86 M20b27 with 88 M20b20 (731) head swap questions

[pfahls1]

I have located a 88 m20b20 731 head and intake manifold that I would like to replace my original head with. Most write ups I see on this topic include using the m20b25 intake with the 731 head out of a m20b23. It looks like the 88 m20b20 I found has the 260 cam in it vs. the 252 in the m20b23. Has anyone used the m20b20 731 head with the 260 cam in conjunction with the m20b20 intake manifold on a m20b27? Is there much difference in the b20, b23 and b25 intake manifolds? How do you guys think the b20's head/intake would suit the b27?

[Madhatter]

Throttle bodies and port sizes are smaller. From what I can see, the 2.0L and 2.3L share kind of similar sized ports and throttle body, the 2.5L one on the B25 is certainly larger and you should use it.

Use the head no problems. Just use the 325i manifold. Physically, externally, as far as all mount points and fuel rail, etc goes, it all fits up fine.

[digger]

I have located a 88 m20b20 731 head and intake manifold that I would like to replace my original head with. Most write ups I see on this topic include using the m20b25 intake with the 731 head out of a m20b23. It looks like the 88 m20b20 I found has the 260 cam in it vs. the 252 in the m20b23. Has anyone used the m20b20 731 head with the 260 cam in conjunction with the m20b20 intake manifold on a m20b27? Is there much difference in the b20, b23 and b25 intake manifolds? How do you guys think the b20's head/intake would suit the b27?

the b20 cam i measured is the same as the b25 except the cam out of the box was advanced a few degrees there was no duration or lift difference at all. the b23 cam has the same basic shape as the b25 cam but slower clearance ramps and thus seat to seat is bigger but overall lobe area is much of a muchness.

what size are the runners of b20 intake? UK guys with B20 often swap the B25 manifold and b25TB to the B20 so this suggests it is inferior.

[JohnH]

I am running an E28 525e fitted with a 731 head,from an E28 520i (M20B20 2/86) together with the M20B20 inlet manifold, albeit modified to accept a 325i b25 throttle body, and running M1.3 . The runner length is the same as the 325i B25 and the diameter is mid way between the eta and B25 though the the outlet flange is matched to the 731 head.

This produces about the same low down torque as the eta, but with much more top end, though its all over by 5500rpm. This may be down to the B20 exhaust manifold with three bolt flange with narrower exit, and E28 eta exhaust system.

In performance terms its about comparable with a European E28 528i up to about 90mph but drops off after that.

[Madhatter]

In the E30's at least, the exhaust manifold on the 320i is no different to the same type of exhaust manifold on the 325i. Same size ports, same everything. The part number is 11621716443. The rest of the exhaust system is the same sized all the way through too, all the mufflers, cat, etc, they just had different sized tips.

Since owning a 320i, been finding they pretty much raided the 325i parts bin to build them. It's almost like they just pulled the 325i motor out and put a 2L right in it's place as they share a hell of a lot of the same components, with only some minor changes to keep the smaller capacity motor running happily.

[JohnH]

Its important to remember that the M20B20 has been through several incarnations.
1. Very early, K-jet and 200 head
2. Slightly later, LE-Jet and 200 head
3. Later, LE2-Jet and 731 head
4. Latest Motronic 1.1/1.3 and ??? head

The exhaust manifolds are a mine field. On RHD examples I believe that there are two major variants, The earlier version with the narrower outlet and 3 bolt flange used by B20, B25 and B27 and the later variety with wider outlet and 4 bolt flange used by Motronic B20 and B25. The inlet flange, port diameter is the same in all cases irrespective of head or capacity. Australian version were also allowed for air injection/EGR.

[pfahls1]

Thanks for the information guy's. I'm checking with the seller to see what type flange is used. I know it's a 731 head out of a 88 euro 320 so I'm thinking he will reply with 4. If that's the case, shouldn't I be o.k. to just find a manifold from a later 325i and use it? The head has less than 100k miles on it and he's asking 650.00 for it and the m20b20 intake. I thought that sounded fair. What do you guy's think?

Its important to remember that the M20B20 has been through several incarnations.
1. Very early, K-jet and 200 head
2. Slightly later, LE-Jet and 200 head
3. Later, LE2-Jet and 731 head
4. Latest Motronic 1.1/1.3 and ??? head

The exhaust manifolds are a mine field. On RHD examples I believe that there are two major variants, The earlier version with the narrower outlet and 3 bolt flange used by B20, B25 and B27 and the later variety with wider outlet and 4 bolt flange used by Motronic B20 and B25. The inlet flange, port diameter is the same in all cases irrespective of head or capacity. Australian version were also allowed for air injection/EGR.

[JohnH]

I wouldnt like to advise you whether $650 is good value or not. From where I'm sitting, in the UK, it sounds like a lot, for a sight unseen head. Are there reciepts and a spec of the work carried out. 731 heads are though away items in Europe.

I wouldnt get hung up about the exhaust at this time. Any exhaust manifold, from any any M20, early or late, LHD or RHD will fit any M20 head. The manifold to head flange and port sizes are all the same. For now use your existing manifold and system.

I would strongly recommend that you upgrade to Motronic 1.3 (from a later 325i) together with all its various sensors, and just run the standard 325i injectors. With the exception of opening up the inlet manifold input flange, this is very much a plug and play exercise.

Heres a picture of my engine tweaked as described above.

[JohnH]

There are a couple of points that may cause problems using the M20B20 inlet.

In the photo below;

1. the knub circled needs to be removed as it interferes with the fit of the metal pipe/cable carrier,

2. this hole is where the brake servo vacuum pipe was fitted needs a plug. It actually has a spiggot here that has already removed.

3. When you open up the throttle body flange it can get very narrow. See the bottom pic for more detail. This may be a good place to build up the flange with JB Weld, otherwise Jim Levie will be telling you to get a smoke test done.

[katgeorge]

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[Madhatter]

An 88 320i already runs moronic 1.3. Side by side, the only difference is the size of the throttle body and ports. The fittings, ports, sensors, etc are exactly the same as a 325i of the same era. Everything fits up the same, right through all the hoses. I haven't flow tested the stock injectors, but I wouldn't be surprised if they are the same flow rate as the 325i ones as the 320i uses a 2.5bar fuel reg vs the 3.0 one on a 325i.

Pretty heavily modified now, but here's the 2l engine back in mine. Was running a 2.5l previously. Needs a rebuild now as its a little tired, but they have swapped over fine.

[JohnH]

Lots of interesting stuff in the pics Madhatter. I like the fuel Pressure Gauge.

[Madhatter]

It's chipped with the miller maf conversion and 19lb injectors, so needed to run even less fuel pressure than normal hence the reg. It's about 30psi at idle. MAF is slightly too big for the 2l though, so if you blip the throttle really quickly it bogs down a little bit.