Topic: Passing Gear range?????
in Forum: C3 Driveline Components
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Ok folks....you too Joel....
I am finally settled on the missing shifter lock issue and since my 78 is
mostly garage kept, I can live with it.
But another issue has got me perplexed.....I was out the other day
cruising on the highway 70mph @3000 on the tach.......I nailed it
expecting to hit a momentus passing gear experience but it just
continued to accelerate. Tried it a few times and no sudden power
explosion? Is it possible that I had already exceeded the passing gear
kick in range??????
BTW I have a SB350@185hp w/a Q-jet and the stock AT.
Thanx in advance for all comments and replies.
mike
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Lifetime MemberManteca, CA - USA
Joined: 11/20/2005
Posts: 3623
Vette(s): 1978,two tone,Metalic Rootbeer & gold
1975 L48 4 speed
Hi Mike. I don't know a lot of what I'm talking about with auto transmissions but speaking from experience, My wifes '78 has the Th350 and acts the same. You have to be going much slower to kick in to passing gear. Seems like it's already in 3rd by the time you reach 35 mph. That's why I'm changing it out for a 700R4 this winter. But from your description, at least you must have some descent hwy gears for an Automatic trans, my wifes car is only doing 60 mph at 3000 rpm (355 gears).
Jimmy B.
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Lifetime MemberModerator
Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20218
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
If you got the trans to downshift at 70mph, you'd wind up hurting the engine, since it would be reving pretty high in 2nd gear @70mph! There are protection systems in the trans that kinda-sorta keep that from happening, tho they can be over-ruled with some tricks. It's best(safer) on a stock engine to not have a passing gear over about 60mph...
There is also a calibration issue within the trans, since some cars have a higher(lower numerically) rear gear ratio, so passing gear at a higher mph is possible compared to a car with a lower(higher numerically) gear ratio. The trans components such as the valve body and the governor will dictate the passing gear limits.
Who the heck are you trying to pass at 70mph anyway?
There is also a calibration issue within the trans, since some cars have a higher(lower numerically) rear gear ratio, so passing gear at a higher mph is possible compared to a car with a lower(higher numerically) gear ratio. The trans components such as the valve body and the governor will dictate the passing gear limits.
Who the heck are you trying to pass at 70mph anyway?
Joel Adams
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Mine is a '79 and similar to your '78. Definitely no kick down at 70 mph.
I think at that speed with those rpm's, vettes like ours are in high gear to stay.
Generally, mine won't give me a downshift over 55 mph.
I like the idea of an overdrive tranny (700-R4 or 200-4R).
Maybe $omeday...
Kevin
Okay guys....I read all of your replies and what you say makes pretty
good sense. Remember one thing and that is I never had a car with
that much power etc. Been driving ("rice-rockets") too long I guess.
I always thought that you can get the tranny to hit passing gear at
any speed. Glad you guys put me on the right track. If I want to
experiment I'll try it at a much lower speed.
This line is for Joel A. I was just tooling down RT 1 and got a wild
hair up my xxxxxxx and nailed it at 70......Was definitely not trying
to pass anyone........hee hee!!!!!!!!!!!!!!!!!
I do know that when my rear pinion seal was replaced the mechanic
told me that according to the seals part number it would only fit
a 74 vette rear, not a 78. So I am not sure of the ratio. I kinda
thought that it seemed to run tight at 70 with the tach at 3 grand.
I would not think that that in itself would make any difference. Seems as
though Kevin is right about it wanting to stay in high gear.
Again gents, thanx so much for the input and I'll be looking for more of
your replies, comments and suggestions.
mike
Lifetime MemberHOWELL, NJ - USA
Joined: 5/18/2004
Posts: 6812
Vette(s): 1979, Targa Blue (72 Color), Pace Car rear spoiler, L88 hood, Dark blue factory interior, 525HP 406, HD 700R4, 370 gears,Steeroids, composite rear spring, TT IIs wrapped in T/A Radials.
Mike, you can get a rough idea of the ratio by doing the following:
Set the rear of the car up on jackstands so that the rear wheels are off the ground
Make an index mark on the bottom of the axle with a grease pen
With the trans in nuetral, have someone rotate the rear wheels one complete revolution
As the wheel is being turned, count the # of times the index mark on the driveshaft comes around.
The last will not be a full turn, so if you wind up the the mark up top, it,s probably 3.55s, 3/4 way around would be 3.70s, and so on.
Paul
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Lifetime MemberModerator
Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20218
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/
Am-Fm/map light National/Regional/Chapter NCRS "Top Flight"
#2-1985 Bright Red/Carmine Cp.L-98/auto
Member: NCRS, NCRS Texas, Corvette Legends of Texas
My '74 has a 3.36 rear, and it runs about 3000 @ 75mph.
The '78 had 4 ratios available:
3.08 (OK code)
3.36 (OM code)
3.55 (OH code)
3.70 (OJ code )
Since your car has the L48/auto engine, you had only the 3.08 and the 3.55 ratios available...Imma guessing that you have the 3.55 gears, since that is the most common ratio for the automatics, and the fact that you are tachin 3000 @ 70mph.
The '78 had 4 ratios available:
3.08 (OK code)
3.36 (OM code)
3.55 (OH code)
3.70 (OJ code )
Since your car has the L48/auto engine, you had only the 3.08 and the 3.55 ratios available...Imma guessing that you have the 3.55 gears, since that is the most common ratio for the automatics, and the fact that you are tachin 3000 @ 70mph.
Joel Adams
C3VR Lifetime Member #56
My Link
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in Forum: C3 Driveline Components
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