Reference this post for the resistor locations: http://speeduino.com/forum/viewtopic.php?p=43863#p43863
The MEGA2560 datasheet says VCC*.6 is guaranteed to to trigger a high input which is 3V with a 5V VCC.
I measured the 2/3 rise time as it gives a 10% margin.
I'm using a UA4C board running a VW ABA 2.0l with the stock VR crank sensor.
I setup the scope on a running engine and measured the stock rise time:
Next I swapped the OUT1 resistor to 1.5K:
And finally a 1.0K:
I left a 1.5K in for my own vehicle, 60-2 trigger with a 7K redline. It should be plenty fast enough. I would have used a 1.7K or 1.8K for my car but I didn't have any.
At 1.0K the signal was noticeably not pulling all the way to ground so I wouldn't go any lower.
I did not experiment with the IN resistors.
In case the images go away:
2.2k (stock) 34.5 microseconds
1.5K 22.5 microseconds
1.0K 14.5 microseconds