Considering buying a dyno...

kb58 wrote: I've had my car run on three dynos, two different DynaPacks and an in-floor DynoJet. After running the car on DynaPack #1 it was run on a DynJet several months later with the exact same setup and similar weather. The DynaPack read "411 hp" and the DynoJet read "325 hp", a difference of about 20%.

DynaPacks usually read closer to DynoJets than that. I ran mine on a DynaPack and a DynoJet within 2 weeks of each other and it made the same power to within 3%, so something else is going on there.

What altitude are you at? There's an SAE standard correction factor for ambient pressure that most dyno software will use, but it can also be turned off. It's possible that the DynoJet operator had it turned off, while the DynaPacks had it turned on.

Some dynos also have a feature where they coast the engine down to try to measure the frictional losses in the drivetrain, so that they can report crank horsepower instead of wheel horsepower. It doesn't work very well so in my experience most people don't use it, but it's possible that the dynojet shops had this turned on.

As for the data logging approach, there is a piece of software called "Virtual Dyno" to do this. There a few big problems with it, one is that it's heavily dependent on road and temperature conditions, so getting repeatable numbers can be tricky. When you're tuning a car you need to be able to see changes of just a few hp in order to nail the timing properly, and achieving that level of repeatability with virtual dyno is hard. You also need to know the aero drag on the car at each speed, and that's a tricky number to nail down precisely. Finally, you typically want to dyno a car in the 1:1 gear (usually 4th in a 5-speed, or 5th in a 6-speed) and most cars are doing at least 120 by the time you redline it in that gear so there are also legal issues.

I've used a land-and-sea dyno a couple times, I didn't care for it much. As with the Mustang dyno and the Dynocom, the numbers seem to be very sensitive to how you set up the run.

The shops I've been to seem to be split about 50/50 whether they put the DynoJet in the ground or use a lift. One shop had the AWD version of the DynoJet -- the second roller is on rails so that you can move it in relation to the fixed one to allow use on cars with different wheelbases.

In reply to codrus:

Operator error with something somewhere.....happens all the damn time. It's why I prefer to set the precise altitude and other factors myself. Heck, I set up the local cycle shop's for their dyno day a few years back....everyone who participated(sans the Suzuki boiz) seemed to appreciate an accurate setup as everything was right in the range of where it should have been.

Evidently the neighboring Suzuki shop was telling people their nearly stock Busa's made 240whp.....lol

Buy the one that you can just adjust the output to what the customer expects or just a hair more. Seems like this is the only way to win.

Having operated a TON I like hub dynos (Mainline & Dynapak) for high power work. Lower power stuff you can get away with less. I don't like tuning wild setups on a Dynojet where part throttle and transients matter, too much cleanup needs to be done on the tuning after at the track and wastes money. For drag, you can do almost everything on a dynojet if you can get it to hook.

Everyone expects hp in Dynojet 248c type numbers, even if those seem high compared to some water/eddy/hydraulic dynos at faster and/or slower sweeps and different gears.

Fun dyno graph: same car, same day, different sweep times. Inertial losses are real.

https://www.flyinmiata.com/tech/dyno_runs/NC_sweep_times.pdf

Keith Tanner wrote: Fun dyno graph: same car, same day, different sweep times. Inertial losses are real. https://www.flyinmiata.com/tech/dyno_runs/NC_sweep_times.pdf

So, which was which? higher HP with the slower sweep time (like using 5th instead of 4th)?

Yup, slower rate of acceleration = fewer inertial losses. Both runs were done in 5th (1:1) gear, we simply told the dyno to take longer. I forget the details - 15 vs 25 seconds comes to mind. IIRC, a 250 rwhp car usually takes about 15 seconds on a Dynojet 248.

That's interesting because I have done some dynojet testing just changing the gear from 4 (~1:1) to 3 (~1.35:1) actually increased hp a little bit with the lower ramp time. Never made sense to me, could just be an artifact of the software vs real.

Paul_VR6 wrote: That's interesting because I have done some dynojet testing just changing the gear from 4 (~1:1) to 3 (~1.35:1) actually increased hp a little bit with the lower ramp time. Never made sense to me, could just be an artifact of the software vs real.

That sounds like the software just isn't correctly accounting for the change in gearing. Keep in mind, on an inertia based dyno like a dynojet, typically a car that can spin the dyno up faster is making more power.

The "road speed" worked right, but who knows. If you plot hp vs road speed it shows up like you would expect with the 3rd gear pull being a few whp higher.

yeah, i never understood why people think that a 1:1 gear is important on the dyno. Cars have various final drive ratios so the 1:1 gear in the gearbox isn't really keeping anything consistent. In fact, if you know RPM (you do) and the speed of the roller (you do) then that is all the gearing calculation you need. The dyno software would be responsible for calculating that gear "overall ratio" based on those two things and then using that to calculate back to HP.

however, I guess a transmission 1:1 gear might have less drivetrain loss?

I've never operated a dyno and only had my car on one, once. And don't know the model it was, but the operator (who has been in the speed business for FOREVER) said he preferred and ran 2nd gear pulls. His stated reasoning was that people used higher gear pulls to show a higher hp reading but it was harder on the car and the dyno and ran the risk of one of those exciting blowouts we've all seen on youtube.

I think the 1:1 gear thing is done for 2 reasons: simplicity, as you only have to account for diff gearing, not trans gearing in the calculations and for possible reduced driveline losses.

The speed thing is part of why I haven't run the Jeep on a dyno. If I wanted to make a pull with the torque converter locked, it has to be done in 3rd gear unless I modify the valve body to allow 2nd gear lockup. And running it up to redline in 3rd means running it to almost 150mph...

In reply to rslifkin:

but like I said, it is much more simple to say RPM of engine = 2500 and RPM of rollers = 1250 and therefore effective gear ratio is 2:1 than it is to try and calculate engine RPM = 2500 X trans gear X rear gear (and doing the work to figure out the rear gear) X size of tire X air pressure in tire X size of roller = effective rear ratio

I agree. But I dunno what some dyno systems (especially older ones) take for inputs, so that may have influenced how people typically used them (and then it stuck, even if it doesn't matter on most dynos).

In reply to KyAllroad:

vewy intewesting

I just had a thought. I'm pretty sure the 1:1 gear thing would have started out with dynojet users. Being an inertia dyno, if you run a higher powered car in too low a gear, you'll get a really fast spinup, which could cause accuracy issues or only getting good data at very high rpm (due to going through a lot of rpm during the time it takes to open the throttle, etc.)

That wouldn't be an issue on any load bearing dyno though, as you can control the spinup rate. So you can make a 15 second pull regardless of the gear you're in.

The 1:1 thing is due to driveline losses, that's all. It's the most efficient gear, and therefore the most consistent.

Calculating engine speed from roller speed is a little fuzzy. You can get close if you know the gearing, but the tire deformation means that it won't be exact. With a hub mount dyno, you lose that deformation so you can get exact engine speed. That was the nicest thing about our Rototest.

When we had the Dynojet, I could usually make a pretty good estimation of the power level just by listening to the run. There's an advantage to doing the same cars over and over

rslifkin wrote: I think the 1:1 gear thing is done for 2 reasons: simplicity, as you only have to account for diff gearing, not trans gearing in the calculations and for possible reduced driveline losses. The speed thing is part of why I haven't run the Jeep on a dyno. If I wanted to make a pull with the torque converter locked, it has to be done in 3rd gear unless I modify the valve body to allow 2nd gear lockup. And running it up to redline in 3rd means running it to almost 150mph...

On my Focus we always did a 3rd gear pull. If we tried a 4th (1:1) pull it would hit the speed limiter before it hit the rev limiter. I suspect this is true with a lot of modern cars.

Hal wrote: On my Focus we always did a 3rd gear pull. If we tried a 4th (1:1) pull it would hit the speed limiter before it hit the rev limiter. I suspect this is true with a lot of modern cars.

That's a good point. I've got the speed limiter raised by 35 mph in my tune (from 115 to 150) just to account for that situation.

We had the dyno operator run one of our builds in 2nd because 3rd would have been over 150.

Ideally you run in the straight through gear for driveline loss reasons, but you gotta do what you gotta do. My Mazda has only ever been dynoed in 3rd because 4th would have it going over 150 on old S-rated tires.

Keith Tanner wrote: The 1:1 thing is due to driveline losses, that's all. It's the most efficient gear, and therefore the most consistent.

It's also typically the strongest, which is a useful consideration when dynoing a turbo Miata. :)

Keith Tanner wrote: Dynocom: Drum dyno, very quick to hook up. Grooved drums means it makes a ridiculous racket and tears up tires. Has limited eddy brake capabilities for resistance, but you can't overwork it. Has had constant problems that "have never been seen before" and are constantly being blamed on installation despite the fact that we've done everything they say and more. Currently requires quite a bit of hand-holding from the operator to make sure the runs are even partially representative and we can't do steady-state. Cannot recommend.

Thank you Keith, this is really helpful information. I was looking at both the Dynocom 5000 and their POD system and came across this post from a Google search. Would you mind sharing what model Dynocom dyno you had problems with and the software age. The reason I was asking is they do have the X series that are a lower grade, and I guess they recently upgraded the software on all models, they have been telling some owners that it fixes some problems, but no one I have asked yet has been willing to spend anymore money with them and has either sold their machines or are just waiting as they think the company should fix their existing problems without a charge. I have pretty much crossed them out of consideration too, but if they have fixed their products recently it would be nice to know.

Yup, slower rate of acceleration = fewer inertial losses.

Which is part of why blanket driveline loss % assumptions are bunk.

If anyone is planning to try to 'adjust' their WHP to CHP with an assumed driveline loss #, just SKIP caring about whether a dyno is accurate, because theres a 1000% chance your assumption of drivetrain loss is complete junk.