Date: Fri, 12 Jan 1996 09:43:23 -0500
From: read_at_engr05.comsys.rockwell.com (Pete Read)
Subject: Redline RPM versus Reliability
John Allen commented on the higher redline and longer stroke
of the '91-'93 M5 engine:
>It's interesting that the longer-stroke motor (S38B36) also
>has the higher redline. Any thoughts?
Yes, I can give you some "rules of thumb" about engine
reliability versus rpm. I happened to research this some
during the past week, so your question is timely.
The Metric Mechanic (MM) Catalog got me started with its
section on engine rpm versus life. Basically, high rpm
operation wears out engines faster, but there's more to it
than that. MM also mentions the critical top piston ring
This sounded very familiar -- so familiar that I searched my
attic for Gordon Jenning's "Two Stroke Tuner's Handbook".
Now, M5s have four stroke engines, but the basics of pistons
and rings are the same for either.
Mean Piston Speed
The best known "rule of thumb" is mean piston speed (average
speed the piston travels at a fixed rpm). It goes like
Mean Piston Speed Result
Under 3,500 ft/min Good reliability
3,500-4,000 ft/min Stressful, needs good design
Over 4,000 ft/min Very short life
Mean piston speed is easy to determine. It's based on the
piston stroke and rpm.
Cm = 0.167 x L x N
Cm = mean piston speed in feet per min
L = stroke in inches
N = crankshaft speed in rpm
Using the 1988 E28 M5 as an example:
E28 M5, Bore and stroke 3.68 x 3.31 in (93.4 x 84 mm)
At 6,900 rpm (factory redline)
Cm = 0.167 x 3.31 x 6900 = 3,814 ft/min
At 7,200 rpm (Dinan chip redline) = 3,980 ft/min
At 6,500 rpm (max constant speed) = 3,593 ft/min
Here are some other piston speeds I figured for comparison.
Redline Stroke Piston Speed
Engine (rpm) in,mm (ft/min)
-------- ------- -------- ------------
BMW 2002 6,400 3.15, 80 3,367
BMW 318i 6,500 3.19, 81 3,463
BMW 325i 6,500 2.95, 75 3,202
BMW 3.0 6,400 3.15, 80 3,367
BMW 535i 6,200 3.39, 86 3,510
MM 4000 6,000 3.70, 94 3,707
MM 3500 6,400 3.39, 86 3,623
BMW E36 M3 6,800 3.38, 85.8 3,838
BMW E28 M5 6,900 3.31, 84 3,814
BMW E34 M5 7,200 3.39, 86 4,076
CBR600 13,250 1.78, 45.2 3,939 (motorcycle)
Notice that most production cars stay below 3,500 ft/min,
while the E36 M3 has the same 3,800 ft/min piston speed as
the E28 M5. With its longer stroke, the E34 M5 piston speed
is even higher than the screaming Honda CBR600 motorcycle!
The two Metric Mechanic (MM) engines also go above 3,500
>From the MM Catalog Engine Specification table, MM uses
thinner (lighter) 1.5 mm (0.059 in) top rings than the standard
BMW 1.75 mm (0.069 in) top rings. Lighter weight reduces
vertical ring acceleration force at high rpm. MM talks
about less "hammering" of the ring groove from the lighter
ring, but it also reduces piston and ring overheating.
How Piston Rings Work
Most people know that piston rings provide a seal for upper
cylinder gas pressure. Have you ever thought about how the
relatively light ring spring tension (about 30 psi) keeps
the high gas pressures (about 750 psi max) from rushing past
Rings don't seal just by their spring properties. Gas
pressure above the ring forces it down against the bottom of
its groove in the piston. The gas pressure also gets behind
the ring, in the back of its groove, and forces it out
against the cylinder wall. Normally this works well.
However, when piston ring acceleration exceeds the gas
pressure holding it in place, the ring lifts upward in its
groove (piston nears top of stroke and is being slowed to a
halt at TDC). As the ring lifts, gas pressure releases
above and behind the ring, causing the ring to hit up
against the top of its groove. When the pressure behind the
ring is released, high temperature and pressure combustion
gases escape down the side of the cylinder wall causing
piston and ring overheating. This hammering of the ring
groove and overheating shortens engine life.
Here's a picture of how it works.
- Normal Condition
Gas pressure in the upper cylinder holds the ring down
against the bottom of its groove and out against the
cylinder wall, forming a seal.
\| v| .
\| :| Left . Right
\| v|------- Cross-section . Side of
\| :.v.... | of Piston . Piston
\|[[] <: |\ . (not shown)
\| |-\----- \ .
\| | \ Ring Groove .
Cylinder \| | \ .
Wall \| | Piston Ring .
\| | .
\| | .
\| | .
2. Too Much Piston Acceleration
Piston acceleration lifts the ring, shutting off pressure
behind the ring and breaking the seal. The ring groove
is damaged by constant mechanical pounding. Hot
combustion g \|v | \ Ring Groove .
Cylinder \| | \ .
Wall \| | rings.
How much piston acceleration is too much for a piston ring?
Well, first figure piston acceleration, and then use another
"rule of thumb" for ring thickness versus piston
The ma A = ratio of rod length, between centers, to stroke
For example, the E28 M5:
L = 3.31 inch stroke
A = 2 (estimated) rod length/stroke
N = 6,900 rpm redline
Gmax = 6900^2 x 3.31/2189 x (1 + (1/2x2))= 89,989 ft/sec^2
Now use the following "rule of thumb" table from the
"Two Stroke Tuner's Handbook" and interpolate to find the
ring thickness for the M5's 90,000 ft/sec^2 piston
Ring Thickness Acceleration
The E28 M5 "rule of thumb" ring thickness works out to 0.057
inches. After figuring this, I hoped the actual rings were
at least as thin as
I quickly measured the M5 piston (with no rings) on my desk.
Using a feeler gauge, the top ring groove measured 0.061
inch. Ring groove clearance is usually about 0.002 inch, so
the M5 ring must be 0.059 inchspeed and acceleration calculations and ring thickness, M5
engine design allows reasonable reliability under high
engine revs, but the word "understressed" doesn't come to
My suggestion is be most careful about sustained high
(redline) rprecommended redline (6,900 versus Dinan 7,200 rpm). Even
shifting at 7,200 rpm won't cause any immediate
consequences, but over time it causes extra wear.
M-engines are pretty rugged. Just don't expect miracles if
you run hard all the time.
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