Muncie 4-speed or 5-speed Conversion?

Very nice set-up, thanks for sharing this, everyone is different to what they like and what feels good for them, FI or carbed?

The 1,800 to 2,000 rpm at 70 mph is a little on the low side for me and the cars I work on, but not for everyone I guess.

 

I have a pretty hot cammed and carburated 406 with a T56 magnum which is 2.66 first and .8 5th, .64 6th and 3.42 gears. The current rear gears are a bit on the high side for this combo, but it's still very usable under all street driving situations. I plan to move to a 3.70 or 3.89 gear. Right now I am doing 2150 at 75 which is on the low side of usability with this motor.
The motor is very happy cruising along at 2300-2500 rpm, so even going to a 3.89 gear puts me in this range up to 75.
And 5th is a nice gear for slower 2-lane highways or when there is some traffic on the freeway limiting you to 60-65 range.
Really depends what kind of driving you do and how much time you spend at high speeds. Where I live in a major CA area the traffic is all buzzing along at 75 so you really do need to comfortably cruise at 70-75 or you are going to be in the slow lane with everybody flying by you. But if you live in a more rural setting and travel on speed limit 55-60 roads most of the time, a 4speed with 3.42-3.55 gears is probably just fine. Figure it's $5K to swap in a TKO or T56 in place of the muncie and you are getting pretty much the same set-up in gears 1-4.

 

I have a 73 that was originally an auto car. I put a 6 speed in it. I found the best route was to buy a donner car. I found a beat up 97 trans am. I wound up putting the LT1 in it as well as the 6 speed. having the doner car gave me most of the parts I needed. There is a bracket that goes behind the brake master cylinder that hold the clutch master cylinder but I don’t remember who makes it. As for the trans cross member if you cut the mount perch off the stock cross member spin the cross member around (left side now right side) and re-weld the perch on it fits perfectly. I wound up putting a 4.11 gear-in my car and because of the 6 speed it handles freeway speeds very well.

 

Ya many years ago we bought a 4th gen burn car for like $2500 and harvested the engine for a CMC race spare and the T56 and all associated parts for the 69 Camaro I had at the time. Donated the car to a fabricator buddy who turned it into a race car roller. The cars these days are cheaper than the parts are worth. No doubt.

 

The one thing you do want to avoid with a carbed car is to not have so much overdrive that you are cruising on the transfer circuits instead of the mains.

 

Could not agree more with this quote, I have had several guys show up with their "hotrods" with new OD auto's and manuals with the wrong or same rear gear, or to deep of an OD for the combo complaining of not so great, or lower than expected mileage (not in the sweet spot) some soggy/driveability issues(booster buffering/puddling) and hot headers and excessive under hood heat (no timing at the cruise RPM). Some I can tune out, some need gear/intake/carb work - change, some just bought the wrong trans/rear gear.

Last weekend I did my mileage test, 85% flat highway, 15% side roads. Kept it at 2,800 RPM, (80mph) on the highway, 171 miles round trip, used 8.64 US gallons, got 19.8 MPG. Car weights close to 4,100# with 2 passengers and trunk junk, 330 SBC 530HP, 306S cam, dual plane with 2 1" custom split spacers, 870 Holley, manual w/.77 OD and 3.73 rear and 27.3 tire. AFR at 15.2 with 48* all in @ 2,700rpm. My best record was 20.4 MPG, same setup but with a "smaller"crane cam and open plenum. Going back to that combo next year, better mileage, and more top end HP!!

 

I’ve been waffling between 3.70 and 3.89 gears for my .64 6th gear for a hot carb 406 motor 26” tire. This last post had pushed me into the 3.89 camp pretty firmly. That still puts me at 2400 rpm at 75 - right in the sweet spot of the motor. This is a demon 750 with single plane intake and timing is still not all in yet but should be in the mid 30s with the vacuum can add.

 

I truly believe you will be better off overall with the 3.89's.

FWIW, and as a close comparative to your car/setup, I ported a set of heads and intake and built/tuned a carb for a customer's car, original 1970 Z/28 with the factory 12 bolt w/4.10's. It's a 400 SBC, 10:1 CR, HR, Dart SHP block, with Dart Platinum Pro 1 AL heads and Super Vic intake/750 Holley w HVH spacer, nice build, not to radical, pump gas, and comes in around 525HP.

He took out the M21 and installed a TKO600 with the .64 5th, at 2,500 with an almost 28" tire he is at 78/9 MPH. I drove the car to tune it up, side roads/highway, so nice, love it!! first gear is a little deep but totally usable and a blast!! No regrets whats so ever. Testing a 650 carb next that I built, may loose 5-7 HP on top, but gains in throttle response and shift recovery may very well be worth it, and may ET better with same MPH, time will tell and show what it wants.

 

perfect - that's almost identical to my engine set-up. I am running a T56 so my first gear is more reasonable 2.66 - that comes in at 10.3 which seems just about perfect for this type of motor.

Cruising on highway in the 2300-2600 rpm range the motor feels just perfect - it's just loafing along up into the main circuit on the carb and at a reasonable timing level w/ the vacuum advance and can run that way all day long, plus it's rather quiet exhaust wise too. Not that this car is a long distance cruiser - but it literally could be no problem. and current timing curve is all in around 2800-3000 but I'm moving to a MSD pro-billet and starting with the 18deg bushing and the blue springs so it will be 18 initial and 36 all in around 2500rpm.

 

The msd is set up to give 10deg already so probably will leave it as it comes for now see how it runs.