Outer Limits 4x4 Board

how much power would a 4x4 drivetrain take?

running 35 inch tyres on my 45. im going to get it dyno's in a few weeks and was just wanting to know the hp of the engine.

im guessing rear wheel hp would be around 65%-75% that of the flywheel.

thanks for your input.

MaccA

The bottom line is, it's impossible to tell accurately without actually fitting the motor up to a flywheel dyno. You CANNOT (despite what 'expert' hot rodders might tell you) just add a % figure to your axle horsepower to get an accurate indication of flywheel horsepower. The drivetrain losses in any vehicle are not a simple percentage - things like which gear you do your run in, the condition of your gearbox and diffs, auto or manual transmission etc etc all effect the difference between flywheel and rear wheel power/torque, and the % loss changes throughout the rev range - and not even as a simple function of rpm. Some losses are a fairly constant HP number (eg friction in diff and gearbox bearings) some change in a non-liear fashion with rpm (friction losses due to gear tooth contact), some are a straight line relationship with rpm (eg inertia of rotating components) If you're using the dyno time to improve tuning then it's worthwhile, but if you're just doing it to find out how many herbs your truck has, you're wasting your time.

cheers

Brian

Plus lose's with highly angled UJs,so factor in your lift.
J Top

dumbdunce wrote:The bottom line is, it's impossible to tell accurately without actually fitting the motor up to a flywheel dyno. You CANNOT (despite what 'expert' hot rodders might tell you) just add a % figure to your axle horsepower to get an accurate indication of flywheel horsepower. The drivetrain losses in any vehicle are not a simple percentage - things like which gear you do your run in, the condition of your gearbox and diffs, auto or manual transmission etc etc all effect the difference between flywheel and rear wheel power/torque, and the % loss changes throughout the rev range - and not even as a simple function of rpm. Some losses are a fairly constant HP number (eg friction in diff and gearbox bearings) some change in a non-liear fashion with rpm (friction losses due to gear tooth contact), some are a straight line relationship with rpm (eg inertia of rotating components) If you're using the dyno time to improve tuning then it's worthwhile, but if you're just doing it to find out how many herbs your truck has, you're wasting your time.

cheers

Brian

i worded it badly. im going to get it tuned on the dyno. then try and work the engine power out from that.

i understand that there are be variables, and theres no exact answer, but most good dyno people should know roughly how much (as thats what i was after).

4wd monthly tested oils a few issues back, using a dyno. new 4.8L patrol was putting out 142kws. from a said 185kw engine.

gearing wont change power, it will only change torque.
double torque at half the revs = same kw

383FJ45 wrote:
im going to get it tuned on the dyno. then try and work the engine power out from that.

that's just it - you can't. You can use a chassis dyno to measure axle horsepower, and you can use a flywheel dyno to measure flywheel horsepower, then compare the figures to determine an overall efficiency curve for a given gear/diff combo, so then you can say 'holy crap, at 1452rpm in low range 3rd gear only 61% of the engine power is getting to the wheels!' but you can't work it backwards, you can only guess. The Dyno Man will probably tell you to just add 25 - 45% to the peak axle power figure to make an approximation of peak engine power, but chances are the magic number you come up with will be far higher than the engine's real flywheel peak power, and the number doesn't mean anything anyway except for poseur value - it's axle torque that makes tractive force and that is what accelerates the vehicle/drives it up the rock face.

i understand that there are be variables, and theres no exact answer, but most good dyno people should know roughly how much (as thats what i was after).

unless the dyno man has done flywheel/chassis comparisons for similar setups, even his best guess is only a guess and could be out by 20% either way (more likely he will err on the high side to make you feel happy)

4wd monthly tested oils a few issues back, using a dyno. new 4.8L patrol was putting out 142kws. from a said 185kw engine.

the 185kW is the power claimed by the manufacturer, it's not a flywheel dyno figure. do you know if it SAE gross/net/corrected? DIN? is that the power it develops on regular unleaded or on fancy Japanese 99RON fuel? The numbers are just marketing unless applied to a specific engine under specific conditions.

gearing wont change power, it will only change torque.
double torque at half the revs = same kw

you're assuming gears are 100% efficient, or at least that every gear set in your gearbox/transfer is of a similar efficiency that doesn't change, which is far from the case. The efficiency of the diff for example is massively effected by the load it is under - hypoid gear efficiency is poor to start with and gets worse as the loading increases. The efficiency of the gearbox is likewise effected by load, and by the chosen gear - the lower the ratio (numerically higher) the lower the efficiency of the gear set. 4th gear is a special case where no gears are employed in the gearbox so the efficiency is exceptionally high - still not 100% as there are losses to the still rotating mainshaft gears and counter gear, bearing losses etc. The gear you choose for your run and the setup of the dyno will have considerable effects on the figures you get out. Chassis dyno fugyres are really only good for comparison - for comparing with flywheel dyno figures to determine the efficiency of the drivetrain for a given set of ratios, but more usefully for comparing power at the wheels before and after modifications and tuning to attempt to accurately determine the value of the modification or tuning. Making up flywheel figures from chassis dyno figures is not a black art, it's at best a guess, at worst just a big fib.

I know this isn't the answer you're looking for but that's the way it is. I'm sure the opinions of some meathead hot-rodders (no disrespect intended, meatheads ) will vary.

cheers

Brian

A good freind owns 1 the latest dyno dynamics dyno's and its pretty funny to see everyones reaction to hp figures. We made a few dyno cars for summernats a couple of years ago, as the dyno really only measures rotational torque vs rpm to give hp (hp=tq x rpm), so a stock car will make way less hp than i with say 5.29 diff gears and small diameter tyres.
as said it is typical for dyno shops to say that you lose approx 30% but if i had a manual gearbox chewin 1/3 of my hp at 1:1 i would rebuild it or take off the handbrake.

After having a dyno and doing a super tourer that usd to get the engines dynoed on an engine dyno and then would check settings on our chassis dyno once engine went back in there are way to many variables to accurately calculate the loss through the drive train. We would have up to 10 % difference from the different compounds of slicks and wet wether tyres as well as tyre pressures.As odd as it sounds even different suspension setups made a difference.
Even though it had a million dollar gearbox and the best of everything in the drivetrain we would have anywhere between 10-20 % loss.
SAM

dumbdunce wrote:

383FJ45 wrote:
im going to get it tuned on the dyno. then try and work the engine power out from that.

that's just it - you can't. You can use a chassis dyno to measure axle horsepower, and you can use a flywheel dyno to measure flywheel horsepower, then compare the figures to determine an overall efficiency curve for a given gear/diff combo, so then you can say 'holy crap, at 1452rpm in low range 3rd gear only 61% of the engine power is getting to the wheels!' but you can't work it backwards, you can only guess. The Dyno Man will probably tell you to just add 25 - 45% to the peak axle power figure to make an approximation of peak engine power, but chances are the magic number you come up with will be far higher than the engine's real flywheel peak power, and the number doesn't mean anything anyway except for poseur value - it's axle torque that makes tractive force and that is what accelerates the vehicle/drives it up the rock face.

i understand that there are be variables, and theres no exact answer, but most good dyno people should know roughly how much (as thats what i was after).

unless the dyno man has done flywheel/chassis comparisons for similar setups, even his best guess is only a guess and could be out by 20% either way (more likely he will err on the high side to make you feel happy)

4wd monthly tested oils a few issues back, using a dyno. new 4.8L patrol was putting out 142kws. from a said 185kw engine.

the 185kW is the power claimed by the manufacturer, it's not a flywheel dyno figure. do you know if it SAE gross/net/corrected? DIN? is that the power it develops on regular unleaded or on fancy Japanese 99RON fuel? The numbers are just marketing unless applied to a specific engine under specific conditions.

gearing wont change power, it will only change torque.
double torque at half the revs = same kw

you're assuming gears are 100% efficient, or at least that every gear set in your gearbox/transfer is of a similar efficiency that doesn't change, which is far from the case. The efficiency of the diff for example is massively effected by the load it is under - hypoid gear efficiency is poor to start with and gets worse as the loading increases. The efficiency of the gearbox is likewise effected by load, and by the chosen gear - the lower the ratio (numerically higher) the lower the efficiency of the gear set. 4th gear is a special case where no gears are employed in the gearbox so the efficiency is exceptionally high - still not 100% as there are losses to the still rotating mainshaft gears and counter gear, bearing losses etc. The gear you choose for your run and the setup of the dyno will have considerable effects on the figures you get out. Chassis dyno fugyres are really only good for comparison - for comparing with flywheel dyno figures to determine the efficiency of the drivetrain for a given set of ratios, but more usefully for comparing power at the wheels before and after modifications and tuning to attempt to accurately determine the value of the modification or tuning. Making up flywheel figures from chassis dyno figures is not a black art, it's at best a guess, at worst just a big fib.

I know this isn't the answer you're looking for but that's the way it is. I'm sure the opinions of some meathead hot-rodders (no disrespect intended, meatheads ) will vary.

cheers

Brian

i asked a question and you answered it (thanks for going into detail of how it works and stuff )

meathead =

DeWsE wrote:
Dumbdance you are a funny man indeed

i dont really care what the numbers are coz its got enough balls to do what i want. like you said, numbers are for poseurs. kw sells cars, torque wins races. i just get bored and think too much. (or not enough)