Torque/horsepower metrics fully measure how quickly an engine can rev?
 

I'm thinking of purchasing a 2011 Ford F-150 with the 5.0 instead of the EcoBoost. While reviews of the 5.0 engine in the F-150 are scant, it looks like it revs pretty well (and is pretty quick), based on some of the recent Ford events for journalists.

My question is: Do horsepower and torque together describe how quickly an engine revs up to its peak RPM? Before you say "torque does that," I'm reminded of big, powerful engines (say, even a ship engine as an example) that may have massive torque, but are slow to increase RPMs quickly despite that huge torque.

So, to put my question another way, if we take any engine, with or without load, apply full throttle and measure how much time it takes to go from idle to max RPM, is the torque completely responsible in determining how much time it takes for that engine to achieve peak RPMs or is this information not completely captured by torque metrics?

Not an engineer, but I would have to say torque isn't the only thing behind quick revving.

Flywheel and reciprocation parts weight, air/fuel mix quality, ignition, intake, port, valve design--all kinds of factors.

And don't forget electronic engine management controlling valve timing, ignition, fuel, etc.

That sounds like a pretty good answer: lots of factors. So, there's more to factoring quickness to rev, i.e., it's not just torque alone as only the "strength of twist" and the "strength to add faster twist," but other factors come in to play.

I'm not an engineer either, and IMO yes, everything you listed is indeed a factor--of making torque; it all contributes to it...but once the torque is "set", you cannot alter those factors and still have the same torque curve; they will all affect it in some way in the end....so for two similar engines I don't think one can say "yes our engines have the same exact torque curve, but since you have 2x the reciprocating weight in the rotating assembly, mine will rev quicker"--the greater reciprocating weight [or any of the other factors] internally "robs" the engine of net measurable torque, so they wouldn't have the same exact brake-torque curve to begin with--a contradiction, in essence.

what I'm picturing in my mind is for example, an engine strapped to an engine dyno where all those factors are aleady set and "seen" by the dynamometer as raw torque output......once all that stuff is predetermined and set, the engine would be endowed with a certain torque curve dictated by all those factors.....for a given amount of load behind the engine, the more area under the entire torque curve-vs-RPM graph, the quicker it will indeed rev to its RPM limit, or HP peak, or which ever predetermined limit you have chosen

for those mathematically inclined, the area under the torque-vs-RPM graph is analogous to a force-vs-distance graph.....force x distance = total work done...in rotational systems, torque is the analogue of force, and revolutions are the analogue of distance....the integral of either curve, is a number that tells you how much total work can be/is done; precisely equivalent to total power done by a system anyway you look at it

just my $0.02

great site you all have here btw

Well I'm no great Gear Head but I can tell you this from first hand experiance.
Just sinply changing from a stock Steel Flywheel to an Aluminum one will greatly change how quickly an engine can rev up and down.

Now the New 5.0 and the 3.7 both are blessed with FULLY counterballanced cranks, they have two right in the middle that are skipped on the older cranks.
This will ensure no High RPM vibration problems. Happy Days !!!


Cheers John

Yup, you're right, and it only adds to my point. The lower rotational inertia (equivalent to mass for rotational systems) of an aluminum flywheel/flexplate will "free up" torque at higher RPMs because there is less torque "robbed" from the engine. This could be seen as an increase in raw measured torque if the engine were strapped to an engine dyno. So you can't have two identical engines with identical torque curves where one engine has a different (lower) reciprocating weight. The one with lower reciprocating weight will indeed produce a tad more torque over time and rev just a tad faster. Now will it be measureable? That would depend on how much load is behind the engine--case in point, an engine in neutral will rev faster than when engaged in gear.