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Impact of running a 3.9 differential in my Bugeye

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Hi guys - the differential in my Bugeye is currently stock (or at least I believe it to be). I acquired a 3.9 some months back and am wondering what difference installing it will make on around town and highway driving.

The current installed differential doesn't make any bad noises or anything, so this would be done solely on any advantages having a 3.9 would offer.

I don't expect to use her on the highway much, but even on the secondary roads down/over here people tend to drive pretty darn fast.

Thanks!
 
still running a 948? or a bigger engine?
 
I've got a 3.9 in the Tunebug. It's a pretty good match for the 1275 that Hap built me, though I'll probably switch back to the 4.22 if I ever upgrade to the 5-speed. 4th gear is a bit too tall to pull the mountain grades around here. But then, I live in the mountains and drive a bunch of steep stuff.

Given that the A-series is a rev-happy motor, I think the 3.9 is a really nice fit if you've built up a decent engine. 65 should be under 4000 RPM if I calculated it properly (my tach isn't reliable, so I'm not exactly sure). Engine certainly seems smooth and happy at 65 in the Tunebug, enough so that I'm not in a particularly big rush to fit the 5-speed.
 
Hi Drew - so is the 3rd / 4th gap an issue with the 3.9 or are you just wishing it didn't require a downshift?
 
3/4 gap is fine in normal driving -- really what I want is a 3 - 1/2 gear for high-speed grades. The MGB's overdrive 3rd is perfect for freeway speeds up a steep grade. A similar ratio in the Tunebug would be ideal. If I didn't live in the mountains where I'm driving between 4,500' and 10,000' it wouldn't be an issue.

That said, I'm generally happy with the 3.9, and doing anything else is real low on my priority list. If the 5-speed option wasn't available I'd be content with the setup as it is now.
 
Go for it! 1098 in a BE ought to handle a 3.9. Its not too difficult to change and if you don't like it you can always change back.

Kurt.
 
I run a 1098 in a square body with a 3.9 and love it. I must mention that I have a 5 speed as well. A 1098 has all the torque you need for the higher gearing. (IMHO). It's such an easy swap you can change back if you don't like it.
 
I run a Datsun 1500 with twin HIF4's and a 5 speed with a 3.7 rear and accelerate up I-90 between Preston and Snoqualmie which is a pretty good hill.
I can hit it at 65mph and be going 75mph at the top.

I don't think this has much to do with the conversation so far but I really wanted to say it...

:jester:
 
I'm just a sucker for that 4.22 rear. Giddy up.
Punching the throttle at 55 is a hoot!
I didn't like the 3.7 ,noticeable loss of acceleration
even with the warmed up 1380.
I think the 3.9 would be a good gear for a daily driver.
It's a one :banana: job so I say give it a try.
easy enough to swap back.
 
Thanks guys - appreciate the info!

One thing that did occur to me is that I should check what is in there now before I swap the 3.9 in. I have no idea what is in there now, can I count tire roations to figure it out?

Thanks again,

Tom
 
Looking at the diff from the input shaft end you will find the ratio stamped on the case on or near the flange that bolts to the housing at about 10:00. May be greasey and hard to find but its there.

Kurt.
 
with a 4.22 you're going 60 mph at 4000 rpm. If your at 4000 rpm. If your going just under 65 at 4k rpm you have a 3.9 already.
 
And a 3.7 = (about) 70mph at 4k rpm in 4th gear.
 
nomad said:
Looking at the diff from the input shaft end you will find the ratio stamped on the case on or near the flange that bolts to the housing at about 10:00. May be greasey and hard to find but its there.

Kurt.
Click to expand...

To see it on the diff in the car, you'll need a mirror to see and be able to read backwards. A 4:22 will be labeled 9/38; a 3.9 will be 10/39. You can see all the ratios here:

https://gerardsgarage.com/Garage/Tech/SpridgetSpecsWS.htm
 
This is what you are looking for.
This is a 4.2
 

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The 3.9 I have found stamped on the lip of the flange as well so if you can't find it you may have to scrape a bit more.

Kurt.
 
Rick b reminds me of the joke where the old man goes to confession and tells of all his sexual actions. The gist of the joke is he isn't catholic, but telling everyone.
That aside, a healthy 1098 will pull a 3.9, with or without a 5 speed tranny. Enjoy....
 
Just did a little driving of my midget today and 70mph is spot on 3000 rpms in 5th gear and a bit over 60 in 4th. 3000 is the point of max torque with the 1275 and that was what I was shooting for. Its why I put the 3.55 gearset in it. I got a 76 junker and need to see what set is in it. Somebody might need it.
 
Kim,

If it's original, a '76 would have a 3.9. The 3.7's didn't go in until late '77.
 
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