Comparison
A Tale of Two Cats
In 1995, the E36 M3 (OBD-I equipped) was spec'd
with BMW # 18 30 1 728 401 pictured below:
From 1996-99, the E36 M3 (OBD-II mandate) was
spec'd with the BMW # 18 30 1 744 162 pictured below:
As you can see, the later model catalyst on the
1996-99 M3s have the unique feature of remaining completely dual all the way
back to the rear muffler whereas the 1995 M3 has a section which narrows down to
a single, 60mm OD diameter section just before the catalysts.
Why Did BMW Do That?
Some people have theorized that BMW merged the
two, larger diameter pipes of the 1995 M3 to allow the exhaust gases to create more heat, to warm the catalysts quicker,
in order to pass strict EPA
cold start emissions. In 1996 and later, E36 M3s employed air pumps to add
more air into the system to allow the catalysts to heat more quickly, thus
eliminating the needless restriction of the single pipe merge.
Mathematical Comparisons Between Two Cats
Notice that the later model catalysts have a
smaller OD which connects to the exhaust manifolds. Below is a comparison
between the two catalysts, before the catalysts, before the single pipe merge:
| Model |
Total
Weight |
Tubing
OD
(before cats) |
Surface
Area
(per tube) |
Total
Area |
| 1995 M3 |
39 lbs. |
50mm |
196.3 sq/cm |
393.7sq/cm |
| 1996-99 M3 |
42 lbs. |
44.5mm |
155.5 sq/cm |
311 sq/cm |
| Result:
1995 M3 area larger by 21% |
This shows that the 1995 M3 cat enjoys a 21%
area advantage over the 1996-99 M3 cats. Larger pipes are great for
overall top end power, but sacrifice backpressure (which negatively affects low
end torque). However, the mathematics get interesting when comparing the
single, merged section of the 1995 M3 cats:
| Model |
Tubing
OD
(at single merge) |
Surface
Area
(per tube) |
Total
Area |
| 1995 M3 |
60mm |
282.7 sq/cm |
282.7 sq/cm |
| 1996-99 M3 |
44.5mm |
155.5 sq/cm |
311 sq/cm |
| Result:
1996-99 M3 area larger by 9% |
This shows that after the single pipe merge,
just before the catalysts, the 1996-99 cats have a 9% area advantage over
the 1995 M3 cats.
In Conclusion
From this comparison, it would support the
notion that despite the obvious displacement advantage of the 1996-99 M3 (3.2L
vs. 3.0L), the exhaust also lends it self to support that the 1996-99 M3 makes
more lower end torque while also allowing freer flowing exhaust gases at high
RPMs. This information would support the argument that the later model
1996-99 M3 catalyst provides not only more lower end torque, but also would
provide the best high end power (freer flowing at maximum exhaust gas velocity).
Submitted by Ben Liaw
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