m20b20 turbo options

[e30sequel]

recently acquired 320i imported from japan, NO FKN SUNROOF;), clean body but with a blown head -- was looking around and want to retain the higher revving nature of 2L so bottom end stays. Now some guy was saying that a 2.5 head (885) will fit but will reduce compression significantly (turbo compatibility or what) negating the benefits of the bigger chambers for NA application. So I am thinking since I have to do the head anyway just slap on the 2.5 head and hook up an ebay turbo kit and maybe some local tuning and I am golden .....right???? any help appreciated ......

[OniKyan]

B20 will not rev anymore than a B25. You're much better off living with the car as it is for now while building a turbo B25 on an engine stand and when it's finished, do the swap.

[einstein57]

The rockers are the big limiting factor for m20s when it comes to high revving. I would love an m20b20 block to boost. I bet it would get great fuel economy and still produce about 300 whp with 18lbs of boost.

[Madhatter]

6800 rev limit, peak power is like 6000rpm too.

Put an eta block under it and keep the 320i head, all the injection and ignition gear is the same as a 325i, but the inlet manifold has port shapes to suit the 731 head.

[e30sequel]

madhatter - methinks you are the one I picked up this idea from (dtm forum) - you were explaining fitment of 2.5 head onto 2.0 and I decided to use this idea as the basis for a very moderate small turbo boost. This would make it as stock as possible and giving me the starting point of #1 lower compression with the bigger head #2 and with a small turbo, it would give me my high revving plus a lil extra torque - your suggestion is a little different this time....mind explainin please before I pull the trigger on a rebuilt 2.5 head I have available locally ..... thanks dude ....much inspired

[e30sequel]

The rockers are the big limiting factor for m20s when it comes to high revving. I would love an m20b20 block to boost. I bet it would get great fuel economy and still produce about 300 whp with 18lbs of boost.

blimey .....but seriously.....yes mr einstein, I would agree thoroughly and this is why the bottom stays -

[Madhatter]

Well, the bore size is much smaller on the 2L block, 80mm vs 84mm. So, not only is the combustion chamber of the 885 head larger than any of the others (731 and 200), but it was designed for a larger piston too. To be honest, Ive never put one on top of the other to see just how much bigger it turns out to be, but I think you'll run into a problem with the head gasket if the chamber is substancially bigger. I was wondering if it could cause problems with the block, but then again, the 200 head is usually used on the eta which has a larger 84mm bore size too. I just dont know what sort of effect it will have being in reverse, the fire ring of the gasket is largely designed protect the surface of the block and contain the combustion gases. So if the gasket is bigger than the top of the bore, it leaves the iron exposed. I know you couldnt run a 320i head gasket as it wouldnt seal properly at all, so there really isnt any way around the above if it has the potential to damage the block.

Honestly, are eta motors that hard to find in canada? If they are and/or you really want to stay with the 2L motor, you might just be better off pulling the 320i head and having it checked out to see if its damaged, then getting it reconditioned as needed and just run the 2L motor. There is quite a bit of meat on the top of the 320i pistons due to the smaller stroke vs the 2.3L, so because it runs a 9.8:1 comp ratio standard, id look at pulling the pistons and having some metal removed from the crown so it ends up similar to the low compression eta pistons. Might find you can have them machined quite cheaply, which would drop the compression ratio and make it a little more boost friendly.

As I said, the 320i runs all the ignition and injection components of a 325i, literally everything is pretty much the same right down to the ecu. They just ran lower fuel pressure to offset the difference in power (ie, the 2L doesnt need as much fuel as the 2.5L), so it seems like a good base to go in any direction.

Give you an idea of what im talking about;

eta piston with a dish in the middle:



The ones in your 320i would normally either be totally flat or have a very slight dome to them. Have them milled down, could drop the compression a full point pretty easily without going to the cost of custom pistons and/or rods. Also means you dont have to try and keep the motor together running a thicker head gasket.

[e30sequel]

that seems like an idea - thank you for dropping this kind of knowledge as we get 0 love/information in N America in general regarding the 2.0. This will help for sure and I will now have to re think this more thoroughly - -- will post the progress

[Madhatter]

obviously just depend on how much it would cost to have them machined, a dish might be more difficult than just having them machined flat (so I would probably just go flat), but I know 100% for a fact the compression height is much larger on the 2L vs the 2.3L which share the same block and rods. If you arent looking at massive horsepower, I reckon this makes the most sense and could get out of it without spending a lot of money that you could probably use to freshen up the 2L motor.

Id start by pulling your head first and just see what condition the motor is in, will probably impact more on which way you go if replacement 731 heads are hard to find over there.