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Discussion Starter #1
At what mph do you shift into 6th?

I have a 72 with a base model 350 crate engine and TH 400 auto trans. I might upgrade power someday, but in the meantime, I want to swap in an OD tranny.

Is the cost of swapping from an automatic to a manual, especially the 6 speed worth the .5/1 ratio? Would I have the torque to use 6th?

The other option is install an automatic 4 speed with OD like the 200R or 700R.
 

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First question is - You didnt mention what gears you have in the car. Need to know that. For anything smaller than a 4.11 rear end gear the 6th gear is a freeway cruising gear. I have a 3.45 rear gear in my car, 0.50 is 6th gear. I put it in 6th at 70mph, too low RPMS under that. 70MPH in 6th gear is about 1600RPM. Worthless gear except for cruising down the freeway to save gas with a 3.45 rear gear. 4.11s will make 6th gear a usefull gear. A definite upgrade for me in the future. Hope this helps.
 

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I go into 6th at 70mph, puts me at about 1700 RPM. The only time I see 6th is when I'm hitting the cruise control.

I'm not sure of the complexity of the swap.,,, but the ZF6 is one tough cookie.:thumbsup:
 

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Discussion Starter #4 (Edited)
ooops , I have 3.08 gears. Right now I'm showing 2800 rpm at 70 mph. So with the 0.5 OD I'd be doing about 1400 rpm.
 

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RedRiderZR1 said:
First question is - You didnt mention what gears you have in the car. Need to know that. For anything smaller than a 4.11 rear end gear the 6th gear is a freeway cruising gear. I have a 3.45 rear gear in my car, 0.50 is 6th gear. I put it in 6th at 70mph, too low RPMS under that. 70MPH in 6th gear is about 1600RPM. Worthless gear except for cruising down the freeway to save gas with a 3.45 rear gear. 4.11s will make 6th gear a usefull gear. A definite upgrade for me in the future. Hope this helps.
I have a 3.42:1 ring and pinion set and as the following photo shows, 71 mph is 1,725 rpm in 6th. As you can see, that produces 29.2 mpg on long stages. Sixth gear is not useless, it results in fewer revolutions per mile, which translates to less wear on the engine. My LS6 will pull 6th as low as 1,500 rpm (50mph), provided you are maintaining a steady speed and not attempting to accelerate. My LT1 engines pulled sixth gear even better at low rpm. But my 1991 ZR-1 was less happy below 1,900 rpms. Sixth gear in the M12 6-speed is 0.56:1 while in the ZF 6-speed it is 0.50:1. Multiply those by the ring and pinion ratio and the ZR-1 has a 1.73:1 final drive ratio when in sixth gear while the Z06 has a 1.92:1 final drive ratio. The ZR-1 also has larger diameter rear wheels/tires which lowers your final drive ration even more.



The LT5 engine was designed to produce torque at higher rpm (that's why it's a dual OHC engine). It actually has less torque at 1,600 rpm than an LT1, an LT4, or an LS6. To get much low end torque at all required the elegant looking dual intake runners that set the LT5 engine apart visually. Long intake runners produce more low-end and mid-range torque, short runners produce more high-end torque. Without the dual length runner configuration the LT5 would have had very poor driveability in normal use. It would have required constant rowing of the gear shift to keep the rpms in the usable torque band.

Refer to Corvette Fuel Injection & Electronic Engine Management 1982-2001 for detailed explanations and torque curve comparisions of the L98, LT1, LT4, LS1, and LS6 engines.

Ray
 

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RT66Z06 said:
Sixth gear is not useless, it results in fewer revolutions per mile, which translates to less wear on the engine. My LS6 will pull 6th as low as 1,500 rpm (50mph), provided you are maintaining a steady speed and not attempting to accelerate.
Exactly my point. If you want to make power in 6th gear, you are going to have to build a stump puller engine in low RPMS to accelerate decent in 6th (taller gear definitely helps). 1500-1600 RPMS in my car will accelerate but not well. Basically for cruising down the highway. One way GM got the mileage out of these cars they needed to get. I would definitely change the final drive ratio since you have 3.08s in there now if you decide to make the swap.
 

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RedRiderZR1 said:
(taller gear definitely helps)
"Taller" means a higher (numerically lower) gear ratio. "Shorter" means a lower (numerically higher) ratio. 3.07:1 is taller than 3.42:1. Taxi cabs have very tall ratios (2.56:1).

Lots of people refer to a low (numerically high) ratio as a "bigger" rear. What is ironic is that the pinion in a "big" rear is actually smaller. In fact, going from a 3.42:1 ring and pinion set to a 4.56:1 set requires installing a different ring gear carrier since the ring gear needs to be moved over considerably to mesh with the smaller pinion gear. Shimming is insufficient to account for the wider spacing.

I had a '68 HEMI Roadrunner in 1968. It was a 4-speed.



I made the mistake of replacing the Dana-Spicer 9 and 3/4 inch ring and pinion set with the 4.56:1 set. Acceleration was great but I ran out of RPMs in the traps in 4th gear and the gas mileage was simply atrocious, not to mention the increased engine noise at cruise.

My Z06 is much faster and has infinitely better driveabilty.

Ray
 

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Ray, I heard from Gordon Killabrew that you should not put a ZF6 in 6th gear below 70mph (with stock diff). I don't remember why... any idea?
 

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Patrick96LT4 said:
Ray, I heard from Gordon Killabrew that you should not put a ZF6 in 6th gear below 70mph (with stock diff). I don't remember why... any idea?
It may have to do with a phenonenon that is usually refered to as "lugging" the engine. If you attempt tp put serious load on an engine that is out of (below) its usable torque band there is a possibility that damage to the reciprocating parts could occur. An example would be going up a grade in sixth gear at 40mph and flooring the engine rather than downshifting to fourth. Engines will normally produce a characteristic noise and vibration when lugging occurs which even Helen Keller could detect and learn to avoid.

There are folks who buy manual transmission equipped cars and always drive in too high of a gear. There are also people who drive in too low of a gear (they are the subject of a Chevy service bulletin concerning excessive oil use). The goal is to drive in the highest gear that the torque curve and prevailing conditions will allow avoiding lugging while minimizing rpms which minimizes wear. If you sense lugging you downshift. Simple. But there is a difference between lugging and "I can't light the afterburner in 6th gear!". Well, no, that's why they put a lever there, so you can select an appropriate gear ratio for the conditions. The raison d'etre of a manual (or automatic) transmission is to provide a means to keep the engine rpms in the usable torque curve (not necessarily the region of peak torque).

Most of the complaints I hear about 5th-6th gears in the manual transmission are from owners who are too lazy to downshift and who should have purchased an automatic transmission equipped vehicle. I suppose it's hard to downshift when one has a cell phone glued to one's ear.

The owner's manual for ZF equipped vehicles has a chart listing recommended minimum speeds for the respective gears (there is also a chart listing maximum downshift speeds). The Z06 owner's manual for 2001 on page 2-32 says that the minimum speed for 6th gear is 50mph with the M12 transmission. The difference between the ZF sixth gear (0.50:1) and the M12 sixth gear (0.56:1) is minimal and one could expect that the minimum speed for the ZF would be slightly higher, but it is unlikely it would be 20 mph higher. Your owners manual should provide definitive data.

If there was any danger of damaging the engine by shifting to 6th gear at 50mph you can bet that Chevy would have erred on the side of caution in the owner's manual by listing a higher minimum speed.

You might ask Gordon Killabrew to cite documentary evidence to support his recommendation.

Ray
 

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RedRiderZR1 said:
Exactly my point. If you want to make power in 6th gear, you are going to have to build a stump puller engine in low RPMS to accelerate decent in 6th (taller gear definitely helps). 1500-1600 RPMS in my car will accelerate but not well. Basically for cruising down the highway. One way GM got the mileage out of these cars they needed to get. I would definitely change the final drive ratio since you have 3.08s in there now if you decide to make the swap.
Perhaps I misunderstood your original point. The inference I drew from "...sixth gear ...worthless gear..." was that you were stating that sixth gear is worthless. I responded that indeed sixth gear is not only not useless, but is very benificial to economy and driveablity when used appropriately. i.e., the laws of physics are very clear on how an engine produces torque (and from that "power", they are two different things).

Rather than building a stump puller that provides enough low end torque to accelerate aggressively in sixth gear it makes more economic and practical sense to downshift to the highest gear that gets the engine rpms back up into the useable torque curve.

Since 6th gear was designed to be used at speeds above 50mph it serves the exact purpose for which it was intended (one of which is saving you the cost of paying a gas-guzzler tax, assuming you bought the vehicle new). Semantics aside, that hardly justifies the characterization of worthless (or useless).

Ray
 
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