[gobrdgo]

Hi everybody. Here’s the engine build plan. 455 cammed 96 heads tri power. I posted earlier about this but wasn’t clear about it. I’m converting my auto 69 bird to 4 speed. Since the m20 and m21 are both 1 to 1 final drive- should I keep the m21 I have or swap for a m20. ? Car will be primarily a street machine. The 3000rpm at 60mph with 3.73 doesn’t sound appealing so I’ll want less aggressive gearing. 3.55 or 3.23. (Custom 9” in the works). So keep the 21 or trade for m20? Thanks a lot. Supposed to make a deal tonight so I’m on the edge of my seat.

[padgett]

Suspect a close ratio M21 is worth more than a M-20 but the 20 is more streetable. I have a M20/3.55./255x70-15s in my Judge and have always wanted an OD for Interstates.

[JSchmitz]

I used to have an M22. I snapped a tooth off of the cluster gear. So much for the rock crusher lol. I picked up an M20 for a great deal. I have 3.23 gears. I like the 2.52 first gear much better than the 2.2 even with a 455. Don't need close ratio with a Pontiac imho.

[leeklm]

"I like the 2.52 first gear much better than the 2.2 even with a 455"

This^^^

I had the 2.52 1st gear ratio in my M20 and while rebuilding it, switched over to a 2.20 cluster. Car had 3.31 rear gears, 27" tires. It sucked. Sold that gear cluster and put a new 2.52 1st gear back in it. Happy again. Plenty of power in my~475 hp 455, but I don't like riding the clutch, especially if starting up an incline, etc.

The M21 with oem 2.20 1st gear would be fine with 3.55 or 3.73 rear gears, but I would not do it if you use a 3.23 or similar rear.

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[leeklm]

Btw, while analyzing your options, simply add the rear gear ratio + first gear ratio to get your final drive number. I.e. in my case 2.52 + 3.31 = a final drive of 5.83 and 2.20 + 3.31 = 5.51.

A final drive ratio of 5.83 in 1st gear was a happy place and a final drive of 5.51 was not (at least for my personal taste).

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[Elarson]

Originally Posted by leeklm

Btw, while analyzing your options, simply add the rear gear ratio + first gear ratio to get your final drive number. I.e. in my case 2.52 + 3.31 = a final drive of 5.83 and 2.20 + 3.31 = 5.51.

A final drive ratio of 5.83 in 1st gear was a happy place and a final drive of 5.51 was not (at least for my personal taste).

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Apologies...not to be a math jerk but you multiply the ratios...not add.

Eric

[b-man]

Dump the M-21.

Run the M-20 with 3.23 gears.

I had a ‘69 400 4-speed Firebird (factory A/C) and that’s exactly the way it was equipped from the factory. A little slip of the clutch to get moving but not terrible, with the torque of a 455 you’ll be golden.

[leeklm]

Originally Posted by Elarson

Apologies...not to be a math jerk but you multiply the ratios...not add.



Eric

Yep, not sure why I was adding

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[gobrdgo]

Thanks for the support fellas

[PDC]

FWIW, I had an M22-Z from Midwest Muncie in with a 3.08 ring gear behind a mildly build FT E-Headed 455 and I loved it. (2.99 / 2.04 / 1.46 / 1:1)

Overall gear reduction in 1st is 9.2. That’s plenty deep. And the wide gear spread is no problem for the torque of a big displacement mill. 3.08 ain’t like true ‘overdrive’ but for the occasional 30-60 mile highway drives at 70 mph, it is completely livable (2600 @ 70 with a 27” tire)

They do make an M23-Z set with essentially the same ratios, but stronger gears for more aggressive driving.

[Half-Inch Stud]

Whoa, wait a darn stinkin minute.

Pick the Muncie that has the 1" diameter countershaft, NOT the 3/4" or 7/8" dia ( leave this alone..) M20/21/22 is drastically a secondary consideration when staring at a small countershaft Dia.

Pick a 3.08:1 gear or settle for the 3.23:1 and pine away for missing a 2.73:1 or 3.08:1.

The 3.55:1 is no fun unless you have a Wagon or GP. You got a bird so tink Highway gears, and be very happy.

I got treed.

[padgett]

Well sorta, thought everyone knew about the weak early shaft (and never a M22). Think it was fixed in 65 and most early ones were scattered all over drag strips.

Also never discussed Saginaw (heavy) four speeds used in Firebirds or the F*rd three speeds in Pontiacs.

[leeklm]

Coworker I am helping ended up with a 1979 super T10 in his 66. With 3.55 rear gears, his first gear feels like a granny gear. I could not fully decode what T10 he has, but assuming it has the 2.64 1st gear. With is modest 389, it starts out reasonably well in 2nd gear...

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[John Milner]

'65 Muncies are all small 7/8 countershaft. If it's all a person had, the small countershaft Muncie's aren't bad transmissions if you stay on the street, run street tires and aren't trying to hook up 500 horsepower. 1st, 2nd and 3rd gear from the 7/8" Muncie will interchange with the later 1" pin Muncies so there are a lot of similarities. I would take a '74 up 2 or 3 ring Super T10 over any Muncie.

[leeklm]

Since we are on the topic... what is the suggested range for a 1st gear final drive ratio?

The guy i am helping has a 1st gear drive ratio of 9.37. Would like to change his 3.55 8.2" rear to a 3.08, which would bring that 1st gear final drive ratio to 8.13, which feels a little low for a 330 hp engine....

Of course another option would be 3.36 rear gears, but does not seem worth the cost and effort to only go from 3.55 to 3.36...

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[ta6point6]

I am with the people saying COIL, a weak coil will cause what you are describing. One other thing to check is the rubbing block on distributor. I had a SBC that the rubbing block on distributor actually wore down and was not opening points wide enough.

[Scarebird]

The M22Z/2.93 combo in my 71 gets things done well.

[i82much]

I have an M23 supercase extra wide ratio with 3.54 gears behind a 505. I love it, but I don't go more than 60 mph. I just drive back roads. My favorite thing about the steep first gear is how easily I can walk it up my driveway going slow enough not to kick rocks up on my quarter panels without having to feather the clutch.

[Formulajones]

If you're going with a really mild 3.23 or 3.55 rear gear, you're going to want an M-20 with the 2.52 first gear.

[69hardtop]

I’ve got a M20/3.55 rear gear with 26 tall tires in my 64 GTO - love overall driveabiity……but I rarely exceed 65mph (sometimes feel myself reaching down for that 5th gear to shift into)