Dynojet technical questions...

K

Knestis

Moderator
A couple of Q's for those of you with real Dynojet experience...

1. What would cause some power/rpm line graphs to be very much wigglier than others? We aren't talking about the difference between smoothing values of 1 and 5, but seriously wavey lines. Is it reflective of something in the test, a setting controlled by the technician, a hardware issue, or actual variation in the "power" made by the car?

2. Have you ever heard of among-run variations of as much as 10-12% and, if so, what might the be attributed to? This is in back-to-back runs - literally - and the weird thing is that the values go up and down. We're thinking it may have been unmetered air getting in, influenced by engine temperature but we'd entertain other explanations - with the engine, conditions, or the test.

Thoughts?

K
 
Kirk, Pics?
I'm no dyno expert but it sounds like to me they have a bad sensor/calibrations issue somewhere.

If you can, run away and go find a DynaPak or Dyno Dynamics dyno (which are the 2 that the dyno experts I know say to use).
 
There's a thread on EFI 101 about out of round tires producing wavy graphs. Basically the tire hops along the roller. Kind of slapping it along. Cylinder misfire and/or detonation can also produce the same effect.

FWIW, EFI 101 forum is a GREAT source of info on anything related to tuning including dyno use.
 
Google Dynapak, call them in Cali, and find the nearest guy with one. It's a real brake type dyno, much more useful, realistic and repeatable.
 
Thread hyjack, but Greg - can you give us the cliffnotes w/o your blood pressure getting too high? I'm curious.

Matt
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I will help Greg out.

DynoJets calculate torque as HP *5280 * square root of cars weight all divided by 9 factorial, which makes no sense. Looking at this calculation, its clear that the Dynapack is better!

Sorry I just couldn't resist. It's Tuesday morning!

:D
 
1. Yes, timing and spark issues, seen it, and have seen it cured - no more wavy lines. Also caused by some other mechanical issues with the rollers as mentioned.

2. Sure it happens, but I've not seen it. A couple years ago a fellow from Horsepowerjunkies took his Camaro to all three Dynojets here in the RTP area to solve this very question. All in the same day - about 4 hours total between them. Difference? About 2% between all of them, it was something like 311, 301, and 309.

We now use the same one each time and I feel it works well for a tuning device to validate changes in a given day. I don't care about the absolute number from one month to the next, but if I was able to vary some parameters on a given day. Comparing numbers from different days and different dynos is certainly full of pitfalls but is unavoidable.
 
...can you give us the cliffnotes w/o your blood pressure getting too high?[/b]
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Briefly as I can...:

Horsepower is a calculated number based on measured torque as a function of RPM (HP = rpm x torque/5252). Therefore, horsepower cannot be "measured." The only way to accurately figure out horsepower is to measure torque and calculate the ponies.

A DynoJet consists of a big roller of known mass, driven by the tires of the car. The rate of acceleration of this known mass is calculated and the horsepower is inferred. Torque is back-calculated from that. Since it is measuring the rate of acceleration of the roller, anything that can affect that rate - without actually affecting torque and thus horsepower - will also affect its inferred values. These include, but certainly are not limited to - wheel and tire size and weight, tire grip levels, rotating mass changes (flywheels, crank pulleys, driveshaft dampers, etc). Sandbagging a DynoJet is a piece of cake: I'd put large heavy wheels and tires on it with crappy generic tires that slip (and maybe even fill up the tires with water...)

On the other hand, a DynaPack consists of two hydraulic units that bolt directly onto the wheel hubs of the car. Rate of acceleration is controlled and torque from the driveshafts is directly measured. Horsepower is calculated from there as a function of RPM.

I'll leave the inferential rants to your imagination. - GA
 
A couple of Q's for those of you with real Dynojet experience...

1. What would cause some power/rpm line graphs to be very much wigglier than others? We aren't talking about the difference between smoothing values of 1 and 5, but seriously wavey lines. Is it reflective of something in the test, a setting controlled by the technician, a hardware issue, or actual variation in the "power" made by the car?


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From my experience this is caused by a problem with the rpm pickup. From what I've seen it's usually caused by the ignition wires. If you have really thick gauge ignition wires the rpm pickup has a hard time getting the signal. At one of the better dyno's I've been to he used a hall effect sensor running off the cam gears to calculate the rpm.

s
 
Kirk,

Did you take it to the local Dyno over off of Chapman St? We just took the Honda (to sort out the new motor) and I noticed the same thing... kinda like my old free lance architectural scetch's.... /\/\/\/\/\/\/\/

One sees everyone else's beautiful smooth chart's and then your's looks like an EKG! Well, it's good to know it wasn't just mine that looked awkward.

I like Stevel's hypothisis... plug wires and spark. It makes sense considering the chart I have (I'm using 8mm Moroso's). It's also possible it could be the tire's skipping on the drum, too. I do know that the way my car runs cold, it barely had any temp in it when we did the later runs.

hoop
 
I fourth the dyno-dynamics or dyna-pack suggestions. They are much better tuning tools, in the right hands.
 
Dyna-pac was used to develop the original control system, and I used a Dyna-pac to dial it in once it was installed in my chassis. Which reminds me, I want to get a copy of the raw data to run some of my own analysis on, being the geky rocket scientist that I am :P

James
 
I'd love to see that derivation...
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It's not a derivation - it's purely empirical, based on comparing steady state dyno measurements versus the inertia dyno. There was a good article, last year I think, in one of the standard magazines (might have been GRM, might have been Racecar Engineering, for all I remember) that chronicled the difficulty with coming up with the rating/correlation.

The dynojet was a tool created by a motorcycle carb. tuner, looking for a solution to jets chosen by steady state tuning which would not provide good driveability. It wasn't created to be a 'horsepower measurement' tool, but a diagnostic tool/development tool. For back-to-back testing, the absolute values don't matter so much as percent improvement in acceleration, or a response, such as a change in afr due to a hardware change.

It makes sense to tune fuel injection tables, which are based on steady state MAP/N or TP/N maps, first with a traditional, steady state dyno (like a dyna pak), and then verify transient tuning either on road or on an inertia dyno like the dynojet. For carburetors, I honestly think the dynajet is a BETTER tool than steady state dyno, because the adjustment 'knobs' are so inter-related.

Choose the right tool for the job, I guess. I wouldn't say 'A' is better than 'B' - each provides a value.
 
.....

It makes sense to tune fuel injection tables, which are based on steady state MAP/N or TP/N maps, first with a traditional, steady state dyno (like a dyna pak), and then verify transient tuning either on road or on an inertia dyno like the dynojet. For carburetors, I honestly think the dynajet is a BETTER tool than steady state dyno, because the adjustment 'knobs' are so inter-related.

Choose the right tool for the job, I guess. I wouldn't say 'A' is better than 'B' - each provides a value.
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But that's the bueaty of the dyna-pak, you can run either steady state or transient modes. When setting my map the run would start when I got above 2200 rpm, then end at 6500rpm. We could have just ran at a constant rpm and changed the load too. The real weak point is the tire slippage factor, and you have to hope it doesn't change while you're testing or during your test. If you're using a dyno-jet and it's working for you then fine don't change, but if you're having issues, then the smart thing to do would be to seek out alternatives.

James
 
I wasn't suggesting that one was better than the other - I have nothing against a dynapak, and appreciate the flexibility. My point was more that a dynojet should be considered a tool for tuning more so than a measurement device.

One of my favorite tuning tools - the towing dyno! Imagine putting a hitch on the race car, and towing it around an oval, dialing in a steady state with a hand controller, and maintaining a vehicle speed at any throttle input you choose; make your changes, change speed and do it again - 0 torque to wide open. Real world airflow, real cooling, if you have a turbo, real world intercooling ...

Ahhh, toys ..
 
Dynojet makes an eddy current dyno now don't they? So simply saying "DynoJet" no longer applies...

We've used 3 different Mustang load bearing dynos in the past year and have been very happy with the repeatability of them (as long as the operators use the right setup values)
 
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