Q&A Forum
An observation. Playing on the YourDyno I notice when cruising performing runs the rpm/mph matches the car instrumentation, however, hold a specific rpm and there is a discrepancy between rpm on car and YourDyno. It was around 400 rpm on a car holding 5000 rpm ( car was around 5400). I repeated the experiment on my old Jeep. Set dyno so 3000 rpm is x mph. Rev counter and YourDyno agree when running up and down and holding part throttles and data runs. However, hold at 3000 rpm YourDyno and rpm on Jeep is 3200 which matches mph difference as well. Thinking this is odd I did parallel testing with Dynocom ComputeX running as well. Identical, the YourDyno and Dynocom read same rpm and mph just that Jeep reads 3200 rpm not 3000. I repeat the part throttle rpms are same on Jeep and YourDyno and Dynocom as are the power runs whether pau or inertia ( I am assuming rpms tally on pau runs so don't take that as gospel as the numbers change fast)hold a specific rpm and the actual rpm is approx 6 or 7% different. Any ideas?
PS very happy with the YourDyno system. The pau contol is brilliant after my son built our own control box. The original was giving out so much interference we could not get the system settled.
Hi,
Glad you like the system!
The Engine RPM is calculated as Roller RPM * gear ratio. So as long a gear ratio is correct, Engine RPM will be correct. You can use a Bluetooth OBD2 adapter to set the gear ratio accurately. YourDyno has a very accurate crystal oscillator to ensure correct Roller RPM reading.
Depending on the Gauge filter settings there is a small averaging delay on the gauges (only affect the visual, not the calculations). The delay is small and it is difficult to look at two gauges at the same time while doing an RPM sweep, but maybe that contributes. Or maybe it is just the car's tacho that is off.
Good anyway that RPM matches on YourDyno and Dynocom (if I understood you right).
I may not have explained myself very well.
If I cruise the system the rpms match to perfection. If I hold a full load specific speed/rpm the rpms/speed no longer match the car. On my Jeep all rpms are same on Jeep and YourDyno and Dynocom until I hold a loaded speed/rpm when the Jeep shows a higher rpm, it does not matter how long I hold the load, on my Jeep 3000 rpm on YourDyno/Dynocom shows 3200 on my Jeep. If I cruise and hold 3000 rpm then the Jeep/YourDyno?Dynocom read the same. This effect also shows up on another car I tested where holding a load at 5000 rpm on YourDyno to measure boost showed 5400 on the car. Both the Jeep and the MG rpms were locked on YourDyno and both YourDyno and cars agreed at all rpms when held part throttle with no load.
I will try and get a video in the new year.
An afterthought;
Could it be that under full load the tyre is deflected enough to change the rolling radius so the rpm/speed measured by the rollers is correct and holding where it has been instructed to (same rpm/speed shown on YourDyno and Dynocom) but the rpm/speed shown in the car is incorrect? Please excuse me if I am talking rubbish 🙂
Good point on the tyre deflection. That's a possible explanation. Another idea is that you have a bit of slip. In both these cases the effective gear ratio changes under full load.
When I have time in the New Year I will run the Jeep again and then let the tyres down quite low and see if the effect has a greater magnitude.
Peter
I had a chance to run the Jeep up today with different tyre pressures.
The dyno was set to hold the speed representing 3000 rpm in 4th gear. Confirmed the unstrapped setting 3000 dyno was same as 3000 on tacho in Jeep.
Run 1 40 psi tyre pressure Jeep shows 3100 rpm
Run 2 28 psi tyre pressure Jeep shows 3150 rpm
Run 4 15 psi tyre pressure Jeep shows 3225 rpm
Looks like it is the deflection reducing the radius of the tyre hence the rpm discrepancy
Peter