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MG MGA - Shifting and Performance

Further to my inquiry about improving performance, perhaps the first question to ask is at what bandwidth does one shift at in these cars to maximize performance.

I assume the answer is "as close to redline as possible"

I tend to shift by ear and pretty much anything over 4,500 rpm seems to flick an immediate unconscious shift switch.

Should I be more agressive? Does higher RPM automatically equal more risk to the engine?
T McCarthy

High RPM means more wear, although lugging an engine can cause wear problems also. High RPM means high piston speeds which increases wear of the cylinder walls and rings, lugging is tough on bearings. To maximize performance you want to shift before you run out of breathing. There is a point in the rpm range mostly affected by camshaft profile and intake and exhaust systems where engine torque will begin to fall off, winding it up past that point has no merit. When you feel it quit pulling, it's time to shift. Normaly on a stock or near stock motor this will happen before redline, if you were to run it on a dyno you will see where the torque curve starts to drop. Running to redline is best left to full race motors on a track, it will reduce engine life as will running anything at it's maximum limits.
J Heisenfeldt

Look at a torque curve for the engine. Best acceleration can be had by keeping the engine running in the uppermost part of the power curve (not the torque curve). For most stock MGs that makes the shift point very close to 5000 rpm. If you warm the engine over a bit with a hotter camshaft the optimum shift point may be at higher speed. If you're not in such a hurry, casual shifting goes well in the 2500-4500 rpm range.

If you want good fuel economy the most fuel efficient part of the power range is the upper part of the torque curve. But that would mean running full throttle at high speed with enough load to keep it from over revving (going up a steep hill or towing a big trailer).

Since the MG has more power and torque than necessary for steady cruising, you can improve fuel economy by reducing the overall final drive gear ratio (overdrive gearing) to make the engine run slower and pull harder closer to the peak of the torque curve. The trick is that higher combustion pressure makes for better thermal efficiency (less pumping loss and wasted energy).

Most original MGs did not have overdrive gearing, mostly to keep the purchase price down. The sports cars were commonly geared for best top speed. That means the final drive gear ratio was selected to match the peak of the power curve to the best top speed possible. The (stock) engine will run about 5500-5600 rpm at top road speed on level road. (closer to 7000 rpm for the Twin Cam).

A numerically lower final drive ratio might be used to improve cruising fuel economy, but at the expense of top speed (not enough engine torque to pull top speed). This is why an overdrive unit or 5-speed is so advantageous, so you can cruise in overdrive for good economy but shift back a gear to have best top speed at higher revs when desired.

The best performance enhancement for you car is driver education. Keep your right foot full down to higher engine speed between shifts and you will get there quicker.
Barney Gaylord

This thread was discussed on 16/05/2008

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