1975 Close Ratio 4 speed
I am interested in the difference between the standard 4 speed and the M21 Close Ratio 4 speed in 1975. What does close ratio mean??
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Re: 1975 Close Ratio 4 speed
Jim here are the specs for the M20 Muncie standard ratio and the M21 close ratio.
MUNCIE M20 -
2.56 - 1.91 - 1.48 - 1.00
2.52 - 1.88 - 1.46 - 1.00
MUNCIE M21 -
2.20 - 1.64 - 1.28 - 1.00
The M20 is easier to get a Corvette going from a dead stop. M21 is harder to start from a stop up an incline. -
In Appreciation of John HinckleyComment
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Re: 1975 Close Ratio 4 speed
It was actually called the Super T-10. From memory:
WR: 2.64, 1.75, 1.34, 1.00:1
CR: 2.43, 1.61, 1.23, 1.00:1
These are still available in the Richmond Gear Super T-10. Richmond bought the design and tooling a long time ago. You can check their Web site to verify and see other available ratio sets.
Unlike the original T-10 and Muncies, the big gap is between first and second rather than third and fourth and the intergear ratio gets progressively smaller as you go through the gears, which is the way it should be. And also as is the case with the T-10 and Muncie first though third can be changed merely by changing the countershaft ratio, which means the 1-2 and 2-3 intergear ratios are fixed.
You can compute the "intergear ratio" by dividing the lower gear by the higher gear i.e. 2.64/1.75 . The shift point divided by intergear ratio tells you where revs pick up in the next higher gear.
DukeComment
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Re: 1975 Close Ratio 4 speed
Jim
As stated you can see the various ratios between them. A lot depends on what you have for engine torque and rear gearing.
Typically the Muncie & St-10's WR came with 336 rear gears and worked well in everyday driving. The nice thing about the WR trannies is you can run just about any gear. I've installed 273's with ST-10's WR up to 411's.
The CR ratio's trannies usually came with 370 411 or 456 gears in higher revving engines. The BB cars w WR used 308 and 336's. The CR trannies are more for road cars keeping the RPM band "close" between up and down shifts.Comment
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Re: 1975 Close Ratio 4 speed
Thanks for your input. Why is the close ratio an option and what advantage or disadvantage does it offer. I understand it is mandatory with L82 and Gymkhana Sus. that would portend a performance advantage----- I need basic thoughts and understanding. Thanks again. JimComment
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Re: 1975 Close Ratio 4 speed
Close ratio made it a bit 'trickier' when coming off the line from a dead stop, but the difference between gears was smaller. That provided an advantage in wheel to wheel competition situations where compression braking was a way of life on the track to avoid braking for various turns.Comment
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Re: 1975 Close Ratio 4 speed
Did you compute the intergear ratios for the two different sets and compare the differences?
The lower it is the more narrow the rev band as you go through the gears.
"Peaky" engines need low value intergear ratios. F1 tranmissions are about 1.10. They shift at 18K and revs pick up in the next gear at about 16K.
If you can tell us what your objective is, the torque/power curves of your engine, and axle ratio we can give you a recommendation.
For normal road driving and an engine that has a suitable torque bandwidth to be called a "road engine" as opposed to a "race engine" the WR Super T-10 with a 3.36 or 3.08 axle is a very good combination that will yield good acceleration and reasonable highway revs.
For an engine that has a torque curve similar to the base engine a 3.08 axle is best. If the torque curve is SHP-like, the 3.36 is the best choice.
DukeComment
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Re: 1975 Close Ratio 4 speed
I doubt that. Each engine-transmission-axle combination had to go through EPA certification testing, which is why the choices were much narrower in the emission era, so GM spent extra money to certify the L-82 with both the WR and CR trans. That EPA requirement is overkill because slight changes in gearing usually had only noise level effects on total emissions, but they did have an effect on CAFE.
The CR trans behind a SHP engine with a shorter axle ratio was offered to replicate as much as possible this common choice of the sixties.
What is the source of your information?
DukeComment
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Re: 1975 Close Ratio 4 speed
I'll try to find the source of that info Duke, I'm not stating that it is the truth, just throwing it out for discussion. The L48 could NOT be ordered with an M21 and there was a $120. surcharge for M40 when the L82 was ordered. No extra charge when ordered with L48.
Strange.Comment
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Re: 1975 Close Ratio 4 speed
Pricing has both a cost and marketing input, and sometimes it can look odd, however, from a marketing standpoint, buyers will usually pay a premium for "performance" options, even though they may add little or nothing to production cost.
If I'm reading the NCRS Spec Guide properly, M40 was either base or a no-cost option on L48, but added $120 on L82, so it can be assumed that the base transmission with L-82 was a manual - probaby the WR. Also, choosing an optional axle ratio with any transmission was $12.
The M40 used on L82 was not the same as L48. I had a higher stall converter and may have had internal upgrades that added to cost. Also, a high stall converter auto will deliver poorer EPA fuel economy than a manual, so the pricing could have been an attempt to reduce demand to a level that would not impact CAFE.
Consider the C5 and early C6. The base tranmission was a four-speed automatic, which was dissed by the automotive press for not being a state-of-the-art six-speed "manuamatic", and the six-speed manual was an option for IIRC $815!
When the Corvette "finally" got a state-of-the-art manuamatic, the six speed manual became standard and the auto an option for about $1200.
When the four-speed auto was standard there was an avaliable optional axle ratio with the auto, but it was "on allocation" because there were a maximum number that could be built and still be within Corvette's budgeted CAFE number. Ordering the optional ratio would often delay production until an allocation was available.
I wonder how GM determined to set the '75 FE7 option at $7. Unfortunately that low price probably resulted in too many FE7 options. It was waaaaaaaaaaaaaaaaaaay to stiff for normal road use and was only suitable for serious autocrossers, and I doubt that many of the 3194 FE7s were ever seriously raced. They should have charged at least 50-100 bucks just to discourage the wrong people from ordering it.
A similar thing happened with Z51 in 1984. It made the car ride like a buckboard, but despite the $600 cost of the option, half of '84 production had it. Big mistake!
The reason I asked the source is because there are so many myths and so much misinformation running around, so it always helps to list sources for information that may be sketchy.
DukeComment
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Re: 1975 Close Ratio 4 speed
my 75 L82 came with WR and 355's.Comment
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