I see a few people running -1.94 tdc offset. Will it run good with just aftermarket exhaust? Or is this when you are running more boost and more modifications? And does pump timing still remain at 3.0-3.5 degrees?

Roughly it sort of goes like this, a deeper explanation is in FAQs & how to set it lots of reading on this topic there.

Timing is computer controlled on electronic IPs; timing will go to where puter thinks it needs to be for a given condition, except during the time set function using a bi-directional scan tool,

During time set commanded the 3.5 timing is the for lack of a better word is "base timing" when a time set is activated commanded/desired goes to 0, & should result in 3.5 avg. of actual.

BTW time set & TDC offset are 2 different adjustments don't confuse them.

The -1.94 is an advance bias added to what computer is requesting at any given time when not in time set (run timing), optic bump is mechanical bias that changes advance reference point in the IP (Dohickey in slot A aligns with Knobbin B device, which is now at B+ 1 mm)):h . Again these are hi-lites more in FAQ, running advanced timing does not harm engine unless going too far, so exhaust mods have no impact on setting. But here is the kicker -1.94 & higher will rattle like a dickens until engine warms up in the winter, -1.5 is a little less agressive and gives good power gain.

Some swear by optic bump, I've never attempted it myself, I have read that it can be a little fussy until you learn what you are doing to get it right. For me the TDC offset is less intrusive, & I have a bi directional tool so it's easier for me to doi it the method I describe in FAQs, now if I had access full time to a Tech 2 then hands down IMO offset learn is only way to go.

Thanks. I have a Snap-on scanner at my shop, and my buddy works for GMC so he has the Tech 2. I just bought the truck a month ago and he brought home the tech 2 and set the offset to what ever is stock. Thanks for the info.

I think the TDC Offset discussion is one that REFUSES to die around here.

Base Timing for ANY 6.2/6.5 pump is 3.5 degrees advanced relative to the crank.
That means, when the IP is at full retard position internally, it should be 3.5* advanced of TDC.

On a mechanical pump with a mechanical advance and no computer, you time it mechanically until the pump is at approx 3.5* advanced position. No computer means you can put it wherever you want and your internal advance will operate exactly the same, but your actual timing will move with the static position of the pump.

DS pumps are a differenct story. The advance is controlled by the PCM. It constantly adjusts advance by moving the stepper motor up or down to get the desired advance. It monitors this by the CAM reference pulse from the Optic sensor and the Crank reference pulse off the tone wheel sensor on the crank.
When you set TDC Offset, the computer will command the pump to full retard internally (0* ADV) and compare cam/crank pulses and the differential between them at this point is the TDC Offset. A 0.00 TDC Offset would mean the pump is exactly 3.5*advanced at full retard. -1.5 means the pump is 5* advanced in crank *'s.

Now that being said, the TDC Offset value IS NOT figured in when the engine is operating. meaning a -1.9 TDC offset DOES NOT give you more Actual Advance than if it were at -0.25. The computer moniters cam/crank pulses to determine and adjust advance position constantly.
Both values are 'good' TDC Offset values, this tell the PCM that whatever advance it commands from the Injection pump, the pump will be able to achieve because the pump is in the right position for its internal advance to reach the entire advance spectrum it might command.

To be continued Folks, I'll not debate it here, some of differences between TDG & I on how it works is semantics I think. I'll discuss off line in PM rather than add confusion to an already confusing topic. Stay tuned for more later.

I was just wondering if -1.94 makes the truck run better. If it is going to hurt anything if I only have modified exhaust. And if the timing still says 3.0-3.5 on the scanner when you do put it at -1.94 so that if the scanner is reading 8 or somethinfg I won't be suprised.

Nope won't hurt a thing, crisper acceleration was the result on my truck,over stock setting, -1.5 is good compromise for more acceleration & less rattle. Not going the path of debate with TDG on why it works , I'll just add that the way I did it in FAQ worked on my truck, since your is a 99 pretty close to my 98 I assume it will work the same on yours

r85sub,
I had a DSG gear set installed on my 94 Blazer. I also set the TDC offset per instructions from DSG to -1.94. I think it really made a difference in the drivability of the truck. It was a lot smoother and the throttle response was crisp. It really ran well in the upper rpm range, say above 2000. Some will say the major drawback was the increased rattle when cold. I really didn't mind the noise and my wife figured the truck was just like me, grumpy in the mornings. I plan on making the change to my 99 when I get the time to swap out the gears.

Tim,
I have been following this subject with interest. I have read almost everything on the subjets here at the place but still have a few questions. For example (and I don't want to hijack r85's thread), does setting the base from 3.5 to 8.5 correlate to a -1.94 offset? Perhaps I'll review the articles again and see if it doesn't make more sense.

Problem is that there is so little public domain technical documentation, and some of the documentation that is out there does not match what actually is occurring or at least in my endeavours. Specifically I'm referring to the GM tech manual which does not reflect the results of what is happening on the 5 different PCMs I've put in my truck while doing R&D.

(It's almost as if the documentation is obscured on purpose so GM tech & others won't attempt to get more performance by tweaking timing, or tech writers did not fully understand it and put down their own interpretation of it, in todays world tech writing is farmed out to low bidders and isn't always 100% accurate)

I thought something might be unique to my truck, but others I PM have had similar result. I don't want to turn this thread into a debate, so I'm holding off on responses until we get this nailed down once & for all.

As far as 3.5 base goes that is only occurs in the commanded time set mode with T2 or Snap On etc., running idle timing 8.5 for example is set by computer, could be higher if PCM thinks more advance is needed, on cold mornings timing on mine has gone as high as 14, and as engine warms up desired timing comes down as does idle rpm. Exact interplay of TDC offset vs. timing is one I'm still wrasslin with to fully understand myself. When I set mine as described in FAQ I did time set check & IP move to "ball park" my timing to have a known starting point, once I got the timing to the 3.5 deg per the manual in time set, then I read TDC offset, which is a different value than timing, since I don't have a T2 to command offset learn I have to do key on key off, APP to the floor method (Only works for OBDII as far as I know), loosen IP mounts and slightly rotate to drivers side to advance, using a T2 you only need to command it with tool and rotate IP only if current IP location does not have enough "Window" to allow for more advanced TDC offset. Hope this helps more than confuses, PM me and I'll try to explain my interpretaion of it better, whether I have the description of how it works right or wrong; what I'm doing works for me.

TDG is in a IP shop and has access to Stanadyne manuals so what he says probably has more validity to my assumptions based on what I have seen on my truck, but I often wonder if the documentation IP shops use is just for repair of IP on a bench or takes into consideration how it reacts in concert with the PCM and vehicle sensors.

I was hoping that the Military manual would offer more insight but the one I have only covers mechanical IPs, I haven't found yet any manual out there that fully explains the electronic IP as part of the engine management system. Even the GM techs training manual is obscure, as it's nearly a verbatim repeat of GM tech manual.

I'll confer with TDG & Diesel Pro both our resident IP guys and see if we can between 3 of us put out a white paper based on realworld and test bench data, for the final how it works reference. Stay tuned.

Oh anybody else that works on GM electronic controlled 6.5s or IPs for a living; PM me I'd like your input as well.

Thanks for the info. I'll have my buddy bring home his Tech 2 and reset to -1.94. He was just here and I was showing him the posts and he said he has never messed around with them. He said they mostly get contractors trucks and always set them to the factory spec. So we'll get his Tech 2 and mess around with it a little and if we come up with anything I'll post it. Thanks again.

TDG is in a IP shop and has access to Stanadyne manuals so what he says probably has more validity to my assumptions based on what I have seen on my truck, but I often wonder if the documentation IP shops use is just for repair of IP on a bench or takes into consideration how it reacts in concert with the PCM and vehicle sensors.
We have Stanadyne manuals and GM manuals. We(I) install almost half the ones we(I) build.
-1.94 won't hurt anything at all, and it puts your advance inside the pump in a very 'comfortable' position during operation, which is the only concieveable difference I can imagine. But the difference is so miniscule if any.

I will agree with what you said earlier about the differences in Tech2 vs MT2500 scanners. The more I play with the dealerships Tech2, the more I want to accidentally run over this stupid Snap-On scanner. It would be great for a general garage, but not for a diesel shop like ours. I'm gonna put that in the request box, but first I have to request that we get a request box ;)

So far autoenginuity scan software for your laptop or palm seems to be on par with the T2, just not as many features but still has plenty.

I can even set it so Homer says "woohoo" when a parameter goes above or below what i specify...lol

...I can even set it so Homer says "woohoo" when a parameter goes above or below what i specify...lol

"I don't care who ya are, that right there's funny!!!" ):h

CR,
I think you set it so it says wooohooo for above your limits and Doh!! for under...LOL:funnypost

Can do that too, to bad the software doesn't accept a longer sound file though. I tried a warp drive one from star trek but it was way to long...