5 cyl interference

[DAWSON-MD]

Jan wrote:

"I saw all the responses from the other listers.  I concur the 10V turbo is
an
interference engine - I have repaired one where the valve contacted the
pistons by cranking the engine with the starter.
As a kid growing up with small block chevy V8s which had valve reliefs
machined into the pistons,  I could never understand the reason the German's
would not use them also.  The small difference in ultimate engine
performance
could not possibly be offset by the very real probability of piston valve
contact.  I know there are people who will argue this point but my response
is - if someone wants to consider themselves a "world class engineer" they
should design a camshaft drive that doesn't fail instead of the current belt
design that looks like it was stolen from a sewing machine.  I take care of
my vehicles and change the belt more often than recommended but I can't
count
how many disappointed VW, Audi and Porsche owners I have talked to who
experience this problem - all which could have been easily prevented.
My .02 DM"


I must disagree on a couple of points.

Those valve reliefs in the pistons, Chevy, Ferrari or any other vehicle, are
designed to provide enough clearance so as prevent valve to piston contact
during NORMAL engine operation. IE, as the piston approaches TDC, that long
duration/high lift cam is still in the process of closing (or opening) the
valve. Break a timing chain in a Chevy and the cam will stop turning which
leaves some valves hanging down right in the same point in space were the
piston is soon to be. Like parking your car on a railroad track, the results
are disastrous. As you suggested, one solution is to design the engine so
that the valves and the pistons don't share any common real estate. A design
trade-off which usually means a lower compression ratio and less power.
    A chain vs. a belt is another design trade-off. Timing chains have a
service life just like a belt does. As each little link and rivet wears, the
accumulated slop adds up resulting in a increase in the length of the chain.
With short chains, such as in a Chevy V8, the increase in length can be
tolerated but in an overhead cam engine the accumulated chain stretch can
easily exceed the limits of a chain tensioning device. When I worked on
Jaguars thirty years ago, I saw many engines that had the timing chain so
loose that it wore a hole right through the timing cover! Cam timing goes
way off as well and there is the very real risk of the chain jumping teeth
on the sprockets. As manufactures went to more efficient OHC engines they
found the belt to be a more reliable and economical alternative. Don't
forget about engine life considerations. Thirty years ago that same mileage
at which you now change belts was usually considered the entire life of a
car. Getting eighty or ninety thousand miles out of a car was extraordinary,
thus no need to change timing chains. Now, thanks to the tree huggers at the
EPA, manufacturers are responsible for low emissions to very high mileages.
Lack of chain stretch (more accurate valve timing, hence more controlled
emissions), ease and cost of maintenance as well as manufacturing costs are
all factors in an engineering decision. Gear driven cam drives, especially
on OHC engines, have their own problems. High manufacturing costs, drive
noise and high mileage maintenance considerations usually rule them out.
Even Ducatti abandon their awesome gear driven cam design for a belt!
    My point is that interference vs. non-interference, chain vs. belt are
just a few of the things that engineers consider when designing an engine.
Manufacturing costs, and now low emissions, usually win out. Ask any
engineer, their job is 95% cost and risk evaluation oriented. Those "world
class engineers" at Porsche/Audi/VW have determined that the "risk" of a
belt breaking is low enough that other priorities take precedence.
    Oh, by the way, breaking a belt in an Audi isn't so bad. If the timing
belt on my 308 slips just 3 teeth I have interference. If it breaks it means
a $8K to $10K rebuild. Ferrari belts are religiously changed at 30K mile
intervals.

Mike
'83 UrQ
'84 4Kq
'87 5Ktq
'77 Ferrari 308 GT4