turbo or no

[jayjack]

i know there is a section for forced induction topics but the question i have isn't directally about forced infuction in itself. ok so i was wondering is it better to have a car with a turbo making like 200hp (such as the saab 9-5 with the v6 rated for 200hp @ 5600rpm) or a nat aspitated car with 200( like the 99' maxima rated with i think 200 @ like 5500rpm). so really my question is if i dropped the engine from the STI in one car and the the engine from a 350z into the exact same car, assuming that they both get 300 hp at like 4500 rpm(not a true just assuming). which would be faster, or would the difference be nominal.

[UncleBob]

as a general rule, the turbo engine will have a higher peak torque, but have a less linear powerband. Its really a question of what kind of 'test' you're doing (there are many ways to define "fast")

Seat of the pants, the turbo engine will likely FEEL faster because of the earlier, and higher peaking torque. But that doesn't mean it'll be faster through the entire RPM range....actually, I'd say its unlikely

Future modification options is another issue, unless you know for a fact you'll never be interested in going any further. The turbo engine could probably be improved apon with little expense/difficulty compared to the NA engine

just some general thoughts

[curtis73]

hp is hp. Its hard to tell which would be faster since I don't have dyno charts in front of me, but if they both put out the exact same outputs (tq and hp) they would be identical. A turbo engine is a naturally aspirated engine 80% of the time. During street driving with part throttle, its not forced induction. That would probably give it an MPG advantage, maybe a weight advantage... at the expense of a little more maintenance and probability for mechanical failure.

But as far as quarter mile times are concerned, HP is the biggest factor. One car with two different 300-hp engines should be able to make the quarter mile in the same time with obvious things like engine weight and traction to consider.

Torque is also a factor in more extreme examples. Comparing a 350z engine to a 2.5L suby isn't going to be much different, but in extreme cases it will. For instance, if you put that 300-hp subaru turbo in a heavy car, then swap it out for a 300 hp LS1, the LS1 will probably beat it due to its distinct torque advantage. Torque is what overcomes inertia from a dead stop and the LS1 would get you off the line quicker. Once you're moving, though, its HP that is doing the work.

[beef_bourito]

a bit off topic but somewhat relevant:

with a 300hp turbo car, it will most likely be a smaller engine. but i'm not so sure you'd get better fuel economy (i haven't seen any real world comparisons though). the turbo vehicle, while smaller in displacement, is still outputing the same power, which means for every unit of time it's using the same ammount of energy. it's also got a restriction in the exhaust causing it to be less efficient.

what would give it superior fuel economy? could it be that because it's a smaller engine you can run a greater load (more throttle) and thus reduce the restriction caused by the throttle body?

[KiwiBacon]

See if you can get the power curve for each engine.
In general the one with a larger area underneath the curve will be fastest. Because on average it can produce more power than the other, even if the peak might be lower.

[Reed]

I'm just throwing this out there...

The biggest appeal of a turbocharged engine is that you can make the power of a bigger 6 or 8 cylinder engine without all the weight. That being said, the STI motor is horizontally opposed and therefore not as light as an I or V engine. Although, being an STI owner, the 2.5T must be the better of the two engines.

[jayjack]

going back to what beef burito said abouth the MPG im not really sure but wouldn't it only produce 300hp at full boost? therefore in street driving you wouldn't have the tubo kicking in most of the time, so you would be using less than 300hp worth of gas, so better fuel ecomony. i think...

[UncleBob]

Originally Posted by jayjack

going back to what beef burito said abouth the MPG im not really sure but wouldn't it only produce 300hp at full boost? therefore in street driving you wouldn't have the tubo kicking in most of the time, so you would be using less than 300hp worth of gas, so better fuel ecomony. i think...

although the turbo is not spooling while cruising, it is still a restriction that robs efficiency. The other issue is most (not all) turbo engines have lower compression ratio than an equiv. naturally asperated engine, which will reduce efficiency overall.

I lost about 5mpg overall when I turbo a bike. Percentage-wise, thats not a huge amount.