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If you want to be absolutely safe as far as valvetrain goes, 3200 max. That being said, many have done 4000 on stock springs. If you do it and keep doing it, its a ticking time bomb.
i pull 4k sled pulling because im a manual i stay in 4th gear and let it eat! ive done about 25 pulls like this and still no problems. (knock on wood).. haha