OD and 3.08 rear end

[Howitzer]

Anyone have experience running a 700R4 and a 3.08 rear on 17 inch tires? I've been doing the math and the RPM's look pretty low...maybe too low.

I'm going to be running a 402 BBC with just the edelbrock performer cam and intake with a 1407 carb, wondering what kind of stall converter i should get for this setup? It's pretty much a stocker after that, no crazy mods.

Thanks in advance!

[kwmech]

700 and a big block......hmm, not gonna last too long unless you do a build up, and the final drive will be a 2.24. Talk about top end

[Howitzer]

Was thinking about going with a Mad Dog level 2.

I also have the original TH400 with a stage 2 shift kit, maybe i should drive that till it dies.

[cdowns]

the few guys i know who tried the overdrive with hi gears like that all said it sucked and was a waste of time and money

[Longhorn 70]

I have had a 700R4 behind my big block Olds for about 9 years now. I haven't driven that much but it has held up for a while. I did add a shift kit, hardened input shaft and other items for durability.

I had a built up TH400 in there before that and drove a little harder then but the longest they lasted was about 2 years.... I got pretty good at changing them.

I know the 700 will go away and I'll have to change it. I'd much rather do it every 9 years rather than 2.

With the TH400 I got around 11-12 MPG with the 700 I get around 15 and it pulls much harder out of first.

[68Gold/white]

Originally Posted by Howitzer

Anyone have experience running a 700R4 and a 3.08 rear on 17 inch tires? I've been doing the math and the RPM's look pretty low...maybe too low.

I'm going to be running a 402 BBC with just the edelbrock performer cam and intake with a 1407 carb, wondering what kind of stall converter i should get for this setup? It's pretty much a stocker after that, no crazy mods.

Thanks in advance!

A lot depends on the engine build...
If the cam isn't too radical, which would keep the opertational RPM's of the engine at it's best in the lower RPM's, you'd probably be O.K.
A stockish build as you mention, would be best...

Which specific cam???

The carb will need some specific tuning, Hot Rod Magazine fine tuned an AFB, this last year in one of their monthly (new) diagnostic articles. The car was an early Chevy II w. a straight 6 in it. The article was technically pretty good. They Bought 2 different types of tuning kits to make it happen, one was for a stock type of tune for the carb, and the other was a high performance type kit (these were not carb rebuild kits, per say, they were specific tuning kits). I am no expert AFB carb guy..

I'd prefer a Quadrajet myself. The answer man for these carbs is www.cliffshighperfromance.com , he has a GREAT book that will allow you to do ALL the work yourself, also...

Remember GM made cars in the 80's with super high geared rear ends (2.29:1, I believe was one gear) to hopefully make the cars get better MPG's, so your thought isn't that far out...
I prefer NON overdrive trannies, myself. A TH400 can be built for a song, compared to an OD unit... and are more duarble (my opinion)...

[Howitzer]

Thanks 68Gold/white.

I ended up going with the Edelbrock 2162 cam, a 2161 intake and a 1407 carb., with stock heads.

Looks like the company i was going to go with doesn't have many good reviews, i'm leaning towards staying with my TH400 and if and when it dies save my dollars for a 4L80E conversion down the road. The 0.75 OD ratio in that unit and the strength of it makes me think it would be a good choice if i can convince the department of war and finance!

[jayoldschool]

The 94-96 Impala SS used a 4L60E (same as a 700R4) with 3.08 rear, along with 255 50 17 tires (same height as a 235 70 15). Highway RPMs at 65 are around 1700.

[90LS1T0Y]

I have that set up with an ls1 and 4l60e in my opinon it is perfect for a truck. I tow with my truck and can use 3rd gear at 80mph with low rpms.

[Ironangel]

Originally Posted by Howitzer

"if i can convince the department of war and finance!"

Oh that's good! That's REAL good!

[hugger6933]

Are we sure that the Impala used a 3;08 rear gear? I had a SS Impala and I thought it had 3;73's but then again that was a while ago. Anyways the main question was about the torque converter any stall in that truck is gonna build a lot of heat with those gears being so tall. What ever you put in I would not dare go over about 1800-2200 and then I would insist on a cooler with an electric fan on it to try to keep the fluid from boiling.

[hugger6933]

I went and looked it up the SS Impalas did come with 3:08's somark down todays date I was WRONG![if we were doing that we would mark up the whole dang calender]I guess I was thinking of my Monte Carlo SS that had 3:73's in it. After thinking it made sense that the Imp SS had the 3:08's because I got like 26 MPG's out of that big road barge. That car was so roomy and would run very well for a 4,200lbs land whale and it was like riding down the road on my couch. One more thing about your combo I am not familar with that cam or its numbers just keep in mind that a cam and a converter are related and a cam that is big can really hurt you on a converter that is a little tight.Also the Impala SS wheels would look cool on one of our trucks[bolt on, you know]

[LockDoc]

-
I don't know about with the 17" wheels but the OD trans and 3.08 gear doesn't work in my Burb. The trans overheats and forces trans fluid out of the front seal. I have been driving it without using the OD and it runs and drives fine that way. All I have to do is decide whether I want to change the trans or rear end. I have a good TH350 or TH400 and I also have a 3.73 Posi. Decisions, decisions...

LockDoc

[hugger6933]

The Impala uses a 5x5 bolt pattern

[jocko]

the eternal question, rear gear or o/d trans... I prefer an o/d trans with a little deeper EDIT: REAR gear (not 1st...) (3.73 seems to be about ideal) - that way, the engine stays in it's power band for more of your trip to 70 mph as compared to a higher gear with no o/d. AND, you retain low end perf and low rpm at speed, without lugging the motor as would be the case with a 3.08 and 700R4 combo.

So, in lockdoc's example, I'd definitely vote rear gear change vice reverting to a non o/d trans because it retains the low end performance of an o/d trans (generally deeper geared in 1st than a 3-spd non-o/d) and the deeper gear itself (3.73 vs 3.08, for example) while still retaining the top end lower cruising rpm capability. Best of both worlds. 3.08 with a TH350 or TH400 could be a little challenged on the torquey end off the line unless it had a big block or warmed small block. Anyway, that's my 2 cents, I would always take more gears in the trans vs. trying to optimize (compromise) on the rear end gearing - rear gear changes alone (with a non-o/d trans) will always leave you wanting better perf on the low end or lower rpms on the high end, depending on the ratio chosen.

[WIDESIDE72]

IMO< the Imp SS could get away with 3.08's and an OD trans due to the fact that it was a slicker package to push down the road. a 67-72 is more of a brick when it comes to aerodynamics.

[Boog]

The big block has the power to haul it on down the road with the 3.08 gears and od. A 3.42 is a real good all around gear for large or small motors with od trans. Of course rear tire diameter has to be taken into the final gear equation also.

[JointTech]

ive read this a couple times now. Why are low RPMs at highway speed a bad thing? Seems like the lower the better your MPG. You have a kickdown that will drop it down if you stomp the gas so whats the issue? I never really understood how a torque convertor worked. I need to go study up on that magic voodoo box.

[hyatt]

I run a 400 turbo bigblock with 273 gears Plenty of lowend because of the bigblock but loafs down the road at 75-80

[Boog]

Originally Posted by JointTech

ive read this a couple times now. Why are low RPMs at highway speed a bad thing? Seems like the lower the better your MPG. You have a kickdown that will drop it down if you stomp the gas so whats the issue? I never really understood how a torque convertor worked. I need to go study up on that magic voodoo box.

Lower cruising rpms make for better fuel mileage and runs cooler down the road. The rpms need to be above a certain point or the engine will lug down. Normally cruising rpm between 1800 and 2200 seems to be in the sweet spot I like to call it as the engines tend to be more fuel efficient in that range. Go too low on the cruise rpms with an od trans and you will get frequent down shifting from od to drive on slight inclines.

[piecesparts]

Things to consider:

The late model engines have fuel injection and computers to lean out the mixture, where a carb will not.

A 700 R4 can be built to hold up to a big block, I have one behind my 383 stroker that has that capability.

The tire size is important to your economy on the 3.08/700R4 mix. Your RPMs will be so low that your engine will actually be out of it's power band and possibly wasting gas, vice saving it. As stated above the top end will be astronomical.

My son's truck lost 5 MPG by going from a 350/350C to a 350/700R4 combo, until we go rid of the 3.08 gearing. His truck would cruise down the highway at 1700 RPMs and work it's butt of trying to maintain speed in hills. A set of 3.73s brought the mileage back to the truck. Your city driving will suck and so will your highway driving in any area that has hills of any size. The engine will always be trying to get enough pull to make it up the incline. The big block will make some difference over a small block, but the theory is the same. Look for a combo that put's your engine/tranny/rear end combo at an optimum RPM and performance will follow.

When setting up a vehicle, I use calcualtions that involves the RPMs, Tire diameter, rear end ratio, and tranny output ratio to balance the system out. My trucks work great at the 2000 to 2500 RPM range. They are no mileage winners, but a 425 hp/4000 lb truck that gets 15 MPG repeatedly is not all bad.

[LockDoc]

Originally Posted by jocko

the eternal question, rear gear or o/d trans... I prefer an o/d trans with a little deeper 1st gear (3.73 seems to be about ideal) - that way, the engine stays in it's power band for more of your trip to 70 mph as compared to a higher gear with no o/d. AND, you retain low end perf and low rpm at speed, without lugging the motor as would be the case with a 3.08 and 700R4 combo.

So, in lockdoc's example, I'd definitely vote rear gear change vice reverting to a non o/d trans because it retains the low end performance of an o/d trans (generally deeper geared in 1st than a 3-spd non-o/d) and the deeper gear itself (3.73 vs 3.08, for example) while still retaining the top end lower cruising rpm capability. Best of both worlds. 3.08 with a TH350 or TH400 could be a little challenged on the torquey end off the line unless it had a big block or warmed small block. Anyway, that's my 2 cents, I would always take more gears in the trans vs. trying to optimize (compromise) on the rear end gearing - rear gear changes alone (with a non-o/d trans) will always leave you wanting better perf on the low end or lower rpms on the high end, depending on the ratio chosen.

Jocko,

That is kind of the way I was leaning. I put lighter weight springs in the back not too long ago so it should come apart pretty easy. I think I will swap the rear out next spring and give it a go....

Boog and Frank are also spot on as far as the mileage and lugging. That was what the burb was doing.

Thanks.
LockDoc

[jayoldschool]

Originally Posted by hugger6933

Are we sure that the Impala used a 3;08 rear gear? I had a SS Impala and I thought it had 3;73's but then again that was a while ago.

100% positive. 3.73s are a popular swap, though. Mine has 4.10s, but I also swapped in a T56. Great combo for performance and has double OD.

[LONGHAIR]

Boog and Frank just saved me a lot of typing. They hit the nail right on the head.

[Ole Greenie]

Okay, here goes my experience:

I have aBBC LS6 with a Bowtie Overdrive Level II and 3.08 gears.

What I like about this is the low RPM at hwy cruise speed. I tend to cruise at 75 most of the time and the motor is at 1800 rpm...not quite the sweet spot for the motor as the LS6 likes RPM.

What I don't like about the 700R4 is that the convertor locks up at 40 mph and at hwy speeds it a bear to get it into kickdown. It has a corvette governor and the TV cable is adjusted properly. Lastly, it is not quite as crisp off the line, partially due to the open rear gear and partially due to the length of the gear.

I am thinking about changing to a 4L80E but I have not done enough research to adapt a TPS to a carburator.

Any other thoughts?

God Bless!