diff gears in relation to dyno output result

[ari666]

hi all. i remember seeing a dyno reading once where the torque curve was something spastic like 755NM, but the HP curved topped out at 200 is HP. some members said it was the result of the car being tested in 3rd gear instead of 4th and was wondering if final drive would have an effect on the figure read off a dyno.

discuss:

[ari666]

no one?...

[HARVZ-6]

Lower the gear = more torque

basic principle, hp= torque/time

torque gets you moving, hp keeps you there

if that makes sense

faster it spins, less torque

[old mate]

my physics knowledge is limited but i'll give it a go.
horsepower is calculated like this.

hp=(torque x rpm)/5252.

gear choice for a dyno run has a huge effect on the torque reading that is given, because of how how the ratio acts as a torque multiplier. as does wheel size. so does the diff ratio. so does a torque converter.
so when you're getting a torque reading at the wheels, its showing the force that is acting on the dyno rollers themselves, not the force that the engine is emitting. its usually called tractive force.
i think you can calculate engine torque on a chassis dyno, but its super hard. heaps of ratios and stuff like bearing drag etc.

ps. im sure i could answer it better but its bed time.

[ari666]

ok, so if the dyno guy didnt ask me what my diff gears were and entered in 3.08 instead of 3.55 would this mean my HP would increase or decrease?

[klampy]

im not sure your horsepower would change, as horsepower is defined as the "rate at which work is done" and torque is related to the turning force of the crank, or something along the lines of the amount of "work" that is available to be accelerated by the "horsepower". if my memory serves me, i could be wrong. stranger things have happened.

[old mate]

i don't reckon the horsepower would be any different, but there would be more torque.

[VNexecutive]

how can the horsepower be the same with the torque reading being different? a car with lower ratio diff gears is gunna give a higher hp reading off the dyno than the same car with standard diff ratios. if i understand the way a dyno works properly.

in theory, if the fact he put in 3.08 diff ratios instead of 3.55 does make a difference, then you would have got a higher reading than if he put in 3.55 instead of 3.08

thats the way i look at it anyways. im sure if im wrong, someone clever will come and correct me.

[Brados]

From my Mechanical Engineering knowledge, Power (KW or HP) is power the engine can create and is usualy constant. Changing dif gears would increase torque but lower the maximum RPM that can be acheive thus the overall power of Engine (Torque*RPM) is still constant. So would have no effect on final maximum HP/KW.

I dont have any Direct experience with Dyno's and testing though so whether this translates to real world experimental data as there are other factors to consider that effect power output such as friction losses etc that vary with RPM. Need a dyno guy to give us the answer to this one. But i would expect it to be a small variation.

[Not_An_Abba_Fan]

Ahhhhh.....the old "pretend" HP/kW figure.

A dyno measures torque, nothing else. It can't even tell what RPM the engine is doing without some input from the user. The dyno then uses the input data to give a torque reading. It then calculates the "power" figure from that using a formula.

By changing diff gears you are changing torque so the "power" figure has to change. But then it also depends on where that peak torque is achieved in the rev range and that will have an effect on the end result as well.

Example, say you pulled 300ft lb of torque at 5000rpm, that calculates to 285hp, but if you went a bit shorter in your diff gears and got 332ft lb of torque at 4500rpm, it still equates to 285hp. But....say it pulled the same 300ft lb at 4500, you would get a drop down to 257hp, even though you haven't done anything to the engine.

[ari666]

ok, so from old mates calculation,

197.8= ( A x 5900) 5252

A= 176.075 flbs @ 5900rpm? seems pretty damn low... 238 NM ... weird since a stock VK black is supposed to produce 361@2800rpm.

but wait.. for that calculation to work, i would need to know peak rpm for torque???? i only know peak rpm for KW. how does that work?>???

[ari666]

ok, now i am full of question marks.

how can it be, that if torque is the ONLY figure really being calculated, HP will quite often go up the rev range quite a lot further. random internet example pics:







a lot of the dyno sheets i have looked at, the torque is completely unrelated to the HP curve. torque generally peaks early and peters off, whereas HP keeps climbing.

[ari666]

so is there no explanation?

[HRT 666]

Thats because HP = torque X rpm so even if the torque starts to drop off slightly but the rpms continue to climb the HP will continue to climb also.
Only if the torque drops off at a large rate as the rpms climbs then the HP will feather off.
Make sence now?
The torque curve will never be UNRELATED to the HP curve as without torque and rpm you will have NO HP.

[ari666]

that makes no sense at all.

so then given my calulation (if i have done it correctly) you saying that somehow i have managed to make the power (KW) increase dramatically over stock, but make the torque decrease ASTRONOMICALLY

ill drop the stats again for those that cbf reading

147.5 RWKW
197.8 HP
176.075 ftlbs torque

[HRT 666]

No that is NOT what I said. For you to end up with any increase in power you will HAVE to have an increase in torque at the same rpm as before.
As it just wont happen with out the torque (unless you kept the same torque and doubled the rpm) and that would not of happened.
Put up the before and after dyno sheets.

[old mate]

HRT's got it. look at this for an example, using the EVO X dyno sheet.

3650 rpm there is near on 320 lb/ft.
(3650 x 320)/5252 = 222.3914 hp

but later on when the torque tapers back to about the same level as before, but at higher rpm it works like this:

5600 rpm and close to 320lb/ft.
(5600 x 320)/5252 = 341.2034 hp.

so the only reason the power figure has gone up is because the torque figure is made at a higher rpm.

[VT-565]

Go to the drag strip.

If it pulls the wheels after a warmup, its got enough torque

[old mate]

ok ari666, i just checked out your dyno sheet. lets do some calculations here to work out whats going on.

its pretty blurry so my numbers wont be 100% spot on. so bear with me.

first lets pic some spots in the rpm range. i might as well do every 200 rpm and then make a graph in microsoft excel. would help my statistics study haha.

im converting from kw to hp at 1.341hp per kw.

2800 = 85.854hp (64kw)
3000 = 100.575hp (75kw)
3200 = 109.962hp (82kw)
3400 = 120.69hp (90kw)
3600 = 135.441hp (101kw)
3800 = 147.510hp(110kw)
4000 = 156.897hp (117kw)
4200 = 162.261hp (121kw)
4400 = 171.648hp (128kw)
4600 = 174.330hp (130kw)
4800 = 182.376hp (136kw)
5000 = 187.740hp (140kw)
5200 = 190.422hp (142kw)
5400 = 189.082hp (141kw)
5600 = 191.763hp (143kw)
5800 = 197.127hp (147kw)
5900 = 197.7975hp (147.5 kw)

alright, so we have all the hp figures and rpms for every 200rpm from 2800 to 5900.
if we apply the formula “torque = (5252 x HP) / rpm” to each of these we can work out the torque at each spot. here goes.

Im converting ft/lb to nm by multiplying by 1.356

2800 = 85.854hp (64kw) 161.0376 ft/lb 218.3670 nm
3000 = 100.575hp (75kw) 176.0733 ft/lb 238.7554 nm
3200 = 109.962hp (82kw) 180.4751 ft/lb 244.7243 nm
3400 = 120.69hp (90kw) 186.4306 ft/lb 252.7998 nm
3600 = 135.441hp (101kw) 197.5934 ft/lb 267.9366 nm
3800 = 147.510hp (110kw) 203.8743 ft/lb 276.4536 nm
4000 = 156.897hp (117kw) 206.0058 ft/lb 279.3438 nm
4200 = 162.261hp (121kw) 202.9035 ft/lb 275.1372 nm
4400 = 171.648hp (128kw) 204.8853 ft/lb 277.8245 nm
4600 = 174.330hp (130kw) 199.0394 ft/lb 269.8974 nm
4800 = 182.376hp (136kw) 199.5497 ft/lb 270.5894 nm
5000 = 187.740hp (140kw) 197.2021 ft/lb 267.4060 nm
5200 = 190.422hp (142kw) 192.3262 ft/lb 260.7944 nm
5400 = 189.082hp (141kw) 183.8898 ft/lb 249.3681 nm
5600 = 191.763hp (143kw) 179.8463 ft/lb 243.8716 nm
5800 = 197.127hp (147kw) 178.5019 ft/lb 242.0486 nm
5900 = 197.7975hp (147.5 kw) 176.0733 ft/lb 238.7554 nm

ok so now we have all the data as an estimate of your rear wheel output. i might make some pretty graphs in a sec for it. i must say though, your engine doesn't have heaps of torque.

[ari666]

I have never been so thankful till right now. thats brilliant, thanks mate. really appreciated.

yep. my engine is shit.

[old mate]

haha no worries, i was bored and was a good excuse to mess around. it wont be 100% accurate but it does give a good indication of the shape and size of the curves.

it must feel pretty linear to drive i reckon.

[ari666]

it must feel pretty linear to drive i reckon.

the weird thing is that its one of the most stupidly happy cars i have ever driven. i dunno how to explain it. even if the graph says its weak, there have been times i would swear that graph is way off.

but yeah, you answered the question above and beyond what id hoped for so rep your way.

[peterg2121]

hey guys i dont know if this is an old post but in relation to diff gears ive had first hand experience and i work as a mechanic they dont increase your torque or your horsepower what happens is basicly you have a smaller ratio so you take off harder only letting your torque kick in earlier so running down the quarter would be great

so say if a car has 200 horsepower and 150nm (nm is newton meters) torque e.g like how tight you do up a wheel nut 90nm if you ran your car on a dyno on the standard set of diff gears say the car on those gears has a max speed of 200kmh

a smaller set of diff gears whitch is good for your drag racing you use the same power same torque but just the car always seems to have torque becouse it doesent require as much to push the car forward on a smaller set of gears

a tall set of gears will help your top speed but they end up being harder to turn and etc

if you look at a dyno sheet with a torque curve reading
1) standard set of diff gears you might say see a say 45 degree diagonal line
2) short set of diff gears you will see the line shoot straight up say a 70 degree line all it is is the power coming on all in one go rather then gradually
3) tall set of diff gears you will see a long line say a 30 degree line

but its a curve not a perfectly straight line

thats what i know dont bag me out just trying to help if you dont understand ask me and i will try and explain it better

[bauer5152]

From my Mechanical Engineering knowledge, Power (KW or HP) is power the engine can create and is usualy constant. Changing dif gears would increase torque but lower the maximum RPM that can be acheive thus the overall power of Engine (Torque*RPM) is still constant. So would have no effect on final maximum HP/KW.

I dont have any Direct experience with Dyno's and testing though so whether this translates to real world experimental data as there are other factors to consider that effect power output such as friction losses etc that vary with RPM. Need a dyno guy to give us the answer to this one. But i would expect it to be a small variation.

I also think so.

[ari666]

here is an interesting fact. the torque curve and HP curve should always intersect at 5252rpm or the dyno reading is false

true story bro.