As Matt has suggested, redline has to do with engine speed, and is almost
always the point in the RPM range when valve float will begin to occur. Valve
float will immediately limit the engine RPM's because compression is failing
due to the valves not closing completely. In some engines, and I don't recall
if any MG's should be included here, a floating valve will collide with the
piston on the up strokes. This is mostly characteristic of newer high
compression overhead cam engines, especially those with high lift cams and
designed-in valve recesses in the pistons.
Red line should be considered the "Never Exceed Speed" for your engine, and
that is if the engine is in decent health. If valve springs are soft, float
occurs even sooner. On the opposite end, an engine built for racing may contain
numerous "trick" components that allow revs to climb above the factory red
line.
The levels of stresses placed on the engine will not change all that much
between 4000 and 6000 RPM at a given road speed, but in order to produce a
higher road speed, an engine must burn more fuel, which produces more heat, and
that in turn puts additional stress on cooling and lubricating systems, etc.
There are too many factors involved in the "speed vs. longevity" debate to see
a clear answer. In my humble opinion, an MG will enjoy running all day, every
day at 5000 RPM with negligible effect on life span. Just be kind to it by
using good oil and a responsible oil change interval. It is much more damaging
to start a cold engine on a frigid winter morning, and instantly rev the poor
thing to yellow line, then immediately drive off at high engine speed. I hate
when engines explode, so I run for cover whenever I hear someone do that. My
engines, whether they were MG's, BMW cars or motorcycles, the Porsche 911S,
etc. all seemed most happy after a brisk drive to clean out the cobwebs.
Jim
Matt Pringle wrote:
> I'd be curious to hear from some of the more knowledgable posters
> regarding this subject. I would have assumed the redline would have
> more to do with valve float, piston centrifugal forces, crank whip, etc.
> than with cooling capacity. I wouldn't think the cooling would be able
> to keep up with any engine running at redline continuously. ie, the
> cooling system would rely on the thermal inertia of the engine to keep
> temps down during occasional ventures into the high RPM's.
>
> Matt
>
> Gordon Bird wrote:
> >
> > The tach on my A has a 'yellow' line at 5500 rpm and the redline
> > at 6000. I do have the original tach in place. Oh yeh, I do have an oil
> > cooler.
> > Gordie Bird
> > 62 MGA
> >
> > **>**>Deal List:
> > **>**>
> > **>**>This whole discussion makes one wonder about the meaning of the
> > **>**>redline is,
> > **>**>anyway. I assume it represented the point at which the cooling
> > **>**>system could
> > **>**>no longer dissipate heat at the rate that the engine produces
> > **>**>it. However,
> > **>**>in going through boxes and piles of used guages at different LBC
> > **>**>events, it
> > **>**>seemed that they all pretty much had the same redline at about
> > **>**>5500. Every
> > **>**>single one had the same redline, although the tachs were
> > **>**>allegedly for all
> > **>**>sorts of different marques with widely varying engines.
> > **>**>
> > **>**>I now suspect that the tachometer backgrounds were all printed
> > **>**>by the same
> > **>**>printer at the same time by Smiths, and BMC just ordered them
> > **>all. Thus
> > **>**>regardles of where the engine should top out, all MG's seem
> > **>to have about
> > **>**>the same redline. (They didn't alter the tach on cars with an
> > **>**>oil cooler,
> > **>**>did they?)
> > **>**>
> > **>**>Regards,
> > **>**>
> > **>**>Charles
> > **>**>'74 Midget
> > **>**>cdsorkin@ix.netcom.com
> > **>**>Bloomfield, NJ
> > **>**>
> > **>
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