What's going on with DIC?

[Randy_the_Hack]

Hi,
I have a Predator 2.0, using it on an '04.5 Chevy Silverado LLY Duramax. When I first installed the +65 hp tune, I saw a good improvement to mileage and performance. I was seeing just under 22.5 mpg hand calc'ed (borne out on a couple of 200+ mile road trips). However, I was seeing DIC report somewhere around 28-29 mpg. I realize that the numbers that DIC report will be off.

Since then, I have changed to +40 hp tune 3-4 times, then back to +65 hp 3-4 times. I've yet to return to stock since I first installed the +65 hp tune.

This last weekend, I did a bansai trip out to Oklahoma from Georgetown Kentucky to see my sons at college. Total miles were close to 1900 miles. I had added a tonneau cover to the truck to improve aerodynamics, hoping to improve mileage. DIC told me I was running in excess of 32 mpg - during one tank, I saw as much as 32.7 mpg reported by DIC. But... when I hand calc'ed the mileage, I was under 20 mpg - somewhere around 19.3 mpg!

While I know that DIC is off with a tune installed, I was under the assumptions that the calculations that DIC performs should make it a reliable trend identifier... so I would have expected that an increase in mpg on DIC from 28-29 mpg to nearly 33 mpg would have corresponded to a percentage improvement to my hand calc'ed mileage. But it didn't; in fact, the percentage that DIC is off changed from approx. 24% more than reality to nearly 33% more than reality.

Any ideas what would account for this behavior? Is DIC just totally worthless with a tune installed? Or is something else going on here? I'm confused...

[mikel]

Yes, basically the DIC is worthless with the tunes installed.It is all based on guesses and commanded fuel flow values, which do not add up right with the tune installed.

Thanks

[Randy_the_Hack]

So... sorry to beleaguer the point... but if the commanded fuel flow values are consistent at a particular speed/engine RPM/etc, even if the total numbers are off, it would seem that the numbers would be somewhat consistent. In fact, that was the case when I first installed the +65 hp tune. Things only started acting in an unpredictable manner after I bounced back and forth between the tow tune and the economy tune a couple of times.

Talked to someone in tech support yesterday on the phone and they indicated to me that the delta between the DIC value and reality should remain consistent. If this is not the case, then you cannot even use DIC to determine where your optimum performance is. In other words (and I did this the other day), you can't pay any attention to the instant mpg's to determine where your best mileage is based on speed. DIC seemed to tell me the other day that the optimum performance was around 75 mph. However, if I take what you've said at face value, not even this trend is indicative of reality.

This seems out of step with most of the opinions I've been getting in related forums out in the Diesel Place. I wondered if I restored the stock tune, then returned to the economy tune, if that would restore some semblance of sanity (albeit inaccurate).

[ubreathe]

I'm not an expert by any means, but have you set your tire size in the predator? If you are getting about a mile of actual distance for each recorded mile then it should be more reliable. I set mine to 29 inch tire height and the actual is 29 3/4. This got me pretty close to actual miles. My DIC records about 16 mpg. I am probably getting about 20. I don't know if this will help

[Randy_the_Hack]

Well... since the tires I am running are stock size, I've not done anything to the tire calibration. I guess I could check things and verify that the size is dead on... but that I think is missing the point. Thanks for the feedback though...