Depends what level you race at to what you call competitive.
On tyres if you race in IKR one race day old tyres from race teams will be fine - we race IKR and for me it doen't matter cos I'm slow anyway, new tyres would be a waste. For my lad, he's quite quick so has a challenge to win on used rubber, but he has done including with a grid of 26 with some on new rubber.
You then use the tyres you used on race day for your next practice day and rotate them that way.
When we raced MSA to stay competitive we ran on new rubber for race day and at the track we race at it tends to make you faster by 2 to 3 tenths, but only for the first race or two. BUT there have been situations where we have gone sloweron new rubber.
The track makes a difference too. Some tracks show less difference between new and used rubber - Wombwell, you might only be a tenth off on used rubber if any.
How the tyres are stored and the overall age of them can make a huge difference to how competitive used rubber can be.
Engine - good for up to about 7 or 8 hours if you are running at the top top end of racing, then a ring will give you the same again, or while it's aprart most put a piston in at top end racing again.
From 8 to 15 hours it will still be quite good but not have the edge.
from 15 to 25 hours it will still run ok and be safe, but will gradually slow.
from 25 hours on you'll be running on borrowed time ref. the top end, the piston comes with very fine grooves in it. These help carry the oil. Once the grooves are worn flat oil control is not so good. this contributes to more heat and stress. A crack will start to develop from the base of the piston skirt, until the skirt breaks up. If you are lucky all you will need is a full rebuild, but normally the piston bits rattling around from 14,000rpm to 0 rpm, break off the tangs on the bottom of the barrel, crank case damage also, rod etc.
I have 3 'mometo' pistons from where we have pushed the hours too far - they went about 30 hours; they have no skrits left.
One of the pistons was caught in time - there a vertical crack in the skirt from the bottom about 15mm long with a tiny hole at the end of the crack. It must have been minutes from failure.
Many say they run rotax to 50 hours plus, but that might be lower rev minimax, or they run lower rpm anyway. Running at 14000 + you need to change pistons by 25 hours.
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Mojo Tyres by 'DJW1' (24 Jul 2016 @ 18:44)
