Silikal@aol.com writes:
> I'm sure Roger Garnett will correct me on this if I'm wrong.
Nah, I'm usually willing to let a lot of people be wrong. Me too, sometimes...
> Eric Lund writes:
> >First, I have been told that the life of an engine at redline under load
> >(i.e. on the road and not on a test stand) is measured in minutes, say
> >18-20 minutes at most.
That would depend on how the engine was built, and what exactly determined
the redline, and if there is extra overhead in the spec'ed number.
The redline of an engine can be determined by many things- usually, it's a
number over which the engine either can't, or shouldn't be run, but you
should be able to run *at* the redline of a production car. My '86 Honda
Civic is almost at 90K miles, and the "redline" is at 6500, but I'm here to
tell you that the ignition cutout is at about 7100 RPM, and I run in that
range all the time, both on and off track!
A better question might be, "What determines redline?"
One thing you could do is run it up, and see how fast you can go before it
blowss! But, on many engines, you just can't- There are many factors. you may
get valve float (valves can't return fast enough, and are still open when
it's time to open again), or you hit a spring resonant frequency, or points
float long before other problems. These act as a sort of built in rev-limiter,
and you may never be able to run fast enough to put your self in danger. (The
redline is set somewhere below the "float point".) Or, there may other design
problems that do show up first.
A stock MGB engine can run at 6500 for extended periods- but usually will
suffer valve float much beyond that. This is tamed with stronger and multiple
valve springs. But the higher spring loads do increase the wear rate of the
cam and rockers.
Limiting factors include-
-linear piston speed
-valve open and close time
-cam profile (affects valve float, and how hard the valves close)
-Oiling capacity
-fuel system
-air flow rate (Inlet, head, and exhaust restricitions)
-resonant frequencys of machanical parts
-rod stretch, and other deformed parts.
Resonant frequency's? Yup- How 'bout a TR3 crank? TR3's have a 5500 RPM
redline- in stock form, the cranks can't take much more, they actually start
bending back and forth like a plucked string, and are prone to breakage.
These days, with careful balancing and an added damper, you can exceed that.
The 2 bearing "twisted wire" cranks on a lot of early British cars certrainly
aren't up for much abuse, but it is amazing what folks did with some of the
Austin 7's, or what MG did with the M, C, and J types.
Rod stretch? Yup- the mass of the piston is going one way as the rod starts
pulling back the other way, and the rod stretches. This can cause breakage.
Metal *is* elastic, after all, but only up to a point.
Or, at the same time, as it goes faster, and piston comes around again while
the valve is trying to close... Eventually the piston can catch up with the
valve before it closes, and the two make contact- bang, bent valves, bang
piston has nowhere to go, bang, the rod breaks.
> Most failure modes of over-revved engines is a thrown rod, and I can't see
> how rings affect that. Usually it's overstressing the rod caps and bolts.
Well, there's lot's of failures. Actually, rods breaking is usually a
secondary failure- they're just along for the ride as it were. Like, if you
run out of oil, and the bearing seizes, as the crank continues to turn, the
rod now rotates with it, and snap! I've seen pistons with valves well
embeded in a piston, due to either over reving, or stuck valves- but, when
that happens, either the valves, piston, or rod is likley to shatter.
Also, wear is accelerated at higher eninge speeds. Some of the early engines
which ran at 1500-2500 RPM just go on forever.
In the end- if you have a modern engine, redline is usually conservative
enough that you shouldn't have a problem. On the other extreme, some race
engines are built to (or beyond) the limit, and running time is measured in
minutes- the closer you run to the edge, the sooner they'll blow.
RG
________
/ _ \ Roger Garnett (Roger-Garnett@cornell.edu)
/ /|| \ \ Agricultural Economics | The Wayward Sports Car Centre
| |\/ || _ | 3 Warren Hall | http://www.wayward.team.net/
| | || | | Cornell University |
\ ||__/ / Ithaca, N.Y. 14853-7801 | (607) 533-7735
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