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Re: Issues of Engine Reliability (LONG AND TECHNICAL BUT INTERESTING!)
Mark,
I found your analysis on engine reliability versus over speed tolerance of
the engine design to be very interesting. You must be an engineer by trade I
assume? Don't worry I come from a family of engineers and I recognize the
thought pattern you expressed to examine the topic. Anyway, your analysis
is interesting with regard to piston ring displacement due to over speed.
Many years ago while in a completely different profession as a service
manager and rep. of a diesel service company, we sold (central distributors
for OEM), serviced fuel injection systems both gas (BMW [Kugelfischer])and
diesel (Robert Bosch, American Bosch, CAV, Lucas, Perkins,Yanmar, GM Diesels
including 671's on up, Maxidyne [as used in Mack Trucks by U.S. Post Office
and UPS] up through large diesels in ships). This included both normally
aspirated as well as turbos. My principal customers were large fleet
operators which included UPS, U.S. Post Office, Scheaffer Brewery [when they
were still in Brooklyn on Kent Ave], Perth Amboy Shipyards, Columbia
University Research Vessel, Exxon Tankers, and many public works operations
(really large engines that you could stand in the cylinder head). In all my
experiences seeing hundreds of tear-downs and rebuilds [which also included
cement mixer engines that would over speed due to down shifting coming off
the large bridges in the NY area and literally blow up or runaways due to
governor failure] extreme high speed was always detrimental to the life of
an engine. The life of the engine, both gas and diesel, was always a direct
result of the speeds at which the engine was run. The higher the RPM up
near redline the more wear and eventual damage or catatrosphic failure.
Needless to say service intervals for lubricants and changing filters were
essential to the life of the engine, which is why I am a strong advocate for
synthetic oils in our VWs.
Basically if you keep your revs under control and don't live in the redline
area (7300 RPM for the VR6 being much to high as you observed) your engine
will live a long service life.
Now don't get me wrong here I love to rev these VW engines just like the
rest of you. All I'm saying is that you have to exercise prudent control of
RPM's. Fortunately as you indicated our VW engines are robust in design to
ensure long life provided that you service the car regularly at prescribed
intervals.
One thing that I was thinking while reading your comments on over speed
tolerance of the engine design has anyone done a teardown of the Honda GSR/R
Type engine? I was wondering how robust that design is compared to the VR6?
The reason I am asking is because the GSR/R Type engine has the highest
production piston speed RPM redline that I know of for production cars. I
know the engines are all aluminum with special cylinder sleeves and all
but what about down below, the main journal bearings, connecting rod and so
on? These areas must under go tremendous stress ordinately higher than the VR6.
My 2 cents...
Dr. Bob
At 05:10 AM 12/4/98 -0800, you wrote:
>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
>
>
>
>