What is the most restrictive part of the L03 Exhaust, the manifold, y-pipe, cat, or pipes?

 

I'd say they all suck equally.

 

being that i have a lo3 and i have done a full exhust on it your in luck... basicy the hole damn thing.... the manfolds anrt that bad if you look at the rest of stuck system, the worst part of my stock system was the y-pipe and cat, after i did headers 3" y-pipe back
i felt a bit of a diffrents but the sound sold me hear for your self

My L03

"Save as" or it wont work

 

They all suck. If I had to pick which one was worse I would pick the y-pipe and manifolds. The cat and cat-back would be a close second. It doesn't really matter which one you choose to get rid of first because they all need to go sooner or later. If you have to choose you are just picking one bottleneck to stick with.

 

nod

 

I've got the unlucky pleasure of having someone replace the exhaust system from the cat back with their own retarded 3" setup. I've also had the pleasure of having the exhaust pipe broke clean away before the cat, giving me a full cutout before the cat and the rest of the exhaust. It is a stock cat by the way.

I can say that with the ECM and mechanical tuning, the lack of backpressure made the car Very boggish. And this is a lack of backpressure with stock manifolds and stock pipe up to the cat.

After i got the pipe rewelded back to the cat, the car felt a million times better. Whether this is due to the ECM being happy with the readings it was getting again or if it's due to some mechanical setup dependent on the backpressure, i'm not sure. But on a stock L03, it seems to like at least some backpressure. And that backpressure comes mostly from the stock cat and some form of muffler.

I never ran the car in a race before and after i rewelded the pipe back to the cat, All i can tell you is on a stock L03, I was still accelerating nicely over 100mph with the cat and such intact. So it doesn't appear the stock pipe is keeping the L03 back much.

when upgrading the exhaust, you'll get the biggest differences from going to a high flow cat and better flowing muffler. You need to be making more than 170hp that the L03 makes to feel a difference by going to larger pipes other than negative. At least that's my experience.

 

No it really is that bad. Have you looked at it? It is barely 1.75" in some of the crimped areas. Backpressure is bad and you never want it. You are getting this confused with scavaging, which can be decreased if your exhaust pipjing is too large. It won't scavenge the exhaust out at lower engine speeds the same way it would if the piping was smaller. However, you should not have this problem with a 3" system.

With just a cat back you should have seen some improvement over the entire power band. People sometimes feel a bog or a "loss in low end" when they add headers and a high flow cat. This isn't because the exhaust is too big but instead its the HUGE change in your volumetric efficiency. This change is not compensated in the PROM and your tune is slightly off. A little chip work will make your eyes boggle with the gains of a full exhaust can yeild. Even with the stock PROM your car should be considerably faster down the track with a full 2.75" or 3" set-up. Most TBI cars see over a 10% increase in the VE tables. That is huge.

 

Like i said, I wasn't sure if the bog was due to the ecm or from some mechanical dependence on the backpressure being there. I never confused this with scavenging, and never brought it up as it's not important, just that the stock tuning depends on there being backpressure, either for some mechanical devices and/or from the ECM. This is in the service manual. Regardless of the purpose of such backpressure, the results of it not being there are usually negative for an otherwise stock setup.

What sort of changes in the ECM are done to compensate for the "bogged down" feeling? please dont say the tuning consists of simply dumping more fuel into the engine.

 

Tweaking to the fuel and spark tables needs to be done. Some time fuel is added but mostly it is a boost in pump shot. The VE tables are also adjusted and are typically 10% off when a full exhuast is added. That is the biggest reason for the bog that some members feel and what the ECM rellys appun. The fueling isn't that big of deal seeing how these cars a bit rich from the factory. The pump shot and VE tables are what need attention. I ran headers along with a 3" cat and cat back without any bog or hesitation. I do know that my gains were limited becuase of the stock PROM.

 

I just dont consider a modification that requires more fuel to maintain the same acceleration as positive, no matter how much acceleration it makes possible that wasn't before. as a daily driver car, sacrificing mileage for "normal driving" conditions isn't a positive effect, WOT is another story.

That was my only concern with compensating the loss of power feeling at partial throttle. Hopefully those changes are mostly just VE table related and dont result in more fuel being used to do the same thing that was being done before the modification.

 

I am not quite sure if I am following you here. You may think you have the same acceleration but I bet it is greater than before. Track times will really show you. The car is just now opperating in a different power band than before. It has changed over the entire power curve and our butt dynos don't pick up on that.

Your mileage shouldn't change that much. Anytime you mod your car you can expect to increase your fuel consumption. Thats where the power comes from. Even at part throttle. However a mod like this is still relatively mild on the scale of power adding and your mileage should not become jeopardized. With prom work your part throttle fuel consumption should remain the near the same as when your car was stock. My mileage only changed when I added headers. A catback does next to nothing to the state of tune of your car. If anything your milegae will improve because you decrease some of the pumping losses in the system without changing the VE much. With an Open element and a 3" cat-back I got 31mpg on every highway trip. I got the same mileage when I was stock. Adding headers changed the tune a bit but I still netted over 25mpg on the highway. Had I had a good tune at the time I would have been right back up to around 28 or so. Exhaust mods are about all you can do to add WOT power without sacrificing much of your part throttle fuel consumption.

Just a thought but are you sure there wasn't anything else that could have been causing your hesitation? Bad 02, CTS, ect ect.

 

I plan on getting a cat-back exhaust, new cat, and maybe headers, but I can’t find an aftermarket y-pipe for my truck. Would the use of the stock y-pipe be that big of a problem? If u know where I can find a Y-pipe please tell me.

 

This isn't 3rd gen material so don't expect this to fly. Check out a few truck forums. I do know that a lot of header kits for trucks allow the rtention of the stock y-pipe. I know JBA makes a header/y-pipe set-up.

 

come on now shifty his got a tbi l03
but then agine shifty has a point, but i welcome you anyways
if you need anyother help feel free to PM me...
this is not a place for trucks just 3rdgens...
theres tons of truck forums out there that are just as willing to help you

 

sounds like backpressure is being confused with delta pulses.

backpressure is bad no matter what. Delta pulses are "sort" of like back pressure... but they're a good thing. In a nutshell, imagine a ball of smoke that gets shoved out of the cylinders right after the exhaust valve opens up. The ball of smoke rolls down the manifolds, through the y-pipe, past the cat, through the piping to the muffler and out to the free world.

Now, think how big of a ball of smoke that is, and how large the tubing is along all of that process to get out to the free world. The manifolds are going to slow it down, they are built so sloppily, that when the ***** of smoke get past the y-pipe that the other ***** of smoke from the other side of the engine crash into it. This results in a loss of power because now you're hanging up the exhaust system.

Delta Pulses (the ***** of smoke) also produce scavenging, most noticed with headers, but still happen w/ manifolds, just not as efficiently, or within the same powerband

 

The whole point of my comment was that backpressure may be depended upon by coding in the ECM or emissions devices on the engine. Of course anything that hinders an engine's ability to pump air is bad, but if our engine bays consisted of just the engine then we'd all have pretty clean and empty engine bays. Last time i checked mine was too crowded to even see the sides of the engine. I was not confusing it with exhaust pulsing or scavenging. Things like EGR and various little parts of the ECM depend on there being a certain range of backpressure/volumetric efficiency, if you get around that then you wont have any problems, but the person said it was stock, so in that situation, simply changing the exhaust system like described would likely give you some undesirable results - until you fixed those things that were dependent on the old setup.


as for engine mods requiring more fuel to make more power ...that's for another thread. Efficiency mods are out there.

 

Assuming that something else isn’t messed up and assuming that you have reasonable driving habits the kinds of mods that we’re discussing here should result in BETTER gas mileage. Changes like headers should make the engine more efficient (lower BSFC) in it’s midrange and significantly improve it’s cruising gas mileage assuming a reasonable cat back behind them. If your mileage dropped I’d suspect that either you’re spending more time using your new found power, there is something wrong with the cat back which is causing the engine to actually see more backpressure or the new O2 sensor location does not allow the sensors to get warm enough in the time period programmed into the prom that youre using (O2 sensor output depends on temp as much as it depends on a/f ratio, if you’re not keeping the sensor warm enough you will get incorrect readings even if the ECM thinks that everything else is otherwise OK to be in closed loop).

 

for the EGR to get it’s designed volume of gas recirculation to the engine it does require the oem backpressure. But that has no impact on gas mileage. The amount of egr has no impact on fuel delivery, the ecm has no means of sensing it.

An exhaust leak could cause you to see worse gas mileage and hurt responsiveness if it was close enough to the O2 sensor and you were getting some reversion through the leak, causing it to read some of the outside air (lean) and dumping fuel to compensate.

 

I agree


I see what you are saying but I guess it is a trade off. We all know that changing parts on our cars could effect one thing or another. If you want performance it comes at a certain cost. It could be undesirable if he mods it to the point where he only nets 20mpg. One just has to prioritize what they want out of their car before starting to mod it. Something as simple as a cat-back will not have any undesireable effect on an LO3. I know you had problems, but yours could have been associated with something else. Running an open exhaust could cause the EGR to not open and close normally but that is rare but could have happened to you. As long as he does not run open headers all the time he should have no functionality problems with the EGR, AIR, AC, Evap, or whatver else is dependent on manifold vacuum.