Why is it so hard to find a 525i donor???

[Hofmeister]

I’m fine with the 3.23 for now while I have the stock M50 with a questionable amount of compression, but I do have an S50 waiting to go in, which should bump me to about 250hp with a tune and some bolt-on mods. At that point I think it would make sense to gear down a bit since the S50 has a lot more torque and makes good power throughout the rev range and not just at the top end. My friend’s S50-swapped 525i has a 3.46 and it’s way too short of a gearing for all the highway driving he does.

No, there’s no place in the US where you can cruise at 100 legally, but on an empty interstate where the speed limit is like 80, 100 isn’t even that much of a stretch. There’s a lot of empty space here in the southwest so it doesn’t even feel all that fast.

I have a full bolt on s52 on my car. 3.23 is perfect compromise
Also note, higher rpm doesnt mean more stressed. These cars are cruising at powerband, leading to being able to run slightly leaner and on less throttle than an engine being dragged at 2k. drive around shifting under 2k and then at 3.5k and youll see what i mean. Even with the extra low end torque of the s52 you can tell its not happy down there. That doesnt include having higher oil pressure and other higher rpm related things. Honestly jon has the only 525 in this thread that could justify higher gears and its cause he actually makes big power with it and it would help push his powerband. Honestly, find a gearing calculator and play with the numbers and youll see how little gains you get but just start falling in and out of powerband
besides big power only reason i see for a 2.93 or 3.46 is if you cant find a 323lsd since they are pretty rare.

[5mall5nail5]

I’m fine with the 3.23 for now while I have the stock M50 with a questionable amount of compression, but I do have an S50 waiting to go in, which should bump me to about 250hp with a tune and some bolt-on mods. At that point I think it would make sense to gear down a bit since the S50 has a lot more torque and makes good power throughout the rev range and not just at the top end. My friend’s S50-swapped 525i has a 3.46 and it’s way too short of a gearing for all the highway driving he does.

No, there’s no place in the US where you can cruise at 100 legally, but on an empty interstate where the speed limit is like 80, 100 isn’t even that much of a stretch. There’s a lot of empty space here in the southwest so it doesn’t even feel all that fast.

Having a 700+ RWHP 525i I have never felt the need to have more gear. If you think that ~50 bhp is going require another gear or much taller rear that's all you, but again, I have no problem cruising at, say, 10 over the speed limit, without feeling like I need to lower my RPM. But that's why it's your car not mine

I have a 3.07 and 0.85 top gear in my 525iT, equivalent to a 2.61 with a non-overdrive gearbox. It has no trouble keeping speed on all but the steepest highway climbs, and it's fine around town, though not what I'd call quick. The car isn't meant for city driving anyway - suspension is tuned for the highway, too many idiots around, should be walking or biking in the first place, aso. 3.23 was put in to meet the expectations of drivers who wanted overtaking power without downshifting. I have the option to do that - stay in fourth or fifth - as well as the option to have a relaxed and efficient cruise. Those with a 3.23 highest gear have only the former option.

As for "legality", all that means is that the people with guns and badges approve of it, or don't. If they aren't around to see it, legality is meaningless. If it harms no one, then I don't care.

3.23 is just a happy medium. The gearbox is gears for acceleration < 3rd gear, overall. If I had a 6 speed transmission behind a ~192hp engine I would want extremely close gear ratios personally. A single overdrive gear is fine, but then having a 3.07 rear as well? The car could be a lot more zippy AND have overdrive, but again, to each their own. I have never felt the need to lower my cruising RPM and I have about 8 differentials and ring gears from 2.93 - 3.73 to choose from.

[dannyzabolotny]

Having a 700+ RWHP 525i I have never felt the need to have more gear. If you think that ~50 bhp is going require another gear or much taller rear that's all you, but again, I have no problem cruising at, say, 10 over the speed limit, without feeling like I need to lower my RPM. But that's why it's your car not mine

I guess for me it’s just cheapness— I figure if I can cruise at a lower RPM I can get better fuel economy. And cruising at a lower RPM would also put less wear on the engine and transmission, wouldn’t it? Most cars nowadays have an overdrive gear, even ones that have less power than a stock M50.

[moroza]

Ehh, simply comparing costs, it's hard to make a case for changing the diff for better economy, unless you're changing it anyway and/or the taller diff and the installation are free. That said, I consider fuel economy a benefit in its own right.

It's about options: taller gearing, especially with a close-ratio box like the 250G or 310/320Z, gives you the option of cruising or the option of having passing power on tap. Shorter gearing forces the latter, doesn't give you a choice.

Powerband is not a one-dimensional factor. Look at a BSFC map and you'll see that energy efficiency depends on percentage of load as well as RPM. Peak efficiency (lowest BSFC) is ~80% load at peak torque, but for most engines, including the M50, that results in far more power than you need for cruising. I don't have a BSFC map for any BMW engine, but looking at comparable 4V petrol engines, for RPMs around 2-4k, and for any given load below ~90-95%, lower RPM will give lower BSFC.

Wear at high load and low RPM vs lower load and high RPM? Good question. Clearly, either extreme - WOT at idle or anything at redline - will wear more than a middle ground. That middle ground has a ton of variables and it's hard to come up with a clear answer. That said, the loads required for cruising - 20-40hp - are so low that it doesn't matter, if you account just for cruising. Of course, you have to account for reserve power unless you want to downshift on the slightest hill. In that case, as long as RPMs are above where it's safe to give WOT (1500-2000 or so), that's an acceptable thing to do. In real-world use, my 525iT cruising in sixth almost never needs more than 50-60% throttle.

[sklatigre]

I just finished a 5speed swap on my 525i. There's a guy on craigslist that has a 525i 5 speed for sale but you're gonna need to piece the rest together unless he has everything else for it. Best bet would be to look for a wrecked one or get that one off of FB Marketplace

https://losangeles.craigslist.org/la...162775402.html

[5mall5nail5]

I guess for me it’s just cheapness— I figure if I can cruise at a lower RPM I can get better fuel economy. And cruising at a lower RPM would also put less wear on the engine and transmission, wouldn’t it? Most cars nowadays have an overdrive gear, even ones that have less power than a stock M50.

Just remember - modern engines/transmissions are a LOT different. Oil pressure at 1800 RPM vs 2500 RPM can be a measurable difference in longevity. Some cars have very advanced systems that regulate oil pressure based on different conditions. They also decouple the alternator. Also, most cars that are cruising at ultra low RPM are still producing very respectable power to do so because of direct injection and inherent advantages. They even go so far as to let go of the torque converter/clutch much more frequently (called sailing). It's not just RPM.

[Hofmeister]

Just remember - modern engines/transmissions are a LOT different. Oil pressure at 1800 RPM vs 2500 RPM can be a measurable difference in longevity. Some cars have very advanced systems that regulate oil pressure based on different conditions. They also decouple the alternator. Also, most cars that are cruising at ultra low RPM are still producing very respectable power to do so because of direct injection and inherent advantages. They even go so far as to let go of the torque converter/clutch much more frequently (called sailing). It's not just RPM.

most of those cars also have small turbos to help with "passing power". idk i get 28mpg at 75mph pretty easily with my 3.23

[5mall5nail5]

most of those cars also have small turbos to help with "passing power". idk i get 28mpg at 75mph pretty easily with my 3.23

Yep, same. I think 3.23, our tire size, and 1:1 5th is perfect personally.

[m60power]

Yep!

My dealer is awesome. I won't even tell you what they gave me - it was ridiculous. I mean, at the end of the day they're just pulling one number up and another down. But still.

BTW not my turbo car. This was my '95 525i daily.

Wowsers! I was just up in Doylestown a week ago at Thompson BMW, and one of their customers had a super clean black E34 525i in for whatever reason. It was lowered on nice M-parallels. Couldn't stop staring at it among the sea of new BMWs on their lot. It really stood out. Asked the tech if it was for sale and sadly it wasn't.

[5mall5nail5]

Wowsers! I was just up in Doylestown a week ago at Thompson BMW, and one of their customers had a super clean black E34 525i in for whatever reason. It was lowered on nice M-parallels. Couldn't stop staring at it among the sea of new BMWs on their lot. It really stood out. Asked the tech if it was for sale and sadly it wasn't.

They took my '95 525i 5spd w/ LSD and 252k+ miles lol

I like Thompson a lot.

[m60power]

They took my '95 525i 5spd w/ LSD and 252k+ miles lol

I like Thompson a lot.

Haha awesome! This particular car was an automatic, 'narrow' grill pre-1995 model year.

Yeah they are a great dealership.