WOT vs Partly OT with Turbo spool up time. Difference in Dyno HP reading. More HP when slowly rolling on throttle

Hey Guys,

My first post here so I'm not sure if this is the right place or the right kind of question to ask?

Some background this is a Dobeck performance Motorcycle dyno, which is an inertia dyno with 3 Air brakes (Two turbines) which are supposed to simulate on road driving better than a normal inertia dyno. The Vrod SE has a modified trask turbo kit fitted. I'm based in New Zealand.

The Vrod is only a 1250CC engine.

When doing a dyno run with WOT you can see on the graph posted (Run #24) The final HP is 151, as the bike gets into the RPM limit quite quickly once the Turbo spools.

Run #25 is done with me slowly twisting the throttle to 100%, giving the turbo a better spool up time and giving the bike better response.

So my question is:

All dyno graphs I see are done at WOT. I'm assuming this is because a vehicle should make the most power at WOT. But what if the turbo lag, and a set rpm limit on an inertia dyno that only allows you to do a non steady state single pull, means that the HP is more when you roll the throttle on?

Is it industry standard to always show dyno results at WOT, or should it show the most realistic power output as graph #25 does? Or will slowly rolling on the throttle always give a higher result in every case on an inertia based dyno?

I hope that makes sense and thank you for any insights

Here is a picture of the setup just for reference/interest.

Applying less than full throttle slows spool since you're reducing intake flow which results in less exhaust mass flow, less drive on the turbo.

With that out of the way, there are other factors which can improve spool. I don't have a confident answer for you, so I'll do my best to give you a few guesses.

Perhaps a dyno load setting or the gearing used for the run was different between tests? Was tire pressure changed? Were any dyno settings changed?

I see you started pull 24 at a lower roller speed, so a lower engine speed. This will typically make spool occur sooner, since you're putting heat and energy into the turbo system longer before you reach a given speed.

In terms of the horsepower being higher, if dyno load isn't being applied in a consistent manner, that will impact the HP reading.

If boost is higher, that will impact the HP reading.

What's the second strip chart (aux 1)? Something appears to be very different between the two runs. If that's boost I think running far more boost on pull 24 is the difference.

Hey Mike, Thank you for the quick response.

Don't worry about AUX 1 that's me messing with a Boost sensor voltage, since that's not displaying the same as the actual boost. Either way don't worry about it.

There is no difference on dyno load setting, no tyre pressure difference.

Pull 24 was started at 38km/h pull 25 was started at 44km/h.

The turbo does build up more boost in pull 25, which is where the HP difference comes from yes. That's because it has a more effective spool up, compared to going to WOT in run 24.

But that's the question I'm trying to get at. Do we use WOT because it gives the highest reading (Theoretically?) Or can we get a higher reading below WOT from better turbo efficiency?

OR am I just seeing a higher HP reading because I am "rolling on the throttle" instead of opening right up.

Does rolling on the throttle affect dyno HP readings compared to WOT on an inertia dyno? So on a stock engine that has 100HP WOT, if I slowly roll on the throttle then will it read 110HP? Or does rolling on the throttle slowly not affect the final dyno reading?

I hope that's making sense

If I'm understanding your posts correctly, pull#25 may have suffered a little less from heat soak, as it was under power for a little less time than #24, which could have meant a denser charge?

Hey Gord.

Imagine all variables are the same for a turbo vehicle on an inertia dyno.

The first dyno run, I open the throttle fully from 10% to 100% WOT at lets say 3000rpm till redline. The dyno sheet comes out as 150HP.

The second run instead of fully opening up the throttle to 100%, I slowly twist the throttle open from 10% to 100 WOT till redline. The HP comes out as 170.

Why is the result different when I slowly open the throttle? And regardless of why this happens, the actual question is: Is it industry standard to just open the throttle from 0 to 100 (WOT) during the power run? And why? What if I make more horsepower by slowly opening the throttle. If I'm looking for a maximum horsepower readout, then does it matter if I go WOT right away or instead slowly increase throttle position?

Perhaps I've mis spoke in my original post. There are two color blocks next to each dyno run. Red and yellow next to run 25, blue and pink next to run 24. Please confirm what the color codes mean as I'm not familiar with this dyno. I want to make sure I'm not making suggestions that don't properly represent the data.

If red is HP for run 25, and blue is HP for run 24, then run 24 had higher output than run 25 throughout. The only reason the peak HP was lower than run 25 is you stopped run 24 early, before the engine reached the RPM where it makes peak power.

Hey Mike!

Sorry I should have clarified/labeled the axes better.

You are correct red is HP for run 25, blue is HP for run 24.

Both cut out at around 8500rpm, see at the end of the line that it curves back down? The actual rev limiter isn't until 9500rpm, but since the curve starts falling I'm assuming the dyno stops recording past peak torque/HP.

Run 24 is not stopped early, both runs cut out as the bike hits the rev limiter. The turbo works more efficiently when I open up the throttle slowly compared to opening to WOT right away. So my question is, both of these readings are the same bike under the same conditions, so which Dyno run do I show the customer? Is it industry standard to just push the gas to the floor, or can I just gently roll on the throttle? Does it matter?

Also thank you for trying to figure out what I'm trying to ask here

Run 24 stops at about 145, run 25 goes to about 180...I'm guessing that's kilometers per hour.

If you perform 2 pulls in the same gear, to the same engine speed, both would end at the same roller speed.

If you're sure you're in the same gear, and going to the same engine speed, the usual cause of one pull engine at a drastically slower roller speed, is wheel spin. During wheel spin you will not get accurate readings.

From what I'm being told and seeing, it looks like you spun the tire on run 24, perhaps not on run 25 because you eased into the throttle. It's also possible the tire spun on both runs, just far more on run 24 vs. 25, but if you do a part throttle pull in the same gear without any wheel spin, then you'll know what roller speed a pull in that gear should achieve at 8500 RPM.

Okay thank you! So it makes sense that easing into the throttle and WOT should still come out as the same HP result at the end, just over a longer run period. Let's say it is tire slip and has nothing to do with turbo lag with the combination of an inertia dyno then let's say the tire will always slip. This is a motorcycle and I've already strapped the ass end down with 4 tie downs. This brings me to the original question:

Does it matter if I show the customer a dyno result where I ease onto the throttle? (If this is the only thing that will prevent wheel slip?)

I would do the test I mentioned at gentle throttle and perfect traction, so you know the roller speed the vehicle reaches in that particular gear, with that tire and pressure, at the peak engine speed you want to test to.

Then you know for a dyno run to be valid, it must reach that roller speed, at the terminal engine speed you chose.

I understand you're focused on how you start the pull in terms of throttle, but when there's a 35 kmh discrepancy at the same engine speed, that means you have tons of wheelspin and the reading isn't valid. How you lean into the throttle may have caused the wheelspin, but the wheel spin is what's causing the difference in the dyno result.

With better traction from perhaps a different strapping method, without wheel spin, then how you roll into the throttle likely would have VERY little impact on peak HP at high RPM.

Do you have pictures of how you strap the bike and how the tire sits on the roller(s)? Perhaps we can offer suggested improvements. For example, tow straps pulling straight down provide a lot more downward pressure than straps that angle forward or backwards. Strapping from the bike chassis compresses the suspension, while strapping the swingarm down causes direct transfer of downward pressure to the tire so it's far more effective.

I often see bikes strapped like the attached picture which shows lots of straps, and the bike is probably stable, but only the one strap off the right of the bike is providing any downward pressure, and it's angled, and it's to the chassis not swingarm.

The other picture shows only 2 straps, but they're to the swingarm so they're far more effective in providing downward pressure on the tire which helps traction. I'm not saying what they did is ideal, but hopefully these images help give a general idea of strapping focused on bike stability alone, vs. strapping to help traction. Ideally your method will provide both.

What gear(s) are you using? The easiest way to minimise any chance of wheelspin is to use the highest gear possible, within the speed rating of the dyno', tyres, etc. As you're in NZ, and presumably using km/hr, those speeds suggest 3rd(?) gear?