Topic: Dynatech LT Stainless Headers & Intermediate Tubes w/Cats.

[Main One]

0.2" difference between actual and stated size really doesn't amount to a whole bunch on the speedo (I could do the math and bore you with it but I won't  ). However every little bit helps when you are not satisfied with how it's accelerating (like we said earlier, you need a bigger or heavier foot!  ). I've found the easiest way to get the speedo correct is to use a GPS unit, set the cruise control on the Interstate, and check to see how far off the speedo is. Adjust it from there until you get it dialed in exactly.

To give you an idea on how little the 0.2" tire size practically affects things, the last I knew the Federal Government allows the speedo to be off my as much as 10%. I never figured that one out except that it must be to allow for the different tire sizes that go on the different models; 10% is a huge discrepancy to me. Think about it; 10% is 6 mph at 60 mph. I also believe it's much more critical to today's computer-controlled vehicles to get it right so that timing, fueling, and all other parameters controlled by the computer are correct. If the computer thinks you are going 60 mph when you are actually going 66 mph you're not getting enough fuel for the speed you are going.

OK, I've bored everyone enough and I really should go back to work. 

Joe,

I think you needed at least another cup of coffee before posting the above. 

The tire size error looked to be 0.8 or 0.98 inches, since they corrected it to 33 after doing a measurement.  I'll let you bore yourself with the math.   

The computer doesn't do fueling (other than when you lift your foot off of the gas) based upon speed, it does it based upon airflow based upon MAF, MAP, RPM and air temperature inputs.

The speedometer issue does however affect shift points.  Those are determined by speed and throttle position.  Fixing the tire size issue can either help or hurt - it would appear that in this case it helped.  Since the tire size was actually smaller than the computer thuoght, the computer would think the truck was going faster than it actually was by up to 3% (there I went ahead and did the math for you  ).  For a given (held) throttle position, once you accelerate through the programmed shift speed for that throttle position, you will get the upshift.  Everything else being equal, even without the WOT shift RPM change, he should in general be seeing all shifts ocurring at about 3% higher speed.

[Ski Edition]

I'll let you engineers haggle out the percentages, but driving it last evening, she seems like a much lighter vehicle and actually has some pep to her step!!  Still more tinkering to do!!  Power is really contagious!!!   

[Main One]

Ski - I've never used a superchips programmer, but I suspect that like with the Hypertech that I used to own, the shift points that you can change are only the WOT shift points.  Since you have found the changes that affect your shift points very positive so far, you may be interested in changing more of your less than WOT shift points via a custom tune.

Question: do you use Tow/Haul mode very often, and if so, how do you like the stock-programmed shift point in T/H?  They are quite a bit different than the standard shift points.  For instance, in my 03's stock tune, it would shift into 4th I think possibly as low as the low 20s mph-wise, but in T/H, not until I believe 55.

A custom tune besides adjusting your part-throttle shift points, can also adjust how "willing" the tranny is to downshift as well as unlock the torque converter.  You may find tweaking these to be to your liking as well.

The 500 RPM shift bump is quite healthy - I'm not sure I know where that puts you.  I believe I'm currently programmed at 6100 rpm with an engine cutoff at 6300.  I was running I believe 6200/6300 but in hot weather seeing the electronic thottle close on me 'cause the tranny wasn't shifting fast enough and I was hitting 6300.  I upped the 6300 to 6400, but then got a little lifter tick that I found annoying, so went from the 6200/6400 to 6100/6300.  I believe after the upgraded tranny goes back in that I will try 6200/6300 once again.   

How all of this relates to Dynatech headers is beyond me???   

[MyBigToy]

I also had my tire size all wrong too.  Nitto says my tire is 33.98 diameter, tire conversion chart show 33.8.  We checked pressure and ensured correct inflation and measured them at 33 even. so we changed that also.

33.98"-33.8"=0.18"

That's the error I was identifying.  I won't argue that I needed another cup of coffee but I think that math adds up.    That's why I find it easier to use a GPS unit because you are relying on the actual speed you are going rather than calculated tire sizes.

Aren't stock WOT shift points 5200? If so, he's now shifting at 5700.