Drivetrain upgrade: 6 speed and new rear gears
 

Want to make some substantial upgrades to my 70 GTO to improve overall drive-ability and comfort. Will soon be looking to swap out the current setup - a mild 400 mated to a M20 4-speed and 2.93 geared peg-leg 10 bolt (8.2) - to my 535 (with a cam change and added EFI setup - estimated 650hp) mated to a T56 6-speed (2.66 or 2.96 first gear?) backed by a 12 bolt rear with ??? gears.

Car will be a street cruiser, with very limited track time. Want to be able to take road trips in the car relatively comfortably. Racing is not a concern.

Looking for opinions on rear gears (considering 3.42, 3.55, or 3.73, or ??) and which 1st gear for the T56.

2.66 first gear, 3.70 rear gear. 26 inch tall tire

I much prefer the gear spread of the ‘close ratio’ T-56 with the 2.66 1st gear. The .82/.64 OD gears are far more comfortable to step into at highway speeds without the annoying big RPM drops and jumps. With a 535, you really don’t need more than 9:1 overall reduction in 1st gear, which makes the 3.42 a great rear gear with the close ratio 2.66 1st gear T-56.

600hp engines do not like low rpms. & will lug @ 70 mph.

First thing you need to know is what speed you can lawfully cruise at and safely. No good to have a machine that will cruise nicely at 100 mph if it's illegal. As Chrisp mentioned, the engine will like to cruise at a certain RPM depending on setup.
I have a 600hp/600 T Trans am with 28" tall tire, TKO-600 with .82 OD with 3.73 gear..It loves all the gears/ and cruises nicely in town and on the highway at 70 mph. Which is actually over the speed limit. Cruises also nicely at 80 mph. Most speed limits here are 100 klms and 110 klms. Approx. 60 mph and 70mph. My engine likes to stay around 2300 rpm. Below that will start to lug.
You need to go to a site that has a calculator and figure out your combo you'll be using.

Good luck
Charles

I've done several 6 speed and rear end gear changes just like you mentioned. Each time we went with 3.73 gears. Works out great and very happy with the results. Cars are very driveble on the highway, and around town

All good advice, though a big stroke engine, unless it's built with higher RPM in mind, is pretty difficult to 'lug'. A 535 can idle at 60mph and not lug on hills.

6 speed with a double overdrive is worthless IMO, especially with a 535, you will never use 1st gear, or 6th, depending on your rear gear. Go to a 5 speed. Will fit much easier too.

Take the extra money your would spend for a 6 speed and get an extreme TKO-600. It will be more usable, and last longer.

The 535 I built was going to go into my 'bird, has a 260/267 @.050 duration, and was going to run it in front of a 3.25 rear gear, using the TKO-600 extreme with the road race gear set, to give you an example. I'm certain it would not have 'lugged'.


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2.66 first gear for sure, and a set of 3.73's and you won't be disappointed. That deal with a .63-.64 overdrive and your average 27" tire is still about 2000 rpm at 65, no where even close to low rpm lugging.

No real reason to worry about lugging a 500-600 or even 700 hp engine. It's really camshaft overlap related, and if tuned properly it can be made to work very well at low rpm cruising. I've done several combos that fall into this category for customers and myself, including one of our own daily drivers, it's perfectly fine.

How much cam does that big engine have?
I'd go a minimum of 3.73, with more gear if it's a pretty "big" cam.
I'd also echo what was said above, I'd consider a TKO. 2 overdrives seems like a bit of overkill.

I went with a close ratio (2.66 first) T56 and 3.50 gears for may 535ci pro-touring '70 Firebird. Not quite together yet though. Most go with 3.70, but I thought I'd like 1st better with 3.50. We'll see. Easy enough to switch to 3.70 or 3.89 if I'm not happy.

I aggravated everyone on this forum ad nauseum about the T56 VS TKO600 debate. I ended up getting the TKO600 Extreme and am glad I did. The Tremec calculator puts my cruise RPM with the .064 final drive ratio at 2143 with the specs I listed.

LOL I'm not sure why that becomes a debate. The rpm drop to a .64 5th in the TKO is not a big deal at all. Millions of factory OEM 5 speed cars that work just fine at it. I've owned and daily driven 3 T56 cars and a couple of TKO's. I almost always found myself skipping 5th in the T56, it was just a gear I never found a use for. Even my wife did the same thing.

People like it, nothing wrong with that, might be great if road racing and hitting a long straight?? I just can't find a good reason in a street car where it's needed.

The TKO's fill the overdrive void fine without the added bulk, fitment, and cost issues associated with the T56. I have nothing against the T56, I do like them, I just don't buy into the argument of needing that extra overdrive gear on a street car. If that's the only reason someone is looking into them over a TKO, save your money.
If you're looking at it from a strength perspective that's a different story. But there are different directions you can go with either trans to fit just about any strength requirement.

If you don't mind me asking...How much was the QuickTime 9 Inch? I ask because that's who UMI suspension guys recommended going with for my 67 Lemans convertible which has a similar setup to your 64. Stroked Pontiac 468 solid roller (606HP, 600 ft. lbs. torque).
I currently have a Super T-10 in it but want to go to an overdrive since I drive it a lot on the highway.

You cant get an apples to oranges comparison because of the endless options but for example I got Explorer disc brakes, Moser axles, Aluminum thru bolt center section, fill plug, jack pad, 1350 U Joint and a lot of other goodies and paid 3100 shipped. You cant touch a 12 bolt or Dana 60 with the same build quality for that

That sounds pretty good. I have a 12 bolt for it but its a 1969 so its a little bigger and I'm trying to optimize as much tire under it I can fit. It's being mini-tubbed right now.

Appreciate everyone’s input. The engine is going to have to be “tamed” a touch... it’s currently 12.5:1 compression and a 276/284 duration cam with .440 lobe lift on a 108 LSA. (Built for the drag car that I’ve realized I need to let go of for now) Obviously that won’t work In this car, so plan to drop compression with either a head swap or dish the pistons, and a cam swap to something in the 260’s duration range and hopefully still have a net lift in the .650-.700 range. So, the whole package can be dialed in together.

As for 6 speed vs 5 speed, I have read some of the discussion about how many say the 6 speed is kinda 1 gear too many (2 od gears isn’t really needed). My concern was strength. At 600-650 hp, I’d be concerned that is pushing the limits and likely would be a short life span for a TKO. I’m not familiar with this TKO600 “Extreme”, so that might be an option...

I forgot to mention that currently I’m running a 28” tire on the car. But, that too can change.

I choose the T-56 magnum when I swapped to a manual. It wasn't about having the extra gear... the T-56 has a reputation for shifting much smoother than the TKO. I went with the .63 OD version and the car has a 3.55 rear. Works great, and shifts like butter.

It just seems like if I was going to spend all that money swapping from an auto to a manual, might as well spend the extra $1k on the T-56.

Good reasoning. Switching from an auto gets pretty spendy and at that point the T56 price really isn't that much more of a leap.

The American Powertrain TKO-600 Extreme is rated at 1000ftlbs, and the mods address all shift quality issues which were reported on earlier models of TKO-600s.

Tremec made updates to resolve the majority of shift concerns with the earlier units, and the Extreme version takes it a couple steps further.

Tremec transmissions use tight clearances, and because of this, there is a break in period. Shifting improves after break in.

Aftermarket shifters can be good or bad, each can have different shift characteristics. Know what those characteristics are before you purchase one.

Many say that the TKO-600 will survive beyond the rated TQ, especially in street cars. Personally, I say chose a trans that has at least 10, if not 20% higher rating than the application you plan to use a trans in. That leaves plenty of padding, and the trans will last longer.

The T-56 is rated at 700ft lbs. There is also several version available that increase the ratings to 1000ft lbs, such as the Rockland 'Tranzilla', and the ATS Extreme upgrade. There are others that claim they make some for higher ratings, but I question those claims.

The TKO-600 Extreme is roughly the cost of a T-56.

Morning brain-dump, hope that helps clear some things up.

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Also, bell housing alignment with Tremec transmissions is critical, and can drastically impact shift quality. And just like installing a cam, you can't just toss one in and not at least check it.


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