I am just south of Nashville in Murfreesboro so it is flat here. I will look to upgrade to the Auto Gear transmission later but I can buy a rebuild M21 from a coworker right now for $500. An extra $1500 does not fit the build budget at this time. However, I will be purchasing the ring and pinion for the rear end though. Wouldn't the 3.08 be a solid option to provide performance and comfortable highway cruising?
I am just south of Nashville in Murfreesboro so it is flat here. I will look to upgrade to the Auto Gear transmission later but I can buy a rebuild M21 from a coworker right now for $500. An extra $1500 does not fit the build budget at this time. However, I will be purchasing the ring and pinion for the rear end though. Wouldn't the 3.08 be a solid option to provide performance and comfortable highway cruising?
The trouble is getting it going from a stop. For most street performance applications, a 10:1 final First gear ratio is usually considered optimal. anything less then 8 would be very hard on the clutch to get the car moving. Your final drive ratio is 6.78, a lot less. Mine is a M20 2.52 first gear and 3.36 rear gear. 1st gear Final drive ratio is 8.47. To me 3.36 is the lowest I would go and I would rather have a 3.55.
The recommendation for a Muncie m20 4 spd is to not go below 3.36 rear gear because of the 2.52 first gear. A m21 has a 2.20 first gear. 3.90 is the recommended rear gear for a m21.
With the set up you are looking at, you will be very unhappy and replacing clutches a lot with a m21 and 3.08 rear gear. This is a set up for failure.
My 3.23 rear m20 combo works well enough for casual driving. Anything more spirited it hard on the clutch. Test and tune night at the track required that I reface my flywheel that I toasted.
I don't think any new rear gears are available in less than 3:55
Would the 3.36 or 3.55 rear gear still allow comfortable highway driving?
Comfortable as it can. You would be running 3600 rpms @ 60 with the 3.36 and about 3800 @ 60 with the 3.55 with stock wheel and tires. But those gears would still suck pulling out with a 2.20 fiirst gear. M21 needs a 3.9 rear gear or you will be disappointed on take off from first.
Muncie 4 spd m 20 or m 21 was not meant to be efficient and if you want good take off, low rpms above 60 mph, you need to look at a 5 spd with a steeper first gear so you can use a 3.06 rear.
I would find a m20 where the first gear is a 2.52 then you would be safe with a 3.36 rear gear. You should be able to find one for about the price as the m21. 3.36 with a m20 would be about as efficient as you could go with a 4 spd Muncie. Unless you go for the auto gear m22z with the 2.9 first gear. Then you would be safe with the 3.06 rear.
You have to really think about the first gear take off so you don't burn clutches up just as important as the final drive rpm output. No need to be efficient on the top end if you have trouble getting to it.
I ended up picking up an LS1 and T56 transmission from an 01 Trans Am yesterday with 55k miles for pretty cheap. Looks like things might go a different direction. I'm going to research adapting the T56 to the 400 and doing the whole swap to make a decision.