built motor vs stock life span
 

so last night im sitting down with some friends and we begin to talk about some car stuff. when the topic of built motors vs stock came up. we began discussing If you were to build a reliable motor for racing lets say 500hp on a sti vs a stock one. if you never raced it and the two cars were drove the same never punished would you get the same amount of mileage out of the car vs a stock one? how about mpg with a tune on the built motor what kind of mpg would you guys think you could get:?: I was under the impression with a reliable motor build you could achieve the same if not better results due to the parts. thanks ahead of time.


im also aware that reliable and race car motor don't walk hand in hand usually but for the sakes of the argument lets be reasonable with this question.

I think a lot would have to do with the actual build on the race motor. A motor built specifically for racing where it's only expected to last a few dozen hours between rebuilds probably won't last as long as a stock motor as a daily driver. But I would guess that most "built" motors, ie motors that have forged pistons and good bearings but is otherwise built to relatively stock tolerances, will likely outlast an OEM motor on the street, if only because subarus' these days aren't exactly the longest lasting motors, at least on a stock tune.

As far as mileage... any built, high horsepower motor should theoretically get better mileage if it's driven conservatively. If a motor makes a ton more power, it's because the volumetric efficiency of the motor has been improved. So that should mean for better mileage. However, the reality is that a) generally no one will drive conservatively enough to counter for the times when they let it rip, and b) there are likely tuning differences to make the engine safe at high power levels that will hurt efficiency when driven conservatively, ie extra fueling for cooling, big injectors that idle rich, etc. So in practice, I would expect worse mileage in general.

What Scott said. If you resolve some of the stock motor's deficiencies, (Piston Ring lands and others) as well as improve components in terms of trueness, balance, fit and even finish (porting, polishing, etc...) without then increasing performance or driving differently, a "built" motor should have a much longer life. Why, because it is BUILT better...

Now if you have gone and changed compression, added boost, reduced tollerences too much, and/or drive like an asshat, all bets are off.