Pretty much, all the ratings are low. You can't make the ratings so low, they automatically S&P. You either get lucky, they blow up, have some sort of mechanical problem and retire, or are just slow as crap the whole race.
Here is what I do. I rate any Start and Parks a zero for reliability. Everyone else is rated 100. Then in the papy_ai I changed the mechanical_failure_chance and mechanical_failure_interval lines.
The mechanical_failure_interval I changed to 245, I think that is half of the original but I'm not sure if my value is the default with the game or not. Then for mechanical_failure_chance I vary it for races, normally between 13.5 and 48.5. For races with a lot of S&Pers I put it at 48.5 and races with not too many I'll put it at 13.5. I normally only change it in increments of 5.
I also upper the chance for engine failure from the default. I've never come to a good ratio between it and the other problems, I tinkered with it a lot and never come up with what seems like the perfect ratio. But that's a personal opinion so you'll have to figure out what you like. But even left at default I think would work for you. I'm a little more of a perfectionist on things.
For my last race at Iowa in the NNS race, 250 laps. These were the laps the S&Pers retired. 0, 1, 2, 8, 9, 10, 13, 20, 37, 43, 69, 73, 75, 89, 102.
Also even with the reliability set to 100 for non S&Pers. That doesn't mean they are immune to having issues. They still have some just that as often.