A while back I noticed that BMW had never made an M version of the m20 motor which was used in the e21, e30, and e28. The motors that did get the M upgrades were:
- (i4) m10 -> s14 [I count this for the m42 as well] (e30 M3)
- (i6) m30 -> m88/s38 (e26 M1, e25 M5 and e34 M5)
- (i6) m50/2 -> s50 (euro e36 M3 and early euro Z3M)
- (i6) m54 -> s54 (e46 M3, late Z3M and Z4M)
- (v8) m60 -> s62 (e39 M5, e52 Z8)
- (v12) m70 -> s70/2 (see: McLaren F1)
What do all the motors have in common. If you say DOHC or timing chains I'll cry. That is beside the point and I'll hear none of it. The answer I want is individual throttle bodies, high compression, headers/high flow exhaust and aggressive cams.
Currently there are three "bolt on" kits available to add individual throttle bodies to your m20. Two are from dbilas and one from Extrudabody.
The "street" kit costs 2,075.00 euro plus shipping.
- You can use the stock ECU with an add on chip
- Looks amazing on the motor
- Is the closest to bolt on and go available
- Members on the forums have decided that the intake is poorly designed and restrictive to the motor
- I also have questions if the intake runners are the right length
- Plenum/airhorns mount with hose clamps
- Most expensive
The other options from dbilas is the "for racing" kit that costs 1,499 euro. (No image available)
- Cheapest kit available
- Bolt on velocity stacks (I believe)
- Place for a second set in fuel injectors (I believe)
- Need a standalone engine management system or need to build a plenum
- I have found no images or documentation on this kit (very little to no information available about it)
- May have brake booster clearance issues
The other option is the kit from Extrudabody it costs $1,999.00 USD.
- This is obviously more cost effective
- Comes with everything you need
- More assembly and modification needed than the "for street" dbilas kit
- I think the extruded and anodized aluminum looks a little out of place on the m20 motor
- I also have doubts that the intake runners are the proper length
- You need a standalone engine management system
Anyways I thought it would be interesting to try my hand at making my own set. Many people have done this before. The first question to consider is choice of throttle bodies. All of the m20 ITB builds I have seen use either motorcycle throttle bodies or BMW itb's from a s38/50/54 motor. The problem I see with the bike throttle body set ups is that they attach with hoses and hose clamps. This has three issues: lots of points for vacuum leaks, a less smooth intake track and potentially a less stable/strong/rigid mounting.
The problem I can see with s38/50/54 throttles is that they are way to big as they are meant for 3.0-3.8L motors and are 48-50mm in diameter which is bad for throttle response and intake velocity. Also they have the wrong port shape so for proper fitting so they need to be cut and welded or some odd adapter plate needs to be made which will result in less than optimal flow.
Eventually I found this diagram of the intake of the m20 head and around the same time I discovered that the Nissan RB26 from the Skyline GTR had individual throttle bodies. As this is a similar sized motor (2,494cc and 2,568cc) I thought they might be compatible. I then found out that they are relatively available and can often be had for around $200 a set on eBay. Also turns out that the space from the center of one throttle to the center of the adjacent throttle is 90mm. This is nearly prefect for the 91mm spaced m20 intake ports. A set came up for a deal one day and I hopped on it. I decided if nothing else I could take measurements and post them up for others considering them as an option. Turns out they are a little too long so either adapters need to be made to push the outside pairs out a few millimeters or a new single axel shaft needs to be made connecting all the throttles.
Next I found a source of 45mm velocity stacks on eBay that has them available for ~$10/piece. This is a steal as velocity stacks generally cost around $40/piece. They are cast with the inside machined.
The newest component to arrive on this long term project was the dbilas manifold. As you can see I am still gathering measurements to try to see how everything will fit together.
This is basically what I am thinking for the runners and intake manifold. The idea is based on the Hartge H27s design but instead of using a small center mounted intake it will be a large V-shaped intake above the velocity stacks. The idea behind this is that it will give more even airflow to each cylinder.
This is really more of a long term project but I have been getting excited and wanted to share.