Quieter upgrade?

[L31MaxExpress]

Got to disagree with that. Manifolds be they headers or cast logs have nothing to do with how much sound exits out the back. The sound waves are contained within the pipe. 151 S-10's and 302 Ford Mustangs came from the factory with SS round tube header type manifolds and they were no louder than comparable vehicles with iron log manifolds. The stangs had a little exhaust note by design. Long tube headers will ring at certain frequencies,normally under WOT but it's not loud,to me it's music.

Chevy V8s and 4.3Ls came with factory tubular shorties as well in some vehicles.

 

[L31MaxExpress]

Is there any difference between the Mexican Sonora and US Tahoe?

Mexican Sonora came with the L31 350 and a 5spd. They were also factory cat-eye when the trucks switched over. GM built the suburbans as cat eyes too.

 

[NMNorsse]

Okay. So I need to use the same number of cats as came with the truck. I like Erik's design but in other threads they've suggested a better y pipe rather than duals back to the cat.

1. Does running them separate to the cat improve the flow?

2. My understanding is the AIR pump sends hot exhaust from the Manifolds to the cat to heat it up quicker. I planned to eliminate it to put in shorties and because my brand new pump makes noise. It's the 4th new one I've put in. The other 3 failed immediately. Cardone is the only company making them and I think they've got a QC problem. Should I plumb the smog system to the cat via an O2 bung even if it means keeping the cast Manifold rather than putting in shorties. Or do the newer cats heat up quicker than Ye Olde pellet cats?

 

[Sean Buick 76]

run two magnaflow mufflers in a row for a single 2.5”

 

[Scooterwrench]

The A.I.R. pump pumps in fresh air into the exhaust stream to;
1. Dilute the exhaust gasses with fresh air and,
2. Super heat the the exhaust gasses like turning up the oxygen on a cutting torch.
Air Injection Reaction was pretty much eliminated by the mid 90's in US vehicles because EEC's created leaner burning engines and cats got better.
I'd toss that aggravating AIR and all of it's related plumbing.

 

[Supercharged111]

The air pump can only promote combustion in the exhaust stream when the mixture is richer than stoich, otherwise there's no fuel for that additional oxygen to burn. When you're stoich or rich, there is no oxygen left after combustion in the engine to promote combustion at the cat. Newer cars that did come with pumps came with electric pumps controlled by the PCM. In the case of my Corvette, it'd only run the pump for around 90 seconds or so. In that time, it also pulled a bunch of timing in certain areas to drive up EGT. Older cars used to use 2 way catalysts. Those ran slightly rich to keep NOx away, and the air pump ran all the time because of that to keep the cats going so they could get rid of HCs. So the only thing that'll happen in your case is that the cat will take longer to warm up if you ditch the pump. Warmup is the dirtiest time an engine runs, so that's why GM wanted the cats lit as quick as possible. Also why they go into closed loop so fast.

 

[L31MaxExpress]

The air pump can only promote combustion in the exhaust stream when the mixture is richer than stoich, otherwise there's no fuel for that additional oxygen to burn. When you're stoich or rich, there is no oxygen left after combustion in the engine to promote combustion at the cat. Newer cars that did come with pumps came with electric pumps controlled by the PCM. In the case of my Corvette, it'd only run the pump for around 90 seconds or so. In that time, it also pulled a bunch of timing in certain areas to drive up EGT. Older cars used to use 2 way catalysts. Those ran slightly rich to keep NOx away, and the air pump ran all the time because of that to keep the cats going so they could get rid of HCs. So the only thing that'll happen in your case is that the cat will take longer to warm up if you ditch the pump. Warmup is the dirtiest time an engine runs, so that's why GM wanted the cats lit as quick as possible. Also why they go into closed loop so fast.

Yep the Vortecs are in closed loop seconds after they start. The PCM actually enables short term fuel trims in open loop. The timing is advanced a ton through a specific coolant temperature range for warmup. With the factory catalyst warmup advance enabled my van runs over 40* advance. The L31 catalyst warmup advance starts around 110F and ends about 150-160F. I find it odd they advance rather than retard the timing. The TPI I had using the 90-92 speed density ECM was the opposite, it would not enable the EST signal until 100-110F and the engine ran on the fairly retarded timing curve provided by the HEI EST modules limp home mode.

 

[Scooterwrench]

The timing is advanced a ton through a specific coolant temperature range for warmup. With the factory catalyst warmup advance enabled my van runs over 40* advance.

To burn off more A/F before the exhaust valve opens?

the engine ran on the fairly retarded timing curve provided by the HEI EST modules limp home mode.

Is that built into the ESC or the ECM?
The reason I ask is I'm going to use a 7427 ECM and toss the external ESC.

 

[tayto]

To burn off more A/F before the exhaust valve opens?

Is that built into the ESC or the ECM?
The reason I ask is I'm going to use a 7427 ECM and toss the external ESC.

different era PCM

 

[Scooterwrench]

different era PCM

Yeah,I know!
That's why I asked.