Weird dyno results

[FryGuy]

We dynoed our car on a chassis dyno yesterday and got some really funny curves that kind of have us stumped. The curves on all the pulls have two very distinct power drops at even intervals. We have several theories but won't really know for sure without further instrumentation.
http://www.niumotorsports.com/run%205.gif

note: our fuel and ignition maps don't really have any abnormalities that would cause anything that drastic and we were also monitoring air/fuel which wasn't really too far off either.

Has anybody seen anything like this? We had the dyno operator confused as well, and he said he's never seen something like this even with all the stuff he has dynoed.

[FryGuy]

We dynoed our car on a chassis dyno yesterday and got some really funny curves that kind of have us stumped. The curves on all the pulls have two very distinct power drops at even intervals. We have several theories but won't really know for sure without further instrumentation.
http://www.niumotorsports.com/run%205.gif

note: our fuel and ignition maps don't really have any abnormalities that would cause anything that drastic and we were also monitoring air/fuel which wasn't really too far off either.

Has anybody seen anything like this? We had the dyno operator confused as well, and he said he's never seen something like this even with all the stuff he has dynoed.

[PedalOnTheRight]

What kind of ignition advance are you running at the top end of your TPS% and RPM scales? It seems like you're loosing power just before each gear shift...although we'd need an RPM vs HP graph to compare. There's a variety of causes for this:

1) Leaks. If you have made your own gaskets surrounding your intake and exhaust ports, and any other location on the plenum behind the throttle body, check to see that there isn't a vacuum leak.

2)a. It's possible that your Dyno results are being affected by bad inputs. Make sure you're using a an Air/Fuel meter (Y band O2 sensor or something). Usually, an AFR of 14 is golden for most of these 4 cylinder 4 strokes. Therefore, you'll need to tune each gear by advancing/retarding ignition or adding/subtracting fuel until you get your AFR correct. Start with fuel maps.

b. A good idea is to do some full throttle pulls up through the gears to get a rough idea of the direction you need to go. Spend a good amount of time on the dyno in 5th gear (or whatever gear gives you a 1:1 drive ratio). Make pulls at various TPS percentages and note the changes in AFR etc. at the areas where you're dropping off in HP.

3) Intake/Exhaust design. If you inadvertantly mismatched the intake and exhaust pulsation within your engine, the effects could be magnified at higher RPM, hence the drop in power. This is a problem that is unlikely to get fixed before competition due to its complexity.

Basically, air, fuel and spark are your variables. You have the air part taken car of with whatever designs you have implemented, so trace the problem to your ignition and fuel inputs. It has been my experience that most of us should be running around 30degrees advance in the upper RPM ranges of your tuning. I believe we run 26degrees BTDC in the lower RPM band.

Your fuel maps need to be smooth transitions between RPM and TPS % points, with the amount steadily increasing and higher values of each.

Ignition Timing is a bit more fickle. You'll find that if you ramp it up evenly with RPM and TPS % that you'll get detonation at high revs. So you may need to take some timing out on the top end. The only way to know is by sight and sound (remove your plugs and see if the look like someone took a pepper shaker to the electrode and elsewhere). Therefore, mess with fuel first to get a smooth curve, and then turn to ignition to fine tune the mixture.

A bit long winded yes, but I hope it helped. Soon you'll be making 81HP like us!

[Grant Mahler]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by PedalOnTheRight:
What kind of ignition advance are you running at the top end of your TPS% and RPM scales? It seems like you're loosing power just before each gear shift...although we'd need an RPM vs HP graph to compare. There's a variety of causes for this:

...

A bit long winded yes, but I hope it helped. Soon you'll be making 81HP like us! </div></BLOCKQUOTE>

To the OP: FYI - Pedal On The Right provides (some) good advice and (much) opinion. Just take this into consideration.

Take a look at HP/RPM graph not HP/MPH. Also, where is the AFR plot?

[BryanH]

I see this all the time on modified 911 engines. If the curves are valid you must have a corresponding reduction in injection time at 39mph and 48mph if you calibrate for a flat lamda curve. Multiple dips can be caused by Ex primary pipes too short, Your Intake runners appear to be large in I.D., this usually causes 1 large dip before the torque peak. could be a combination of the two.

RMIT fan club

[B Hise]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">I see this all the time on modified 911 engines. </div></BLOCKQUOTE>


ah varioram...

[FryGuy]

thanks for the suggestion guys. There are no gear shifts in this pull, and it is an identical graph to the hp vs rpm. I just used speed because the rpm graph had some discontinuities in it due to a poor signal pickup on the dyno.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by BryanH:
I see this all the time on modified 911 engines. If the curves are valid you must have a corresponding reduction in injection time at 39mph and 48mph if you calibrate for a flat lamda curve. Multiple dips can be caused by Ex primary pipes too short, Your Intake runners appear to be large in I.D., this usually causes 1 large dip before the torque peak. could be a combination of the two.

RMIT fan club </div></BLOCKQUOTE>

That was one of the things we were thinking about. We are running much shorter runners than we ever have in the past, and were thinking we might be hitting some weird first and second wave frequencies. If this is the case, we should be able to log pressures drops in the plenum that would reflect this correct?

[Marshall Grice]

For what it's worth (admittadly not much) i second the intake/exhaust tuning error. We used a 01-03 gsxr 600 and had a somewhat similar looking curve. I don't have a curve but you can actually hear the flat spot on the dyno(VIDEO (http://www.csupomona.edu/%7Efsae/video/dyno_vid06.wmv)).

our first power peak was at ~8500 rpm, is yours similar?

[Grant Mahler]

How much timing are you running?

[Akos]

Fry guy,

I hate to be the one to point this out, but your HP curve by the looks of it is due to bad intake and exhaust tuning.

I could almost bet you are running a short (20" or so) 4 to 1 header with long intake runners.

If you have some room on your intake to change the length, it can be usually be adjusted to make the hole less obvious.

I'm not sure if you run simulations on your setup, but it looks like you are hitting anti-resonace on both intake and exhaust at that point.

[Dan Deussen @ Weber Motor]

FryGuy,
You mentioned that you had some problems with the rpm pickup of the dyno. Was is at the same areas wher the hp dips are? If so, maybe your ECU has a problem with the cam and/or crank sensor values and therefore outputs some bad signals to the coils which could cause the power dips and at the same time the troubles with the rpm pickup of the dyno! Also vibrations could distrurb some contacts (coils, injectors, sensors, etc.) at the rpms where you are having the power dips.

[PedalOnTheRight]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Grant Mahler:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by PedalOnTheRight:
What kind of ignition advance are you running at the top end of your TPS% and RPM scales? It seems like you're loosing power just before each gear shift...although we'd need an RPM vs HP graph to compare. There's a variety of causes for this:

...

A bit long winded yes, but I hope it helped. Soon you'll be making 81HP like us! </div></BLOCKQUOTE>

To the OP: FYI - Pedal On The Right provides (some) good advice and (much) opinion. Just take this into consideration.

Take a look at HP/RPM graph not HP/MPH. Also, where is the AFR plot? </div></BLOCKQUOTE>

Opinions? How So?

[Buckingham]

How does that graph correlate to your designed powerband? Example: If 25mph = 4000 rpm and 65 mph = 11000 rpm and you designed your vehicle to be driven between 8800 rpm and 11000 rpm (55-65 mph), don't lose sleep over what happens below 8800 rpm.

Also, what engine?

[FryGuy]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by dhaidinger:
How does that graph correlate to your designed powerband? Example: If 25mph = 4000 rpm and 65 mph = 11000 rpm and you designed your vehicle to be driven between 8800 rpm and 11000 rpm (55-65 mph), don't lose sleep over what happens below 8800 rpm.

Also, what engine? </div></BLOCKQUOTE>

Its a yamaha R6. the peak occurs right around 10500 rpm which is almost exactly where we designed it for(we designed for 10750). We are going to due some testing tommorow and monitor a variety of things to see if we can't narrow down what me be causing this. Im kind of thinking there are some intake and exhaust tuning problems, but hopefully the data we can get we tell us for sure.

[VFR750R]

4-2-1 exhaust or 4-1?

The software isn't using RPM signal for HP if its not working right, is it? You did the math for Speed instead?
I know it's obvious but we can't see your torque curve; do dips in the HP curve correlate to dips in torque curve?

Do you have anyway to measure tire slip? Or clutch slip? Did you try rolling the car off and then remounting it to see if it changes?

You may already be doing this but having the cars rear tires slightly ahead of the rollers when strapped in will help the tires bite in the rollers under load.

[Neil S]

I've worked on dynojets for 5 years now and they always seem to have weird issues all the time. Just today we had RPM drop out, torque flat line at 1500ft/lbs, and tach readings bouncing all over the place. There are a lot of little things that mess up the readings on the dynojets.

If the graph vs. RPM isn't continuous then HP readings always get messed up looking even when graphed against speed (since it is a function of RPM).

Do you know what model dynojet you were on, mainly wondering what weight drum it was and if they had another motor mounted to it to initially spin the drum.

[Bete Noir]

Trying to analyze engine performance based on horsepower curves can be very misleading. Much better to plot torque vs rpm, along with lambda.

Double humped torque curves are very common when resonant tuned induction and exhaust pipes are used. If they are designed to tune at the same rpm, then a large single humped torque curve is presented. If either intake or exhaust, or pairs of cylinders are not tuned to be in phase, then a double humped curve will result. If the phase shift is properly designed, it can produce a wider torque curve than can be achieved when stacked in phase.
The key is to look at the resonant tuned characteristics of both inlet and exhaust and each pair of runners or pipes.