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Transmission - Muncie or T10

Mrace43

New member
Joined
Jun 19, 2014
Messages
4
Location
Pickerington, OH
Corvette
1975 Bright Blue Coupe
New to the forum:

Recently dropped a 383 stroker, 430 HP ( at the crank ), 470 lb ft torque into our 75 Vette. The car still has the stock 4spd (I believe it's the M20). Is this tranny going to grenade on me if this car is run hard at all? The original L48 motor was only 165HP. If so any suggestions to upgrade? I would really like to keep the 4 speed. Can it be upgraded (by rebuild) or do I need to start with something else? Also was wondering if this really was a Muncie transmission. I always thought they were T-10s. Any help is much appreciated. Thank you.

Matt
1975/Blue L48 - Dad purchased new in 1975
 
Matt
75's and later were not Muncie trans, they were Borg Warner Super T-10's and had better ratios but were no stronger then a Muncie. Neither will hold up to the power level you have if you hammer on the car. My son is on his 2nd 75 and blew up 3 ST-10's pushing the car hard. The auto gear super case is stronger but we went with a Liberty Gear faced plated 5 speed with a Mark Williams 1350 slip yoke.

Once you get the trans setup the next weak link is the 10 bolt, outer axles, and joints.
 
Transmission

Thank you very much GTR1999. So is there any kit to beef up the existing T10 or do we have to start over. Was also wondering what the Gymkhana suspension was that year? It is listed as a $7 option. There was an off road suspension that was $400 - pretty steep for 1975 - even more than the optional L82 engine I believe. I was also trying to confirm if the car has the 3.36 gear ratio. I think the only options for the L48 4 spd that year were the 3.36 and the 3.08. I have the original window sticker and it doesn't list anything so I assume that means it's a 3.36 as the 3.08 likely would've been listed since I believe it was an "option". Does that sound correct? Thanks.
 
If you don't push the car hard that ST-10 will be ok, but then why build a 383? You can get an iron mid-plate to beef it up and the after market performance group once sold a cast iron ST-10 that was stronger but I suspect you have the stock aluminum ST-10. Again my son blew those up with about the same power level and a lot of abuse, iron mid plate wasn't going to save it. You can call Paul up at 5 speeds.com. He knows trans inside and out and may be able to guide you on how well a super case will hold up, better then a stock case but we haven't pushed one.

We had a Gymkana in a 75. It came with stiffer shocks, springs, and a rear sway bar.

The base diff gear for a 4 spd was a 336. There may be only 3 mph difference between a 308 & 336 gear and couple hundred RPM. You can always mark the pinion and rotate the tires counting the # of turn in one rev to get close to the ratio, 3 turns= 308, 3.33= 336, 3.5= 355 etc.
 
Transmission

Thanks again GTR 1999. Here's a quick background on the car so some things might make a little more sense. My dad has owned the car since new. He ordered it in 1975. 6 years ago he took the car in for an oil leak. He was told the engine needed to be removed. (I'm assuming it was a rear main or something). He asked the shop about doing some performance upgrades while the motor was out. They suggested a crate motor would be the better route and sold him a "300 HP" motor from Jasper Engines. The car was a major disappointment as far as power. The car spent the next 5 1/2 years in and out of different shops trying to figure out why the car wasn't making power only to find out that the original shop had actually replaced his engine with a stock replacement motor with a mild cam upgrade thus making about 180 HP. Consequently he no longer had his stock motor thus depreciating the car while still making virtually the same pathetic power. The car took a lot of abuse while it was in those different shops mostly by corvette corner in Columbus. My dad was sick of the car and disgusted with it and wanted to get rid of it.

My parents stay in Florida for the winter and while they were gone I decided to try to get the car fixed up and working properly. Believe it or not after contacting the original shop (it had been almost 6 years) admitted their mistake and offered to replace the motor with a Blueprint 355 CI motor making 375 HP from Jegs at no cost. I asked them if I could upgrade and do the 383 roller motor because I thought it would be more durable and less maintenance. After all since the car would never be stock again anyway (since they had junked his stock engine) I figured why not give it some more power. I ended up having much of the interior replaced and also re-did several other parts of the car including new wheels (stock rally's), tires, rear half shafts, bearings, e-brake, u joints, front bushings, tach, clock, a/c repair, door seals, t top seals, floor mats, paint chip repair and several other misc parts. I bought a new cover for it and parked it in his garage and left it there for him to discover when he got home from Florida. He was so thrilled. He's loving the car again and we plan on doing several car shows and cruises together like we did when I was a kid.

So while the motor is no longer original, (which I didn't have any control over the shop discarding it), the rest if the car is still all rebuilt to stock specs which is my preference. This is why my first choice would be to stick with the 4 speed. I know that's a very long winded answer to why build a 383 and worry about keeping the stock tranny but that's one of the main reasons.

The other reason is even more silly than that. I've always loved the whine that the T10 transmission makes. I was 8 years old when my dad bought the car. It was the talk of the neighborhood. Even 165 HP seemed awesome in 1975. The sound that tranny makes evokes childhood memories for me so I guess it's somewhat sentimental reasoning for wanting to keep it in the car. Okay, I told you it was silly.

Although for the most part the car won't be run really hard my dad has said on several occasions that he would love to take it to the drag strip just once to see what it will do. My guess is it would make the mph of a high 12 second car but E.T. In the 13 second range due to the 225/70 tires. In any case I don't know if it would survive 2-3 passes down the drag strip or not. I would launch it gingerly and granny shift it. But I'm wondering if the RPMs alone would be too hard on it. Thoughts? The original motor has a 5200 RPM redline. The new one is made to shift at 6500. Do you think the high RPMs alone will kill this thing? I assume I should upgraded the half shafts when I had them replaced.

Was the gymkhana suspension really only $7? What was the difference between it and the "off road suspension" that was $400? I'm assuming it must have been quite a bit based on price difference.

Thanks again for reading my thread that's actually more like a book. 😊 By now I'm sure you're thinking, this is the "quick" background???
 
If your M20 (which is an RPO not a transmission type) is in good condition and you're not drag racing or track day road racing, it will probably be fine, but I'd make sure you're using a premium lube such as Red Line MT90. If the trans is not in good condition, I'd have it overhauled.

On the other hand, if your Dad is a frequent drag racer, autocrosser or track rat (but, it doesn't sound like he will use the car like that) the M20 likely will not have the durability you need. If that's the case, and you insist of sticking with a four-speed, your options are likely limited to a modified Super T10.
 
Shock loading breaks parts. In addition to the trans the other weak links are the 10 bolt posi case and the outer axles. I have changed many that were broken from abusive usage.

I don't think the higher redline will break things as much as a hard launch with hook up will.

I think I would put the car on the road and drive it and see how it goes. If you break the trans or posi you can address them at that time, in the mean time enjoy the car and the time with your Dad.

I am going through my son's 12 bolt diff now, stepping up to 1480 axles and U-joints. He broke the 1350's along with the 10 bolts, ST-10's, drive shafts, and 1/2 shafts. Breaking Spicer solids was a result from the last trip to the track.
 
Just curious, Gary, what kind of torque does the engine your son's car make? Clutch car or automatic? Race only or street/strip?
 
Hib,
It makes about 400hp at the wheels. Not sure on the Tq rating he has the dyno sheet. It is a manual, now a Liberty Gear Tremic, face plated. He ran different tires on it from street to slicks, it is a street/strip car. It was running in the 11's and probably would have just made it into the high 10's that day but he sheared both inner and outer spicers on the LH shaft at the line.

With hard street use the diff went first then the trans until I built him the 12 bolt using Tom's parts. The ST-10's just can't handle it and cracked open like an egg. Muncies are no better. He almost went with the super case M22 but opted for the Tremic setup.
 
Transmission

Thank you Gary and Hib for the very helpful info. I think you echo my thoughts which is to just enjoy the car and address the issue if and when it breaks. Thank you both again.
Matt
 

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