# Horsepower from acceleration

1. Nov 29, 2006

### gt5816v

I'm trying to calculate a vehicles bhp based on acceleration.
I'm using RPM time stamped in milliseconds.

Tire Dia= 0.62738 meters
R&P= 3.23
3rd Gear= 1.66
Mass= 1500 Kg

Wheel speed= Engine RPM/(R&P*Gear)
Velocity= WS*Tire Dia*0.0523598
M*(Dia/2)*1000*((v2-v1)/(t2-t1))=Nm
Nm*0.001341022/5.3618= Ft.lbs
Ft.lbs*RPM/5252=hp

Does this seem correct? Obviously this does not factor for aerodynamic losses.

Time RPM
426273 860
426897 984
427193 1066
427487 1143
427785 1245
428080 1350
428375 1463
428669 1577
428964 1705
429258 1823
429556 1951
429850 2093
430145 2238
430439 2400
430735 2541
431030 2715
431324 2852
431617 3013
431912 3156
432207 3314
432503 3471
432798 3646
433094 3827
433388 3956
433684 4154
433977 4299
434273 4458
434569 4613
434865 4789
435160 4950
435456 5085
435751 5240
436053 5382
436348 5521
436643 5657
436938 5770
437234 5919
437529 6039
437824 6176
438120 6307
438414 6403
438709 6538
439005 6616
439300 6721
439596 6799
439891 6920
440192 6981
440780 7228

Thanks,
Joshua

2. Nov 29, 2006

### Staff: Mentor

The times are not in milliseconds, but other than that your approach looks correct. When you correct the units on the timestamps and graph the HP versus time, what does it look like? Are you making your own in-car dyno? Cool.

3. Nov 29, 2006

### gt5816v

Thanks for the reply. It's for a pda application that reads info from the OBD2 port. The time stamp should be ms... It's not relative to real time, just a timer. It does not reset when logging begins, perhaps that's the confusion?

What's kind of odd is that one car is providing very believable numbers but 2 others do not. They read very low at all speeds so I don't think it's a wind resistance issue. I have real dyno numbers from all the vehicles for comparison.

These are the specs on the 2nd car:

Tire Dia= 0.60452 meters
R&P= 1
3rd Gear= 5.3865
Mass= 1200 Kg

Time RPM
45620087 874
45620704 969
45620998 999
45621293 1056
45621585 1125
45621879 1187
45622169 1236
45622462 1297
45622755 1382
45623048 1472
45623341 1519
45623632 1598
45623925 1685
45624219 1768
45624511 1866
45624804 1955
45625099 2053
45625395 2133
45625687 2224
45625982 2311
45626275 2405
45626569 2479
45626862 2583
45627156 2671
45627449 2760
45627744 2860
45628049 2950
45628343 3059
45628636 3148
45628930 3237
45629223 3328
45629517 3425
45629811 3518
45630104 3614
45630397 3704
45630692 3783
45630986 3876
45631281 3971
45631575 4065
45631870 4172
45632164 4261
45632458 4356
45632753 4447
45633049 4525
45633344 4611
45633639 4702
45633933 4798
45634228 4893
45634522 4973
45634817 5059
45635112 5145
45635406 5306
45635702 5333
45635994 5400
45636288 5492
45636583 5570
45636878 5672
45637172 5737
45637466 5845
45637760 5919
45638053 5987
45638349 6068
45638643 6131
45638938 6200
45639231 6284
45639525 6356
45639819 6434
45640114 6501
45640407 6574
45640702 6649
45640997 6706
45641292 6789
45641588 6833

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4. Nov 30, 2006

### Staff: Mentor

Yes, that makes more sense that it's just the delta that matters. I couldn't understand the absolute ms reference. So you are saying that in the following:

that the delta-T for the 2nd and 3rd datapoints is 617ms and 294ms? That's pretty low-performance sampling, eh?

As for the graphs, I'd isolate the torque and HP graphs for each situation, to help debug the problem. It's pretty easy to look at the torque and HP graphs together to see if they look reasonable or not.

5. Nov 30, 2006

### gt5816v

The sample rate is only about 3hz That's the best the OBD2 has to offer on ISO vehicles....
The first sample is usually junk. I posted the raw values but for graphing that initial value is discarded.

Delta ms.
617
294
295
292
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296

Those screen shots are each of one vehicle and compare the actual BHP to the acceleration based HP. As would be expected at ~60mph a significant divergence begins. The second chart never lines up though. I think the formulas are correct and either the gear ratio is off or data from the vehicle is being interpreted wrong.

Best regards,
Joshua