Will I blow up my M21 4-Speed? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Dub J · 03-01-2009, 02:13 PM

Im worried that the M21 Muncie 4-speed that I bought isnt gonna last behind my small block 350. I was reading some horror stories on the board about guys that tear their M21 4-speeds all to hell when they power shift or dump the clutch (which I plan to do). I really like the idea of a straight shift in my truck, and dont really want to spend $2000 on a TKO500. Whats the next best alternative? Will I definitely blow the muncie up? I'm building a 350 .60 over with patriot 64cc aluminum heads, Comp 286 cam, Edelbrock Air gap intake, shorty headers and 750 double pumper. Im thinkin this motor will put out atleast 350-400 HP right? Is that too much for this lil tranny?

Has anyone put a T56 in from a Late model camaro? I'd love to do that, and saw somewhere that it could be done but couldnt find anything on the board where anybody has done it.

Chevymann1968 · 03-01-2009, 02:16 PM

This is some info i would love to find out about as well!!!! I think that it would last!!! It should unless you start to run drag radials or slicks then i think that you will have a problem!!!

bhoeger70 · 03-01-2009, 02:19 PM

It is really hard to break a m21 behind a small block. I had a m20 behind a big block in a chevelle and the only way I hurt it was doing stupid stuff (shifting without using the clutch for one).

697498chevy · 03-01-2009, 02:26 PM

Had a factory 4 speed in my '67 Chevelle many years ago . Did a bit of power shifting , even blew up the motor once . Never had any problem with the trans . My 396 had headers , Eledbrock , Holley , etc .

Dub J · 03-01-2009, 02:30 PM

Really cause I was reading threads about guys tearing the teeth off the gears every so many thousand miles. It kinda got me worried. Do any of you guys know what 4th gear will cruise at on the highway say if your running 3:73 gears and 20 inch tires? It shouldnt be 3K at 75 or 80 should it? Im trying to get an idea of what I should do here, either stick to my Muncie or try to go 5 or mabye even 6 speed if I can get my hands on one for cheap enough.

truckpoorpackrat · 03-01-2009, 02:32 PM

If you are planning on dumping the clutch / speed shifting you will probably tear it up. Why don't you put a lenco or other race trans in it, then you will be here asking about the driveshaft or axles. Sorry, but if you abuse it something has to break.

BurnoutNova · 03-01-2009, 02:34 PM

yes it will blow... send it to me before you ruin it!

I think it will be fine... some Muncies have had way more ower and abuse put through them than you will with your 350, and they lasted.

**special-K** · 03-01-2009, 02:39 PM

These Muncies were made for this.They were muscle car transmissions,tough as nails.That M21 will hold up better than any car 5spd.

truckpoorpackrat · 03-01-2009, 02:42 PM

Guess I should ask what you have done to the motor (cam, nitrous, blower)?

nowdays some smallblocks put out more power than bigblocks.

Dub J · 03-01-2009, 02:46 PM

350 .60 over with patriot 64cc aluminum heads, Comp 286 cam, Edelbrock Air gap intake, shorty headers and Holley 750 double pumper.

thirdstreettito · 03-01-2009, 02:47 PM

My dad had a 68 Camaro with a 425hp 350 and a stock M-21. He never blew it up. He put 10's of thousands of miles on it and had no problems. I guess it all depends on who is driving and how they drive/shift.

Dub J · 03-01-2009, 02:54 PM

Well I dont plan on intentionally pushing the limits of it, but If I were to be involved in a lil street scurmish, I wanna know that I can bang on it (within reason) if I need to and not disentegrate the guts inside.

Dub J · 03-01-2009, 03:00 PM

How much stronger is the M-22 Rockcrusher, than the 21? I know their supposed to have straight splines which are supposed to be better I hear.

truckpoorpackrat · 03-01-2009, 03:20 PM

You should be OK & parts for it should be reasonable .

**SactoJim** · 03-01-2009, 04:27 PM

Beat the hell out of one in my 68 Camaro for years. Managed to break the input shaft off one night when a couple friends were in the back seat and I said, "watch this"! Extra weight made it hook, and...... SNAP!

Big advantage in a truck is lack of rearend weight. If you load that thing up with a bed full of dirt and then doing 4,000 rpm clutch dumps, you will probably find any weak points in the tranny.

ChevLoRay · 03-01-2009, 04:43 PM

My memory says the thing most people complained about with any Muncie transmission was the stock Muncie shifter. Hurst made a lot of $$ selling shifters to replace the stock ones. Seems the shifters got the blame for missing shifts and 86-ing the gearbox as a result.

Dub J · 03-01-2009, 04:53 PM

What RPM do you think I'll be at in 4th gear driving around 70-80 mph on the highway with 3:73 gears on 20" tires? Anybody know?

t&tkennels · 03-01-2009, 05:03 PM

had a buddy with a '68 camaro, small block ( 500 +hp) ,4.88 rear, m&h tires.thousands of miles of full throttle shifts with his m21, never had a problem except wrinkling his quarter panels!

russellmarling · 03-01-2009, 05:05 PM

I doubt that you will blow it up. I have an M21 behind the built 454 in my truck and I'm not easy on it. Haven't had any problems with it yet. If you're worried about RPMs at 70mph with 3:73s you will probably not be happy without overdrive. I hope you mean 20" rims, not 20" tires- that would be really small. What is the outside diameter of your tire?

t&tkennels · 03-01-2009, 05:07 PM

i think your be turning around 3,200 to 3,500 rpms at that speed . just a guess?.

Dub J · 03-01-2009, 05:26 PM

Their 20" rims 255 40 in the front and 275 40 in the back. Dont know what that equals out to in actual size, but 3500 RPMs is what I figured it would be. I guess its gonna cruise at around 55 or 60 then huh? Oh well atleast I'll have good power from light to light. This thing probably will be a pain on those long rod run trips wont it?!

**special-K** · 03-01-2009, 06:07 PM

Nah,you see muscle cars,hot rods,and old stuff in the right lane on interstates all the time.3,000 rpm is cruisin' by old standards.You`ll roll along at 65-70 ok.

russellmarling · 03-01-2009, 06:07 PM

Yes, it will be a pain to cruise at 70 on the highway. Look up board member shifty if you want to do a T56 swap. He installed a LS1/T56 in his truck a year or two ago. Keisler Engineering makes a kit that relocates the shifter further forward which would put it in a better spot for our trucks. They have a lot of hydraulic clutch related stuff too. I would like to do a T56 in the near future to make my truck more highway friendly. The M21 will be great if you're not on the highway though.

russellmarling · 03-01-2009, 06:17 PM

Not trying to contradict you Special-K, we wrote our posts at the same time. Dub J I also forgot that you have 3:73s- I have 4:11s so my highway RPMs are probably a good bit worse than yours will be.

cstm68 · 03-01-2009, 07:07 PM

Quote:

Originally Posted by Dub J

Their 20" rims 255 40 in the front and 275 40 in the back. Dont know what that equals out to in actual size, but 3500 RPMs is what I figured it would be. I guess its gonna cruise at around 55 or 60 then huh? Oh well atleast I'll have good power from light to light. This thing probably will be a pain on those long rod run trips wont it?!

a 275/40/20 tire is 28'' tall a 255/40/20 i wana say should be around 26''

03-01-2009, 02:13 PM	#1
Dub J Junior Member Join Date: Mar 2006 Location: Nashville Posts: 80	Will I blow up my M21 4-Speed? Im worried that the M21 Muncie 4-speed that I bought isnt gonna last behind my small block 350. I was reading some horror stories on the board about guys that tear their M21 4-speeds all to hell when they power shift or dump the clutch (which I plan to do). I really like the idea of a straight shift in my truck, and dont really want to spend $2000 on a TKO500. Whats the next best alternative? Will I definitely blow the muncie up? I'm building a 350 .60 over with patriot 64cc aluminum heads, Comp 286 cam, Edelbrock Air gap intake, shorty headers and 750 double pumper. Im thinkin this motor will put out atleast 350-400 HP right? Is that too much for this lil tranny? Has anyone put a T56 in from a Late model camaro? I'd love to do that, and saw somewhere that it could be done but couldnt find anything on the board where anybody has done it.

03-01-2009, 02:16 PM	#2
Chevymann1968 Registered User Join Date: Sep 2008 Location: Houma LA Posts: 193	Re: Will I blow up my M21 4-Speed? This is some info i would love to find out about as well!!!! I think that it would last!!! It should unless you start to run drag radials or slicks then i think that you will have a problem!!! __________________ 1968 GMC Rebuilding Stage 2001 Camaro SS SOLD

03-01-2009, 02:19 PM	#3
bhoeger70 Registered User Join Date: Sep 2007 Location: Grand Forks ND Posts: 434	Re: Will I blow up my M21 4-Speed? It is really hard to break a m21 behind a small block. I had a m20 behind a big block in a chevelle and the only way I hurt it was doing stupid stuff (shifting without using the clutch for one). __________________ "Beer is proof that God loves us and wants us to be happy" Ben Franklin 67 Chevy 1/2 lwb, 67 GMC 1/2 lwb (parts) Last edited by bhoeger70; 03-01-2009 at 02:19 PM.

03-01-2009, 02:26 PM	#4
697498chevy Registered User Join Date: Jan 2009 Location: South Carolina Posts: 265	Re: Will I blow up my M21 4-Speed? Had a factory 4 speed in my '67 Chevelle many years ago . Did a bit of power shifting , even blew up the motor once . Never had any problem with the trans . My 396 had headers , Eledbrock , Holley , etc .

03-01-2009, 02:30 PM	#5
Dub J Junior Member Join Date: Mar 2006 Location: Nashville Posts: 80	Re: Will I blow up my M21 4-Speed? Really cause I was reading threads about guys tearing the teeth off the gears every so many thousand miles. It kinda got me worried. Do any of you guys know what 4th gear will cruise at on the highway say if your running 3:73 gears and 20 inch tires? It shouldnt be 3K at 75 or 80 should it? Im trying to get an idea of what I should do here, either stick to my Muncie or try to go 5 or mabye even 6 speed if I can get my hands on one for cheap enough.

03-01-2009, 02:32 PM	#6
truckpoorpackrat Registered User Join Date: Dec 2008 Location: joplin mo. Posts: 272	Re: Will I blow up my M21 4-Speed? If you are planning on dumping the clutch / speed shifting you will probably tear it up. Why don't you put a lenco or other race trans in it, then you will be here asking about the driveshaft or axles. Sorry, but if you abuse it something has to break.

03-01-2009, 02:34 PM	#7
BurnoutNova Registered User Join Date: Mar 2004 Location: CA Posts: 2,057	Re: Will I blow up my M21 4-Speed? yes it will blow... send it to me before you ruin it! I think it will be fine... some Muncies have had way more ower and abuse put through them than you will with your 350, and they lasted.

03-01-2009, 02:39 PM	#8
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,852	Re: Will I blow up my M21 4-Speed? These Muncies were made for this.They were muscle car transmissions,tough as nails.That M21 will hold up better than any car 5spd. __________________ "BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~

03-01-2009, 02:42 PM	#9
truckpoorpackrat Registered User Join Date: Dec 2008 Location: joplin mo. Posts: 272	Re: Will I blow up my M21 4-Speed? Guess I should ask what you have done to the motor (cam, nitrous, blower)? nowdays some smallblocks put out more power than bigblocks.

03-01-2009, 02:46 PM	#10
Dub J Junior Member Join Date: Mar 2006 Location: Nashville Posts: 80	Re: Will I blow up my M21 4-Speed? 350 .60 over with patriot 64cc aluminum heads, Comp 286 cam, Edelbrock Air gap intake, shorty headers and Holley 750 double pumper.

03-01-2009, 02:47 PM	#11
thirdstreettito Resident of Here Join Date: May 2008 Location: Wesley Chapel, FL, USA Posts: 7,716	Re: Will I blow up my M21 4-Speed? My dad had a 68 Camaro with a 425hp 350 and a stock M-21. He never blew it up. He put 10's of thousands of miles on it and had no problems. I guess it all depends on who is driving and how they drive/shift. __________________ ~Bradley~ 2004 Black GTO LS2/T56 too much to list Miss having a truck

03-01-2009, 02:54 PM	#12
Dub J Junior Member Join Date: Mar 2006 Location: Nashville Posts: 80	Re: Will I blow up my M21 4-Speed? Well I dont plan on intentionally pushing the limits of it, but If I were to be involved in a lil street scurmish, I wanna know that I can bang on it (within reason) if I need to and not disentegrate the guts inside.

03-01-2009, 03:00 PM	#13
Dub J Junior Member Join Date: Mar 2006 Location: Nashville Posts: 80	Re: Will I blow up my M21 4-Speed? How much stronger is the M-22 Rockcrusher, than the 21? I know their supposed to have straight splines which are supposed to be better I hear.

03-01-2009, 03:20 PM	#14
truckpoorpackrat Registered User Join Date: Dec 2008 Location: joplin mo. Posts: 272	Re: Will I blow up my M21 4-Speed? You should be OK & parts for it should be reasonable .

03-01-2009, 04:27 PM	#15
SactoJim Registered User Join Date: Oct 2005 Location: Sacramento, CA Posts: 1,970	Re: Will I blow up my M21 4-Speed? Beat the hell out of one in my 68 Camaro for years. Managed to break the input shaft off one night when a couple friends were in the back seat and I said, "watch this"! Extra weight made it hook, and...... SNAP! Big advantage in a truck is lack of rearend weight. If you load that thing up with a bed full of dirt and then doing 4,000 rpm clutch dumps, you will probably find any weak points in the tranny. __________________ 72 Cheyenne SWB Fleet Nut & bolt frame off in progress...99.9999% - supercharged 383 - 4 wheel discs - relocated fuel tank - My Build Thread



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03-01-2009, 04:43 PM	#16
ChevLoRay Old Skool Club Join Date: Jun 2001 Location: Benton, AR "The Heart of Arkansas" Posts: 10,880	Re: Will I blow up my M21 4-Speed? My memory says the thing most people complained about with any Muncie transmission was the stock Muncie shifter. Hurst made a lot of $$ selling shifters to replace the stock ones. Seems the shifters got the blame for missing shifts and 86-ing the gearbox as a result. __________________ Member Nr. 2770 '96 GMC Sportside; 4.3/SLT - Daily driven....constantly needs washed. '69 C-10 SWB; 350/TH400 - in limbo The older I get, the better I was.

03-01-2009, 04:53 PM	#17
Dub J Junior Member Join Date: Mar 2006 Location: Nashville Posts: 80	Re: Will I blow up my M21 4-Speed? What RPM do you think I'll be at in 4th gear driving around 70-80 mph on the highway with 3:73 gears on 20" tires? Anybody know?

03-01-2009, 05:03 PM	#18
t&tkennels Registered User Join Date: Sep 2007 Location: creedmoor n.c. Posts: 188	Re: Will I blow up my M21 4-Speed? had a buddy with a '68 camaro, small block ( 500 +hp) ,4.88 rear, m&h tires.thousands of miles of full throttle shifts with his m21, never had a problem except wrinkling his quarter panels!

03-01-2009, 05:05 PM	#19
russellmarling Registered User Join Date: Jan 2006 Location: Highlands, NC Posts: 138	Re: Will I blow up my M21 4-Speed? I doubt that you will blow it up. I have an M21 behind the built 454 in my truck and I'm not easy on it. Haven't had any problems with it yet. If you're worried about RPMs at 70mph with 3:73s you will probably not be happy without overdrive. I hope you mean 20" rims, not 20" tires- that would be really small. What is the outside diameter of your tire? __________________ '70 GMC SWB step-side- 454, 4 speed, air bags (in progress) '09 CTS-V M6 '03 Sierra SWB Z71 '82 Yamaha Seca XJ650R Turbo! '04 Yamaha FZ6

03-01-2009, 05:07 PM	#20
t&tkennels Registered User Join Date: Sep 2007 Location: creedmoor n.c. Posts: 188	Re: Will I blow up my M21 4-Speed? i think your be turning around 3,200 to 3,500 rpms at that speed . just a guess?.

03-01-2009, 05:26 PM	#21
Dub J Junior Member Join Date: Mar 2006 Location: Nashville Posts: 80	Re: Will I blow up my M21 4-Speed? Their 20" rims 255 40 in the front and 275 40 in the back. Dont know what that equals out to in actual size, but 3500 RPMs is what I figured it would be. I guess its gonna cruise at around 55 or 60 then huh? Oh well atleast I'll have good power from light to light. This thing probably will be a pain on those long rod run trips wont it?!

03-01-2009, 06:07 PM	#22
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,852	Re: Will I blow up my M21 4-Speed? Nah,you see muscle cars,hot rods,and old stuff in the right lane on interstates all the time.3,000 rpm is cruisin' by old standards.You`ll roll along at 65-70 ok. __________________ "BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~

03-01-2009, 06:17 PM	#24
russellmarling Registered User Join Date: Jan 2006 Location: Highlands, NC Posts: 138	Re: Will I blow up my M21 4-Speed? Not trying to contradict you Special-K, we wrote our posts at the same time. Dub J I also forgot that you have 3:73s- I have 4:11s so my highway RPMs are probably a good bit worse than yours will be. __________________ '70 GMC SWB step-side- 454, 4 speed, air bags (in progress) '09 CTS-V M6 '03 Sierra SWB Z71 '82 Yamaha Seca XJ650R Turbo! '04 Yamaha FZ6

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