dent but its a 13 year old truck so I wasn't going to send it to the body shop. Insurance cut me a check for 1200 and I knew exactly what to do.
Out went the origional 700r4 in favor of a rebuilt one with a shift kit, extra clutch packs and a corvette servo. Then out came the origional 3.42 ring & pinion, replaced with a 4.10 set.
The desicion to go to 4.10 instead of 3.73 took a long time to think out, lots of match, until I was certain that 4.10 was what I really wanted. All the acceleration you can possibly get out of the 2.8. After driving it a few hundred miles, I am really glad I didn't go for the 3.73's. It just wouldnt have been a big enough jump from 3.42. What was once a bogged down bird is now snappy and quick as can be (for a 2.8)
I got a little bonus with my particular setup. You'd think with 4.10 gears my mileage must have taken a good dive. I know I was, so I was pretty surprised when the new gears were in and my math at the pumped (which is corrected for the speedo difference) showed a gain of 4MPG. This really puzzled me until I remembered how 2 years ago my dad and I had rebuilt the 2.8. During the rebuild, I had insisted on getting an aftermarket camshaft put in. I really didn't know what I wanted, so I just got what seemed to be the best I could get without having to modify the computer. That was all before I really had any idea what I was doing, I got the idea for the cam somewhere and went with it hoping to gain a few HP. Now, two years later that cam turns out to be by stroke of dumb luck to be a perfect match to these 4.10 gears. Aha! It all makes sense. I realized I didn't "gain" 4 MPG, I recovered 4MPG from when I rebuilt the motor and put in a cam that has a different range of efficiency RPM's than stock. Doi.
The stock cam and gears were designed to be efficient at highway speeds, around 60 MPH and 1900 RPMs. Little did I know two years ago that the new camshaft I was putting in reached maximum efficiency somewhere between 2100 and 3500 RPM, while I was still using stock gears that put me at 1900 RPMs on the highway. I wrote off the slight MPG loss as part of the rebuilt motor being broke in. Then this spring in came the 4.10 gears which put highway RPMs up into the efficient range of the cam and I got my 4MPG back. Whoopee!
Even if its technically more of a recovery than a gain, putting in a gear set that makes the car so much faster while getting better mileage is like christmas. I wasn't sure I'd ever see the say when my 2.8 was chirping the tires in 2nd gear, but its here

In about another month I will be headed to the drag strip to see just what the car is capable of and will post back the results. Im pretty sure just by the feel of it that I would have no trouble keeping up with, if not beating the same year stock 305 firebird. We'll have to see about that.
Now if only I could afford a turbo setup... maybe if security runs into the other side of my truck too I will have the fastest V6 firebird yet

 

[Mark42]

Update 01/19/2011:

Have had the 3.73 gears and Eaton Truetrac differential in the car since 12/4/2009 (about 13 months and 16,000 miles). This modification is without a doubt the best that can be done to a street car. The car performs so much better. Acceleration is improved, downshifts almost always put the motor in a higher HP rpm, and the car just rockets off. Off the line acceleration is improved and the car is doing 0-60 noticeably faster than before. My tests using a stop watch and the factory (reprogrammed) speedo show 0-60 in 7 seconds. And of course the fun factor has increased 1000%, as tire spinning is easy to do with just a stab of the throttle, and heavy throttle through a turn will lock up the rear and cause the rear to hang out through the turn.

A nice perk is that the fuel economy was not affected at all over the long run. In fact, doing strictly highway driving with a light load, the economy is over 30 MPG's. The car averages around 26-27 mpg depending on the amount of highway/city driving done. So the initial fuel economy figures listed in my earlier post were probably low do to "high spirited" driving with the new gears. A Magnaflow Cat-Back dual exhaust was installed, and that may have made up for any lost economy.

The gears were Summit brand, so the actual manufacturer is unknown. Summits racing gears are strong, well machined and have a break in coating that apparently works well. But they are not as quiet as the OE Pontiac gears. A few months after the gears went in, an aluminum drive shaft (from a Trans Am) was installed and it reduced the "white noise" sound from the gears to practically nothing. I have to point the sound out to passengers for them to notice. I believe the reduction in noise is due to the aluminum drive shaft, because aluminum does not transmit sound as well as steel. So noises from the differential gears are not transmitted up the drive shaft to the trans where they are heard inside the car. The aluminum drive shaft is a nice compliment to a rear gear change. Less HP is needed to spin the aluminum shaft at cruising speeds, and less PH is needed to get the shaft spinning from 0 to 60 mph. Also note that the aluminum shaft gives about a 12 lbs weight savings over the steel shaft, and it is also balanced to a finer spec than the steel.. But the down side of an aluminum shaft is it is just not as strong as a steel shaft. For the basic street car, this is not an option. Even for a bracket racer, the shaft should perform without twisting/failing.

The Eaton Truetrac has been a fabulous posi. It runs silent in all but the most extreme conditions. In rainy weather, the Truetrac keeps the tires planted well with no skip/slip under normal throttle. No hop, shudder or wiggle under normal driving conditions. Give a bit more gas, and the Truetrac will start to lock, and cause the typical rear "kick out" with even more throttle. Under heavier throttle, it locks solid. The only time I have heard noise from the differential itself is when the two rear tires are the extreme situation. Such as one tire on dry pavement, the other tire on ice. This happens in my driveway during winter months. The Truetrac will make a "singing" or bell ringing sound when differentiating at the max. Slight application of the parking brake for a moment, or slightly applying the brakes causes the differential to lock, and all is well.

I have towed a 1700 lbs boat/trailer combo with the Firebird. The Truetrac will lock on the wet boat launch ramps and let the car pull the 1700 lbs load out of the water with no wheel spin or hop. Just smooth throttle until the car starts to move and up it goes. It does help to keep some weight in the rear for extra traction. Right now the tongue weight is only about 90 lbs, but the car tows just fine.

The differential was filled with Mobile 1 synthetic gear lube when the gears were installed, and after the break in period, the lube was drained and refilled. Will change the differential lube again after 30K miles pass. The small volume of lube means it is inexpensive and well worth the time to keep fresh Synthetic in the housing. Plus adding a drain plug makes the job a breeze.

One annoying problem did show up after the Truetrac was installed. Under heavy throttle the car would get very bad wheel hop. The whole car would vibrate and shake and the rear tire would be hopping down the road like a basket ball. This really impacted acceleration and pushed 0-60 times to about 10 seconds. Did a lot of research on F-body wheel hop and fixes and the popular choices are Polyurethane bushings, Lower Control Arm Relocation Brackets, boxed or tubular Lower Control Arms, and Moog solid hard rubber bushings.

The poly bushings are so hard that suspension bind is an issue and has been documented to cause stress fractures in the welds of the shock mounts and LCA mounts on the axle tubes. There were also a few "cold flow" problems with the poly bushings, so I crossed them off the list. Bought a set of Lower Control Arm brackets. These bolt-on brackets lower the mounting point of the LCA so there is more leverage by the LCA to prevent twisting and hopping of the axle under torque. I never got to installing the LCA mounts, so I don't have first hand experience with them. Also read a few times that the stock Lower Control Arm is designed to twist so that suspension bind is reduced. The stock LCA's are more than strong enough for a street car. Boxing them (welding a panel to the bottom to make them square tubes) or replacing with heavy round tube LCA's contributes to suspension bind and traction loss because the axle can not easily ride at an angle to the body. The OE U shaped LCA's are U shaped to encourage a small amount of twist or flex so the axle can easily move as it needs to. Unless you are drag racing, its best to keep the stock un-modified LCA's in your daily driver.

The solution I chose was the Moog solid, hard rubber bushings. Hard enough to prevent excessive deflection that was the cause of the wheel hop I was experiencing, but soft enough to not have the bolt holes stretch out and become sloppy. The Moog bushings work fantastic. The rear stays planted on all surfaces. From a stand still, a quick stab of the throttle will leave 60+ feet of posi rubber on the road. When driving the kids to their friends houses in slippery winter time, the 'bird handles well. In snow, the rear will fishtail around if too much throttle is applied. But generally, the rear grips and gets the car moving.

The PCM re-programming by PCMForLess.com was a very good choice. The runs fine, there are no performance issues, and the changes to the shift points make driving really fun even though the trans is an auto. Shifts are firm but were not very consistent. I took the Bird in for a trans flush at about 45K miles. A new filter was installed and the trans was filled with Valvoline Synthetic Dexron 6 trans fluid. After about 300 miles, the shifts were noticeably more uniform, and after 6K miles, they are very consistent. Gone is the "sticky" or "shudder" feel that would happen sometimes under half throttle. Now its really nice, not neck snapping, but firm, quick and smooth. A big change from the OE setup.

All in all, the gear change was the best mod to the car to date. And I strongly recommend a full trans flush and fill with the Valvoline synthetic Dexron VI fluid. What a difference!

Regards,

Mark

 

[Sonjiin]

Well you have me sold on completing this mod to my 94 Bird this spring.

I read your post in December and have been contemplating it and have asked a few friends and the consensus was its not worth it for a V6, to just sell it and save for the V8 and then do it. But i like my little V6 and after looking into the parts/ costs and gains of this I think its well worth it.

Thank you for the write up!

 

[305 Formula]

I've been thinking about doing a posi/LSD unit in my 90 Formula 305 in the spring/summer and some lower gears. It has 2.73s stock & its kinda a dog. I think i want to go to a 3.73ish gear, not 100% sure yet. I don't want to kill my mileage too badly & i don't want it to be screaming going down the highway.

Does anyone know the 4th gear ratio of a 700R4?

 

[Injuneer]

I'm not so sure I agree with your thoughts on the stock LCA's metal structure twisting (deforming) to reduce bind. Its the bushings in the stock LCA's that are shaped to permit the LCA to twist relative to the bolt through the mount. If you look carefully at the stock rubber bushings, they are solid in the "front" and "back". Those are the surfaces that bear the compression load during accel and braking. But the tops and bottoms of the bushings are cut away, to allow the "eye" of the LCA to displace under torsion, as the body starts to "roll" under heavy cornering loads.

There are new designs that overcome the bind issue, while still using the more solid polyurethane bushings. This is one example:

http://www.hotpart.com/shop/index.php?l=product_detail&p=85

Additionally, you can use a spherical rod end on one end of the LCA. By putting the rod end on the axle end of the LCA, you avoid the typical noise and harshness problem of putting the rod ends on both ends of the LCA.

 

[Mark42]

I'm not so sure I agree with your thoughts on the stock LCA's metal structure twisting (deforming) to reduce bind. Its the bushings in the stock LCA's that are shaped to permit the LCA to twist relative to the bolt through the mount. If you look carefully at the stock rubber bushings, they are solid in the "front" and "back". Those are the surfaces that bear the compression load during accel and braking. But the tops and bottoms of the bushings are cut away, to allow the "eye" of the LCA to displace under torsion, as the body starts to "roll" under heavy cornering loads.

There are new designs that overcome the bind issue, while still using the more solid polyurethane bushings. This is one example:

http://www.hotpart.c...uct_detail&p=85

Additionally, you can use a spherical rod end on one end of the LCA. By putting the rod end on the axle end of the LCA, you avoid the typical noise and harshness problem of putting the rod ends on both ends of the LCA.

I agree there are many solutions in the poly bushing set to reduce bind caused by basic poly bushing design, such as the spherical design you mention. But to me the greatest draw back is the cold flow issue where the poly changes shape under pressure, and does not recover.

Regarding the lower control arms flexing as part of the design, that is not my idea. I read that on another forum where F-bodies were being modified for handling improvements in road racing. That is where I read the U shape control arm is a specific design that allows flex without metal fatigue. I can easily see how it can perform that function, especially when you consider that the Moog solid (hard) rubber bushing is the same as the stock LT1 bushing, and they really don't afford much flex, especially when compared to the uber-soft star bushing installed in V6 models. The U shaped control arm can't help but twist under extreme conditions. Probably not at highway speeds but at low speed driving where big pot holes, bumps, and other irregular road surfaces are encountered.

Just relaying info that appeared to be sound info to me. I can't say its a fact.

 

[joesperformance]

Is the Eaton TrueTrac on good on the auto's or good for manual's as well? I need to also find out what gears I have on my 3.8l v6 5spd, 98 bird.

 

[Jorgens]

+1 for the manual guys. I never thought much about the rear end cause im an economy buff as well. But this post gives some inspiration about both. That may be the research topic this weekend. Great post!!! Great Read!!!! Great fun for the 6'ers!!!

 

[hkydude1990]

Just came across this thread to read up on rear ends, and after reading everything, I share the same question as Joe. Will all of this work for a manual transmission, or are there differences that I would need to look out for? I'm dead set on a posi unit for my 3.8, but since everything here was listed for an auto I just want to make sure I'm not going to spend unnecessary money on something that won't work well with my car. I want to make sure I have all my bases covered so that the swap goes as good as it can.