calculating correct dyno data.

[AMXstocker1]

i'm not sure if this can be done or not but we went to a chassis dyno awhile back and they didn't hookup a lead to measure engine rpm they let the computer back calculate off of gear ratio and rear wheel rpm. and i thought at the time this was sketchy info at best because our car has a 8 inch 6000rpm converter in it that at the very least is going to take some time to catch up so long story short we only made 2 pulls becuase of issues but the dyno said 440hp and 650lbs of torque at 4500rpm and this after i just got finished watching the tach hit 7000rpm and the shift light come on thats set to that rpm so my question is, is there a way mathematically to calculate the correct reading for the actual engine rpm? i was trying to work it out but i'm not sure how to do it accuratly simply taking the torque # of 650 and multiplying it by 7000 and dividing it by 5252 gives something like 820 hp which is absurd for our combination.

[rustbucket79]

Torque converters multiply torque. Not sure how it works but I think max multiplication is at full stall wheels locked then as acceleration continues the multiplication decreases.

I'd like to know if this is factual.

[MadBill]

With a high stall converter, chassis dyno numbers are useful mostly just for test to test comparisons; there's just too much unpredictable slippage.

[AMXstocker1]

agreed i just wish they would've used another way to track rpm like a lead on the #1 plug wire or something anyway the engine got freshened over the winter and should be going on a engine dyno next friday. so we will get real data.

[gruntguru]

If you don't know rpm forget the torque number on a chassis dyno. OTOH the power number doesn't care about gearing. Chassis dyno power = engine dyno power minus losses.

You can calculate flywheel torque from the power number and the rpm. So 440 x 5252 / 7000 = 330 lb ft plus losses eg 20% loss => 330/1.2 = 396

Torque converters multiply torque until the stator starts to freewheel above which rpm the torque out = engine torque.

Yes Rustbucket the torque multiplication drops with rpm until it reaches 1.0, then stays at 1.0 from there up.

[AMXstocker1]

i still don't think thats right i guess we well see what it actually makes at the flywheel. i don't think you can take one side or the other and change the rpm factor and get any real info. you would have to back calculate to newtons or watts then move back forward i'm not sure what the dyno sees and sadly that motor didn't get down a drag strip last year so i don't have that to go off of.

[John Wallace]

440 HP = 328 kW

Same thing just different name.
(S.I. or metric or imperial etc)

[digger]

If you don't know rpm forget the torque number on a chassis dyno. OTOH the power number doesn't care about gearing. Chassis dyno power = engine dyno power minus losses.

You can calculate flywheel torque from the power number and the rpm. So 440 x 5252 / 7000 = 330 lb ft plus losses eg 20% loss => 330/1.2 = 396

Torque converters multiply torque until the stator starts to freewheel above which rpm the torque out = engine torque.

Yes Rustbucket the torque multiplication drops with rpm until it reaches 1.0, then stays at 1.0 from there up.

i wont pick on your claims of 20% as that is a can of worms but 330/1.2 is not 396. nor is 330 at wheels going to be 396 at flywheel assuming the losses were 20%. 330/0.8 = 412.5

[Stan Weiss]

330 * 1.2 = 396

Stan

[digger]

330 * 1.2 = 396

Stan

not how it works

80% is 330 so 100% is 412.5

[gruntguru]

Correct. 320/0.8 = 412.5.
Apologies for the stuff up.

[rustbucket79]

If you don't know rpm forget the torque number on a chassis dyno. OTOH the power number doesn't care about gearing. Chassis dyno power = engine dyno power minus losses.

You can calculate flywheel torque from the power number and the rpm. So 440 x 5252 / 7000 = 330 lb ft plus losses eg 20% loss => 330/1.2 = 396

Torque converters multiply torque until the stator starts to freewheel above which rpm the torque out = engine torque.

Yes Rustbucket the torque multiplication drops with rpm until it reaches 1.0, then stays at 1.0 from there up.

Thanks for the confirmation.

[slo-svt]

I'm a strong believer in using the inductive pickup or obd connector if your not using it to datalog. I have seen too many instances where the engine speed calculated by the dyno is incorrect. That IMO is one of the main contributors to the reason chassis dyno numbers are all over the place. You can see two cars with the same induction tract and one dyno says it made 40 more hp and peak 3-400rpms higher. I have even seen this on a manual cars. Other contributing factors include tire type, air pressure in the tires, how tight the straps are, ect. Chassis dynos are great for tuning. I would assume the new hub dyno's eliminate many of these problems but I haven't had the luxury to try one yet. Don't get hung up on the numbers. Go the track and weigh the car. You will have much better luck figuring everything backwards from that.

[digger]

I'm a strong believer in using the inductive pickup or obd connector if your not using it to datalog. I have seen too many instances where the engine speed calculated by the dyno is incorrect. That IMO is one of the main contributors to the reason chassis dyno numbers are all over the place. You can see two cars with the same induction tract and one dyno says it made 40 more hp and peak 3-400rpms higher. I have even seen this on a manual cars. Other contributing factors include tire type, air pressure in the tires, how tight the straps are, ect. Chassis dynos are great for tuning. I would assume the new hub dyno's eliminate many of these problems but I haven't had the luxury to try one yet. Don't get hung up on the numbers. Go the track and weigh the car. You will have much better luck figuring everything backwards from that.

Wrong engine rpm affects "effective/derived torque" not power you can get a power figure without engine rpm. It uses roller surface speed as x-axis in mph or km/hr

[slo-svt]

I'm a strong believer in using the inductive pickup or obd connector if your not using it to datalog. I have seen too many instances where the engine speed calculated by the dyno is incorrect. That IMO is one of the main contributors to the reason chassis dyno numbers are all over the place. You can see two cars with the same induction tract and one dyno says it made 40 more hp and peak 3-400rpms higher. I have even seen this on a manual cars. Other contributing factors include tire type, air pressure in the tires, how tight the straps are, ect. Chassis dynos are great for tuning. I would assume the new hub dyno's eliminate many of these problems but I haven't had the luxury to try one yet. Don't get hung up on the numbers. Go the track and weigh the car. You will have much better luck figuring everything backwards from that.

Wrong engine rpm affects "effective/derived torque" not power you can get a power figure without engine rpm. It uses roller surface speed as x-axis in mph or km/hr

Could you go into more detail on how the calculations are performed.