Anatomy of the "Baby Six" HiFlo ST Sport Engine - Pt 2 |
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| The Challenge
We've offered ported cylinder heads for "Baby Sixes" since
the Winter of 1986. The complete engine proved time consuming to develop
due to four major weaknesses:
- Cam belt failures
- Connecting rod failures
- Head cracking (at the number 5 cam journal)
- Failing head bolts
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Cam Belt Rod Problems
Cam belt & rod problems have been reduced or eliminated by redesigning
the piston. The high silicon content forged piston with semi slipper-skirt
design and deep valve reliefs, is about 18% lighter than a stock piston
to reduce the load on the connecting rods & crankshaft. The valve
reliefs (eyebrow cuts) are deep enough to accommodate up to 11mm of lift
without the valve hitting the piston. If your cam belt fails, these pistons
won't bend the valves.
The Pistons
Our pistons have lightweight low tension rings with a three piece oil
ring design. We chose Alusil (high silicon content aluminum) pistons with
vented skirts because of their ability to run at tight cylinder wall clearances.
Because of their high strength, these pistons can take more side loading
on the skirts without collapsing the piston. It is not that uncommon to
pull stock pistons out of ETA engines and see some slightly collapsed
with about .004" to .005" piston to cylinder wall clearance.
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Metric Mechanic "Baby Six" piston. Lightweight high silicon
content forged pistons with deep valve reliefs. |
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Failing Head Bolts
The factory head bolts will often be stress cracked. These 12.9 grade
bolts (comparable to a grade 8 USA bolt) have straight cut shanks. BMW
has superseded this bolt to a Torx head bolt that is threaded about 1/3
the length of the bolt. This threaded area is used to produce bolt stretch.
When the ETA head heats up, the bolts offer very little give; so occasionally
they'll break. We use a special 8mm socket head bolt (Mercedes style)
with a cut down shank similar to the 4 and 6 cylinder head bolts. This
type of head bolt gives with head expansion, instead of fighting it. The
valve cover needs to be clearance for these special socket head bolts. |
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Special Socket Head Bolts. |
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Head Cracking Solution
To correct head cracking problems that are due to restricted water flow
at the #5 cam bearing journal, we modify the head gasket to push more
water coolant up into this area. To further reduce the possibility of
head cracking, we avoid using and head with a casting date older than
7 years. |
"Baby Six" Head Gasket Modification
We've enlarged the holes to increase water flow to the #5 cam bearing
journal. |
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Summation
Our HiFlo ST Head is ported for a 19% flow increase & given Surface
Turbulence™ for efficiency. When this head, with an early 323i or
325i cam, is combined with our 2.8 liter bottom end (short block) the
result is..
One Dynamite Engine!
We've raised the rpm limit in the computer to 5800. The engine puts out
185hp and has a strong usable power band from 3200-5800 rpm. We also recommend
a rear gear change to either 3.25:1 or 3.45:1. |
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Photo illustrates how disaterous engine damage can
be when the cam belt fails. Notice the ripped off teeth on the cam belt
and broken off valve heads sitting in the #2 and #3 cylinders. |
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