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Hello group!
Just picked up a nice project...1968 L79 4 speed. When looking at the Muncie stamp, I don't see the A, B or C letter designstion to ID it as an M20, M21 or M22. Did the forget these often? I am guessing I need to look at the spline ro know for sure....any insight?
Looks like a 1968 Dec 15th stamp...
Steve
it wasn't until the 69 model year that the A , B, C suffix was added to the stamp.
you have a muncie built nov 15th 1967 for the early 68 model year.
your spline rings on your input shaft will tell you the m20 or m 21 designation so yes you have to pull the tranny to find out.. or do some fancy math with your car speed/rpm/rear gear/ tire height calculation
Joe is right; no suffix on 68 Muncie for M20, M21 or M22.
Thought for you on cking for M20 or M21 and how I do it; put the transmission in 1st, jack it up and rotate the rear tire exactly 1 turn and see how many times and part of 1 turn the drive shaft turns at the trans. Ck the gear ratio for a M20 and M21 for 1st and you will be able to tell which one you have. M20 1st is 2.52:1 and M21 is 2.2:1. This is enough difference to be able to tell by rotating the tire exactly one turn. Hope this helps.
when in 1st gear, your engine must rotate ( rear gear ratio X transmission gear ratio ) times for every Tire rotation. assuming no slippage. then you have to calculate ( tire height X PI ) to get the distance the tire goes in one revolution.
multiply your engine RPM to get number of tire rotations per minute, to get distance to get miles, to get time ( per minute) and then on to miles per hour use your phone to get speed.. ( don't trust your car's speedometer), convert inches to feet, to miles..
plug in the 2.52 and the 2.21 into the equation both. whichever agrees with your phone's speed ( or closest) is your 1st gear ratio. take your prevagen before you attempt this.
Now my brain hurts and i think it might be easier to pull the tranny and count the rings on the input shaft.
Joe is right; no suffix on 68 Muncie for M20, M21 or M22.
Thought for you on cking for M20 or M21 and how I do it; put the transmission in 1st, jack it up and rotate the rear tire exactly 1 turn and see how many times and part of 1 turn the drive shaft turns at the trans. Ck the gear ratio for a M20 and M21 for 1st and you will be able to tell which one you have. M20 1st is 2.52:1 and M21 is 2.2:1. This is enough difference to be able to tell by rotating the tire exactly one turn. Hope this helps.
wouldn’t that tell you the rear end ratio? Not the trans ratio.
I see a VIN stamp on the trans. Is it original to the car? The standard setup for an L79 was M21/3.73 rear. Check was the code on the rear is would be another good clue
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My L79 came with the M21, it was what the guys were told was the racing ratio, so ya, doesnt mean much and was **** at a drag strip...not that your car is the same. two ways with the rear wheels off the ground, i prefer the whole car....quick jacks!!!
2 ways to do it
Easiest way for you is to get a friend to help. Youvare under the car while he runs around doing everything
Take off your distibutor cap, mark where it is pointing.
You climb under the car Wrap a piece of painters tape around your driveshaft so it overlaps.
Cut the tape square and lay it on a smooth surface and mark a line 2-1/2 inches and mark it as m20 and a line 2-7/8 inches and mark it m21.
With the tranny in nuetral, Have your buddy turn the rear wheel foward so you know which way the driveshaft will spin.
Put the tranny in first gear
Put the tape on so that as the driveshaft spins it will come to the m20 mark first, then the m21.
Using a remote start switch have him use it and rotate the engine 1 revolution while watching the distributor rotor go one revolution.
you watch the tape....when he stops at one revolution your are staring at your tranny first gear ratio. You should see m20 first then m21 if the tape is on correctly
Put the cap back on, remove the remote start trigger, done...
You want the math....your stock 68 driveshaft is 2 inches, mutiply that by pi which is 3.14. Gives you 6.28 which is how long your tape will be and the shaft circumference. Divibe that by the two gear ratios gives you 2.85 and 2.51 and those are your two marks
Second way, you do it all yourself climbing under and out from under you car a bunch of times
Last edited by Rescue Rogers; Dec 15, 2024 at 09:29 AM.
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ANother way to tell and already knowing you have a 3.73 rear end is at a stop sign, rev to 1800 rpm and let the clutch out normally, if it chugs and wants to stall i bet its a m21.....that first gear wants higher revs to get going. I had to be above 2000 to look like i knew how to drive
If you have a 3.55 rear you will be slipping the clutch foooorrrrreeevvvveeerrr
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All you need to do to swap your tranny from a m20 to m21 is change the input shaft and cluster gear...ptetty easy. Watch Paul Cangialosi youtube at https://m.youtube.com/@GearBoxVideo/playlists
wouldn’t that tell you the rear end ratio? Not the trans ratio.
I see a VIN stamp on the trans. Is it original to the car? The standard setup for an L79 was M21/3.73 rear. Check was the code on the rear is would be another good clue
Good question, I think you are correct. I must be thinking how to determine the rear ratio when the clutch is in neutral.
If the clutch is out and you are engaged in 1st gear, maybe I am thinking you would need to pull plugs, roll the Corvette rear tire on the ground exactly one turn and see how many turns the harmonic balancer rotates. The engine harmonic balancer would rotate more with a 2.52 M20 relative to 2.2 M21, right? This would be similar to Rescue Rogers distributor example where you are looking at engine rotation?
Last edited by 20mercury; Dec 15, 2024 at 06:18 PM.
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Originally Posted by 20mercury
Joe is right; no suffix on 68 Muncie for M20, M21 or M22.
Thought for you on cking for M20 or M21 and how I do it; put the transmission in 1st, jack it up and rotate the rear tire exactly 1 turn and see how many times and part of 1 turn the drive shaft turns at the trans. Ck the gear ratio for a M20 and M21 for 1st and you will be able to tell which one you have. M20 1st is 2.52:1 and M21 is 2.2:1. This is enough difference to be able to tell by rotating the tire exactly one turn. Hope this helps.
the driveshaft turns the same amount at the transmission end as it does at the differential...you are thinking how to figure the diff ratio
Originally Posted by Tiger Joe
wouldn’t that tell you the rear end ratio? Not the trans ratio.
I see a VIN stamp on the trans. Is it original to the car? The standard setup for an L79 was M21/3.73 rear. Check was the code on the rear is would be another good clue
Yes....this should have ended here but i couldnt stop myself after this
Originally Posted by 20mercury
Good question, and I agree rear ratio is part of the equation. Since the rear gear ratio would be constant in this example for only considering the M20 or the M21, I think you are only looking for a relative difference in number and partial number of turns and not counting turns per se. So I think I have this right, all other things being equal, the M21 would turn the rear wheel more than the M20.
Nope...you blew it...you arent going to see the transmission turning anything on the other side if its in nuetral so you can turn everything unless.....noooo.....
Dude seriously...you think that turning the rear tire, you would be able, with your normal human arms, be able to turn the differential, driveshaft, transmission in first gear, to count one full revolution of the motor, then do all the math backwards to figure it out....seriously. okay prove me wrong, without a calculator tell me how many fingers you have....
Sorry...had too. You are correct that from the engine side to the drive shaft side through the transmission you can figure out the rotational differenceof the 2 first gear ratios....thats what i did a while back.....you cant do it in reverse and stop at the driveshaft transmission interface.
If you had the transmission out you wouldnt need the drive shaft....just spin the input shaft and count rotations on the output shaft
the driveshaft turns the same amount at the transmission end as it does at the differential...you are thinking how to figure the diff ratio
Yes....this should have ended here but i couldnt stop myself after this
Nope...you blew it...you arent going to see the transmission turning anything on the other side if its in nuetral so you can turn everything unless.....noooo.....
Dude seriously...you think that turning the rear tire, you would be able, with your normal human arms, be able to turn the differential, driveshaft, transmission in first gear, to count one full revolution of the motor, then do all the math backwards to figure it out....seriously. okay prove me wrong, without a calculator tell me how many fingers you have....
Sorry...had too. You are correct that from the engine side to the drive shaft side through the transmission you can figure out the rotational differenceof the 2 first gear ratios....thats what i did a while back.....you cant do it in reverse and stop at the driveshaft transmission interface.
If you had the transmission out you wouldnt need the drive shaft....just spin the input shaft and count rotations on the output shaft
U are right I was thinking rear ratio.
Now I think I could pull the plugs, put the trans in 1st and mark the location of the harmonic balancer, you come over and we push the Corvette on the ground one tire rotation and see how many times the harmonic balancer turns and back figure out what 4sp we have since the whole system is locked up at that point. Correct? When are you coming? LOL's!
OR, before we drop the trans, maybe remove the shifter arms, pull off the trans cover and count gear teeth.
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Oh jeez, i took me two hours to figure out the math before and now we have to figure in the rear differential, do all the other math but its different becuae you are moving the engine one tire revolution....the engine could spin 8 or 9 times.....ohhh Im tired already!!!!
Spark plugs in or spark plugs out....I am old ya know
Oh jeez, i took me two hours to figure out the math before and now we have to figure in the rear differential, do all the other math but its different becuae you are moving the engine one tire revolution....the engine could spin 8 or 9 times.....ohhh Im tired already!!!!
Spark plugs in or spark plugs out....I am old ya know
I am betting not as old as me and I am certainly not wiser than you! HA HA!
Come over anyway and I will buy you a beer! My head hurts from thinking about this!
2020 Corvette of the Year Finalist (performance mods)
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I think you sell it and buy a Ferrari 308GT...might be easier. You could take off the dizzy, put it in first, crank the motor while the dizzy goes around once, mark how far you traveled. Then do the math for wheel rotation for your tires circumference.....who said we ...would never need math???!!!
Crawdads shrimp Gumbo and beer.....you may see me sooner than you want!!!!!! Then we could cruise over and see Tony the Doorgunner!!!
I know Tony and Arg0413 both and both are great people! Maybe you could help me knock out a couple of Corvette projects I have to complete. My better half told me if I die and leave her all this Corvette stuff, I am going to be in BIG trouble!
Seriously, if you pass through Louisiana, let me know. Cruising the Coast on the Miss Gulf Coast is a great excuse to come this way, 1st part of Oct every year, 9000+ vintage autos older than 1989 I think. Fun time! Lots of Corvettes there too.
Last edited by 20mercury; Dec 15, 2024 at 07:45 PM.
The letter for the Muncie transmissions, as stated, started in the 1969 model year, but not right at the beginning. From what I have observed, you start seeing the A, B, or C on transmissions with a build date of November 1st (I believe that’s the letter S code) NOT Car build date, but the trans build date code.
Missing Muncie ID letter
