325i Sport Gearbox upgrades / swaps

[Mikey_Boy]

Hello folks,

Yes - that old chestnut of gearbox upgrades is rearing it's ugly head again. Currently I run an '89 325i sport for trackdays, but planning a stroked n/a motor for when the current lump goes bang. Whilst it's out, I am thinking it would be good to upgrade the box as well, despite the fact the current box is considered 'bulletproof'. I have read on here that various options are available from doing nothing (apart from rebuilding) to E36 gearbox swaps? (6 speed E36 M3 box??)

I have noticed that even on the sport box the gap between 2nd and 3rd gear is a bit large, so looking to address that. I do understand that lowering the diff ratio will help and this is something else I am considering. Any advice here on a good compromise? I like to drive to and from the trackdays so something that gives a good cruising ability would be nice!

My question is - does anyone (like Quaife) do a closer ratio gear set? What options are there to swap? Also, I am not certain what boxes will fit the M20 engine with the minimum of fuss?

Any pearls of wisdom much appreciated!

Many thanks!
Mikey_Boy

[GeoffBob]

Hi Mikey,

I think that this thread here will answer most of your questions.

The box with the most ideal ratios for your application is a Getrag 262CR, although please note that:

A 262cr is quite tricky to fit to an M20 engine. You will need the bellhousing from a US Spec 528e and these are rather thin on the ground.

SFAIK, a set of aftermarket straight cut close-ratio gears is available from the metric mechanic to fit the Getrag 265, Quiafe will probably be able to cut something to your specification as a special order, but will cost you an arm and a leg.

Be careful with regard to your final drive ratio. The spacing between gears on a daily driver are typically so wide that raising your diff ratio (say) to bring 5th gear into productive use on the straightaways usually confines 1st and 2nd gear to the pits and paddock. And lowering the diff ratio to enable cornering in second will often shift 5th too high for use on the straights. Unfortunately, a 5 speed box from a daily driver seldom delivers more than three useful gears on track, which is why I set about redesigning the gearbox on my track car. You can see the design of my box (including ratios) here. Questions welcome.

[Theo]

I've just had an idea regarding the fitment of a 262cr to an M20... I've got a belhousing here from a 325i automatic and I really don't think it would be that hard to make an adapter plate to mate it to the gearbox.

[ross_jsy]

They do a straight cut gearset you say

Can't seem to find it on the site, but god I would love that for the sound alone.

[e21Jason]

Bell houseing to fit the 265cr to the m20 are about $200 from the states as they are used in the manual us spec 528e in 1981/2

Jason

[GeoffBob]

They do a straight cut gearset you say

Can't seem to find it on the site, but god I would love that for the sound alone.

Don't quote me on that , but I definitely saw something on their site when I last browsed it. I'll have to try find it again.

[Mikey_Boy]

Folks,

Thanks for all the info - GeoffBob - you are a star! Please do keep us posted on the gearbox redesign adventures...

Seems I need to go-a-bellhousing-huntin'!!!

Cheers,
Mikey_Boy

[Mikey_Boy]

One last question for GeoffBob and e21Jason - let's assume I am successful with my hunting - with the US 528 bellhousing and 262 gearbox, will the prop bolt straight up? Also, any issues with the gearshift to get it to fit?

Many thanks!
Mikey_Boy

[GeoffBob]

Hi Mike

In response to your PM:

See attached a plot of your vehicle speed in mph as a function of engine rpm and selected gear. I am assuming that you are shifting up on your red-line of 7000rpm.



Notice how your engine speed stays between roughly 5000rpm and 7000rpm in all gears. This is what you are looking for out of a close ratio gear-box. Sadly, I had to build my own gear-box to get exactly this as there isn’t one ”aoff the shelf”a that can do this for my engine.

With a 43:11 (3.91:1) ratio diff you'll make it up to 125mph (201kph) at 7000rpm in 5th gear. Unless you plan to tackle any tracks with particularly long straightaways then I think this is ideal, so long as you have sufficient traction to handle the increased torque at your drive-wheels due to the higher overall ratio of your drive-train. If so, then your acceleration will be phenomenal

Unfortunately, you will have to rev your engine to 3900rpm in order to do 70mph on the motorway in 5th. This is not ideal. If you plan to use the car for both road and track then I would seriously look at fitting a 45:13 (3.46:1) diff which will drop your engine speed to 3500rpm at 70mph, with a top speed of 142mph. This should do you well for both road and track. You may even be able to use 1st gear on very tight corners, hairpins etc.



If in doubt, stick with your current diff and try it on the track, I think that you will be fairly impressed provided your wheels don’t spin. Semi-slick tyres at correct temp and pressure an absolute must ”“ you won’t get the full benefit of this setup with normal road tyres. If you find the car a little too ”arev-happy”a for your liking then consider a 3.46:1 diff, but I wouldn’t recommend lower than that for on the track.

Best of luck
Geoff

[Mikey_Boy]

A public thank you to Geoff for all his help with this! Latest from me is that I now have a 262CR gearbox being stripped and re-built - I have also managed to find a 528e bellhousing from the USA for my M20 engine - eBay came up trumps!

As soon as I get the box ready I shall post a separate thread as I install...

[GeoffBob]

You're welcome Mike. I can only imagine how you felt when that bellhousing arrived from the US. Quite a unique position you are in.

I look forward to hearing how she performs.

Geoff