5.3 or 5.7, which would you do?

[slowburb]

I can't remark on the cam. Are the Comp rockers self-aligning? Have you confirmed fitment under the stock valve covers? The 2500 exhaust is comprised of two catted headpipes (cats are significantly bigger than half ton cats), which are offset from each other so the cats are each the same distance from the heads, and they terminate with the same dual pipe flange as the half ton pipes, where they connect to a flanged dual pipe into the bus-sized DI/SO muffler, to single tailpipe. The tailpipe on my Suburban was 2.75" and stainless steel. The half ton headpipes are 1-7/8" while the 3/4 ton pipes are either 2.5 or 2.75 inches. Otherwise, I lost you when you started talking about back pressure. I don't know what that is.

 

[OutlawDrifter]

Exhaust systems do no need back pressure. They need velocity.

Back pressure is a myth that started about the time everyone was putting straight pipes on tbi's and having 02 sensor issues. Swap in a heated O2 and the problems went away.

I repeat, back pressure is a myth.

 

[slowburb]

Exhaust systems do no need back pressure. They need velocity.

Back pressure is a myth that started about the time everyone was putting straight pipes on tbi's and having 02 sensor issues. Swap in a heated O2 and the problems went away.

I repeat, back pressure is a myth.

They need optimized velocity, matched to the demands of the engine's ability to pump air, and I'd say the term "backpressure" originated well before 1987.

 

[OutlawDrifter]

They need optimized velocity, matched to the demands of the engine's ability to pump air, and I'd say the term "backpressure" originated well before 1987.

i've got quite a write-up typed on another site if you'd like me to get all scientific and start throwing around formulas. we can bill nye this thing to the ground if necessary.

yes i'm sure the term was used before then...my point is, this became a common term when automakers were using non-heated 02 sensors. straight pipes allowed the exhaust to flow with less restriction, causing less heat to warm up the O2 sensor. this in turn led people to believe backpressure was not only needed, but a good thing.

 

[slowburb]

Hahaha, we can leave Bill Nye out of this one. I'm with you man, and agree that backpressure is a marketing term as opposed to a scientific condition of flow dynamics.

 

[BoominSVX]

I saw pics of the two side by side on google. Didn't take long to find. Okay, so if we stray from the term "back pressure" and put on this new sexy sounding term "optomized velocity", the point is still there. A pipe too large will adversely effect the engine torque number. I don't care to get into the math of it, but what I understand is pipes can be too big, and they can be too small. I'm not building a drag strip racer here. By the time the cooler exhaust reaches the tailpipe, it'll still make it through the 2.75" just as easily as the hotter gases make it through the dual 2.5" pipes straight off the headers at 5x the temperature. We can Bill Nye it to death, but I'm not sure how to determine the velocity of air that will be cycled through this air pump at specific RPM's, nor am I sure of the temperature loss throughout the travel of the exhaust/muffler. I also have no idea how to determine where the velocity of air finds the sweet spot for torque vs air. Finally, I have no idea how you would measure real world air volume through an entire exhaust. I DO know that my tires look flat when winter comes fast. They always have less pressure at 20 degrees than they do at 80. That part makes sense obviously. I also know that opening an exhaust larger than required for high RPM flow (which I don't plan on doing) can cause a LOSS of torque. Call it by any name you'd like, but there is such a thing as too big, and I don't wanna do it. There's also a term for when 2 cylinders fire on the same side of the engine, and it starts with an N, but I can't remember it now. Supposedly having a collector chamber helps this phenomenon pass with minimal effect of built up velocity. I passed high school and stopped there. Trigonometry and algebra are out of the spectrum of common sense, so I didn't pay attention. Was this the moment my teachers all warned me about in high school? Hmmm, didn't hurt nearly as much as they said it would.

Still going with my previously posted plan. Found like 6 sets of headers on CL for around 50 bucks. One even stainless. I'll call and look at them later. Gonna be tied up for a bit. Oh, and yes Slowburb, I know I need self aligning as the last time I took the L31 apart, I did not see guide plates. Even if I did, the 1.6 RR needs the rod hole enlarged to 1/2", so I'd want guides on there.

 

[sewlow]

"Trigonometry and algebra are out of the spectrum of common sense, so I didn't pay attention. Was this the moment my teachers all warned me about in high school? Hmmm, didn't hurt nearly as much as they said it would."
LMFAO! This had me on the ground!!!
Me. "I put in the same #'s as you, but my answer is different. Why?"
Teacher. "The answer is not important. The procedural method is."
Me. " But If I used the same #'s, & the same procedure, I should get the same answer!"
Teacher. "Not necessarily."
Me. "Well, isn't the procedure the requirement to get the right answer?"
Teacher. "No"
Me. "This is non-sensical gobbledy-****. Where's the logic?"
Teacher. "Fail!"

School. The place where you do you're daily penance for being a child in the government's squirrel cage.

Apologies for the sidetrack! Lol! You're statement opened some old wounds!

 

[slowburb]

I saw pics of the two side by side on google. Didn't take long to find. Okay, so if we stray from the term "back pressure" and put on this new sexy sounding term "optomized velocity", the point is still there. A pipe too large will adversely effect the engine torque number. I don't care to get into the math of it, but what I understand is pipes can be too big, and they can be too small. I'm not building a drag strip racer here. By the time the cooler exhaust reaches the tailpipe, it'll still make it through the 2.75" just as easily as the hotter gases make it through the dual 2.5" pipes straight off the headers at 5x the temperature. We can Bill Nye it to death, but I'm not sure how to determine the velocity of air that will be cycled through this air pump at specific RPM's, nor am I sure of the temperature loss throughout the travel of the exhaust/muffler. I also have no idea how to determine where the velocity of air finds the sweet spot for torque vs air. Finally, I have no idea how you would measure real world air volume through an entire exhaust. I DO know that my tires look flat when winter comes fast. They always have less pressure at 20 degrees than they do at 80. That part makes sense obviously. I also know that opening an exhaust larger than required for high RPM flow (which I don't plan on doing) can cause a LOSS of torque. Call it by any name you'd like, but there is such a thing as too big, and I don't wanna do it. There's also a term for when 2 cylinders fire on the same side of the engine, and it starts with an N, but I can't remember it now. Supposedly having a collector chamber helps this phenomenon pass with minimal effect of built up velocity. I passed high school and stopped there. Trigonometry and algebra are out of the spectrum of common sense, so I didn't pay attention. Was this the moment my teachers all warned me about in high school? Hmmm, didn't hurt nearly as much as they said it would.

Still going with my previously posted plan. Found like 6 sets of headers on CL for around 50 bucks. One even stainless. I'll call and look at them later. Gonna be tied up for a bit. Oh, and yes Slowburb, I know I need self aligning as the last time I took the L31 apart, I did not see guide plates. Even if I did, the 1.6 RR needs the rod hole enlarged to 1/2", so I'd want guides on there.

All the calculations and all the algebra, trig, and calculus aside, I'll go with GM powertrain engineering. There's no way they put those headpipes on 2500/3500 trucks for them to LOSE power over the half tons. Not to mention the countless threads I've read all over the internet claiming power gains from swapping half ton headpipes for the bigger ones.



With regard to the rocker arms. There are three ways to control pushrod alignment in an sbc, and none can be used with any other at the same time. If you're going to use guideplates, you cannot run self aligning rocker arms. If you're going to open the holes up for clearance to run the 1.6 rocker arms, you do not "need" guideplates. The sself-aligning rocker arms will still do their job.

 

[BoominSVX]

LOL! Sewlow understands me. Remember, "you're not always going to have a calculator with you in the real world". I look around from where I sit. You know what I have within arms reach? A cell phone. What I don't have is a pencil.... or a piece of paper. Truthfully, this is my flashlight but it also functions as a cell phone.

Slowburb. I know these guide plates will work, but the whole point of moving to roller rockers is to reduce friction. We're not going for 6k revs. Springs are cheap and self aligning are the same price even if I buy new, so no worries. The engine is ready to pull, but I'm done for the night. I'll pluck it tomorrow and start prepping parts. I still have to do all the basics. Front and rear main, T stat, (water pump is fairly new), plugs, wires, cap, rotor, and might even do a little gasket matching and polishing. I think we're safe with this setup. The plan is still as previously stated. I'll be going to the 2500 exhaust and will determine the best layout when I have both sitting on the ground in front of me. Larger Y-pipe and shorty headers included. I picked up what you put down brother. My point was that there really is no point in taking the truck to a shop and having 3" mandrel bent exhaust custom built as it could hurt this mild build anyway. Maybe if this was a 454 it'd matter a bit more. The pipe off of my duramax turbo could be 5" and it still wouldn't hurt as the turbo is still in the same place. I also have a full 5.3 exhaust out back already that I might see if I can pull some pipe from. I know I HAD a set of shorty headers, but they are either covered by pine needles or rotted out and returned to the earth. I can't find em. They didn't have an EGR bung, but I can just weld one on if I can find them. I'd keep the EGR only because it doesn't hurt to have on the highway at 70mph. They can be problematic, but they can also save fuel in an already heavy fuel hog and I hope to tune her clean. I'll probably even bring it to Tharf and have him tune it on the wideband afterward. This thread will be updated when I get some more done.

 

[skylark]

i've got quite a write-up typed on another site if you'd like me to get all scientific and start throwing around formulas. we can bill nye this thing to the ground if necessary.

yes i'm sure the term was used before then...my point is, this became a common term when automakers were using non-heated 02 sensors. straight pipes allowed the exhaust to flow with less restriction, causing less heat to warm up the O2 sensor. this in turn led people to believe backpressure was not only needed, but a good thing.

I wouldn't mind seeing your Bill Nye stuff. I like to learn. Would you mind posting a link?