An approach to an objective classification model for used engines.
Gentlemen,
In my efforts to document my M88/3 engine rebuild, I had to describe a classification model for used engines as part of an objective comparison between all the options (RMFD engine, rebuild, used engine).
Although this model is open to debate, it can be used to determine the risk of used engines that are offered on the market. I have defined five classes (I to V) that determine the quality of an offering.
At first, I have made a selection table based on mileage, availability of a traceable service history and the actual compression test results. Based on these parameters, the risk of a used engine is determined into one of five classes (I, II, III, IV or V).
Classification of a used engine
Mileage
0 to 50000
km
Compression
90 to 100
80 to 90
<80
%
Traceable service history
yes
no
yes
no
-
Risk classification
Class I
Class III
Class III
Class IV
Class V
Mileage
50000-100000
km
Compression
90 to 100
80 to 90
<80
%
Traceable service history
yes
no
yes
no
-
Risk classification
Class I
Class III
Class III
Class IV
Class V
Mileage
100000-150000
km
Compression
90 to 100
80 to 90
<80
%
Traceable service history
yes
no
yes
no
-
Risk classification
Class II
Class III
Class III
Class IV
Class V
Mileage
150000-200000
km
Compression
90 to 100
80 to 90
<80
%
Traceable service history
yes
no
yes
no
-
Risk classification
Class III
Class IV
Class IV
Class V
Class V
Mileage
200000 to 250000
km
Compression
90 to 100
80 to 90
<80
%
Traceable service history
yes
no
yes
no
-
Risk classification
Class IV
Class V
Class IV
Class V
Class V
An engine with more then 250k km is reaching its end of life (EOL) and is therefore a risk class V.
The next table translates the risk classes in work that can be expected.
Risk class
Quality
Running
Preventive repairs
Imminent repairs
Description
I
Excellent
Yes
No
No
No outstanding issues, visually excellent throghout
II
Good
Yes
No
No
Minor (cosmetic) issues that do not require repairs
III
Average
Yes
Likely
No
Repairs only needed to improve reliability
IV
Below average
Yes
Yes
Likely
Extensive repairs needed; overhaul should be concidered
V
Poor
No
Yes
Yes
Overhaul is needed; full rebuild must be concidered
This is not an approach to asses the absolute value of an engine, only classify the risk one may expect.
Constructive comments / remarks for further improvement are welcome.
The missing variable (and one that is very difficult to quantify) is how an engine has been used. Low miles and documented servicing is great - but if the motor was pulled out of a car that saw considerable time running at high RPM - or thousands of hours idling in traffic - the likelihood of future problems (and an early rebuild) rises considerably. Similarily, a very low mile motor that was only used in 2 minute trips to the corner store and never warmed up (or worse - abused when cold) may not turn out to be as reliable as the history and mileage alone would suggest.
Who owned the car and how they drove it is at least as important as how far they drove it and how well they took care of it.
Whilst every eventually can never be covered , it is useful to have a standard and guide in relation to used engines .
Car values , afterall , are based on certain criteria and an engine is a fundamental part of the car .
An engine's value has to be based on it's potential longevity and core components , as your guide suggests .
There have been high mileage , abused donor vehicle , engines sold for a pittance recently . Does this mean that a superb , quality , engine should also be worth the same ?
Thanks for your well structured guide and insight .
D
__________________
Better a day as a tiger than a lifetime as a sheep !
Interesting exercise Ray. What category would you have put your M88 engine in? Knowing now that it had some questionable head work.
Based on mileage and lack of traceable service history, it was a class IV engine upon purchase. I have always felt something with the engine was not OK.
Quote:
Originally Posted by woods
The missing variable (and one that is very difficult to quantify) is how an engine has been used. Low miles and documented servicing is great - but if the motor was pulled out of a car that saw considerable time running at high RPM
With the proper maintenance, warming up and cooling down, sustained high RPM driving is not a problem for S38/M88 engines. As long as there is a traceable service record with proof of the necessary (preventive) maintenance that is carried out, the risk is significantly reduced.
Quote:
Originally Posted by woods
- or thousands of hours idling in traffic - the likelihood of future problems (and an early rebuild) rises considerably. Similarily, a very low mile motor that was only used in 2 minute trips to the corner store and never warmed up (or worse - abused when cold) may not turn out to be as reliable as the history and mileage alone would suggest.
I admit this is not a conclusive model, and there are exceptions, but generally one can assume that neglected engines show little or no service history due to which thet are always is classed III or lower, thus in need of work.
Interesting classification Raymond, though I do agree with Woods about the importance of the kind of use the engine has had. I see no problem with track and autobahn driven engines if servicing is up to scratch. Mine has seen both in it's lifetime. Hell, I drove about 700kms today from Kiel (Germany) to Brussels (Belgium) in my old M5 3.8 with over 240,000 kms on the clock. She ran like an absolute dream, and despite traffic and numerous kurrywurst stops it took less than 6 hours.
Do you honestly believe that these engines are not built for much more than 250,000kms? I've seen plenty with far more on the odometer (many have travelled double what the figures in the dash say) being trashed around without mechanical issues and others that after some 100,000kms were treated to an AT engine from BMW.
Whats the link? Poor maintenance? Unequal engines? How come some S38's went pop with hardly any mileage and others go strong up to the 400,000 km mark?
Interesting classification Raymond, though I do agree with Woods about the importance of the kind of use the engine has had. I see no problem with track and autobahn driven engines if servicing is up to scratch. Mine has seen both in it's lifetime. Hell, I drove about 700kms today from Kiel (Germany) to Brussels (Belgium) in my old M5 3.8 with over 240,000 kms on the clock. She ran like an absolute dream, and despite traffic and numerous kurrywurst stops it took less than 6 hours.
Do you honestly believe that these engines are not built for much more than 250,000kms? I've seen plenty with far more on the odometer (many have travelled double what the figures in the dash say) being trashed around without mechanical issues and others that after some 100,000kms were treated to an AT engine from BMW.
Whats the link? Poor maintenance? Unequal engines? How come some S38's went pop with hardly any mileage and others go strong up to the 400,000 km mark?
Questions, questions, questions.
Members views much appreciated!
Lantz
I think this is an important question in the attempt to classify S38 risk. I applaud Raymond's system which would represent an 'ideal' means by which to classify engines were one able to control for the variables, which may be unknown, that cannot be controlled for - the 'safety factor'.
When an S38 goes pop after 60k miles can it be due to understandable variability in manufacturing or is it always due to some variation in use/abuse? My observation would tend to suggest that the S38 is an extremely robust design, with a degree of over engineering compared to more recent M engines and excellent quality control over the components, and that premature failures are more than likely attributable to neglect/abuse; this being the case the proposed model looks dependable were it practicable to administer. Compression test results are not always available with used motors, for example.
__________________
If you focus on quality, you don't need to worry about quantity.
Do you honestly believe that these engines are not built for much more than 250,000kms? I've seen plenty with far more on the odometer (many have travelled double what the figures in the dash say) being trashed around without mechanical issues and others that after some 100,000kms were treated to an AT engine from BMW.
I agree that the S38/M88 engine family is robust, but that aspect has to be treated as a separate point then longevity. In my M88/3 engine, measurements proved that the average cylinder bore wear rate has been 2E-7 mm/km over 155k km (~100k miles). The maximum wear can be calculated by subtracting the piston-to cylinder wall clearance of an new engine from the maximum allowed value that is specified by the manufacturer. Based on these figures and assuming that piston wear is negligible; the short engine would reach its EOL (end-of life) not before 515k km (~320k miles). However, this is an average value that ONLY applies to the short engine. To determine a typical value for the entire S38/M88 engine family, we need much more empirical data from other engines to obtain a distribution list. Until I have one, I will not state that the wear rate of my M88/3 engine is a typical value that is a benchmark for others.
You must also consider many other parts that are subject to wear. Even the cylinder head wears out in time. In the case of my M88/3 engine, the cylinder head has completely been worn out after 155k km [1]. Just recently, I received an Email from a German E28 M5 owner whose M88/3 engine suffered from a similar failure as mine with a service life that did not differ much from mine.
S38 engines will not be different. Granted, some of the M88’s design weaknesses have been addressed such as timing-gear train, the diameter of the valve bucket seats, stronger crankshafts (as from the S38B36), but the cylinder heads themselves are from the same design. In fact, documented cases of S38 engine rebuilds can be found on the Internet and in the most cases, the rebuild occurred with mileages far less then 250k km. These days, only true enthusiasts considers a RMFD engine from the OEM supplier or a rebuild that is fully documented with underlying proof for the work that has been carried out and the decisions that have been made.
Even when you monitor the market, record the mileages and create a distribution list, E34 M5’s with more then 250k km are rare and if one creates a distribution list, offerings with more then 300k km are an exception and can therefore be ignored in the model. But most importantly, E34 M5’s with a catastrophic engine failure and a high mileage are generally not rebuilt, but broken apart for the remainder of their parts. Most often the recorded mileages of these failures is located in the second halve of the distribution list that starts at zero and ends at the maximum possible engine life based on the wear rate of my M88/3 engine. With other words, the failure rate starts to rise at mileages above (515k km /2) = 257k km.
References:
[1]: M88/3 Engine rebuild for #231, see this thread.
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