How do you log the rpm and time it properly with velocity?

I use ELM327 Bluetooth OBD2 reader to log OBD2 data (RPM,speed,boost etc) directly.

alternatively : www.gtechpro.com

GTech Pro is much more expensive than 1 dyno run.

They're IPhone apps than can calculate hp based on accelerometer data like Dynolicious & Track Day. Same concept as GTech Pro, much cheaper, pretty accurate, but only hp value no torque value.

Iphone apps are crap. I've tested it and the results were far from actual after dyno. The gyro is not industrial spec tool. They rely on acceleration of the movement (which has a high tolerance) and GPS data (which is also crap AGPS & low frequency) to work out the estimation value. If you drive a 200hp car which weight less than 1 tonne, your HP indication will be higher due to power to weight ratio.

Dynolicious & Track Day doesn't use GPS data. Probably it boils down to putting the correct info (vehicle weight etc), putting the Iphone in correct orientation (and stationery), and no jerking movement. Or the apps don't count aerodynamic drag / rolling resistance.

I'll stick to my OBD data then

If you retune your car ten times due to bad tuning/mechanical failure/upgrades, it's a whole lot of different story. Mind you, I am the sole distributor of gtechpro in SEA

I don't think any reader that only monitors acceleration will be accurate as there's no other input to the data.

Important parameter other than acceleration should be time synched rpm. With this 2 data you should be able to tune the engine properly, even if the weight data is not available, just you won't get the real horsepower but good enough for tuning.

Use a videocam to record the RPM, speedometer and a stopwatch. Calculate offline during video playback.
Get the car rolling at top gear, and do full throttle until desired top RPM is reached.
Only works for manual.