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Hi all. My 91 M5 has been losing a LOT of coolant without any signs of internal or external leaks. Chris (bmwdirtracer) has helped me troubleshoot/diagnose the issue multiple times with leak down tests, pressure tests, etc and we still haven't been able to find anything. Previous owner had overheating problems that were "cured" with a new radiator, but I'm beginning to think either the head warped or the head gasket is going (and perhaps evades leak down test while engine isn't under load).

Long story short, I'm going to do a top end rebuild, possibly doing rings and conrod bearings. I have heard a lot of conflicting advice whether or not to go OE for the valves and bearings vs getting VAC performance valves/ VAC performance conrod bearings. Any further advice would be appreciated!
 

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I'm going to temporarily sit this one out, and allow all the extremely knowledgeable guys at this forum to post their thoughts, because I want to do the best possible, for my very good friend, and magnificent driver, bavarianstig. This car is not used on track; Michael has a 135 for that. But he's an extremely accomplished driver. The M5 has ~150k miles, Michael wants another 150k miles without further major work beyond this rebuild.

Please, everyone, research first, and then give your best advice. He deserves our best, although he's new to the forum. I'll be redoing this motor.
 

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Hi all. My 91 M5 has been losing a LOT of coolant without any signs of internal or external leaks. Chris (bmwdirtracer) has helped me troubleshoot/diagnose the issue multiple times with leak down tests, pressure tests, etc and we still haven't been able to find anything. Previous owner had overheating problems that were "cured" with a new radiator, but I'm beginning to think either the head warped or the head gasket is going (and perhaps evades leak down test while engine isn't under load).

Long story short, I'm going to do a top end rebuild, possibly doing rings and conrod bearings. I have heard a lot of conflicting advice whether or not to go OE for the valves and bearings vs getting VAC performance valves/ VAC performance conrod bearings. Any further advice would be appreciated!
I can't comment on VAC vs OE

+1 on the head gasket / slightly warped head.
 

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A good standard well built S38 is a great engine .






















That said , technology and precision / tolerances have improved since the original engine was designed and well manufactured aftermarket pistons , rods , valves , MLS Head Gaskets , injectors etc are now all available at reduced costs .










I would ask whether I want the M5 to be a good solid original example i.e. a Classic , or if I am seeking out and out performance .
 

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I agree with David... but one thing i do not recommend.. and that is MLS,,, if that is used,, the block surface must be 100%,,,,,

S38 is just an engine,, and not complexed at all,, timing in the cams is dead easy.. and they are both 264° (( the same also on B38))

i would say,, either a cracked head or blown HG ,, do not drive the car at all IMO...

when working on the engine ,, take it out of the engine bay,, all access when working on,, around etc etc is million times easier ,,

I would recommend a full gasket.. change everything,, even the bearings,, they are not expensive,, clean it up ,, new water-pump,, new thermostat,, just do it once..

i know it cost more,, but it is idiotic to have the chance and opportunity ,, when everything is open not to do it well

the car is what ,, close to 25 year old,, and i am sure it is a wise idea to change all the gasket and seals,,

the S38 is a strong robust and well built engine... can last forever with good maintenance ,, regular oil change ..etc etc

OT,,,

i have a picture thread here of my B38 re-build,, and almost all mechanical things are the same
alpina.123.is : S38B38 Engine rebuild | 31.08.2013

B36 do not have oil cooled piston from underneath,, as the B38
 

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Agree with David and Sveinbjörn. I would definitely use VAC parts, but keep the factory specs as much as possible. There is something to be said for a stroker or a VAC "stage 3" head or whatever, but factory specs with current technology available in the aftermarket is how I would proceed if I were in your position.


As already mentioned, this is an opportunity to replace/refurbish parts on the bottom end, I would replace the timing chain guides, overhaul the intake system o rings and so on. The bill will add up as it always does, but it is an investment in long term reliability and driving enjoyment.
 

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I'd like to ask the opinions of you very erudite gentlemen, in Michael's behalf:

Without any sign of trouble from any other item, which items would you replace, prophylactically?

Valves? Guides? Springs? Retainers?
Oil Pump?
Pistons, with Bore, or just hone and rings?

Of course, I have my own thoughts....but the collective wisdom of this forum's best is a magnificent resource to draw from, when I want to do the best job possible for Michael, without unnecessarily spending his money. Given the cost of S38 items, and the very high quality of the original parts, I don't want to "throw" parts at the engine, if they won't make it better and more reliable.

I have advocated: Rings with a quick hone, conrod and main bearings and bolts, full gasket & seal set, head to excellent machine shop, 3-angle valve job, BMW labeled water pump, new valve keepers, timing chain and rails, Euro S50B32 tensioner.

Many thanks for your thoughts, my friends!

Chris
 

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I would add an oil pump to the list, as well as send the injectors out to be tested and refurbished if necessary, and also to replace all vacuum lines and intake system gaskets and seals while the head is under construction.

As for the pistons, rings and bearings, I am also very interested in the opinions of the community. I was recently at the Tokyo Motor Show and was able to see Mahle's exhibit and their new piston designs. I wonder if any are applicable to the S38, and how much they would cost relative to an OE set. I have also been told that custom Wiseco pistons are a cheaper alternative and can offer improvements over stock. I am far from being in a position to understand the pros and cons, so I am very interested to hear what you all think. Experienced engine builders please continue to chime in.
 

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I bought Forged pistons from VAC,, CP made... the cost was 1100 us$

just to mention ,, the gain vs oem pistons,,

is that B38 is on realoem 2800 us$...

CP is more than 50% cheaper,, and the weight difference is 100 gram per piston,,, that is a LOT in rotating mass 600 gram 6 pistons 100gram i 0.353 pound so 1.5 pounds in saving

i also bought ARP.. studs,, and i do also recommend that.. but S38 is a steel block so it is NOT a MUST,, but better,, if the block was an aluminum then it is a must IMO

i would buy all gasket as Victor Reinz or similar oem,,,

main bearings are the same in ALL IL6 Bigblock,,

rod bearings are the same in

S14,,
M88,, EURO E24 and E28
S38B35 US E24/E28
B36

B38 have special rod bearings,, ONLY oem and the price is mind blowing

Oil pump.... no, i would not change it,, it is solid and can last forever,, but VAC offers a solution for higher pressure,, if you want

but all rails ,, yes,, and the timing chain... NO. it is double on S38 and top quality,, if the engine is on oem rev limit then there is no reason IMO

tensioner,, well some say yes,, but it is better to have a new i agree on that,, rotor and cap,, ONLY BOSCH,,, others will start to misfire etc etc after some time ,, i have BAD experience with other brands

Clean suround is the most important thing...,, just like a surgery room,,

OT,, i glasblasted some things and it looked just like new, you can paint also those things you want,, and be individual (( i did back in the days)) and that can be cool if the color combo is what you like most !!

but good luck,, and it will be nice to see what you will do

regards
 

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Others know more than me about the tech, but if one wants 150k miles post rebuild I think there is a very strong argument for stock specs, or even OEM parts. I suspect there are very few non stock spec engines that have done 150k miles by now, so I doubt what evidence there is to prove longevity.


(my point is not that aftermarket engines blow up - there is just such a small dataset its impossible to prove anything).


All the best with it.
 

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Others know more than me about the tech, but if one wants 150k miles post rebuild I think there is a very strong argument for stock specs, or even OEM parts. I suspect there are very few non stock spec engines that have done 150k miles by now, so I doubt what evidence there is to prove longevity.


(my point is not that aftermarket engines blow up - there is just such a small dataset its impossible to prove anything).


All the best with it.

Great discussion, you bring up pertinent points :) For me, the visible or memorable S38 builds are strokers or turbos, where even the latest and greatest in the aftermarket are pushed. I think that is where part of the skepticism? in the aftermarket comes into play.

The M88/S38 is a special engine for sure, but the miasma of the bavarian engineer-gods just has to be wafted away from time to time. There are some unique features, but it in reality there isn't anything too unique that makes it incomparable to other engines of other makes. If factory specs are maintained, there should be no reason for aftermarket pistons to go the distance of the OE piston, or more. If you expand beyond the S38 and look at the longevity/reliability produced by, say, CP pistons in other engine applications, the record is outstanding.
 

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As there have been some discussions about pistons,,,

The ONLY reason.. for recommending other pistons ,, like Wiseco CP JE et,cetc vs oem is the cost between oem and aftermarket,,

also it is no question that the aftermarket pistons are a better quality
in almost all cases as they are FORGED material,,
 

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My engine was rebuilt by a good buddy (and the previous owner) @ 220,000 chassis miles (blown head gasket). We're both convinced the drivetrain was replaced at some point, but the records that far back are lost, and we don't know how many miles were on the engine. Regardless, it just needed a comprehensive top-end. The cylinders, etc, were still good as new. He replaced all of the top-end wearables mentioned above, added the S50B32 tensioner, etc etc, but for the bottom end I think he just poked around, checked torque, and resealed. He even bought a couple VAC parts, like bearings IIRC, but ended up selling them when the bottom end proved solid.

The thing rips at 5280 ft, though I'll soon get to try it at sea-level this Thanksgiving. Over the last 5,000 miles I've only had to top it off one quart, and have netted 25mpg hauling stuff and azz in the mountains. By all accounts it's a healthy engine, but I wish I knew the actual mileage.

I think at 150k documented miles, you can do the top end (still an easy $5k bill DIY) and just do a health check on the bottom, and probably net another 50-100k miles on the stock bottom end. Doing the bottom end at the same time as the top is obviously easier, the engine being out, etc, and should actually be cheaper than the top-end unless you go crazy stroker.

At first I was bummed my friend didn't performance build the bottom, but one thing I (theoretically) dislike about S38 strokers (at least most of them) is that one definitely trades a bit of the high-revving nature for added low end torque. If I want more torque down low, I'll go for a V8 or turbo--I'd prefer to keep and even magnify the addictive S38 rush to redline, and add torque up high without altering the power band.

Plus it may just be the internet, but it seems strokers (and even factory B38s) have more issues. YMMV.
 

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Others know more than me about the tech, but if one wants 150k miles post rebuild I think there is a very strong argument for stock specs, or even OEM parts. I suspect there are very few non stock spec engines that have done 150k miles by now, so I doubt what evidence there is to prove longevity.


(my point is not that aftermarket engines blow up - there is just such a small dataset its impossible to prove anything).


All the best with it.

Au contrare.

The factory pistons and parts are a compromise. Cast Pistons are fragile.

Forged aftermarket parts will last MUCH longer than a factory s38 assuming all assembly and other aspects are equal.

The s38 was only meant to last 100k miles....

They almost NEVER make it past 180k if they're used as they were meant to.



Sent from my iPhone using Tapatalk
 

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Some S38b38 engine parts that are NLA now like the crankshaft! No short engines can be bought now either

Who said an S38 was only meant to last 100k! thats cobblers
 

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Yeah, me too. While I'm sorry to hear that this masterpiece engine uses cast pistons, I am quite sure that Motorsport division had absolutely no "planned obsolescence" in mind when they built these motors. After all, they are a magnificent modification to the M30, which, in my opinion, is the most durable engine BMW ever made.

Michael's car is coming to me on Wednesday, to begin the rebuild referred to here. Now that you guys have corrected my misconception that the S38 had forged pistons, I kind of want to talk Michael into some Wisecos or CPs.....what's your thought, Michael?

https://store.vacmotorsports.com/cp-pistons-bmw-s38-piston-kit-35-36---38-l-p1290.aspx
 

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I actually believe that the stock pistons are high quality items, they also have steel inserts in them.
If they are within spec and check out OK I would re ring them (order rings directly at Mahle).
 

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if i had the money and was rebuilding my 3.8 id have some funky JE pistons and arrow lightweight rods or similar to replace stock, a nice lightweight flywheel and the whole balanced. youd probably save 100grams per pistion and the rods then the saving of the fly would at a guess shave 8 maybe 10kgs off the rotating mass on the bottom end. Which on this sort of enigne would no doubt transform it.

I did this a few years ago with a stock M20 B25 i had. I fitted an M52B25 crank and rods from an M52b28 and a light fly. The rods where 100 grams lighter, the crank about 6 kgs and the fly 3kg over stock parts. I kept the stock lawnmower spec camshaft and a BBR starchip and it was magic! it ran and idled like stock you'd of never know if you weren't familar with how a bone stock engine drove
 
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