engine and drivetrain 5-speed followup

[Guest Anonymous]

I got some great feedback yesterday from the group about transmission choices and rear ends but there's one thing left that I must ask.

When all you guys upgrade to the 245 OD 5-spd AND add a 3.91 LSD doesn't that give you a net lower ratio for your top end? Therefore it would not be .81:1 it would be more like 1:1??? Just curious exactly how much the 391LSD brings down the overdrive gear?

Also when you go to the 391 who do you bring your speedos to to recalibrate them? (or are you guys just guestimating?)

9.5 comp, 292 cam, lightweight flywheel with 4-puck clutch (stock PP), bav-auto header, urethane all around, Ireland swaybars, Billy HD's X 4, ST springs X 4, vented fronts & rear disc upgrade, 38/38, etc.

Thanks

[Grover]

I have the set-up that you are talking about and actually have a graph showing the different actual engine speeds at a variety of rpm's for both a 3:64 and 3:91 diff. Since I don't have a clue how to scan the document, if you want to give me a fax number I'd be happy to send them to you. As far as top-end you may be correct. However, when my speedo is reading 95mph, it feels plenty fast enough to me. Lastly, on the speedo calibration you can probably find some on this in the archives since I know it has been discussed; you'll need to send it to a certified facility and I want to say it was in $150-200 range to recalibrate it.

[Guest Anonymous]

My curiosity is mostly because I'm trying to decide between the 364 & 391 (or maybe a 345) LSD and either a CR or OD transmission. If I keep the 364 and use a CR it will be exactly the same as what the 4-speed is now at top end, just with more gears in the middle.

If I go with a 391 and the OD will it not be the same because the 391 will negate the OD 5th gear???

[Senna27]

Since I don't have a clue how to scan the document, if you want to give me a fax number I'd be happy to send them to you.

Do you have a digital Camera? If you do, take a picture and post it!

BTW, what does your tack read at 95 MPH with the 5 speed OD and 3.91? Also, what size tire/rims do you have?

I've heard all sorts of nasty stuff (reliability issues) with the CR 5 speeds. Apparently they're rather fragile!

[Grover]

Alright, I try to snap a photo tonight and post it. As for what rpm my tach reads at 95mph, I can't honestly say that I remember, but I'm going to guess in the 5k range, it certainly wasn't high enough that I was worried about red lining.

[Beaner7102]

Look at the speed/diff/tyre ratio guide on the FAQ

http://www.bmw2002faq.com/content/view/56/32/

That way you can see what speed/rpm you should be doing.

Beaner7102

[jpcapoc]

The 3.91 will only partially compensate for the OD transmission. The driveshaft with a 3.64 spins 93% as fast as one with a 3.91 (3.64/3.91 = .93) therefore your top gear with the OD and 3.91 is the equivalent of .86 to 1 compared to a 4 speed and a 3.64 (.8/.93 = .86)

[john_a]

If I keep the 364 and use a CR it will be exactly the same as what the 4-speed is now at top end, just with more gears in the middle.

If I go with a 391 and the OD will it not be the same because the 391 will negate the OD 5th gear???

The answer is that the OD trans with a 3.91 will still offer the "overdrive" advantage (like a numerically lower final ratio) compared to a CR with a 3.64 diff.

There is a very simple way to solve for this.

Just take the top gear of the trans multiplied by the diff ratio.

0.81 x 3.64 = 2.95

0.81 x 3.91 = 3.17

0.81 x 4.11 = 3.33

0.81 x 4.22 = 3.42

Compare the above to the trans' with 1:1 top gear (CR or stock 4 spd.)

1 x 3.64 = 3.64

So if you have a standard 4 speed or CR 5 speed, then you will still lower your engine RPM for cruise speed in top gear by changing to a OD, even if you change to a 4.22 diff at the same time!

[Lupin3]

Of course, in addition to the transmission's fifth gear and your differential's gearing, you'll also need to consider your wheel size in calculating both your final drive gearing and your speedometer adjustments.

Going from a 13" to a 15" wheel can make a substantial difference in your actual gearing. Your gearing will essentially be lowered (numerically), resulting in slower acceleration but fewer highway rpms and better gas mileage. The tire sidewall you add to those wheels will further affect this calculation.

As far as the 3.91's affect on your final gearing with an OD, the final gearing will still be less than 1:1 since by defintion the the OD lowers it below 3.91. Whether that will be a lower final drive gearing than a four speed or CR with a 3.64, I don't know. I suspect it will be - multiplying 3.91 by .81 yields 3.16 - though my math has been known to warp both space and time so I'm probably wrong to calculate the numbers this way.

If this is close, however, it may represent a good compromise between the CR and an OD with 3.64 gearing. You'll get better acceleration than the CR without 3.91, and lower revs. On the other hand, you'll get slightly slower acceleration than a CR with a 3.91 diff, but much lower revs and longer engine life.

If efficiency and gas mileage is more important, match an OD with a stock 3.64.

[Guest Anonymous]

Has anyone put in a 3.45 rear end? I thought the Turbo cars came with the CR 5-gear and 3.45 rear end??? I have stock E21 turbine style wheels (13") but was planning on going up to 15" turbine style with a low profile tire. I'm thinking that the 3.45 with larger tire and CR will not be a Butt-stang 5.0 killer off the line but will be quite fun in the turns and on the highway.

How come there does not ever seem to be talk about going up to that 3.45 & only down to 391, 411 or 422?

[gracer]

Turbo cars came with a 3:32 rearend. They came with either a 4 speed or a 5 speed CR. G