ok i thought up of a method to find the best rpm to cruise at but i dont have the neccesary data to do a test

basically my theory and assumptions go like this ...

if you have a dyno graph for the fit (with a/f ratios) then you have the horsepower at the wheels at any given rpm. in a fixed gear (5th cuz we're cruising) then we know the amount of horsepower we have at any given speed

now as i am unaware of the exact amount of drag the fit creates (dont have c/d, or frontal area) i'm just going to assume that the factory rated 180kph is the theoretical maximum (horsepower at wheels is all used to overcome aerodynamic and rolling drag)

assumption 1: assuming total drag is proportional to the square of speed we can figure out the amount of horsepower needed to maintain a certain speed at any given speed in 5th gear

now we have amount of horsepower we make at full throttle at a specific speed AND amount of horsepower required to maintain that speed, this gives us the amount of "extra" horses we have at any given rpm

assumption 2: this is a big assumption and i know it's not right but ive come up with no better substitute .... throttle input is directly proportional to horsepower output ... meaning 100% throttle gives 100% of the WOT horsepower figure (duh) .. and 50% gives 50% of this WOT horsepower figure at that rpm

so we can now express the amount of throttle needed to maintain speed at any given rpm, to go one step further we multiply this by the air fuel ratio to take into account mixtures vary throughout the rpm range (although at part throttle this figure will almost always be at or close to stoich).

the problem with my theory is roller dynos arnt really accurate and vary alot with many variables so to do this reliably we need either an engine dyno or a chassis dyno (one that attaches to the hubs), in addition the drag characteristics of each car vary even if no modifications have been made (a dirty car is less slipery then a clean car), and ofcourse my major assumption that throttle input is directly related to horsepower output (by throttle input i mean % of throttle movement in throttle body, not the pedal)

anyways i cant think of a better way of figuring this out other then a real life test which would be arduous and painstakingly long and inaccurate due to multiple confounding factors and random uncontrollable variables .... i know this is a really random topic and really really long read but any input would be greatly appreciated

 

The sweetest spot for any engine (Fit incl.) is the one at which power line and torque line are crossing ( power/torque graph ) In case of Fit 1.5 VTEC that's at app. 3700 rpm - VTEC is on at 3500 rpm. Try cruising in 5th at that rpm (on the flat road) and you will feel how engine is relaxed and yet you will have nice speed .

 

the power torque graph can intersect at different points if you use different units, the standard here is lbs-ft for torque and hp for power and it always itersects at 5252rpm

i really wanna get my hands on some hard data instead of conjecture as im sure we all have different opinions on where it's best to be based on experience but that's worth jack squat without the numbers to back it up

i personally believe that it's best to be either at 100km or 140kph and nothing in between just because ive had high MPG numbers when i cruised at these two speeds but have gotten terrible numbers cruising at the 120-130kph range, this makes no mathematical sense until you realize that there are confounding factors and variables which were not taken into account. no matter how many real life trials you have you're only gonna get close to the "real" answer, what i'm proposing is using deduction to figure out where this and so that we have something to aim at, as opposed to flying blind and by the seat of our pants

 

Heres a suggestion...see if any of the member who have the scangauge can get some data on fuel flow for you, in 5th gear, minimum throttle (only enough to maintain speed) and over a range of speeds (45-75). If you can get the fuel flow data, it will then become an exercise in math to find your optimum cruise speed. There are quite a few 'hard' numbers you can use, like the known qty of fuel in tank, the engine's displacement, rpm vs speed in 5th gear, speed/dis/time, etc.

EDIT: I'm assuming you're wanting to find the speed to cruise where you are maximizing your fuel efficiency and your velocity made good (getting there sooner). Usually these two are at odds w/ each other... but there should be a point where you get the best of both worlds, assuming constant cruise on level ground (continous known load on engine).

 

Desregard which unit we use ( Hp;Ps;KW - lb ft ; Kgm ; Nm ) power and torque line in the graph, will intersect at 3700rpm. Like max. power is at 5800rpm and max torque at 4800rpm - whatever units you like to go. RPM's are phisical constanta (revolution per minute) and only engine load.

 

um ... no it doesnt, horsepower and torque are interrelated and by that relationship any change in unit will affect where they intersect on a graph

 

Absolutely. Power = (Torque * RPM)
Torque will have a unit of meaure, as will power. In the case of Horsepower, James Watt somehow decided that the perfect horse could do 550 foot-pounds per second of work, and the Unit of Horsepower was born. An explanation on how we get to the 5252 constant value used in the HP formula can be found at Howstuffworks "How do you convert engine torque to horsepower?"

If we invented smaller unit, (Ponypower?), our cars would have a higher number of max PonyPower, the intersection point on the graphs would be different, and the actual engine output would be exactly the same. The intersection point between torque and horsepower has nothing to do with efficency, it's simply a function of how the units of power are defined.......

 

Aren't we talking about a couple different things here?

1. Best RPM for performance
2. Best RPM for fuel economy

 

just fuel economy, performance is a whole other story cuz

 

Performance makes no diference when cruising other than top speed, so I would assume we are talking about fuel economy. Once you start to accelerate that's a different matter, but in that situation there is no optimal RPM because the RPMs are going to change as you accelerate.

 

You are wrong. Numbers are relative. For example, 1 orange and 1 dozen eggs have nothing in common, even tho they share the number "1". 1 dozen eggs is 12 eggs. 1 orange could be 0.5lbs, etc. Units does matter b/c they are trivial and not real. In some units, the power/torque graphs never intersect.

I think you are confused. The intersection corresponds a "constant" k in the equation Power = RPM * tq * k

To answer the original poster, I figured Torque curve is not a good way to figure out engine characteristic. Torque is also based on throttle. Lead footing it at 2000 rpm may yield higher torque than feather footing it at its peak RPM 4800rpm. Although, Drive by wire might manually adjust the throttle such that you cannot use 100% throttle at 2000rpm, b/c honda feels it is a waste of energy to yield so much torque at that low rpm.
other countries like japan use...um..."watts" ("watt", like "newtons", was not dubbed by the person its named after)


edit: i remembered something. if you use hp and lb*ft curve, it ALWAYS intersects in the same RPM position regardless of engine, because of the definition of horsepower.

 

Power and Torque are (engine) outputs proportional to RPM. and corelated to each other. For math. calculations and diagram drawings they MUST be from the same "basket" of units - Imperial or Metric. Numerc value will differ betveen Imperial and Metric , but ABSOLUTE value will be the same !!!! Lets take Fit/Jazz L15A engine as a model for this execize:
Max.Power (5800RPM) 110 Hp = 80.85 KW
Max.Torque(4800RPM) 14.6 Kgm = 143 Nm = 105.39 lb ft
As you can see nominal values (numbers w/out units) are all different, but when they are placed in the SYSTEM OF UNITS they are equolized.
Have you noticed by now, that Power and Torque are produced at given RPM-s. 0 rpm = 0 Hp 5800 rpm = 110 Hp concecvently any RPM point (value) between 0 and 5800 has coresponding value in Hp or Kw or Pound feet per sec - Same applys on Torque output.
Power and Torque are constant in their absolute values ( we can convert them trough system/standard of units) and variable only against RPM-s

 

??

i'm not quite sure what you're trying to say, you can express torque in whatever unit you want, and power with whatever unit you want, it makes no difference

the definition of a horsepower is different from the definition of a watt, one hp = X lb-ft per minute while a watt = Y lb-ft per minute, if you transpose these 2 graphs onto the torque/rpm graph you get 2 different intersection points

if you use watts and newtons versus hp/lb-ft you get 2 different intersection point, etc.

the fact of the matter is the intersection point of power and torque gives no useful measurement or insight to the potential of the car's engine as it is the same for all engines

there are many useful attributes of a dyno sheet but the intersection point is not one of them

where max power occurs, where max torque occurs, the shape of the torque and power graph, the area under the curve (this is a biggie), the curvature of the graph (how linear power delivery is), just to name a few

what i'm proposing is you take the dyno read out, and you transpose a horsepower requirement vs speed (based on being in highest gear) onto the horsepower vs rpm graph to see how much excess horsepower you have at any given time, coupled with the air/fuel ratio data we can extrapolate the amount of fuel used for each stroke and by extention the amount of fuel per minute (that is if my assumption that X% gives X% power at that rpm). with the amount of fuel used per minute at a given speed we can then plot the amount of fuel used for any given distance at any given speed

it's got alot of holes, the 2 big ones being that my assumptions were off (which i'm sure they are but i'm assuming it's a relatively good approximation for what actually happens given thermal efficiency is relatively constant iirc and VE only degrades marginally)

 

side note ... why the hell am i in business and not engineering ...

 

One thing to consider, more waste is created at wider throttle positions. 50% of WOT may yield 60-70% of HP. You can find references to this fact, Im just lazy. SO...I guess at this point believe me if you like. Just think that with 1/2" throttle you get decent response, and floored, their is only a mild difference. Food for thought...

 

Being an Engineer , I can see clearly way you are not in engineering.

 

While I am very impressed and, dare I say, blown away with your claim as an engineer, engines are simply less efficient with the throttle wide open. Otherwise the same horse power/second/unit of fuel would be the same throughout the throttle position and RPM. That would truly be a miraculous feat of engineering. I happen to know of several vehicles that claim 80% of their hp being generated at half throttle (I think that Hyundai had a whole promotion based on the increased power with less fuel). It would be naive to think that everything works on linear curve that has no variances for drag based on speed of the engine. If you can find some real evidence other than you are some sort of engineer, oh sorry you capped it; Engineer, I will stand by simple logic and four years of physics/diff eq.<img>

 

My comment was to KnifeEdge and his 'side note'

 

Wow! I sincerely apologize for my sarcasm and sinicism. I have been grumpy (have a newborn and averaging 2 hours/sleep a night) and have been quite sick. Again my apoligy is for you, and I hope you can accept it. I was rude.

 

No problem - honestly . I know how it is (how it was, my Son is 21)
All the best to everyone on your end, specialy to newborn