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Issues of Engine Reliability (LONG AND TECHNICAL BUT INTERESTING!)
Engine Life
Hello everyone! Mark Radelow here In an effort to bring some technical
discussion to the table I want to bring up the topic of engine life. I
know I know, it's been discussed before. But after perusing some
technical engine documents I came across something interesting. We all
know that revving your engine high is definitely hard on the motor. The
question is how hard is it. Well I came across an interesting article
that talked about engine life, specifically related to piston head speed.
It was talking about the top ring area and that you need to be especially
careful how fast your piston goes in relation to this. Well I look at
some posts and stuff and a guy was talking about a discussion in one of
his tuning books about this. Basically it breaks down like this.
I copied this from a post on the BMW list. I don't know who did itSorry
if you know who you are:
<quote on>
1. 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.
Gas Pressure
:
:
\| :
\| v
\| :|----------------------------|
\| 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 gases get past the ring, overheating it and
the piston.
Gas Pressure
:
:
\|:
\|:
\|: |----------------------------|
\|v | .
\|: | Left . Right
\|v |------- Cross-section . Side of
\|:[[[]] | of Piston . Piston
\|v \ |\ . (not shown)
\|: |-\----- \ .
\|v | \ Ring Groove .
Cylinder \| | \ .
Wall \| | Piston Ring .
\| | .
\| | .
\| | .
\| |----------------------------.
\|
\|
\|
<quote off>
The result? Increase wear on the piston, piston ring, rods, crank, etc.
etc. etc.
Thanks whoever made the great diagrams!
Okay simple enough. Now comes the question of determining at what RPM
this occurs. I turn to an article out of Gordon Jenning's "Two Stroke
Tuner's Handbook". True we have a four stroke, but the principle should
remain the same. Anyway, the formula is as follows to determine the
maximum speed allowable before things turn bad:
Cm = 0.167 x L x N
Cm = mean piston speed in feet per min
L = stroke in inches
N = crankshaft speed in rpm
This gives you the actual piston speed in feet per minute. Then you go to
this handy little table (I don't know how the determined it, but they did)
to figure out where your engine stands.
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
Okay, this is all good and well, but how does it apply to our car Well
let us see:
VR6 Engine
Stroke = 90MM (out of technical manual)
90MM = 3.54 inches (courtesy of handy HP 19BII Calculator)
Okay so plug this stuff into the formula:
First with stock rpm
CM = 0.167 x 3.54 x 6500 (I think this is stock redline)
CM = 3,842.67 ft./min
Okay that isn't SO bad. We have already determined the VR6 is a fairly
robust motor and is quite well built. Now let's figure it out for two
tuner chip rpms (6900 my P-Chip, 7300 Garrett's)
P-Chip
CM = 0.167 x 3.54 x 6900
CM = 4,079.14 ft./min
Garrett
CM = 0.167 x 3.54 x 7300
CM = 4,315.61 ft./min
Okay now it is true these speeds are reach for only a few seconds
(providing you don't hang at the rpm) but there can be no doubt that
revving above the stock redline can, and will, be harmful to the life or
your engine. Even if the rings float just once, hot gases would escape to
the bottom end. Figure this, let's say you hold the engine at 7,300 rpm
for 2 seconds.
7,300RPM = 121.67RPS (Revs per second, 7,300/60)
Divide by six to figure out the revs for each cylinder
121.67/6 = 20.28.
That means that each cylinder has its rings float and gases escape to the
bottom end 20 times (providing the rings are indeed floating). Or a
total of 121 times for the whole engine! All in the space of 2 seconds!!!
I don't care what anyone says, if the rings are floating damage and/or
increased wear is happening.
Now these formulas are just the opinion of people, but I think that even
if you add a bit of safety into the formula even then 7,300 rpm would seem
to cause extreme wear on your car. Here are some examples of other cars
with what would be considered OVER BUILT engines.
Redline Stroke Piston Speed
Engine (rpm) in, mm (ft/min)
-------- ------- -------- ------------
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
Just for comparison
CBR600 (bike) 13,250 1.78, 45.2 3,939
Okay, I know they are all BMW engines, but they are ALL M engines. And
not one of you, even Wally ;), can argue these are not well built engines
built to take a load of stress. Hell the rumor is that BMW picked old M3
four cylinder engine blocks right off the factory line to make F1 engines
out of them. I find it funny that only one of them goes over the danger
zone (barely), but even then it's at ridiculous RPM and not even close to
what the VR6 pistons are doing at that speed. Plus this is with a Dinan
Chip that STILL recommends maximum constant engine speed be limited to
6500 RPM.
What's the point of all this? Well I am not gonna be as happy to spin my
motor to redline anymore. I think this shows a very good valid point that
the tuners don't do all their homework; including Garrett. I will be very
weary of what they called reliable from now on. It also shows you with
stroker kits out there should be even MORE careful.
Now will this reduce the life of your car noticeably? Maybe yesMaybe no.
There is no way to be sure. But if you are like me and would like the car
to actually live BEYOND it's powertrain warranty then it is definitely
something to consider.
One thing, I am not an expert so I would like some responses from the
group (Todd, Don, etc. etc.)
I am sure this is gonna start something
Mark Radelow
(MEGA FLAME SUIT ON)
1997 Jetta GLX "Dear Lord What Have I Started" Version