Why does my LS sound like #$% after 21/2" duals, no cats and flowmasters? Good AFR's

[Covert1]

What's up with my truck? 2008 1500, 4x4, 5.3l, ly5, small cam 218/224 .552 @ 113.0, heads milled .030, ported and polished heads int & exh. After getting my tune pretty straightened out where i happy with it the only thing missing was that sound of flows on a SBC so i took the exhaust off and deleted the cats, built a new system with 2 1/2" duals and 40 series flows, deleted rear o2's, no CEL, engine pulls good still but man it sounds like sh$%. Popping in the exhaust at idle (14.3-15.3afr) and at a certain rpm (1500-2000ish) it sounds like the baffles are broken loose in the mufflers with a really tinny sound to it. Wide open about the same with 13.0afr. Truck has manifolds not headers. Is there something that is in the tune that could cause this or is it how the LS sounds? Sure isn't an old 350.

[5FDP]

Sound is not tune related unless it's misfiring horribly. Engine design and firing order will play a roll but you can't control that.

Straight pipes with flowmasters sounds terrible on these things if you ask me. Headers would just make it even more raspy and loud.

[Covert1]

Ya, i'd have to agree. It's a completely different sound compared to an old small block. I was thinking last night about the design of the cathedral port heads and how they might make a different sound and then add the fact that fueling is controlled a great deal as to when it is injected and you would have a whole different sound. I was just wondering if the injection boundary and EOIT might have a roll in changing the sounds. There's 28.9* difference between my cam and the stock one. I was curious if adding or subtracting the number from the boundary would change the tone because it would move the relationship of the firing events around? I had read several posts saying unless you had a really big cam that you didn't need to mess with the boundary but in my mind 28.5* difference in overlap is a lot of wasted energy if the fuel isn't getting put in the right time along with ignition. Any thoughts on it?

[Black LS1 T/A]

Put some race cats in there and the raspiness will go away. At least that is what happened for my Trans Am.
I HATED the sound with just headers and mufflers.

[GHuggins]

Ya, i'd have to agree. It's a completely different sound compared to an old small block. I was thinking last night about the design of the cathedral port heads and how they might make a different sound and then add the fact that fueling is controlled a great deal as to when it is injected and you would have a whole different sound. I was just wondering if the injection boundary and EOIT might have a roll in changing the sounds. There's 28.9* difference between my cam and the stock one. I was curious if adding or subtracting the number from the boundary would change the tone because it would move the relationship of the firing events around? I had read several posts saying unless you had a really big cam that you didn't need to mess with the boundary but in my mind 28.5* difference in overlap is a lot of wasted energy if the fuel isn't getting put in the right time along with ignition. Any thoughts on it?

Who said injection timing wasn't worth messing with is full of it. I change it even with stock cams and if it's popping at idle your burning fuel in the exhaust. Whether it's due to the cam or misfiring is up to you to determine.

[Covert1]

Who said injection timing wasn't worth messing with is full of it. I change it even with stock cams and if it's popping at idle your burning fuel in the exhaust. Whether it's due to the cam or misfiring is up to you to determine.

It was in a post i read through yesterday, it didn't make sense to me that it wouldn't make a difference. You should be able to finesse it for better low or high end power depending on the timing shouldn't you? Like degreeing a cam when building an engine (ADV or RET) depending on where you want your power band to come on.

So my question is then, the stock cam had -33.5* overlap and my new one has -5.0* overlap so there is a difference of 28.5* positive between the 2. So do i add or subtract the # from the boundary & ECT table? It's backwards in the E38 if i remember right, so to adjust i would subtract the 28.5* so it would fire the injector sooner in the opening event?

[GHuggins]

I'm probably sticking my foot in my mouth, but I'm pretty sure all of the 4th gen platforms had injection timing the same with some ECM's like the E78 giving more options - I really like that ECM after looking through the cal more by the way... Basically adding to boundary retards it / subtracting from ECT retards it / adding to rpm advances it... Boundary is best for rpm break points... Don't retard it too quickly - make a nice slope - end around a 595 to 620 boundary setting up top around 50some hundred rpm point then flatline from there... Add to boundary in the lower rpm areas to stop the popping - may need more timing too - injectors, build and everything will play into this... Adding a spreadsheet to help illustrate things...

[Covert1]

I'm probably sticking my foot in my mouth, but I'm pretty sure all of the 4th gen platforms had injection timing the same with some ECM's like the E78 giving more options - I really like that ECM after looking through the cal more by the way... Basically adding to boundary retards it / subtracting from ECT retards it / adding to rpm advances it... Boundary is best for rpm break points... Don't retard it too quickly - make a nice slope - end around a 595 to 620 boundary setting up top around 50some hundred rpm point then flatline from there... Add to boundary in the lower rpm areas to stop the popping - may need more timing too - injectors, build and everything will play into this... Adding a spreadsheet to help illustrate things...

Unless i missed something somewhere in my setup mine isn't the same. I attached a copy of the stock tune and both boundary and ect have what i thought to be some weird #'s. As you said everything i've seen in here on the forum has had them the same all across the board. I want to say 520 was the #. The spreadsheet and your insight are very much appreciated. Spreadsheet is f'n awesome btw. Can't wait to use it when i get to my other pc. Again thanks and i'll post what happens after i get the #'s input and play with it a little more.

[GHuggins]

The spreadsheet wasn't made by me - it was a group effort Most have injection timing setup from the factory to advance with rpm in the boundary and/or rpm table and then retard with temperature in the ect table...

[Covert1]

Just noticed i forgot to upload the tune.2008 silverado LY5 stock tune.hpt2008 silverado LY5 stock tune.hpt

[GHuggins]

That's normal injection timing for most ls platforms... They advance it with rpms...