M30 vs. M90

X

x_atlas0

Well-Known Member
Messages
1,845
Reaction score
178
Location
Clarkston, MI
I've been looking for the last few days over dicussions about the M30 and M90 engines. (bimmerforums, bigcoupegroup, firstfives, the lot of them) I've been attempting to determine which would work best for a twin-screw kit I've been cooking up.

As far as I can tell, the M90, being significantly oversquare, makes more low-end torque, but dies above 5k or so due to head design.

The M30, however, being a significantly more square engine, (in B35 trim) means it had more even torque over the RPM spread and started to croak at abut 5500 RPM, although I haven't been able to tell if that is because of the larger valves or the displacement setup.

I know the M90 has the advantage of being easily stroked with a M30B35 crank to bring it to ~3.650L, but it also has the disadvantage of head upgrade restrictions and the use of a distributor, whereas the M30 has the later "distributorless" Motronic 1.3 setup and larger valves later in life.

Given how the M90 is based off the M88, I also don't know if the M88 head (not the S38 head) is a bolt-on affair. If it is, that is a very significant advantage to a M90-based engine.

So any other pros/cons between the M30B35 and the M90?
 
You have more or less covered all the angles yourself. one thing to bear in mind though is that forced induction is a great leveler so valve sizes etc aren't as important as with a N/A application, that isn't to say that the forced induction engine wouldn't benefit from bigger valves cos it would it's just that you just need to tweek the boost up a bit to get the same result. The main reason that the M90 stops producing power a little over 5K rpm is the induction manifold won't allow it to, see here http://e9coupe.com/forum/viewtopic.php?t=867
I'm putting my neck on the line here but I'll wager that if you force feed an M90 it'll make good power well beyond 6K.
Choose which ever engine you feel is best but don't discount the M90 especially if you allready have one/
 
Just another few of things to bear in mind

1...you need to get the compression ratio right otherwise you will end up with chronic pre detenation (pinking, pinging).
2...If your going for high revs you may consider "racing" valve springs to avoid valve bounce
3...Think about using water injection at high boost
4... An intercooler (air - air) or a charge cooler (air - water)
5... Heat shielding and under bonnet (hood) cooling. A turbo engine gets very very hot!
6... Speaking of which an electric water pump is a good idea to keep the the coolant moving even when the engine is idling after a hard run and reduces the problems of heat soak.
7... Large oil cooler

Most of the above is done to rally cars, mine included, to increase power and reliability (unlike my spelling!)

Malc
 
Just another few of things to bear in mind

1...you need to get the compression ratio right otherwise you will end up with chronic pre detenation (pinking, pinging).
2...If your going for high revs you may consider "racing" valve springs to avoid valve bounce
3...Think about using water injection at high boost
4... An intercooler (air - air) or a charge cooler (air - water)
5... Heat shielding and under bonnet (hood) cooling. A turbo engine gets very very hot!
6... Speaking of which an electric water pump is a good idea to keep the the coolant moving even when the engine is idling after a hard run and reduces the problems of heat soak.
7... Large oil cooler

Most of the above is done to rally cars, mine included, to increase power and reliability (unlike my spelling!)

Malc
Click to expand...

The M30 and M90's 9:1 CR is quite acceptable for mid to mild boost, even non-intercooled. I wouldn't go above 7psi without a chiller, though. The timing wouldn't have to be pulled until above 14psi, based on what others have done with the M30.

Protection against valve float is certainly a good idea.

The method of forced induction I am considering is a twin screw supercharger, not a turbo. Everybody does a turbo, and I like the linearity of the power delivery with a super. I know it will be forever less efficient.
 
Superchargers are nice I must admit. Your right about the "boost" delivery from them.
Have you thought about using a G-lager (spelling) the wierd type of compressor that VW put on some Golfs and Corrado's?
Supposedly the design has been around for ages but only in the last 10 - 15 years has the engineering technology been around to actually make them where they work well
Malc
 
If I ever get around to add forced induction, I'll be using one of these:

http://www.rotrex.com/
Click to expand...

Why use a centri supercharger? The adiabatic efficiency may be slightly higher than a twin-screw, but then the boost is linear to the RPMs, so you won't get much more power at less than 4k RPMs.
 
The decision is primarily based on packaging issues. Its already a tight fit under the hood and the Rotrex is a nice, compact unit that can bolt up in place of the a/c compressor. I'd run an intercooler in front of the radiator and have it tie into the stock D-Jet air filter housing so its fully stealth. 8) The only thing I haven't quite figured out yet is the custom crank pulley to run a serpentine belt for the charger, but keep the v-belts for the water pump, alternator and power steering pump.

I'm not sure where you would physically fit a twin-screw while retaining the L-Jet/D-Jet log manifold. I'm visualizing a typical Eaton blower like on GM sixes -- I'm sure there are other options I'm not aware of -- but I can't see how that would fit without some major re-plumbing of the intake. Maybe you could fit it down low on the passenger side, but it sure would be tight. Plus, my motor already has plenty of low-end grunt -- but a major boost in torque and hp from 3-6k would be ideal. Like everything else, its all a compromise. . .
 
I am not sure I agree with your assessment of the M90 Vs M30 torque curve.

I have a 1980 635 with an M90, and a 1988 with an M30B35 (the higher CR version)

The M90 clearly has much more low end torque than the M90. The M30B35 comes alive at about 3500 RPM, and goes well from there to 5.5K, and which point it levels off. The M90 (L-Jet) Starts pulling hard at about 1500 RPM, and goes all the way to 6.8K (as far as I have ever pushed it - Redline is at 6.5). It never goes flat, and if yours does it is not tuned right.

The contrast between these two cars is dramatic. The 88 is silky smooth and super refined.. but there is not really very much magic going on under the hood. It feels like a doctor's weekend car (and, as I recall, the first owner WAS a doctor...) Even with a 5 SP manual with conventional gears it is no match for my 1980 Euro with a 265 and 3.07 LSD...

The Euro is harsh, mechanical, noisy, and fast as blazes. I let fellow coupster Terry Sather(?) drive it and he managed to chirp the tires going into 3rd on the freeway on-ramp. This is simply a well tuned L-Jet M90. In my mind this engine in a coupe would be amazing!

I have kicked around the idea of supercharging, but honestly, unless you are ready to mess up the coupe by going to 275 rear tires you will never get all that power onto the ground in a light coupe.

My 3.453 cents

S
 
I have never driven a M90 car, hence I was asking around about the engine.

It sounds like the best option is to do a B35 head on a M90 block, which gives a CR of ~10.5:1, and a 3.65L displacement, coupled with the larger B35 valves.

I figure, with a Megasquirt system and a MLS headgasket to lower the CR to ~9:1, the engine should be quite stout.
 
It never goes flat, and if yours does it is not tuned right
Click to expand...

Hmmm I'm not too sure weather to take umbrance to that statement or not but I would like to ask you (ScottAndrews) does your M90 use the standard inlet manifold? If it does then I can guarentee that it will produce peak torque at 5-5.5K rpm I'm not saying that it won't rev beyond that but it's a mathematical certanty that that is roughly where peak torque and power will be. Period. Period. Have you ever had your power and torque curve plotted by a proper rolling road?
As for not being tuned properly hmmm. The tendancy here is for me to assume that you are asserting that I and everyone else(even BMW) don't know how to use a fuel pressure guage, timing light and a C.O. meter which I would find offensive if I weren't wise enough to realise that in making the statement " it is not tuned right" you haven't got a clue what you are talking about.
I'm sorry for being so abrupt but its a bit silly to come out with a statement like that when BMW themselves quoted peak torque at 5200RPM.
I suggest that you read the thread linked to below, in it you will see that the author has spent a lot of time and a lot of money building an engine to what esentially was group 2 racing spec but it still peaked at 5400 RPM.
Maybe they didn't know how to tune it properly either.

http://e9coupe.com/forum/viewtopic.php?t=867
 
I will do a full build story on my motor in the near future, its been a hard engine to get right. The 3.5 motor is very sensitive to intake design, and we could still find loads more torque and hp by trying different intake lengths and different placing of the injectors. I have now decided not to fit the engine in the car, it would be such a shame to spoil the originality of my old CSL, I`ve just unearthed loads of its history and have some pics of HRH Prince Charles driving the car in 1973 and loads of pics of the car from 1972- 1975 are on the way to me. I am going to go through the car in the winter and replace all the suspension bushes and brake pipes, as they are still the ones that were fitted when new and rebuild the motor that's in it. It may just need a head rebuild, it still uses very little oil but smokes way to much on over run. I have also decide to have the car resprayed for cosmetic reasons, it still has an awful lot of original paint, front wings, roof, boot lid, 1 door have never been painted and have started to show 35 years of use.

I have decided after looking for a car to race next year to build my self a really nice Alexander Calder `Art Car` replica and Finnish developing the the engine. I have a CSL to look in London next week end, its scruffy but complete and with what the seller wants for I would recoup most of the cost of it by selling of the bits I wont need.
marc
 
Marc,
Probably your valve stem seals have gone, especially if the compression is ok
Head has to come off to fix, but you should not have to strip the rest of the engine
Malc
 
I remember when I saw some pictures of racing CSLs, they had an intake manifold like this, shaped sort of like a V:
[Broken External Image]:http://www.topsecretjpn.com/products/surgetankrb.jpg

Now, to make that work on a CS or CSi, something would have to be done with the brake booster. This intake manifold design has also shown up in just about all the formula SAE cars I have seen. I think they use the same idea of using the engine's resonance properties to gain some additional pressure as the exhaust. Although, in the exhaust, it is used to help seat the valves better, rather than force some additional air into the engine, as it would be with this style intake.
 
You could actually throw away the brake booster (servo in Brit speak) and go for an adjustable biase pedal box and two master cylinders.
This is very common on rally cars and as long as you get the relationship of the volumes of each master cylinder and the calipers it is operating correct, plus the leverage from the actuation levers and rods you will end up with a brake pedal with "feel" and requiring very like extra effort when pressing the pedal.
I should know I have built and used such a system in my "resting" Lancia Delta Integrale. I am helping to build a historic class Saab 99 Turbo rally car the same way.
Malc
 
You could actually throw away the brake booster (servo in Brit speak)
Click to expand...

Or even fit the hydraulic servo system as fittid to 5s,6s and 7s of the eighties eh malc :wink:

Thanks for the pics btw everything was connected up ok so still trying to work out why its playing silly buggers.
 
The bakes appear to be soft (like they are extremely over servod) but work great, after a while the pedal gets firmer and firmer (brakes still work very well) untill they start to bind. After a while (could be 10 secs upto 5 mins) they release themselves and become soft again. I'm begining to think that I may have two different problems which is confusing matters somewhat.
It could be that I have made the link rod between the pedal and the transverse link too long, this could be why they start to bind after the (possibly faulty???) accumulator has managed to build up pressure.
I spoke to a guy who was a BMW teccie when this system was current and he said that the system never really gave any problems except for the accumulators so thats my best guess at the moment. I'll have to bite the bullet and order a new accumulator, its a big pill if it's still the same afterwards though.
 
You must log in or register to reply here.
Back
Top