in my 51 iv got a small block with muncie 4 speed and iv just set up the rear suspension with a 9" housing, i don't currently have a centre for it but I'm trying to work out what will be the best ratio for it, its going to be driven a lot so i want something that will sit nice on the freeway. any opinions?
It really depends on your tire size. Generally for anything to be driven on an open road regularly, I'd stick to no lower than a 3:50. Perhaps even a 3:25. Both will give you decent performance, yet still be livable on the highway.
cheers iv got pretty big tyres on it, they are g78 15's. so do you think something like a 3.08 would be to low?
Here's a dohickey to play with. http://www.richmondgear.com/101032.html You can find what you need here.
i ran 11's with 3.08's and a muncie m-21 but had a short tire 24 tall it would run 115 in third and would go thru the lights at 127. and done real well on the highway.
3.00 or even 2.79 if you want a crusier. If you have an M-20 with a 2.54 1st, it will help you out in town, or when you what to show the traffic youre the boss. If you have the close ratio 2.20 1st, you give up a little from the stop light. I have a 2.79 and I like it fine on the highway, 460 lift cam in a SBC with a T-10. (2.64 lst gear) 1850 RPM at 65 MPH with 31" tire.
Whats your first gear ratio in your trans first gaer multiply first gear and read end ratio you want close to 10 much less it will be hard to get rolling hard on your clutch
Slightly different animal but I have a 327, PG and 9" in my '52. It also has G78 15s. It runs 3.00 rear and is nice to drive pretty much anywhere. Probably not so good on the strip but I don't need that. Also with the 3.00 rear the original speedo is bang on accurate. Nice bonus. Pete
also consider what equipment is in the engine, what's the cam like? 2.75 gears are nice on the highway, should give you around 2700 rpm at 75 mph. But with a normal Muncie 1st gear ratio of 2.5 or 2.2, you will have to slip the clutch some to get moving. You need to decide what RPM you want to run on the highway, and how the car will perform at low speeds, and figure out what's best. Fortunately there is a wide range of gears available for the 9" rear, although some are only available used. btw I have an accurate speedo with whatever gears I put in my 9" rear, because I also know how to put the proper gears in the transmission to make the speedo correct.
You are right about the clutch slippage Jim. I'm loving my 2.70 gears behind the Nailhead and Muncie in my modified. Really nice to drive on the highway. But it is a bit of a pig around town. Warm clutch getting off the line and back to first for tight corners on suburban streets. In a heavier car the clutch could really be an issue. Hoping I can get my left foot behaving a little better before I do some damage.
At exactly 2700 RPM with G78 15's doing 65 MPH cruising speed you'll need a 3.46 Rear end ratio that is 2700 RPM x 28 inches diameter divided by 65 MPH divided by 336 = 3.46
If you want it all, you're gonna have to put another gear in the transmission, but for comfortable highway driving I'd try somewhere between 3.08 and 3.31. You need to figure out where your engine is comfortable running down the highway and then gear it for the speed you want. Good example is 3.08 with a 27" tall tire at 70 MPH is around 2700 RPM's. Larry T
The problem with a Muncie is that, even on the wide ratio box, the span of ratios between 1st and 4th isn't very wide. So, unless you have so much power you don't need much first gear, you can't get optimum first gear and the best cruise gear from the same axle ratio. For a street car you will need to decide what compromise suits you best, then live with the trade-offs. With many of the newer transmissions the opposite is true. Street car transmissions made for engines with less power and/or narrower power bands have ratio spreads that are wider than ideal for the typical hot rod.
I had a Muncie behind the 283 in my 48 years ago when I put the first of several V8's in it. 3.0 gears were way too tall and rough on clutches. I'm not sure what gear ratio it had but with N 50's on the back it ran 3000 rpm at 80 on the highway. I ordered the only "new" car I ever owned a 69 Olds Cutlass with a 350R M21 Muncie and 3.42 Posi and that was a great all around combination. It was quick through the gears and still could cruise at 80 easily. That was with G60 14 tires on the back. Still the engine setup and cam has a big impact on rear gear selection. Too much cam and a 2 something gear ration and the thing won't pull a sick whore off the pot but it will cruise at 80 easily once it gets there.
Most of the "driver" engines I've had seemed to like running down the highway somewhere between 2500 and 3000 rpms(closer to 2500 seems better) and the new stuff with overdrives are closer to the 2000/2500 rpm range. You probably know what RPM you're comfortable running down the road. If you don't, it won't take long to figure out. I've run several cars with lower gears, but with 3.73's, I ended up cruising at 65 MPH/3000 RPMS. It wasn't a real nice highway car. Larry T
I had a 4.11 banjo in my modified and it was doing 2800 at 60mph. I didn't like it. Didn't have enough in reserve for passing and such. With the 2.70 in the QC I'm doing about 1900 at 60 and can drop back to 3rd to pass no problem. Too tall around town though. I'm going to get a 3.38 set of gears I think. No point having the quickie and not experimenting a bit.
after talking to a few people including the person i got the muncie off (who had it in a A coupe with a small block and a nine inch), and a few diff/tranny shops iv decided to go with 3.55:1 LSD considering the size of my tires and how I'm planning to drive the car i think not should suit me well, if not i can always change it but il see how it goes. cheers for all the input its been very helpful
I've been running my model A with 3.25 rear ratio with a Muncie M20. Rear tires are 700 x 16's. Launch does not stress the clutch and with the tall 4th gear, gives comfortable highway cruising. Just a nice combination for street use.
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