TheSamba.com :: View topic - Advice wanted on big nut gearbox on 1600 sp

Tue Mar 16, 2010 2:30 pm

Hi,

I have a 65 standard running a freshly re-built 1600 single port over here in the UK. Currently running standard height, stock transmission, reduction boxes (small-nut). Standard 14 rims with truck tyres - so quite a big rolling radius.

A friend has offered me a big nut complete gearbox with approx only 30,000-40,000 miles on it.

As I understand it, and I'm still only running in the engine, I will top-out at about 55-60 mph as things stand, but have good torque and speed up hills.

I surf a lot and me and the family will be on the motorway (freeway) a lot.

Will the bignut box give me a cruising speed of around 65 mph, all things being equal? It's only a bit more, but it would make a difference. Or will I find long hill-climbs more of a crawl, or be screaming in third because a tall 4th is too much of a grind?

Any advice/experience much appreciated. I've said I'd let me mate know by tomorrow, so i need to make a decision. At the moment, I'm inclined
to swap to big nut (the money's the going-rate for the Uk, and not really the point)

Thanks,

Ben

Tue Mar 16, 2010 3:23 pm

I had a '67 sundial with a 1500sp and big nut trans. Drove it around California, including some of the steeper hills in San Francisco and it did just fine. Was also fine on the highway. Also had a '64 panel camper with this combo and worked well. I have had a couple of straight axled buses, but I am planning the same engine/trans combo as you (1600sp with big nut) in my current bus.

Tue Mar 16, 2010 3:26 pm

i am running the set up you are asking about... bus runs fine... used to run up hills slower than me , but once i sorted the carb out it was fine... i have just asked the question about running 14" rims as i am running 15 at the moment...

Tue Mar 16, 2010 3:31 pm

Thanks guys, that's what I thought after reading around it.

Could do with a bit more info on speed though - is it likely to be happy round 65 (or even 70 mph)?

Could do with that little bit more so I'm not sitting in with the artic lorries all day on the motorway.

Ben

Tue Mar 16, 2010 3:49 pm

I have a big-nut trans with a pretty mellow 1641cc engine. It drives fine at 70 mph on the freeway, although I don't push it much faster than that. Maybe 75 at times, but usually I cruise at 70.

Good luck.

-Greg

Tue Mar 16, 2010 3:52 pm

Thanks Greg, that's what I need to know. I'll go for it.

The 1600 I have just had built seems to have good compression and is lively.

Ben

Tue Mar 16, 2010 4:04 pm

Ben,

In my personal opinion, the idea of a bus 'happily' going along at 70 is hard to obtain. Yes it can be done, and has been. But, if you want to keep the bus stock, expect 60-65.

The big nut will be an improvement over the small nut, with everything else being equal. But its on the order of like 2-3mph more than what you are getting now. The gear multiple is changing from 1.39 to 1.26, not a big change.

The other slight advantage of going from a small to big nut, is that the rear brakes are slightly larger.

Tue Mar 16, 2010 4:10 pm

Interesting. I dont want to be flogging the engine or whirring the t*ts off the gearbox. 2-3 mph seems hardly worth the effort, and my current setup seems happy (non-whinny or screechy, no jumping out of 4th etc). Everything else is stock.

I know plenty of straight-axled and IRS'd buses will do 70 no problem, but I dont have the money for that and my setup is totally stock (I like the comfy ride and the height, though corners can be a Leaning-tower-Of-Pisa).

Maybe I should get used to slip-streaming the big lorries and be happy with what I've got, or at least run it under load for a few months to get a feel for it?

Ben

Tue Mar 16, 2010 8:28 pm

I went to the technical section on The Samba, and used the RGB calculator. It looks like my numbers were a little off, not 2-3 mph, but more like 5-6. Little better, but still not draw dropping.

If the tranny was bad, I would say go for it, but otherwise pass.

Tue Mar 16, 2010 8:47 pm

Khopkinsx wrote: Ben,

In my personal opinion, the idea of a bus 'happily' going along at 70 is hard to obtain. Yes it can be done, and has been. But, if you want to keep the bus stock, expect 60-65.

The big nut will be an improvement over the small nut, with everything else being equal. But its on the order of like 2-3mph more than what you are getting now. The gear multiple is changing from 1.39 to 1.26, not a big change.

The other slight advantage of going from a small to big nut, is that the rear brakes are slightly larger.

X2 I have had all the poss gearing straight 3.88/89th to a stock 60 Mnt Gear S/C Stock 4.12 with lg nut 1.25 RGB. is hard to beat you mite need a bit more power. These KIDS that say they run 70/75 mph are either nuts or full of shit..A split window bus is a 55/62 mph ride.. oh ya the brake thing is good, you cant hardly buy parts for a small nut, the big nut brake is as good or better than a C.B. disk brake

Wed Mar 17, 2010 1:45 am

Khop and Alaskan...thanks for that. That's what I need...real figures.

5-6 mph is worth it for me. It's the difference between being sat there frustrated, and over-takign the speed-restricted lorries that sit on our motorways all day long. Plus, cruising will be better if I go to a 1776 at some point.

The bigger brakes will help (family in the back, plus surfing gear)

Thanks for all your help guys.

Ben

Thu Mar 18, 2010 12:13 pm

Bignut gearbox went in...seems quieter and faster at higher speeds, with later gear-change (still running in the engine, so didn't push it in 4th).

Only thing a bit odd is I noticed when reversing a long way to park up, that it thunks a fair bit going into reverse - no problem with reversing itself or finding it or slip or anything - just a big clunk. Didn't do it the first time, but did after that. Could it have anything to do with the gear guide plate positioning? Or do I just ignore the thump?

Look on the bright side, I certainly know when I'm in reverse :)

Or it could be my crappy technique, thoguh I'm pressing it down properly, and finding the path is easy.

Ben

Thu Mar 18, 2010 5:41 pm

Ben Scammell wrote: Or do I just ignore the thump?

Don't ignore the thunk. It will ruin it. You're clutch or pilot bearing or hub spline is hanging up. Hopefully you can just adjust the clutch cable tighter. When the gear oil is cold it does that too and you have to wait longer to pull back into reverse. Don't let it happen.