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Which pumpkin do I use?

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djk

djk

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I'm starting clean-up and getting ready for full restoration of my '60 bugeye, and in all the boxes of parts, it turns out I have 4 pumpkins. One is 9/39 (an orignal), one is 10/38, and 2 pumpkins are 9/41. Which would be a good choice to use?

Also, I have 2 sets of leaf springs; one has 15 leafs total, and the other has 11 leafs total. Is there a benefit to using one over the other?

Thanks,
-Debra
 
They normlly came as the 9/38 being a 4.22, a 10/39 being a 3.9, and a 9/41 being a 4.55 (from a Morris Minor).

As for which to use. I guess that depends on your engine torque. The 4.22 is a good choice for the 948/1100 and the and 3.9 is good for the 1275, being standard configuration. The 4.55 was used with the lower powered and heavier A35 and Morris Minor (34 - 37 hp). Large torque increases would push more towards the lower numerical ratio.

I have also a rather varied choice at home being a 11/41 = 3.73, 10/39 = 3.9 and a 9/38 = 4.22. I will probably use the 4.22 because I'm using an 1100.

The 4.55 are good for race cars where; 1. low-end torque is sacraficed for more high-end torque which translates to horsepower. 2. engine speeds can be in the 7-9000 rpm range.

For street use, torque is your friend. Therefore if you have a 1275, the 3.9, if the 1100, it depends on the torque. Also the 3.9 would give the 1100 better mileage ... and worse acceleration.

I would suspect that one set of springs is not of BMC manufacture. They may well have the same spring rates.
 
Debra do we have pics? Another Bugeye gona get the treatment. Nice.
 
I'll be using a 1275 - I was driving the 422 with a 1098, and I enjoyed it a lot. I do some amount of highway driving, but usually just to get me to the hills where I really enjoy my sunday drives. I'm leaning toward the 9/38 or the 10/39. Do you know how much the 9/41 are worth? I suspect someone who's racing would be interested in them.

You can check out my restoration pic and stories at:
blog.brokenroads.com
Click on Restoration Photos for lots of photos.
 
I debated a while back about what differential I would run on the '62 Sprite when it's finished. I had the 4.22 that came with the car and is in good shape as well as a 3.9 that I got from a friend. I settled on the one I didn't have, the 3.7 because I plan to do a lot of road miles. The quest for the 3.7 has been a long one; Ebay prices have gone crazy. I finally snagged one last week; more than I wanted to pay, but...

While sorting through the decision I put together a spreadsheet that provides MPH @ RPM for all three of the combinations mentioned and for the stock 4 speed as well as the B210 (Rivergate) 5 speed. Keep in mind that tire circumference is one of the variables; mine is listed on the chart.

It's too large to post here, but you can see it by clicking the URL below:

https://www.raysmg.com/rays_mg_midget_whatzit3.htm

Ray
 
I have a 3.9 on a mod 1275 and am happy with it. I had used a 4.2 before and it was not as nice as the 3.9.

Pat
 
Which it won't if the wind resistance is higher than the forward thrust... which it won't be because the car won't go fast enough for the wind resistance to be higher than the forward thrust. Unless it goes down a cliff. /bcforum/images/%%GRAEMLIN_URL%%/jester.gif /bcforum/images/%%GRAEMLIN_URL%%/lol.gif
 
You miss the point. The chart I pointed to merely illustrates the relationship between RPMs and speed. Period.

To incorporate wind resistance, forward thrust, frontal area, curb weight (what happened to drag coefficient), etc. was not the intent. Most of us have little/no control over those variables. We do have control over choice of differential.

Ray
 
One of these days you can plug some actuals in there as well.
 
Already have, Jack. I'm running the '73 Midget with the 3.9 diff and Rivergate 5-speed. My '66 had a 3.9 and 4-speed. And, I drove the '62 with the 4.22 and 4-speed.

I'm keeping all three of the diffs until I have a chance to sort out the Sprite. My guess is that I will probably wind up running the 3.7 or 3.9...of course, if an autocross were in the plans, the 4.22 would only be a 30 minute job.

Of course, I do have to remember not to drive if the wind is blowing, not go up-hill, and put in gas...but other than that, it works pretty well /bcforum/images/%%GRAEMLIN_URL%%/smile.gif

Ray
 
I've got the original 4.22 and a 3.90 to choose from, for my modified 1275 and 4-speed. I'm going to start with the 3.90, and see how the new motor pulls the mountains around here. The new-found power should be equal to it.

Nice sheet, Ray. I wonder how much difference smaller tires would make -- I'm running 55/80R13 on the Tunebug. By a sheet I have handy, that's a circumference of 22.76".
 
Drew,

If you go to the spridget-tech website under A-Series Engines, at the very bottom of the page, he has a spreadshhet on comparing various drivetrain ratios to the 155/80-13 tires.I compared my Datsun 5 speed with 4.22 and 155/80-13 tires to this chart and the numbers were almost identical to the chart on both speed and RPM.
 
I ran a 4.22 with a slighty modified 1275 for about 3-4 yrs. I really enjoyed it, but I finally found a 3.9 and was amazed at how much less "busy" the motor was on the highway. It did cause you to downshift on the road on grade sometimes that I didn't have to with the 4.22 rear end. Now, about the SC motor, it seems to be perfect. I have a later speedo that just happens to GPS out to the correct speed, so I will stay with the 3.9. I think if I didn't have the 5 speed, I'd run the 3.7.
 
what size tires are you running on the SC with the 3.9 punkin?

Pat
 
14" 185 60s Yokohomas. Kent Prather sells Circle wheels and we had them built with different offsets front and back. They set under the wheel wells. They are within a 1/4" the heigth of the original tire size. Moss in their semiannual pub with the articles several yrs ago ran a couple articles with original tire sizes and dimensions and modern tire sizes and dimensions. They are a good fit and provide a great ride.
 
""""""I'll be using a 1275 - I was driving the 422 with a 1098, and I enjoyed it a lot. I do some amount of highway driving, but usually just to get me to the hills where I really enjoy my sunday drives.""""""

With the higher torque and HP of the 1275 (the exception will be off idle to about 1800 rpm) the 3.90 will probably be better, reducing the rpm at a given speed about 8%. Even better would be a 3.70 gearset if you can find one, which would lower the rpm/given speed yet another 7%.


""""""" I'm leaning toward the 9/38 or the 10/39. Do you know how much the 9/41 are worth? I suspect someone who's racing would be interested in them."""""

a good 4.56 (9-41) is bringing about $200. I suggest moving that one down the road as you prolly never will use it.
 
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