S38b35 motor rebuild

Specific conversations and info for the BMW E28 M5 and M535i.
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Project007
Posts: 2
Joined: Mar 11, 2024 8:17 PM
Location: New Jersey

S38b35 motor rebuild

Post by Project007 »

Hi All,
I am new to the site and would love to get some helpfull information, Im, currently having my s38b35 motor completly rebuilt but would love to get a little more pep.. any recommendations??
I would like to beef up the motor without going tubo or crazy I want to keep it naturally aspirated.. any help looking to get an additional 30 to 50hp

Thx
turbodan
Posts: 9166
Joined: Jan 09, 2007 10:19 PM

Re: S38b35 motor rebuild

Post by turbodan »

Welcome

The best gain with no trade-off would be a set of quality headers. NA tuning generally requires moving the torque curve upward and comes at the cost of midrange power, drivability and fuel economy. Headers are pretty much the only exception other than a good performance chip.

Raising the compression a bit might be worthwhile. This also increases power across the board but you may run into issues with fuel quality. If you build a motor that's not content to run on pump gas I think any gains would be hugely overshadowed by the cost and inconvenience.

The S38 could surely benefit from some port work. There's always something to be had in a mass production engine. Cost/benefit is diminishing here though. This isn't cheap HP.

If you do go a little hotter on cams you can raise the compression a bit to offset the lower dynamic CR. The best way to capitalize on significant changes to the power curve would be with standalone injection. The stock DME will not be ideal for anything other than stock cams, same goes for any off the shelf performance chips. These are tuned to optimize the stock engine and will be suboptimal for anything else.
Project007
Posts: 2
Joined: Mar 11, 2024 8:17 PM
Location: New Jersey

Re: S38b35 motor rebuild

Post by Project007 »

Thanks for the quick responce.

Since this platform is derived from the m88 why can't I just bore the head get compression up to 10.5 add bigger pistons and rings, fit the car with cams from the e34m5 add new headers, Intake and new exhaust.

Does this sound right or am I all over the place?
Wish I could put this in better context not sure if my approach is correct or not...Help!
turbodan
Posts: 9166
Joined: Jan 09, 2007 10:19 PM

Re: S38b35 motor rebuild

Post by turbodan »

The only thing I don't like about boring oversize is the compromise on sealing area between cylinders. In the S38B36 BMW increased the stroke for extra displacement. In the B38 they increased stroke again but only bumped the bore up by 1.2mm. I suppose you could go that far at least, even with the stock stroke you still get a little bump.

What would be really neat would be to adapt the B36 intake system. There are certainly gains there. You could put together B36 cams, the B36 intake, a ~1mm overbore and standalone injection to run it. I believe controlling the intake could be done easily with megasquirt. It would operate a lot like VTEC, simple on/off control by RPM.
szacsi72
Posts: 224
Joined: Nov 13, 2022 3:41 AM
Location: Hungary

Re: S38b35 motor rebuild

Post by szacsi72 »

One tip:

Watch out for the main bearings and keep them in order and clean!
At least in EU, it's almost impossible to find any in 00 size. Even king only sells 0.25 and 0.5 (never ever grind a bmw crankshaft.)
I normally never do this, but - if they are in good shape - I would reuse them.
Or else, you need to buy 14 new shells from BMW, which, at least here, directly from BMW costs 800USD, which is absolute nonsense for a set of mains.
CanadianMiniFan
Posts: 129
Joined: Oct 16, 2011 6:31 PM

Re: S38b35 motor rebuild

Post by CanadianMiniFan »

Why would you never grind a BMW crankshaft. The clearances required are known, it’s standard practice for a whole variety of engines being rebuilt.
turbodan
Posts: 9166
Joined: Jan 09, 2007 10:19 PM

Re: S38b35 motor rebuild

Post by turbodan »

BMW does a fantastic job finishing, polishing and hardening the journals. You would be hard pressed to find a place that will produce result than what you took out of the motor as is.

Like a reground cam, lot of places do it, just not very well.
CanadianMiniFan
Posts: 129
Joined: Oct 16, 2011 6:31 PM

Re: S38b35 motor rebuild

Post by CanadianMiniFan »

I agree theres a lot of machine shops that put out subpar work, but the idea that nobody could grind, polish and harden (nitride?) a crank properly still doesnt click with me. Dont get me wrong, if you have a good crank and bearings, keep it stock, but if you cant get bearings or the crank is scored, worn oval etc, I'd rather have a crank reground undersize than a borderline standard journal. Appreciate your reasoning and reply, not looking for an internet slap fight, just have never heard of somebody recommending to never grind a certain make of crankshaft.
turbodan
Posts: 9166
Joined: Jan 09, 2007 10:19 PM

Re: S38b35 motor rebuild

Post by turbodan »

I generally have an aversion to what I would consider doing too much during a rebuild. Lot of guys go further than is necessary and I don't think it's beneficial, not to mention the additional cost.

In my experience with clapped out e28 engines, if it hasn't spun a bearing the crank should be mint.

I believe they are nitrided at the factory.
EuroShark
Posts: 1477
Joined: Feb 12, 2006 12:00 PM
Location: Colorado

Re: S38b35 motor rebuild

Post by EuroShark »

I have to agree with leaving these BMW cranks alone unless they really need to be worked on. Nitrided or not, they're very sturdy and beautifully finished. I've dealt with machine shops that have given me cranks back missing as much as a thousandth after "polishing", and that's really not great.

As for power: compression, cams, headers, and if possible, modern fuel injection. Bumping compression makes sense if you need to work on the bores and can't reuse the original pistons. Otherwise, it's probably the least bang for the buck on its own. Cams are probably where you'll pick up the most power, even with stock headers, but a better set of pipes (euro or Supersprint) will make the most of the improvement from the cams. Eliminating the AFM and being able to take full control or fueling and ignition will free up the last of the real bottlenecks and allow you to make the most of any mechanical modifications you make.
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