b25 - what's all the fuss?!

Post Fri Dec 19, 2014 1:00 am

jimbom30cab wrote:I'm looking for a 3.91 if anyone has one for sale?

PacerPete or some of the traders / breakers on here might have one
Also try FAB Recycling
Have you ever thought of New Quaife LSD from Birds

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Post Fri Dec 19, 2014 2:13 am

The b25 is a slug and will get hosed by most cars these days. even a proper 2.7M20 and basic 24V swap is pretty low down the pecking order with the plethora of turbo engines on the street.

The E30 is not about all out speed its about fun and a balanced package

Post Fri Dec 19, 2014 9:46 am

The E30 is about drifting / oversteer

Post Fri Dec 19, 2014 12:20 pm

Apart from the 325 donk being a larger capacity, isn't it more fuel efficient too?

Post Fri Dec 19, 2014 12:47 pm

pony wrote:The E30 is about drifting / oversteer

Yours might be, mine's about smooth cruising and looking good.

Post Fri Dec 19, 2014 12:49 pm

Yes, it can be when driven gently (I regularly saw 30mpg average out of mine), except that you're more inclined to want to use the extra performance.

Post Fri Dec 19, 2014 1:38 pm

The B25 is more thermally efficient and has lower BSFC - in English, makes more power for the same fuel input.
A B28 is the same jump again.

Comes down to stroke length on these, a B25 is really short and a B20 is F1 short.

Rog, If your 320i has not had a change it should be nearly the shortest one fitted like mine was. 4.something.

The vin number into BMW fans should tell you.

The new diesels are fast not because of their HP, it is simply a result of the "area under the curve".
They make more torque everywhere in the rpm range, torque turns the wheels.

Anyone who has ridden a 125 super sport bike know they are fun and quite fast but totally gutless out of the right rpm range, which is why a lower powered 4 stroke can accelerate nearly as fast.

As for the early/late b25 power figures being the same, the early engine is hobbled by the management. Late management is a start, retrofitting m50Nv stuff really works as a few people have done.

Post Fri Dec 19, 2014 1:51 pm

Fitting later management to early engines won't produce more power

Post Fri Dec 19, 2014 2:16 pm

pony wrote:Fitting later management to early engines won't produce more power

Of course it wont, thats why BMW didnt bother fitting injection/management to M10 engines for example

Post Fri Dec 19, 2014 2:34 pm

Hairy screech said the early engines are hobbled by engine management
Not sure what he means presumably means power is restricted

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Post Fri Dec 19, 2014 5:00 pm

I gather that some M10 engines were fitted with Motronic for the 318i.

The earlier Motronic 1.1 is OK, power delivery is pretty good, but they tend to run a little more 'raucous' than the more refined later Motronic 1.3, due to less memory mapping points, so a cruder curve for ignition timing and fuelling.

You could liken this to the graphics on say, a 1980s ZX Spectrum compared to the graphics on a PC.

Post Fri Dec 19, 2014 6:58 pm

Andyboy wrote:
Brianmoooore wrote:If you're buying one, go for a 3.76. My touring was changed to one of these when the gentle whine from the original diff. suddenly changed to a very loud whine, and although I didn't notice the reduction in acceleration, I did notice the improvement in fuel economy.

Seconded. I had a 3.91§ in a 325i Touring manual (they were all 3.91's iirc) and I replaced it with a 3.64 I got for free. The difference in motorway cruising speeds was very noticeable, both in economy and engine noise.

And they drive so much better for it in all situations i think. I'm a 3.64 diff man myself no matter the body shape. Thats 2500RPM in 5th@70mph. Not bad for a 25 year old car

Post Fri Dec 19, 2014 10:24 pm

Speedtouch wrote:Yes, it can be when driven gently (I regularly saw 30mpg average out of mine), except that you're more inclined to want to use the extra performance.

in city driving the b25 is embarrassingly poor with economy, not that it stoppped me from doing it though....

the b25 is nice and smooth and even compared to many modern cars they are really good. newer ones just seem to have a harshness about them in comparison

Post Sat Dec 20, 2014 12:47 am

Speedtouch wrote:I gather that some M10 engines were fitted with Motronic for the 318i.

No. K Jet up to mid '83 and LE from there.

The only real difference between 1.1 and 1.3 is that 1.3 is set up to run a catalytic converter, having less timing advance and a weaker mixture. This can be rectified with a suitable custom made chip.

Post Sat Dec 20, 2014 12:56 am

3.64 is a good all rounder with 24v and e30 box ... i was very happy with one fitted to the m50 and m52 using both an m40 box and an m42 box as engines stepped up .

i have a question while we are here about the e36 evo six speed box...is it direct or over drive 6th meaning is another 3.64 diff an option ?

Post Sat Dec 20, 2014 7:19 am

Simon13 wrote:
Andyboy wrote:
Brianmoooore wrote:If you're buying one, go for a 3.76. My touring was changed to one of these when the gentle whine from the original diff. suddenly changed to a very loud whine, and although I didn't notice the reduction in acceleration, I did notice the improvement in fuel economy.

Seconded. I had a 3.91§ in a 325i Touring manual (they were all 3.91's iirc) and I replaced it with a 3.64 I got for free. The difference in motorway cruising speeds was very noticeable, both in economy and engine noise.
And they drive so much better for it in all situations i think. I'm a 3.64 diff man myself no matter the body shape. Thats 2500RPM in 5th@70mph. Not bad for a 25 year old car

Me too. My 3:64 with the 2.7 is a very nice cruiser but it still has enough grunt to pull it past that ratio.

Brian, it's actually a 3:73 not a 3:76. 3:73 was the choice Alpina made for their C2 2.7's IIRC (Si could confirm this) and was also the ratio used in Auto Convertibles when new. May also be manuals as well. However, when I went from the 73 to a 64, did I notice much difference? Certainly not enough to worry about.

Post Sat Dec 20, 2014 8:12 am

Doh. Sold my 3.64 to Rav

Always felt the touring was a bit busy at motorway speeds but I can't see me pulling the back end to bits again anytime soon

Post Sat Dec 20, 2014 10:24 am

magpie wrote:3.64 is a good all rounder with 24v and e30 box ... i was very happy with one fitted to the m50 and m52 using both an m40 box and an m42 box as engines stepped up .

i have a question while we are here about the e36 evo six speed box...is it direct or over drive 6th meaning is another 3.64 diff an option ?

Its an overdrive Mags, 5th is direct so its the MPG gear and gets the revs down on the motorway

My e36 B3 with this box and a 2.93LSD is great, 70mph in 6th is a sexy 2200rpm. 90 is around the 3000rpm mark. Which means i get low 30's mpg on a run

The C2's used what ever ratio the donor car had, my 2dr had a 3.64 from factory which i've retained. But my old touring had a 3.91 which i swapped out for a 3.73 iirc

Post Sat Dec 20, 2014 11:32 am

Mick, 6th gear is 0.83. Ratio. So a 3.64 is perfect.

Post Sat Dec 20, 2014 12:02 pm

its not really is it, 3.15 or 3.25 would be good. Even a 3.46 would be too far imo. 3.64 will just make all the gears tight and 1st gear nigh on useless

Post Sat Dec 20, 2014 12:27 pm

Depends what you want Si, I have a 3.73 fitted and its about right for how I want it to drive. 3.64 lsd would be just perfect so am going to fit one in the new year.

I also tried a 3.25 and 3.46 on the same car (same set up as Mick) and yes the cruising revs were lower but it took the edge off the fun factor.

All depends what you want.

Post Sat Dec 20, 2014 12:46 pm

"edge" wtf! You're talking about making it drive like a normal car or turing it into a 18mpg tyre shredder because the gearing is all wrong.

If you want a 6 speed box whats the point of it if you can't use all the gears?

Post Sat Dec 20, 2014 1:10 pm

34mpg on the last tank

Post Sat Dec 20, 2014 1:11 pm

Tyres all look ok

Post Sat Dec 20, 2014 2:07 pm

Is this a track day car Jim?

Post Sat Dec 20, 2014 3:32 pm

No, just for fun weekend driving and the occasional trip to work Dan.

Post Sat Dec 20, 2014 5:44 pm

E46 M3 has a 6 speed box with 5th the direct drive and 6th and over drive.

It has a 3.62 ratio diff and there is a BMW Motorsport 4.10 ratio available which a couple have been fitted at ML.

First a still usable, just.

Post Sat Dec 20, 2014 5:57 pm

E46 wheels/tyres are huge in comparison to E30 wheels which will offset the ratios of the diff/gbox

Post Sat Dec 20, 2014 6:02 pm

DanThe wrote:E46 wheels/tyres are huge in comparison to E30 wheels which will offset the ratios of the diff/gbox

Good point, the circumference is 120mm bigger on the e46 when comparing a 205/50/16 to a 225/40/19.

Post Sat Dec 20, 2014 6:08 pm

I'l be the first to admit the gearing is short and a 3.25 would be ideal for normal driving. With that fitted it was doing just under 2500 rpm at 70mph, there is a pic on my thread.

However, I wanted a pacey e30 which is why I went down the m52 route and a shorter diff adds to the driving fun and the whole point of the build in the first place. I wanted more of a pocket rocket than a motorway mile cruncher.

And actually 1st gear is fine , I use it just like any other car. There is instant torque and power in any gear and cruising at the speed limit in 6th is just under 3k rpm. Mpg is fine, better than an M20 and traction is ok unless being stupid.

Why don't you have you have a drive Si at the next meet and see what you think?

Post Sat Dec 20, 2014 6:23 pm

Right just looked at mine, I have a 4.45.

What kind of driving experience can I expect with this mated to my now B25?

Post Sat Dec 20, 2014 6:24 pm

Hair raising !!

Post Sat Dec 20, 2014 6:26 pm

jimbom30cab wrote:Hair raising !!

In what respect Jim?

Post Sat Dec 20, 2014 6:29 pm

It should give very lively acceleration, at the expense of top speed.

Post Sat Dec 20, 2014 6:29 pm

A brilliant one.
About 3500 rpm at 70 and rubber marks everywhere.
great for around town.

Edit - oh and the ability to pull a dead range rover+ caravan with top box and 4 people in it off a junction like it was nothing special.
Poor guy had no idea how an "old estate" managed that one.