I was curious if you had any idea about the ability of the stock S38b36 crankshaft to take higher rpms such as a 8000rpm redline. Im not asking if the stock engine can do this as I know there will be issues with the bearings valve springs, etc. I am asking because I am about to begin an engine rebuild project and am curious to know if the stock crankshaft from the 3.6 if knife edged (mainly to reduce some mass) can withstand being spun to 8000rpm with peak power coming in around 7500rpm. IIRC the crankshaft is forged and with the other components of the engine sorted and balanced correctly do you think there would be an issue with the crankshaft or woodruff key at these rpms and yes the engine must last past 4,000 miles preferably around 150,000 and I dont see how this could be an issue if built correctly and properly maintained. I would ask this question on m5board.com to Farrel as he seems to have a good understanding of the s38 but for some reason I cant post after I registered so I have been stuck simply reading as much as I can, not nescessarily a bad thing. Thanks for your input im all ears!

The total harmonics do not allow for such a lofty RPM. It will 7600 for SHORT periods. By reducing mass and extremly fine ballance it may hold but not for extended time.

Unesential is correct. It cannot be done. The reciprocating mass of the engine design does not allow for such high revving figures no matter what you do. If you want to rev that high, it is more feasible to consider and engine transplant of an engine designed to do that work.

Ok this is what I was looking for, kinda but are you telling me that the engine itself cannot take this because it will hit its harmonic frequency and blow itself a part? If so I understand the STOCK engine cant do this. Or are you telling me that the crankshaft itself cannot take the rpm because it will reach its natural frequency or are you referring to the combined mass of the connecting rods, crankshaft, pistons, flywheel, and harmonic balancer (vibration dampner)? If so again I stated I am refering to the crankshaft itself not to the complete reciprocating mass as I understand these parts will ahve to be replaced. I would think if the stock crankshaft cant be lightened enough to reach the rpms I stated because of lack of material or for some other reason a billet crankshaft could be made that would be lighter and more finely balanced is this not true? And having the other components as light as possible and more importantly the vibration dampner matched to the frequency induced by the engine at the stated rpms I dont see a reason that the engine could not see these rpms reliably?

Again Im not looking to spin the stock engine in any form to 8000rpm I am getting ready to begin a performance engine rebuild that will be my replacement engine for the 93 M5 and I have the resources to do it right (or have someone else do it right as I dont have the time to do the rebuild all by myself) and I am just trying to get some opinions on the stock crankshaft and its ability to take the rpm with a new dampner if nescessary and by no means am I trying to say you both are wrong just trying to work through ways of accomplishing my goal with u both. As for an engine transplant I would never even consider doing it I would just buy another car, haha which after this thread and the amount of money I will be spending some would see as the best alternative but for some reason I love the rawness of the s38 and the overall understated looks and grace of the e34 m5. By the way puppypilgrim I love the B10!

Bump!

Well thanks everyone but I got my answers I needed and an engine builder. Thanks!

Talk to Andreas at PPF if you ever get stuck.

He seems to spend most of his days building street 1000hp S38's for crazy scandinavians to drive in the snow.

There's guys on the FI forum running 8000rpm on M10 engines, I'm sure the S38 can take it.

I found this funny yesterday, apparently a new S38b38 from BMW is about $18K!

Again Im not looking to spin the stock engine in any form to 8000rpm I am getting ready to begin a performance engine rebuild that will be my replacement engine for the 93 M5 ...but for some reason I love the rawness of the s38 and the overall understated looks and grace of the e34 m5. By the way puppypilgrim I love the B10!

Seems to be a contradiction. IF you intend on building a new engine for whatever purpose then you no longer have your S38 as it will now be whatever you have made. I am sorry that I could not help further. It is possible to accomplish many things when redesigning.

As unesential stated, you will incur primary harmonic resonance at 7600rpm with the stock crankshaft. The crank is a forged steel item, but I have no idea how it takes to knife-edging and other techniques for weight reduction and at what rpm the crank would undergo primary harmonic resonance. A vibration damper tuned to meet this resonance wont help because the resonance will overcome the damper.

Talk to Andreas at PPF if you ever get stuck.

He seems to spend most of his days building street 1000hp S38's for crazy scandinavians to drive in the snow.

There's guys on the FI forum running 8000rpm on M10 engines, I'm sure the S38 can take it.

I found this funny yesterday, apparently a new S38b38 from BMW is about $18K!

M10 is an inline 4 whereas the S38 is an inline 6. The crankshaft of the S38 is thus roughly 50% longer and this is where the problem arises.

B38s are still available from BMW? They used to be, but I thought the supply was exhausted about 3 years ago.

M10 is an inline 4 whereas the S38 is an inline 6. The crankshaft of the S38 is thus roughly 50% longer and this is where the problem arises.

B38s are still available from BMW? They used to be, but I thought the supply was exhausted about 3 years ago.

I was thinking more of the bottom end not the top :). I was joking anyway since really high strung engines need rebuilding very often it hardly meets his criteria plus I think he's going forced induction so won't need the wild rev limit.

I read on a forum yesterday a guy in the UK had enquired about a new engine and he was quoted £9k and £4200 for a 6spd gearbox. I guess they could be rebuilding you one and call it new since they have various parts lying around.

De witt and unesential thanks for your input as always I figured a way out through the problem. But I do have one more question how did you come to find out the resonance specifically for the s38 was there an issue with these engines before cracking the crank or what? I take it the crankshaft for the S54 is quite a bit shorter than the S38 as it is based on the baby sixes and that is one of the reasons it is capable of revving to its higher rpm? Just interested in this subject, thanks again.

It is fairly common to hear of unlimited chipped motors self destructingabove the aforementioned RPMs. Fahey actualy made a repair sleeve for the end of the cranks where the balancer would shear off the woodruff key. So yes there is a histoy of these problems. You also need to remeber that this motor is based on an older block design, it has been around for some time and is race developed already. Other issues to be concerned with when increasing your output are the cooling of the heads.

I have actually been in a E34 M5 when the woodruff key decided it had had enough the vibration damper worked its way loose and the postion sensor couldnt get a reading, I would say that this is the least that could happen. Do you think it would be possible to redesign components for this area or is it possible to use Frank Faheys repair kit to address this problem before it ever surfaces, I was kind of under the impression that it was more of an M88 too S38B35 problem. I understand the engine was race developed and that it is an older design and I respect that, it is one of the only true M engines out there that was developed to go racing. I also understand that with increased output comes increased heat. I also know that the S38B38 head had the cooling ability increased some how, did they simply enlarge the coolant passages? I am already planning on running the tropical water pump and pulley for the smaller diameter to get more coolant circulation. I still havent figured a way to directly address the head, any suggestions? Also at anytime if you need more information about what I am setting out to do let me know. Im still not sold on going with a billet crank for weight savings as I trust the BMW forged crank more and I would rather limit the rpm than risk having the crank go boom at the wrong time. I was told by DefactoM6 to contact Chris Stickley to do the engine rebuild and I will have to see what his opinion is of increasing the rpm limit and if it is worth it reliability wise, again 50 horsepower and a broader power band are easy to give up if it means the engine will last longer. I am planning on having the engine completely balanced but again if the S38 has problems at 7600rpm or so do to design constraints then it most likely wont do anything towards gaining rpm. Do you think it would be wise to talk things over with Fahey to get his input on options as well? Thanks again.

I was thinking more of the bottom end not the top :). I was joking anyway since really high strung engines need rebuilding very often it hardly meets his criteria plus I think he's going forced induction so won't need the wild rev limit.

I read on a forum yesterday a guy in the UK had enquired about a new engine and he was quoted £9k and £4200 for a 6spd gearbox. I guess they could be rebuilding you one and call it new since they have various parts lying around.

Well the M10 has been a proven powerhouse when turbocharged, the greatest BMW engine other than the S72/3 is the M10/13. Paul Rosche looked 2002 owners with well seasoned engines much like aged meat, and used those for the M10/13. Seemed to be pretty reliable, for a 1200+hp engine displacing only 1.5l nominal, I mean, reliable for a few hours operating time isnt bad, right?


have actually been in a E34 M5 when the woodruff key decided it had had enough the vibration damper worked its way loose and the postion sensor couldnt get a reading, I would say that this is the least that could happen. Do you think it would be possible to redesign components for this area or is it possible to use Frank Faheys repair kit to address this problem before it ever surfaces, I was kind of under the impression that it was more of an M88 too S38B35 problem. I understand the engine was race developed and that it is an older design and I respect that, it is one of the only true M engines out there that was developed to go racing. I also understand that with increased output comes increased heat. I also know that the S38B38 head had the cooling ability increased some how, did they simply enlarge the coolant passages? I am already planning on running the tropical water pump and pulley for the smaller diameter to get more coolant circulation. I still havent figured a way to directly address the head, any suggestions? Also at anytime if you need more information about what I am setting out to do let me know. Im still not sold on going with a billet crank for weight savings as I trust the BMW forged crank more and I would rather limit the rpm than risk having the crank go boom at the wrong time. I was told by DefactoM6 to contact Chris Stickley to do the engine rebuild and I will have to see what his opinion is of increasing the rpm limit and if it is worth it reliability wise, again 50 horsepower and a broader power band are easy to give up if it means the engine will last longer. I am planning on having the engine completely balanced but again if the S38 has problems at 7600rpm or so do to design constraints then it most likely wont do anything towards gaining rpm. Do you think it would be wise to talk things over with Fahey to get his input on options as well? Thanks again.

I would say that the problem with the crank key is mainly a B35 and M88/3 problem, but that was because they basically used inferior steel and did not use any processes to increase the strength of the steel. One guy in an article I think in the OCT 98 issue of Bimmer Mag called it basically "butter knife steel."

The problem is that once the long crankshaft just has so much rpms and all of those forces acting upon it etc, it doesnt matter what the key is made out of, whether it be a 50 rockwell A tested steel or made from diamonds (j/k), the crankshaft will snap in the middle.

This is approaching the extent of my knowledge of possible outcomes with regards to what you are proposing. DefactoM6 has pushed the envelope with his B35 and basically created a B38 that revs like crazy and dumps fuel into the exhaust manifold with every shift. Fahey is a solid name and an expert. Korman's knowledge primarily lies with the M10 later iterations such as teh S14, but the basics of the S14 are the same as with the S38 M88 etc.

As far as cooling goes, there are a few factors to consider, but mainly: Altitude. CO Spgs is what, 6000ft elevation on average? Air density is reduced, the capabilities of a radiator are reduced, but you dont creat as much HP. So there is an analysis to be considered. Idle hp is the biggest problem. I have an FFR kit cobra that has a 450hp 347ci (5.3-5.4L) engine, it runs a massive aluminum radiator and an electric puller fan. No clutch actuated fan accessory. I do not run an oil cooler and the engine only has a problem in stop and go traffic conditions. As long as it doesnt get too much above 90-100C water temp, im good. Oil temps, eh. It runs normal.


Moral: talk to fahey, at least I would. I did my own work on rebuilding my S38s, but when it comes to modifications, thats a different story. The only mod I did was install an aftermarket Autronic DME on my AFM S38B35....now it runs like its got 20 more hp. Whether it does, thats up for debate.