engine and drivetrain My impressions of the OD Tranny Vs. the old CR

[Guest Anonymous]

Awhile back, I posted about my plans to swap an OD 5-Speed for the dog-leg CR box in my 2.5L S14-powered '73 tii with a 3:64 LSD (http://www.m2bmw.com/cooper.htm). Many of you were very helpful in providing your opinions, both on the pro and the con side. Well, I went ahead and did the conversion and figured that I would share my impressions. Basically, the only thing that I like about the car now is the fact that I can carry on a conversation with my wife at 90 mph whereas before it was pretty deafening at that speed. Otherwise, I just HATE it!! The car literally feels like a DIESEL compared to what it was like before... I don't get it, but the acceleration just feels incredibly sluggish. I literally thought that either the throttle cable had slipped and I was only getting 3/4 throttle OR that the clutch was slipping. I never realized how much of my car's explosiveness was attributed to the CR box and I'm a little confused: help me out here, but I thought that the first 4 gears were the same ratios in both boxes?! In any event, before the car was just a BEAST whether erupting out of corners or when downshifting from 5th to 4th on the freeway (it seemed that within seconds the speedo would be smacking against the 120mph limiter!). Now, when I try to downshift from 4th to 3rd with the hope of reproducing that type of acceleration, first off the revs are just too high and second, it still doesn't pull anywhere near as hard as before. Another thing I noticed: When accelerating from 1rst to 2nd and then to 3rd, even if I shift very quickly so the revs don't get a chance to fall too much, the car just feels SLOW. I guess everything is relative and I'm sure it's still a fast car, but I'm really surprised by how much more tame it feels now. The bottom line is that I built this car as a back-roads warrior and the times that I take it on the freeway are pretty few and far between. In other words, the occasional benefit of low-rev freeway driving seems minimal compared to the reduction in fun on windy roads. I'm glad that I kept the CR box! Any thoughts as to why the OD transmission resulted in such a decrease in quickness would be appreciated. Some of you suggested the OD box in conjunction with a 3:90, but wouldn't that just put me back at the same highways rpms as with the CR box but without the entertaining interaction of the close ratios? Anyways, live and learn...COOP

[c.d.iesel]

put the 3.90 diff in report back

it's a quick swap

[ColinK]

From bimmers.com....

Getrag 235/5, optional close-ratio 5-speed for the 2002:

street version: rally version: race version:

1st: 3.368 2.71 2.30

2nd: 2.16 1.84 1.56

3rd: 1.58 1.38 1.28

4th: 1.24 1.13 1.09

5th: 1.00 1.00 1.00

vs. Getrag 245/4, '80-'82 320i overdrive 5-speed

1st: 3.68

2nd: 2.00

3rd: 1.33

4th: 1.00

5th: 0.81

The FAQ page on trannys and diffs may also help.

Glad to hear you didn't get rid of the CR.

[inkatouring]

It is not OD vs. CR that makes the difference. It is the combination of the OD and the 3.64. With the CR and 3.64 you effectively (slight differences in spacing between gears) have an OD with a 3.90.

[Tbone]

Just looking at the top gears:

A 1.00:1 top gear and a 3.64 rear end, final drive is 1.00*3.64=3.64:1

An overdrive top gear of .81:1 and a 3.64 rear end-->.81*3.64=2.95:1

Couple that large drop in torque delivery to the ground when in 5th and the wider gear spacing that pushes the upshift rpm drop even further away from your peak torque at the crank, the OD box should feel like the parking brake is stuck on (after you've had a close relationship with a CR box)

The best answer would be a close ratio 6 speed, with 1st through 5th similar to the ratios in your CR box AND and overdrive 6th. I'll keep dreaming about that one!

[harrym2]

Hi Coop

Do you think that the cr kit that can be had for the od box would be a good idea ?

That way you can have a fast car plus a good car for the motorway in one

[BLUNT]

Just looking at the top gears:

A 1.00:1 top gear and a 3.64 rear end, final drive is 1.00*3.64=3.64:1

An overdrive top gear of .81:1 and a 3.64 rear end-->.81*3.64=2.95:1

Couple that large drop in torque delivery to the ground when in 5th and the wider gear spacing that pushes the upshift rpm drop even further away from your peak torque at the crank, the OD box should feel like the parking brake is stuck on (after you've had a close relationship with a CR box)

The best answer would be a close ratio 6 speed, with 1st through 5th similar to the ratios in your CR box AND and overdrive 6th. I'll keep dreaming about that one!

what about an e34m5 euro dogleg box? or did they not have a 6 speed?

if they did that would be the ultimate behind a hot 2.5. of course youd have to butcher to get it in

[NickVyse]

5th in the CR box is the same as 4th in the OD - IE one to one. I think you'd need a 4.10 to feel the same explosive urge, but as you say you're losing the benefit of that OD ratio on the highway. I've never driven an M2 with the OD box, but always thought it was a poor combination for the s14, and it would seem your experience confirms this.

Good luck with the 6 speed!

Nick

[harrym2]

what about an e34m5 euro dogleg box? or did they not have a 6 speed?

if they did that would be the ultimate behind a hot 2.5. of course youd have to butcher to get it in[/quote.]

Some e34m5 did come with a 6 speed as did some e34 540 and e36 m3 . Wonder if they could be made to fit ?

[NickVyse]

what about an e34m5 euro dogleg box? or did they not have a 6 speed?

if they did that would be the ultimate behind a hot 2.5. of course youd have to butcher to get it in[/quote.]

Some e34m5 did come with a 6 speed as did some e34 540 and e36 m3 . Wonder if they could be made to fit ?

there's no easy answer - search s14.net for many threads relating to this.

[BLUNT]

what about an e34m5 euro dogleg box? or did they not have a 6 speed?

if they did that would be the ultimate behind a hot 2.5. of course youd have to butcher to get it in[/quote.]

Some e34m5 did come with a 6 speed as did some e34 540 and e36 m3 . Wonder if they could be made to fit ?

well the big 6 box would bolt up. i imagine youd have the same clearance issues in the tunnel as with the 265. but if the e34 s38's came with a 6 speed that would be a nice option if you had enough motor to pull it off

[harrym2]

How much engine do you think would be needed .

[NickVyse]

what about an e34m5 euro dogleg box? or did they not have a 6 speed?

if they did that would be the ultimate behind a hot 2.5. of course youd have to butcher to get it in[/quote.]

Some e34m5 did come with a 6 speed as did some e34 540 and e36 m3 . Wonder if they could be made to fit ?

well the big 6 box would bolt up. i imagine youd have the same clearance issues in the tunnel as with the 265. but if the e34 s38's came with a 6 speed that would be a nice option if you had enough motor to pull it off

to quote John of M3 Tech fame (taken from s14.net):

Hi all,

I know the e34 sounds nifty to some...

Ive run the numbers several times with the e34 6 speed,

and the gear ratios are just not suitable because the

rpm drops between gears are too much.

It is very important to understand that changing a diff ratio

will not change the rpm drops between the gears...IOW,

the operating bandwidth on the gearbox does not change.

the operating bandwidth of the gearbox has to be matched

to the powerband of the engine. This is extremely important.

diff ratios are merely used to set top end speeds, or to

bias a certain track to use either the lower gears or the higher

gears.

When I look at a gearbox, I look that after each gear change,

and assuming a shift rpm of 8000 rpm, the rpm does not fall

below 5500 rpm. MINIMUM. More ideal setup would not

fall below 6000 rpm on a gear change. The e34 box does not

satisfy this condition.

What does this mean to in everday usage to me?

on my car using a CR gearbox the 3.72 1st gear is MUCH

too short. It does not make sense to use a short gear

above 4.0. This gear needs to be around 3.0 to start

to make it usable -- on a 5 speed with 1:1.

On a 6 speed with 6th 1:1, ditto. If 6th were 0.81,

then we need to make a correction.

A good rule of thumb is to start from 6th 1:1 and work

your way backwards on the ratios.

Ive said this before, but the gear charts previous shown,

which do notheing more than plot a gear ratio vs. gear number

tells us absolutely nothing. You can not look at such a graph

an guess much about performance, other than to say

one gear is a bit shorter on 1 box than on another.

What you need to do is compute the speeds and rpm

drops given some max rpm. Then you need to think

a bit about your usual track and which gears are normally

used in the key sections. For street usage, you may want

to check the ratios under operating conditions at lower rpm.

e.g. "what if I drive 100 mph in 6th gear, what rpm will I

be turning", how large are my rpm jumps on shifts around

4000 rpm (street) vs rpm jumps at 8000 (or 9000) rpm (track).

Danny brought up a good point: you cannot just mix things

up with gears. There are particular gear sets available

and it has to work. On motorsport gearboxes they have

all the gears to rebuild ratios to pretty much whatever

you want. It just not cheap.

His analysis of the end price is correct. You must also consider

things like bearing longevity, there are certainly cheap bearing

that might last 10k and then there are more expensive ones

that will last 100K. So even if you can build the box for

4000 euro using cheap parts, that doesnt mean you will

be happy with it for long. This is sort of the same reason

a motorsport diff gear costs twice the price of the street

version (and motorsport offers both). The former is of

higher quality and will last longer. Similar reason why your

stock rebuilt diff unit isnt the same as the motorsport diff

unit, I can certainly tell the difference.

if you still think the e34 sounds nifty then go spend the

4000 on one and I will tell you I told you so when you

are unhappy. A 6 speed must be usable. deleting a gear

makes no sense. Its like the tremic 6 speeds with 0.5

or 0.57 top gear, these boxes are nothing other than

4 spees with 2 overdrives. You waste your time with it.

John

Whole thread can be read here:

http://www.s14.net/forums/showthread.php?t=13538&highlight=speed

[harrym2]

This is a good read. Thanks