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Was reading 1976D (Sheeps Clothing) which has an article from Jack French, which I seem to remember also seeing in the Companion perhaps too.
It stresses for drivers NOT to increase throttle for down changes, and to use synchromesh.
"For three speeds 'it will grunt but also put up with it".
i'm sure there's been discussion on good gear changing on the new forum, but what do people think of the above? Is it only for tuned engines? I can see that it might change the dynamic with Phoenix and the like, but what about for standard-ish old engines?
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What reason/s are given in the text for this Jon?
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its in an article about how to get more power and how to prevent it all falling apart... and I can see the theory of transferring stresses onto the cheaper component (my words there, not his). No explanation from him though...
Other tips are
dont race engine in neutral
dont rev up when changing down
lightening the flywheel reduces torsional loading etc.
You should be able to find it in the Green Companion.
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The pistons and little end exert several hundred times weight. The up pistons exert far greater accel/decel force than the down. The reasonably exact figures are a simple calculation. With throttle the firing load counters the difference to some degree. At light throttle the bend on crank is considerabel (although because pulses are not sharp as from firing, engine not unduly harsh). So it is not just the rev for the gear change but slowing at high rpm with closed throttle in the gears which is destructive. (If you can counter the wind, it is prudent to coast down 60 mph gradients.) I cant see how the double declutch blip would be worse than following overrun revs, except that it likely to higher rpm.
(When the crank went on my car the final damaging let go was on the overrun)
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28-05-2018, 10:07 AM
(This post was last modified: 28-05-2018, 10:10 AM by Alan.)
I don’t think he meant you can bang the gears in at any speed and expect the engine to take it. More that you should match vehicle and engine speed by allowing the vehicle to come down to engine speed rather than the other way around.
Bob’s right. When they fail, conrods almost always fail in tension, not compression, and it’s high revs, low load that does it.
Alan Fairless
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28-05-2018, 10:14 AM
(This post was last modified: 28-05-2018, 10:15 AM by Ruairidh Dunford.)
I also imagine this is [Jack’s] advice for those wishing to race?
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Theory doesnt really consider whether you have a crash helmet on though, surely. The bit about going down hills is something we can benefit from - but whether its sensible in practice on a steep hill might be very different!
So, coasting means taking out of gear (so you are less in control) presumably? Rather than holding in gear with no accelerator and achieving engine braking? I'm not sure I'd want to do that unless it was very straight...
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He believed that high rpm at little or no load achieved not much but shortening of crankshaft life. Yes, he was talking about racing, I remember he wasn't impressed by high revs in the paddock. Whumping he called it if I remember right without looking it up.
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I dunno if there are any out there curious as to why upward pistons exert so much more force than downward. Some books mention mysterious “angularity of the crank“ with little elaboration. I have explained to other car club members in the past...
Imagine an engine 4 inch stroke (2 inch crank throw) with a conrod just 2 1/8 long. From 90 to 270 deg the piston would barely move but from 270 through 360 to 90 the piston would move near the whole 4 inches up and down. Clearly the acceleration/deceleration at the top half rev is much greater than at the bottom half rev. In the case of the Seven with less extreme conrods the upward force is about 2/3 increased over the downward and as the latter is several hundred times the weight of the parts, increasing with the square of revs, the difference of two up piston assemblies together cf the two down together is very considerable. With the throttle open, on the power stroke the several hundred psi acting on the piston opposes. Although the noise level is much greater, crank at speed on the level may be less stressed than at the same speed downhill with closed throttle.
The topic recalls the exquisite sounds of motor racing from my boyhood. Some of the cars here were prewar GP relics (later sold for millions) with modest brakes and the sound of the double declutch down changes to slow for corners was very much part of the scene. I used to delight in emulating in my Seven until I learned better.... and fitted semi Girling brakes.
