2003 Tahoe 5.3 Flex a dog when accelerating

[Mudsport96]

To add a little bit. The power loss i see was 10%. Could be a coincidence that it was about a 10% increase in tire diameter.
Lawrence Tolman dynoed his stock lm7 silverado 2wd and put down 231hp/296tq.
So if we use the same percentage increase in diameter against an lm7 stock hp, we get 205hp/263tq to the ground with 35s. A lost of 30hp and tq is like always being hooked to a trailer.

 

[Burban22]

Fellow 2003 5.3 Flex Burb owner. Interested in doing upgrades similar to yours -- tune, exhaust, 33 inch tires and was wondering what, if anything you would do different.

Already did 2in up country lift, planning on 285 70/17 on factory Z71 rinse and efan conversion. Thinking about HPtuner vs BB tune, though concerned about the learning curve (and/or making a mistake in tuning that will **** the engine).

Any advice appreciated!

You will notice the acceleration drop going from stock tire size to 33" (285/70/17) It's not terrible but my tuned burb has to downshift a bit more if I drive 70mph or lower on the Interstate than with the stock tire size. I have 3:73 gears but picked up a wrecked burb that has 4.10's! A big job to swap front and rear axles but 4.10 and 33" are a great combo. 33" and 3:73 are ok, but any larger and you should regear. We have a '04 Yukokn XL Denali in the family with the 6.0. Had that tuned by BB and it woke it up quite a bit as well. I'd recommend any competent tuner, doesn't have to be BlackBear. I run two different sizes of tires and wanted BB to tune another PCM so I can switch them when I change from summer to winter tires which are different sizes. They refuse to do that even though they did the same thing for me on my '97 k1500 but that was in their earlier years. I'd have to send in my PCM for them to change the tire size, can't use one of those speedo correction odb2 programmers since the tune is locked by BB.

 

[555hp]

You will notice the acceleration drop going from stock tire size to 33" (285/70/17) It's not terrible but my tuned burb has to downshift a bit more if I drive 70mph or lower on the Interstate than with the stock tire size. I have 3:73 gears but picked up a wrecked burb that has 4.10's! A big job to swap front and rear axles but 4.10 and 33" are a great combo. 33" and 3:73 are ok, but any larger and you should regear. We have a '04 Yukokn XL Denali in the family with the 6.0. Had that tuned by BB and it woke it up quite a bit as well. I'd recommend any competent tuner, doesn't have to be BlackBear. I run two different sizes of tires and wanted BB to tune another PCM so I can switch them when I change from summer to winter tires which are different sizes. They refuse to do that even though they did the same thing for me on my '97 k1500 but that was in their earlier years. I'd have to send in my PCM for them to change the tire size, can't use one of those speedo correction odb2 programmers since the tune is locked by BB.

Thanks! Would you recommend staying with the 265 70 17 then (with my 3.73s that I do not lean on changing)?

 

[Burban22]

Thanks! Would you recommend staying with the 265 70 17 then (with my 3.73s that I do not lean on changing)?

I like the look of 33 but you can’t beat acceleration and mpg with stock tire size. Also I can feel the difference in unsprung weight between the two tires. Aka harness on bumps, expansion joints. Stock tire is prob 35-40lbs per tire and 33” 55-65lbs per tire.

It’s not a huge difference but noticeable. Do what you really want as these don’t get great mpg or acceleration as it is anyway lol.

 

[Mudsport96]

I like the look of 33 but you can’t beat acceleration and mpg with stock tire size. Also I can feel the difference in unsprung weight between the two tires. Aka harness on bumps, expansion joints. Stock tire is prob 35-40lbs per tire and 33” 55-65lbs per tire.

It’s not a huge difference but noticeable. Do what you really want as these don’t get great mpg or acceleration as it is anyway lol.

We used to joke about a friend of mines 79 step side chevy. Had a HEALTHY 406 in the 500hp/400tq range. It had 4.11s and he ran 35 inch boggers. We would go to the local 1/8 mile track and bolt on a set 30 inch tires all the way round and he would run it. Called it the "20 minute big block swap", because it made such a difference in acceleration. He could go through the traps at about 86mph with the 30s but only hit about 70 with the 35s.

 

[Burban22]

We used to joke about a friend of mines 79 step side chevy. Had a HEALTHY 406 in the 500hp/400tq range. It had 4.11s and he ran 35 inch boggers. We would go to the local 1/8 mile track and bolt on a set 30 inch tires all the way round and he would run it. Called it the "20 minute big block swap", because it made such a difference in acceleration. He could go through the traps at about 86mph with the 30s but only hit about 70 with the 35s.

Lol. Yep tire diameter and weight definitely affect performance!

 

[76Nick]

Ok guys sorry I dropped off the thread, I've been in FLA on vacation and just got back a couple days ago.

Are you able to see a graph or just an instantaneous reading?

I was finally able to get this but I doubt it will help. My scan tool isn't the greatest but it's better than not having one at all. I grabbed my knock retard. It's 101 seconds long from a dead stop and getting on an on ramp for the highway up to 70 mph. I obviously kept recording after I reached 70 mph. I just sanwiched a rubber hanger on that touching tailpipe so I will have to take a new reading to see if that helped.

 

[76Nick]

I know the thread has moved onto gears and other things so I have been wondering....is it worth switching out my 3.73's for 4.10's if I'm running 35" tires? I'm not trying to race anybody, I just want to be able to keep up with highway traffic and get up to speed in a reasonable time. Also tow a travel trailer and a utility trailer now and then.

 

[Marky Dissod]

... is it worth switching out my 3.73 for 4.10?

Yes, even if you weren't using 35" tall tires, GM undergeared every vehicle with a V8 since the 70s.
Oh, wait -

... if I'm running 35" tires?

Moreso, Yes. You not only need to make up for GM undergearing it,
you also need to make up for the heavier tires with even more rolling resistance.

... just want to be able to keep up with highway traffic and get up to speed in a reasonable time. Also tow a travel trailer and a utility trailer now and then.

Gottdamnt, YES.

 

[mikez71]

That does seem to be a lot of knock retard, though not to the degrees of retard like I had with my loose motor mount bolts.

To the extent big tires will affect performance, could that be causing knock retard?

 

[Marky Dissod]

That does seem to be a lot of knock retard, though not to the degrees of retard like I had with my loose motor mount bolts.
To the extent big tires will affect performance, could that be causing knock retard?

Possibly? Engine load is always slightly higher than expected when your vehicle is wearing taller heavier tires with greater rolling resistance.

He should check to see if the knock retard is false knock by trying 89 or 91 octane.
If he gets THE SAME mount of knock retard in both cases, it's definitively false knock,
and he should check for a physical resonance / loose bolt / anything that could be making a noise that another vehicle would NOT make.

 

[76Nick]

That does seem to be a lot of knock retard, though not to the degrees of retard like I had with my loose motor mount bolts.

To the extent big tires will affect performance, could that be causing knock retard?

Possibly....I hadn't even considered that. I need to find the time to get it back out and check the retard again with that rubber I shoved in to dampen the exhaust pipe and to from there

Then I gotta start looking at 4.10's lol

 

[76Nick]

He should check to see if the knock retard is false knock by trying 89 or 91 octane.

I'll give that a shot too next time I fill up. When I put my cat back on the new pipe touches the rear crossmember after the muffler. I wondered if that may be what is causing it so I sandwiched a spare rubber hanger in there and now have to take another reading.

 

[Mudsport96]

You could also try running it almost empty and put half of a tank of e85 in it since it is flex. That is 100+ octane. Should stop any knock you have.
Also. Does your scanner have the ability to see what the alcohol content sensor is claiming it sees?