which 6-speed

[v8coupe]

Well with my recent upgrading of things I'm beginning to fear for the 727. I know they are ok trans, but I fear 300hp will eat it up towing. I will not rebuild it if it dies so I think I need to start shopping for it's replacement. I want a stick, always did but options were slim.

So the question is which 6-speed to go with. I really like the idea of an Eaton FSO-6406 but not sure it's worth the extra weight. I like the feel of the G56 and that I can throw a gear vendors OD on it for a double overdrive setup. Then there is the NV5600 as well. Truck is used primarily for hauling my racecars on a 2k lb open deck. Plans are not to race it, pull with it, or off road it. Towing and commuter use, so mileage and durability are key.

[Philip]

Well if you have a 727 trans. That means you have 3:07 gears. You don't need double OD. With a 3200 governor spring you can top 100 MPH right now. With a 6 spd that means you can do 100 in 5th and still have one gear left.

Let your billfold tell you what trans to use. A good used 5600 is going to run 2.5 to 3k by itself. Do not buy a small input shaft 5600. A used G56 can be found for 1 to 2k. Some times even cheaper. Both will require a clutch for a 1 3/8's input shaft. The 5600 uses the same flywheel as the 4500 as far as I know. The G56 uses a special conversion flywheel for a single disk. The flywheel has a raised offset on the engine side of the wheel. This stand off is 3/4" tall. It moves the clutch deeper into the bell. The G56 has two different OD. The G56 behind the 6.7's has a .74 OD. The 5.9 G56 has a .78 OD. I would recommend the .78 OD ratio. With a .74 OD and towing. You might not be able to use OD till 75 MPH with 3:07's.

[BILTIT]

I sure like my gear vendors OD, i only use it as an extra gear and not for splitting though. I do not use it much when towing heavy as it loads the engine pretty hard. I do use it everytime i am unloaded (or just pulling my boat, etc) and on the highway, i run at 82mph at 1800 with the GV and 3.54 gears (33" tires).

[v8coupe]

I'm liking not keeping the 3.07's out back. I want an LSD and rear Discs which are both more difficult on a D71hd then others. Probably going to look for a AAM 11.5 in a 3.42 from a late 3rd gen or 4th gen.

I would also likely go with an aftermarket clutch kit on either transmission. Likely a street/towing twin disc since I do plan on more power. The eaton is a nice tranny stronger then either of the OEM options, but difficult to find all the parts for. You need an SAE bell housing, flywheel, and shifter assembly.

Now will a G56 take a 1000 ft-lbs I seem to get mix results. I am ok with having to pit some upgrades on it to male it stronger.

[oldestof11]

If you get a twin disk clutch, I recommend a Valair. Southbends like to crack and break the center plate. Southbend uses a cast steel piece whereas Valair uses a steel piece. I havent seen a Valair break but I have seen more than a few SB's break. Same tow ratings, same HP limits. These guys also sled pull. The Valairs haven't died where the heat breaks that center plate.

Just my $.02.

[BILTIT]

I have had good luck with my valair dual organic disc.

[v8coupe]

Valair is one of the options I was looking at. Would a 3.73 with the gear vendors still be useable towing? Plans are hx35 for now, already have the HX52 to make twins.

[Philip]

Now will a G56 take a 1000 ft-lbs I seem to get mix results. I am ok with having to pit some upgrades on it to male it stronger

At that torque range your into the high 400 HP range. The G56 gets border line at 500 HP. Also trying to tow with 400 + HP is going to take some work. The EGT's would be the issue. Up graded IC, turbo, injectors, fuel system and so on.


If all your towing is a something on a flat deck car trailer. You don't need 1,000 foot pounds of torque. Set it up in the 250 275 HP range. That would put you around 550 to 600 ft lbs range. It would tow good. Plus the drive train would like it better. There isn't to much to up grade to reach that level eather.

[BILTIT]

Valair is one of the options I was looking at. Would a 3.73 with the gear vendors still be useable towing? Plans are hx35 for now, already have the HX52 to make twins.

I have no issues towing 12k but i run a he351cw. With that much weight i don't use the GV, you want to keep your rpms around 1500+ when towing. Anything under 1500 and the motor really starts to work and vibrate etc. I do use the GV with light loads (2500lb boat), motor doesn't even notice that.

[v8coupe]

I'm well aware of the requirements for sustained high HP diesels. An hx52/35 combo will spool quickly and provide 500hp worth of air at 60psi easily. The trick is fueling it correctly. I'm not new to diesels, just the 12v. I am currently building a 350hp 2.0L tdi racing vw so egt and fueling requirements are things I understand.

If I ran the numbers right with the 6.7L .74 od and a 3.73 R&P 60 mph would be ~1750rpm nice cruise speed. 70mph is ~ 2050 rpm little higher then I'd like. At that speed the gear vendors would be ~1600 which should be great with a turbo package that can excessively flow there. Running lean is the key it's much easier to make 300hp with low egt's with excessive air. Better to over build and detune, then under build and overfuel.

[oldestof11]

FWIW, my best mileage has always been 1900-2000 rpms. So running 70mph with a load at 2000 rpms would be a sweet spot.

Also, I don't know why I didn't think to say it before. Rebuild parts for the NV5600 are getting scarce as it is no longer supported by New Venture. This was a problem back around 2010. They also don't like to tow heavy in 6th gear.

[v8coupe]

FWIW, my best mileage has always been 1900-2000 rpms. So running 70mph with a load at 2000 rpms would be a sweet spot.

Also, I don't know why I didn't think to say it before. Rebuild parts for the NV5600 are getting scarce as it is no longer supported by New Venture. This was a problem back around 2010. They also don't like to tow heavy in 6th gear.

I had read NV5600's were getting difficult to find parts for, but that they also took a beating.

Oldestof11 torque delivery will dictate mileage. It takes x power to drive x load at x speed. A properly designed low rpm twin setup will make more power down low changing the sweet spot. Goal is 1000ft-lbs with under 1400° egts doable but difficult.