Any of you know of a owner that has toyed around with and actually performed a conversion on their early model e31 with the M70 w/4 spd auto to a M73 w/5 spd auto?

Lot s of work, But Doable, On RF, The guy that go's by 8Sumthing put a S70 in his!

:eek: Doable? Yes, but you better refrain... ...or use the latest V12-version of E65. Doable too.

I wonder if the N73B60 engine can mount up to a 6spd? I would be a really awesome conversion on a 850i, but is it not really high compression and they have something like a direct injection to them that is different from the rest of the bmw v12 engines?

I wonder if the N73B60 engine can mount up to a 6spd? I would be a really awesome conversion on a 850i, but is it not really high compression and they have something like a direct injection to them that is different from the rest of the bmw v12 engines?

Actually...11.3:1 compression..pretty high IMO. Won't fit. Oil pan a huge issue. Plus its wider. Killer motor though. I would take the 7 speed over the 6.

I was wondering for us m70 guys, if we rebuild an engine, wouldn't it make sense to use am73 crank, it would be cheaper than trying to find a s70 crank, or can you just swap the cranks?

It's not that easy, If it has a bigger stroke, You will need shorter rods or pistons to keep them from banging into the valves, Now! If you do all that, Might as well go a Liter up. What i'm saying is if you get that involved with the tare down, It will be a waiste to go for 20-50Hp, I would say 150 more atlease, I would! If the heads and the crank come off, I would squeeze out the most that the block could handle.

I was wondering for us m70 guys, if we rebuild an engine, wouldn't it make sense to use am73 crank, it would be cheaper than trying to find a s70 crank, or can you just swap the cranks?

more involved than that...

As was said by 8eights and TxGR8White, far more complicated. BMW has made changes to their engines that prohibit them or parts in a lot of instances that they are not backward compatible.

The crank from a M73 will not "fit" into the M70 for a lot more reasons, some could be over come but at a lot of effort. eg. The crank nose (snout) of the M73 is completely different then the snout of the M70, it would impossible to use the vibration damper from the m70 on the M73 crank and the damper is where the engine DME gets its rotational timing from.

In reality, the M70 or the M73 is not a rebuildable engine, yes one can replace pistons but you are taking a chance that they will not seal. The cylinders can not be bored out, only a light scuff is possible, and as was said, you would need shorter rods to keep the pistons from going out the top of the cylinder bores,and even if you could find shorter rods, then the bottoms of the stock pistons would hit the counterweights of the crank!

I could go on and on, and it has been investigated long ago, the bottom line is, don't even think about it! If you want a M73 powered E31, sell yours and by a one. Nothing on a V-12 re-do would be cheap.

IMHO, Pure waste of money. Go buy a 96+ 850Ci instead.

The intent of this thread is removing the m70 and placing the m73 instead. Modifying a m70 with m73 internals is a waste of time. As an example, taking a 305 out of a Camaro and putting in a 350.

The intent of this thread is removing the m70 and placing the m73 instead. Modifying a m70 with m73 internals is a waste of time. As an example, taking a 305 out of a Camaro and putting in a 350.


All you need is the engine, transmission, electronics, exhaust (partial), driveshaft. Need to make sure the motor has ALL of the goodies on it (knock sensors, air injection, etc.)
Alot of work for little gain - better off buying a M73 equipped car in the first place (probably cheaper in the long run).

*very similar to those that spend big $$$ to make the 8 go "quicker", take that money and buy a used 'Vette....

I have the engine the trans and computers. I have the drive line and the exhaust.Here's what I don't have........ the wiring harness/s. Here's the deal, I have a M73 ready to rock and roll. I also have a M70 with restoration needs that is in the car now. I want to yank it out and put in the M73 so the car can be driven. The M70 wiring difference is on the cooling system and the exhaust, everything else is the same. As far as the transmission, Im not sure about the wiring but the driveshaft is different but again I have all that.

As far as vettes...I had a 87 Callaway...80's are over, comfort is in.

Why do you think some of us are drawn so intently to the 850csi. For some reason even though I have a very nice one, a decent one, and many other nice cars, I still want the euro version csi very badly. Thought about buying a nice M6 but for some reason the 850csi is like an enigma. Some days it drives me a little nuts,... why do I, and many others even care. I could buy a nice corvette, or viper, but why do I .. many others always come back to the 850csi. They're not even all that fast. One of those things that make you go HMMM. Anyone?? Buler?

As far as vettes...I had a 87 Callaway...80's are over, comfort is in

Great car, but no comparison to the C7 in terms of ride comfort.

Anyone?? Buler?

850twin, make yourself 850triplet!

http://www.simplyluxuryimports.com/webtemplate.aspx?iid=405042

I have the engine the trans and computers. I have the drive line and the exhaust.

I think if you listen to the folks here, you (should) realize that you've done the easy part - making the parts work properly will be a nightmare. The only (repeat "only") reason to do this is because you really (really) like wrenching and see it as an end in and of itself. Nothing wrong with that, but even if you do get it working right you'll still end up with something that's worth just about zilch.
If you think that you're a) saving money or b) going to have something really special I'm afraid your'e mistaken.

I want to yank it out and put in the M73 so the car can be driven. It would be far (far) easier and quicker to simply pull the M70, do whatever work it needs and put it back in.
If you're intent on using the M73 bits, keep your eyes peeled for a later car needing the same treatment as your old one

The M70 wiring difference is on the cooling system and the exhaust, everything else is the same. As far as the transmission, Im not sure about the wiring but the driveshaft is different but again I have all that.

Totally wrong! The electronics is all different, the DK motors on the M73 are stepper motors whereas the DK on the M70 are analog. The DMEs and the EML are different. Does the engine harness/transmission of the M73 plug into the early model E31? don't know, I am sure there are some differences. If your M73 came out of a '96 or later, it would be programmed for OBD II, it is going to have fuel evaporation controls in the DMEs and wiring + sensors would need to be added or you will get a check engine light. SMOG laws in your neck of the woods?

This would not be a plug and play conversion for sure. Even if you do all your own work, it will be a lot of $$ for stuff that you haven't thought of.

On the other board you mentioned someone replaced the M70 with a S70, I was involved in that and it does work because the S70 electronics is the same as the M70 in all respects except the "K" switch.

4 speed auto to 5 speed auto will have to be integrated into the body electronics as well.

I still go along with the previous suggestion, sell yours and get a later model with the M73 and then you will have a spare engine and trans.

Totally wrong! The electronics is all different, the DK motors on the M73 are stepper motors whereas the DK on the M70 are analog. The DMEs and the EML are different. Does the engine harness/transmission of the M73 plug into the early model E31? don't know, I am sure there are some differences. If your M73 came out of a '96 or later, it would be programmed for OBD II, it is going to have fuel evaporation controls in the DMEs and wiring + sensors would need to be added or you will get a check engine light. SMOG laws in your neck of the woods?

This would not be a plug and play conversion for sure. Even if you do all your own work, it will be a lot of $$ for stuff that you haven't thought of.

On the other board you mentioned someone replaced the M70 with a S70, I was involved in that and it does work because the S70 electronics is the same as the M70 in all respects except the "K" switch.

4 speed auto to 5 speed auto will have to be integrated into the body electronics as well.

I still go along with the previous suggestion, sell yours and get a later model with the M73 and then you will have a spare engine and trans.

I said wiring difference. I know that the dk motors are different.I know they have 4 02's ...etc.Question is about the wiring. Its a 95 OBD1. Since you were involved in a project of this sort did the guy re-use his harness from the M70 to make his S70 work or did he purchase the S70 harness. Thats all I need to know. I'm not using anything(no electronics or computers) from the M70. As far as the trans, its not neccesary to change..was just an extra project since I'm going to rebuild the motor might as well do the trans.

I said wiring difference. I know that the dk motors are different.I know they have 4 02's ...etc.Question is about the wiring. Its a 95 OBD1. Since you were involved in a project of this sort did the guy re-use his harness from the M70 to make his S70 work or did he purchase the S70 harness. Thats all I need to know. I'm not using anything(no electronics or computers) from the M70. As far as the trans, its not neccesary to change..was just an extra project since I'm going to rebuild the motor might as well do the trans.

I am not sure what the S70 conversion would have to do with a M73 conversion?

The M70 harness was used and the S70 DMEs were used.

Trans change? I think you need to evaluate that as well, the 4 speed 4HP24 will not work with the M73, the reason is the crank sensors are located at the vibration damper of the M70 and at the flywheel for the M73, there is no area on the 4HP24 bell housing to locate the crank sensors for the M73 flexplate where the toothed wheel is located. Further, as I believe I mentioned earlier, the damper of the M70 will not fit on the M73, the crankshaft snout is completely different.

The way I see it is if you were to find a way to make or configure a crank sensor set up at the front of the M73 damper (perhaps an after market toothed wheel) you could drop in the "block" of the M73 and use all the old M70 stuff. But that means that the existing M70 DK and intake manifold would have to be used. The M70 electronics would not have a knock sensor circuit and wouldn't have timing/fuel curve for the 10:1 compression the M73 has. Would work but wouldn't be optimal.

Ok..now we're getting somewhere...in order to make it work I would have to use the 5 spd trany with the M73.....if the harness worked on the S70 it will work on the M73.....Here's the next issue....M70 has more coolant sensors and has 2 less 02 sensors. How do I go around that?

Ok..now we're getting somewhere...in order to make it work I would have to use the 5 spd trany with the M73.....if the harness worked on the S70 it will work on the M73.....Here's the next issue....M70 has more coolant sensors and has 2 less 02 sensors. How do I go around that?

The harness for the M70 and/or the S70 will NOT work for the M73, where did you get that idea?

The entire reason that the CSI (S70 engine) was not imported to USA after '95 was because it would not meet OBD II, in order to do that all the electronics had to be changed!, That is what happened to the M73, it is designed from the get go to be OBD II compliant, that is why nothing electronically including the harnesses from the M70 AND S70 will work on the M73.

Also most things such as the instrument panel, OBC ABS, ASC etc, interface with the engine electronics were changed from the M70/S70 to the M73.

Sorry for the confusion. Appreciate the input. I thought the early 95's manufactured 10/94 were analog, eml not egs. That's why I ask!

Sorry for the confusion. Appreciate the input. I thought the early 95's manufactured 10/94 were analog, eml not egs. That's why I ask!

I don't want to be confusing, the M73 and its associated electronics was designed from the beginning to be OBD II compliant so even thou the M73 engine was offered before '96, BMW know and planned for the upcoming OBD II with the introduction of both the M73 V-12 and the M62 V-8.

You may have already gotten word on this from Scotty M73 from the other board, if you don't have a complete donor car so that you could change all the other electronics boxes besides just the engine controls, plus the needed harnesses, you are asking for BIG trouble and a high $$$$ cost!

The biggest problem answering these questions, it is difficult to see where it is all heading, it is a bad idea to make "assumptions" on what BMW did.