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Over-revving and Gearing.

Subject: Over-revving and Gearing.
Date: Sat, 14 Aug 1999 13:52:36 EDT

"It's attitude, remember?  You just have to be willing to replace the rod
bearings occasionally. "

Some ramblings inspired by the discussion above:

The stock A engine will get into valve float at around 6250 rpm, depending on 
age and state of valve springs, but that is well past the useful range, 
except, as Barney stated, when you want to avoid a shift. That occurs more 
frequently in slaloming, but sometimes happens in racing as well.

Something that I see repeatedly is people, even experienced racers, mistaking 
valve bounce (when you get a sudden drop in power) with an engine 'falling 
off the cam', ie going above the useful power range of the breathing of that 
particular engine (which is normally pretty gradual.

I have heard people saying such things as "This engine keeps making power all 
the way up to 6500 rpm". What they really are observing, is an engine that 
peaks at perhaps 6000, and then falls, but not precipitously after that. When 
I point out to them that if they shift lower down, they will hit the torque 
peak in the next gear, actually have faster acceleration times, and wear and 
stress the engine less, it usually falls on deaf ears. I guess revving it 
high just sounds so good to them.

Because of the pretty wide ratios in the stock A gearbox, you actually have 
to run it up fairly high in second in order to hit the right torque plateau 
in 3rd. If the ratios were better chosen for racing, as in the factory close 
ratio box, the situation is immeasurably improved.  A stock Twincam with a 
normal transmission will accelerate to 100 mph in around 41 seconds. Leave 
the engine alone and put in the next rear end set up - 4.55 instead of 4.3, 
and fit the CR trans, and the same car will do the ton
 in 30 seconds flat (R&T test). Part of that is certainly the diff gears, but 
much of it is the CR transmission.

Just to get my shot in at those later, slower MGs (MGBs, weren't they?), a B 
does that run in over 42 secs.  And to further put things in perspective, a 
mildly tuned Twincam (YRX 210), with stock compression, 
42 mm Webers, the CR box, and a 4.1 diff (note that this is a step in the 
WRONG direction, but the car was set up for long track work), did the 0-100 
dash in about 25 secs., using only 6500 rpm (power peak on a stock engine is 
6700, on that one, perhaps a little higher, and for best acceleration, 7000 
should be used).

To be fair to the B, what passed for a full race car back in the 60s would do 
a second or so better than that, using a 4.3 rear and 14" tires.  God knows 
what a current B would do, but then while you can get just better than 160 
bhp out of a B (with 16:1 compression, and an engine life measured in the 
order of an hour or so), you can get 180 out of a Twincam using slightly 
lower rpm, and considerably less compression, so.......

The long and short of it, is do not buy a CR box for driving around on the 
street - it will be tedious, as first is so high. Do buy one if you are 
racing, and consider one for slalom, but remember that once you are out of 
first, there is no going back for slower turns, so the option of a stock box 
and over-revving once in a while may result in better times.


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