Performance characteristics of TDC offsets

[Spindrift]

Assuming that my truck is OBD-II vintage, what performance characteristics can I expect from a -1.5 TDC offset vs. -1.94 TDC offset under a no-load and load condition?

[Billman]

You there RJ...Hello...

[rjschoolcraft]

I can't speak for the spcifics he's asking about. What I know is that going from -.7 to -1.94 picked up about 2-3 hp across the board for my Suburban. I know that tbogemirep runs at -1.5 and likes it, but I don't have any numbers to compare. I was kind of hoping that Kennedy might see this and chime in.

[Spindrift]

I was also hoping that jd would speak up. He had recommended that the TDC Offset for drag racing should be -1.5deg, minimum. I'm hoping he can provide the reasoning behind the recommendation.

[kowsoc]

I remember reading about some testing done at various settings....I believe Charlie Peterson was involved. A search should find it. A setting of -1.50 had given the best fuel economy. I don't remember how the power was compared to a more advanced -1.94. My truck is at -1.85 and I wouldn't change it....fuel ecomony is great. If I had a Tech II I would mess around though. ;)

[gmctd]

I stated the -1.5deg figure as the minimum required setting over the factory value to improve power.
Actual recommended value depends on whose reprogramming you are running.

My thoughts are -1.5deg for lower rpm torque - pulling a load where no downshifting is done above 45mph, and -1.94deg for increased horsepower in higher rpm ranges.

[eracers999]

Well i just recently set my TDC offset from -1.14 to the -1.94 and back to a setting of -1.58 here is what i have learned. Yesterday i hauled 2 loads of trees back to back, 70 mi round trip, temp 84, road speed 74 to 80, scaled at 16k + each load. The -1.94 builds more heat, builds it quick, seems to overwhelm the cooling system, 230 in shot order, egt went up 50 to 75 deg. it starts better, better throttle responce, has good torq all the way thru, Makes this engine rattle a bit much for my liking at freeway pulling speed. I dont have the 97 cooling mods yet, but do have the d max fan and clutch mod. Inbetween loads i whipped the tech1 on it and reset TDC off set to -1.58 and off i went, this engine in this truck likes this setting better because it can more easily manage the heat, the timing of -1.94 seems too much for it. It pulled impressivly with both settings, i am of opinion that it pulled better in the rpm range of 24 to 2600 better set at the -1.58 .
Kent

[charliepeterson]

A few years ago I did some testing with JC about this subject TDC settings.

-1.94 is aggressive. It builds EGT's faster on long hard pulls with no intercooler. It starts the truck quicker. You're basically using the fuel more efficiently. Cold starts do "rattle" a little more. I personally think the truck gets off the line slightly better too.

-1.50 is really close in performance. EGT's build slightly slower. Without an intercooler any setting is going to get you to the red zone. Smoke might be slightly less here as well.

Either of these settings or anything in between is much better than the factory settings. Fuel mileage is about the same with these settings. It's all about performance!

[Billman]

Spindrift

If your main goal for the TDC Offset is for your trip to the Dragstrip, I think your splitting hairs before you establish a baseline.

By all means, Do what makes you happy.

From my experience down the strip with ANY car/truck, a consistentcy must be obtained before making any changes. Changes must also come 1 at a time. This way improvements can be monitored easier.

I have seen the differences in what it takes to turn a 15sec. car into an 11sec. screamer and scrape .5 of a second off a 9.20 car. Let me tell you, 4 seconds off a 15sec. car is much easier.

While 3-4 horsepower with TDC Offset changes IS an improvement, I think more ET can be shaved off by learning how to actually drive the Burb down the strip.

Things like starting line technique, tire pressures, shifting, etc. can show BIG reductions in ET.

How about a little Alcohol in that WMI?

[Spindrift]

Billy,

I started this thread to solicit performance characteristics while under load (towing) since this is what I'm building the truck around. I couldn't agree with you more regarding my need to practice technique on the strip, first, and then worry about tuning next. However, something (somebody ;) ) tells me that my dip into drag racing will be short lived.

I'm running 30% methanol in my water tank now and I've learned that the kick I get when the dilution rate is more, say 40 - 50%, isn't worth the pre-ignition at those higher methanol concentrations.

Kent/Charlie,

That's exactly the kind of info I was looking for, but hadn't been able to find in any of my searches. Thanks.

[ 08-30-2004, 05:21 PM: Message edited by: Spindrift ]

[DmaxMaverick]

TTT for Joe Bleaux