Southern California 2003 Silverado SS

dcairns

Member
Joined
Jun 17, 2026
Messages
8
Reaction score
32
Location
SoCal
Hello everyone. Original owner of a 2003 Silverado SS. Radix Supercharger and SnugTop shell and a BedRug on the truck bed. After breaking the 4l65e for the fourth time, I recently upgraded the 4l65e transmission to a 4l80e transmission. Tow my 1996 Fleetwood Prowler trailer with the SS, and it does fine.

HPIM0476.jpg

HPIM0328.jpg

LostCoast01194.jpg
 

tmpass

Active Member
Joined
Aug 9, 2025
Messages
27
Reaction score
58
Location
MA
It should have been the 4L80E From the factory... much more durable and I think the closer gear ratios in the 80 are better

Great color on that... would be nice to see a video with some blower whine :)
 

stutaeng

Well-Known Member
Joined
Feb 9, 2022
Messages
956
Reaction score
2,469
Location
Dallas, TX
@dcairns is your Silverado SS a 2wd or AWD? And agree with @tmpass 100%

I did a 4L80e swap on my 99 Silverado 2wd when I swapped the V6 to the 5.3. Swap was pretty straightforward (almost too easy TBH). 4x4 would be more challenging, and AWD is the most difficult because of that hard to find NP149 input shaft. I love my 99; it's a joy to drive :) . Will pass it on to my oldest son someday.
 

dcairns

Member
Joined
Jun 17, 2026
Messages
8
Reaction score
32
Location
SoCal
All 2003 SS's were AWD, so yeah, more complicated. Needed a adapter to mate up with the transfer case. The adapters are out there, just need to search a bit. Also drive shafts needed to be changed, longer in front, shorter in back due to the 4L80e being a bit longer. A re-tune of the computer shift points was also done and it shifts like it came that way from the factory.

Like you, I intend to drive my truck to the grave... or at least till the DMV says I can't drive anymore :D I don't want anything newer, I like stuff I can fix myself.
 
Last edited:

DenaliSS

New Member
Joined
May 26, 2025
Messages
3
Reaction score
5
Location
Left Coast
Hello, I have the sister truck to yours-2003 Sierra Denali. From what I can tell the only difference is front end sheet matal and the AWS/ all wheel steering. Question If you ever cruise at. The legal limit (as if I do) at speed limit or at 70mph what’s your gas mileage? I am thinking about a Paxton supercharger and what to see how it affects thins. Thanks in Advance DB
P.S. This is my second Sierra Denali drove the first till engine died and needed a paint job.
 

dcairns

Member
Joined
Jun 17, 2026
Messages
8
Reaction score
32
Location
SoCal
If I try to drive gently, freeway gas mileage can touch 16mpg at 65mph, but as soon as hills or traffic come up, it gets down to 13-14mpg. Around town it is more like 10mpg with stop and go waiting at traffic lights. Going up mountains with the trailer, 1-3mpg is common o_O
Towing in general (flats and hills and mountains) works out to about 9mpg.

The Radix supercharger is bypassed for mild driving so it has little effect on mpg until the manifold vacuum drops and the supercharger starts whining sound (a lovely sound, if some what fuel-ishly expensive sound :)) I don't know if the Paxton does the same bypass thing, although I would guess it does. As I recall the Paxton is a centrifugal supercharger which makes it's boost in the higher RPMs, vs the Radix which is a roots type and gives boost right off the line. But it has been a long time since I shopped that info.

Minor quibble between the Denali and the SS, according to what I find on the internet, in 2003 the Denali had the LQ4 6 liter 325hp and the SS had the LQ9 6 liter 345hp.
I have often wondered how it would be to have the all wheel steering. Would be handy in tight parking situations. But I hear it adds weight and complication to things

My engine is still running strong, but the paint is needing help. Clear coat is flaking off on the tops. I am saving up for a re-paint.
 

DenaliSS

New Member
Joined
May 26, 2025
Messages
3
Reaction score
5
Location
Left Coast
If I try to drive gently, freeway gas mileage can touch 16mpg at 65mph, but as soon as hills or traffic come up, it gets down to 13-14mpg. Around town it is more like 10mpg with stop and go waiting at traffic lights. Going up mountains with the trailer, 1-3mpg is common o_O
Towing in general (flats and hills and mountains) works out to about 9mpg.

The Radix supercharger is bypassed for mild driving so it has little effect on mpg until the manifold vacuum drops and the supercharger starts whining sound (a lovely sound, if some what fuel-ishly expensive sound :)) I don't know if the Paxton does the same bypass thing, although I would guess it does. As I recall the Paxton is a centrifugal supercharger which makes it's boost in the higher RPMs, vs the Radix which is a roots type and gives boost right off the line. But it has been a long time since I shopped that info.

Minor quibble between the Denali and the SS, according to what I find on the internet, in 2003 the Denali had the LQ4 6 liter 325hp and the SS had the LQ9 6 liter 345hp.
I have often wondered how it would be to have the all wheel steering. Would be handy in tight parking situations. But I hear it adds weight and complication to things

My engine is still running strong, but the paint is needing help. Clear coat is flaking off on the tops. I am saving up for a re-paint.
Yes the engine did have less HP. I always wondered why they did that. The whole point of the Sierra Denali was for trailer towing. The rear steering has 3 selections. Lock straight, 4 steering and 4 wheel trailer steering. Pulled a trailer a lot when I was working, could turn in very tight places. Like you said it make parking almost anywhere Super easy. I have tried to help with the HP Issue by adding aftermarket dual exhaust, Volant CAI, Accel Coils, Granitilli Plug wires, etc. I have a TBSS manifold to install when I do the cam, springs, Rockers change(streetable). Then the supercharger. Your comment on little boost at cruise does interest me. So I am going to look into that. I think it was you who said I will drive this truck till they have to pry it from my cold dead hands . Also that’s for your quick response.
DB
 

99Sierra2500

Well-Known Member
Joined
Apr 12, 2025
Messages
533
Reaction score
1,185
Location
Indiana
Yes the engine did have less HP. I always wondered why they did that. The whole point of the Sierra Denali was for trailer towing. The rear steering has 3 selections. Lock straight, 4 steering and 4 wheel trailer steering. Pulled a trailer a lot when I was working, could turn in very tight places. Like you said it make parking almost anywhere Super easy. I have tried to help with the HP Issue by adding aftermarket dual exhaust, Volant CAI, Accel Coils, Granitilli Plug wires, etc. I have a TBSS manifold to install when I do the cam, springs, Rockers change(streetable). Then the supercharger. Your comment on little boost at cruise does interest me. So I am going to look into that. I think it was you who said I will drive this truck till they have to pry it from my cold dead hands . Also that’s for your quick response.
DB
I think the engines have different cam profiles.
 

Marky Dissod

Well-Known Member
Joined
Aug 10, 2025
Messages
246
Reaction score
532
Location
(718)
An aluminum-head LQ4 is pretty much an LQ9, except with dished pistons & a 9.4:1 scr, vs the LQ9's flat top pistons & a 10.1:1 scr.
Interesting fact: After '01, LQ4 LQ9 & LS1 all share the same replacement cam part number: 12561721.
Oversimplifying a bit: an LQ9 is basically a cheaper LS1 for GM, while moving the LS1's torque/power curve a wee lil bit lefter for towing/hauling.

(I'd upgrade an LQ4 with Gen3 5.3L heads, or an LQ9 with either Gen3 or Gen4 5.3L heads.)
 
Last edited:

Latest posts

Members online

Top