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fastricky

5 speed + 3.45?

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14 minutes ago, fastricky said:

 

Thanks! It's similar to the chart on the FAQ which is what I went by to get my info...

Yes, I like that the google spreadsheet is interactive, and you can find your speed at a given RPM for each gear and even adjust for tire and wheel changes. 

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I didn't know they were hard to find or anything, I've got a 3.45 gear set in my shed just doing nothing...

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(edited)
44 minutes ago, Spyke said:

I didn't know they were hard to find or anything, I've got a 3.45 gear set in my shed just doing nothing...

AFAIK US-bound cars didn't come with 3.45 gearsets.  I've heard--but have never been able to confirm--that very early US tii's had 3.45 diffs.  But I remember examining the diff of a 3 digit US tii (VIN 2760xxx) in a junkyard many years ago and finding a 3.64 diff.  And the factory parts book doesn't list a 3.45 as coming on a US spec car...

 

mike

Edited by mike
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I think the E30 & E36 318i had an optional 3.45 small case LSD. You should be able to swap the guts into your case.

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Yes, the E30 diff could offer such a gearset...I have a E30 320i with 3.45.

I find the 3.64 too long with the OD 5-speed, but I don't motor along at 90. I hate the gap between 1st and second, drives me nuts...so I would never put a longer gearset even with 150hp.

Andrew

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3.4x diffs come in E36s with 1.8's and manual transmissions.

 

So the gearsets are easy to find.   In the US, a surprising number of ti's 

got LSD's, so the LSD carrier is also not rare.  

 

But then you'll need/want an E21 case to put the carrier in, and THOSE are

starting to become a bit harder to get hands onto...

 

t

 

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Installed the 3.45 diff in my tii (original was a 3.64). IMHO the combination of the 3.45 and the 5-speed is outstanding, just what I was hoping for: it dropped the cruising revs by 250 - 275 rpm. MPG is now @ 30 from 26. Much quieter experience at high speed and the added LSD feels great. I also far prefer how the car launches from 1st gear now, which with the 3.64 I found to be too short a gear. Only catch is the speedo is now off by @ 4mph at high speed.

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(edited)
1 hour ago, fastricky said:

MPG is now @ 30 from 26.

 

1 hour ago, fastricky said:

Only catch is the speedo is now off by @ 4mph at high speed.

 

Of course these two things are related. Meaning, the mileage is not what it seems unless the speedo is accurate.

 

(says the guy with an inaccurate speedometer, who's happy to see 23 mpg at the pump)

 

I like the "launch" with my 3.9, but then I barely get across the intersection and it is time to shift into second.

Edited by '76mintgrün'02
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Hi,

By decreasing the gear ratio on the differential, you magnify any gaps in the gearing of the transmission...or is it not that noticable? So if you have a CR box with the long first gear and install a 2.xx, you elongate 1st even more, while the gaps between the other gears grow only slightly because they were close to begin with...or am I wrong in my thinking?

Andrew

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4 hours ago, Oldtimerfahrer said:

Hi,

By decreasing the gear ratio on the differential, you magnify any gaps in the gearing of the transmission...or is it not that noticable? So if you have a CR box with the long first gear and install a 2.xx, you elongate 1st even more, while the gaps between the other gears grow only slightly because they were close to begin with...or am I wrong in my thinking?

Andrew

Put a bit of this between your cheek and gum and chew on it a while.

DURING a gear change the transmission output shaft/driveshaft rpm does not change, only the input shaft/engine speed changes from one gear to the next. So there is no change in engine speed gaps no matter what the differential ratio.

Road speed is in your mind, discard it.

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Hi,

I don't have the need to cruise at 85, but am annoyed, especially on inclines, by the gap between 1st and 2nd on the standard box...so I have been contemplating a shorter final drive...which would move the shift between 1st and 2nd into a better powerband for the engine... I had a CR box in an E12 and did not enjoy it around town, so I put the CR box off my agenda a few years back for my 02. Now I guess I kind of regret "upgrading" to the OD box...small step sideways. On the track it's the start/finish straight on my track where shifting to 4th will happen at some point.

Andrew

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2 hours ago, Oldtimerfahrer said:

Now I guess I kind of regret "upgrading" to the OD box..

The 1-2 gear ratio spread in the 4 spd box is 3.7% greater than the 5 spd OD box.

 

Both the 1st and 2nd gear ratios in the 5spd OD box are lower than the 1st and 2nd gear ratios in the 4 spd box, probably to handle the E21 (a heavier car) from a standing start.

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14 hours ago, jimk said:

Put a bit of this between your cheek and gum and chew on it a while.

DURING a gear change the transmission output shaft/driveshaft rpm does not change, only the input shaft/engine speed changes from one gear to the next. So there is no change in engine speed gaps no matter what the differential ratio.

Road speed is in your mind, discard it.

 

What?

 

If you switch to a numerically lower final drive, you increase the minimum practical vehicle speed for each gear, while bumping the rpm floor up from where it was previously. This makes it necessary to accelerate in a lower gear longer before upshifting than was previously needed to avoid lugging with the numerically higher final drive.  The rpm spread between gears increases as rpm increases, and running up to higher rpms effectively increases the spread between adjacent gears. This very much affects drivability regardless of where the shift points should be at a flat out sprint.

 

tldr: Yes, changing the diff can amplify gaps.

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(edited)
8 hours ago, Jimmy said:

Yes, changing the diff can amplify gaps

How does the driveshaft speed change while changing from one gear to the next (depress clutch, move gearshift from one gear to the next)?  That is the rpm spread, road speed/differential has no influence during the gear change.

If the driveshaft speed changes, you must be spinning the rear wheels on the roadway.

Edited by jimk
sp

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Well, I would think it would make a difference, though I will not go into calculating the gearing.

I am standing at the base of a hill...waiting for the green light to start a hill climb.

Scenario 1 OD box with 3.64 diff

I drop the clutch at 2600+rpm and my lsd does its thing and the tires transfer everything to the pavement. At some 6200, I shift in 1,5 seconds to 2nd. I have not powershifted, the engine resumes its work at a pitiful 2000 rpm due to the hill and gap in gearing,  at speed x. Because I am well below the sweet spot of the engine, it takes a few seconds to get back to the powerband and y seconds to reach redline again. 

 

Scenario 2, same box longer final drive

My assumption was that with a longer final drive (say 2.78)  my land speed at the shift point to 2nd gear is marginally higher, but when shifting, the rpms I start with in second will be even lower due to the higher overall gearing. Hill doesn't help, meaning that I will be further away from where I want to be on the rpm band. So any speed benefit is lost by bogging around in the cellar.

It's like carrying speed through corner's - entry speed is less relevant than speed exiting the apex. The further away I am from 3000rpm when 2nd gear is engaged the slower I will be. If I'm at 2800 I will be there in a second or two and If I'm at 2000rpm it will be 5-10 seconds to reach 3000 from where I quickly get to 6000.

I guess that intuitively the cr box is the right solution, but a short final drive would lower top speed and lower the gaps between gears.  A longer one should exacerbate. 

Andrew

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