Bizarre Dyno result - sawtoothed flow

mikequig · 03-05-2008, 01:15 PM

I don't come to the community very often for help, but today's visit to the dyno left me/us befuddled.

The motor is a recently top-ended w/new P's and C's Euro CIS 3-liter. These motors normally show between 190-200 HP on this chassis dyno. On all the runs (4 pulls) instead of the nice smooth lines that we normally see it yielded these jagged lines on the HP and torque curves which showed up to a 5 HP drop/increase from moment to moment along the RPM range. Just prior we ran up a 3.2 L motor and all was normal, thus no issue w/the dyno itself.

I have dyno'd a number of these motors and the dyno is my a** was telling me that the car indeed was not pulling with the robustness that a Euro should.
Indeed, the lack of snap in the motor on the straightaways at Pueblo sent me looking at ignition and cam timing (spot on) and fuel delivery (injectors are newer and flow to spec). These items were good to go when we came to dyno it. The numbers were sub-par but I am out of answers.

SO, has anyone ever encountered a HP/Torque curve that looked like this? If so, I would love to hear what you found.

Thanks in advance
Mike Quigley
Euro 3.0 911 Class F or E or whatever

SC-targa · 03-05-2008, 01:21 PM

Is the mixture too rich?

DUK · 03-05-2008, 01:25 PM

Ignition timing jumping or not maximized, is my .02.

mikequig · 03-05-2008, 01:32 PM

I should have mentioned that the air/fuel was a tad lean at around 14:1 at it's worst - probably not a deal killer in itself though we will richen it up some.

I wondered about the ign. timing changing but the dizzy is bolted down tight, and we got full advance before taking it to the dyno. Perhaps I should change it out for a known good one and even put the timing light on it during a pull.

I'm suspicious of a vacuum leak perhaps causing the flapper to pulsate in the intake - most common CIS issues have been dealt with like boots/injectors, new rubbers on the intake tubes etc.

Mike Quigley
Euro 3.0L Class F or E or whatever

Scott R · Scott R's Garage

Here is mine, 3.0L fresh engine, everything rebuilt:

MotoSook · 03-05-2008, 01:34 PM

I'd put a scope on the timing and if the fuel ratio looked unsteady, and if the fuel ratio looks unsteady, I'd put a gage on the fuel pressure.

MotoSook · 03-05-2008, 01:37 PM

Quote:

Originally Posted by mikequig

I'm suspicious of a vacuum leak perhaps causing the flapper to pulsate in the intake - most common CIS issues have been dealt with like boots/injectors, new rubbers on the intake tubes etc.

An intake leak can't be ruled out, but the fuel pressure dampens the flapper a bit, me thinks it would have to be a big open/close leak to cause the flapper to be erratic. That's why I said check the fuel pressure.

Steve@Rennsport · 05:07 PM

"Smoothing" not switched on in the dyno software.

I see that all the time looking at other's plots.

mikequig · 03-05-2008, 04:32 PM

Steve, I'd gladly go along with that analysis on "smoothing" but we had just rolled a 3.2 Carrera off the dyno prior to my pulls. It was just a matter of opening a new file. The program was still running from the prior pull.

Mostly I'm just thinking that somehow this motor with good leakdown after a recent top end is just now producing the HP and torque that it should. Seeing the look of this curve has me looking for a smoking gun of some kind.

Thanks to you all for your well-considered and speedy replies on this.

Mike Quigley
Euro 3.0 911 in Class F or E or whatever

ssetek · 03-05-2008, 06:01 PM

smoothing is a band aid for some dynos poor sampling, that does look like a sampling issue with the dyno or some interference with the tach pickup

Jess · Jess's Garage

Mike

I really don't think it looks all that different than mine. The numbers you got are respectable. I think you're good to go for TWS!

Here's mine from a different dyno but with similar spikes.

Jess

ssetek · 03-05-2008, 07:47 PM

if you look at the top right you will see the sae correction factor and 0 smoothing most "egojet" graphs with smoothing at 5

richde · 03-05-2008, 08:48 PM

Quote:

Originally Posted by mikequig

Steve, I'd gladly go along with that analysis on "smoothing" but we had just rolled a 3.2 Carrera off the dyno prior to my pulls. It was just a matter of opening a new file. The program was still running from the prior pull.

Mostly I'm just thinking that somehow this motor with good leakdown after a recent top end is just now producing the HP and torque that it should. Seeing the look of this curve has me looking for a smoking gun of some kind.

Thanks to you all for your well-considered and speedy replies on this.

Mike Quigley
Euro 3.0 911 in Class F or E or whatever

180hp at the wheels, or around 207 at the crank...does this not sound right? If there were a problem with the car, you wouldn't even approach these numbers.

Dyno setup problem, nothing wrong with the car.

Quicksilver · 03-05-2008, 09:03 PM

As Steve and richde have alluded to, none of the jaggedness of the plots are larger then accuracy tolerance of a dyno. Dynos aren't accurate enough to create a meaningful plot full of that kind of bouncy detail. It really is some kind of aberration that the smoothing function of a dyno is designed to clean up.