I've seen a few threads on here discussing back pressure and catalytic converters. I'd like to change out the exhaust on my 89 (L98) and eliminate the pre-cats and the cat. However, since I'm not doing it myself, the muffler shops say I shouldn't go without the cat since the engine needs the back pressure. I'm coming to you experts. Is the catalytic converter a necessary evil? Thanks

 

You don't need back pressure though might want to find a smarter bunch of guys than the current muffler shop. If smog checks and compliance is required in your state legally they can't remove at least the main cat and possibly precats.
Dave

 

You dont need back pressure, you need exhaust scavenging...

 

the main cat acts like a X pipe and aids in scavenging, make sure you add one or a cat. , back pressure in a stock chevy or any engine is a good thing for low end torque, an old sayin 'go big for horspower, go small for torque' still applies. I worked in a performance exhaust shop for years. if you are going to keep it mostly stock , dont go bigger than 2 1/4" or possibly 2 1/2" . you will drive arround wondering where your gas milage went, some chevys wont run right in stock form without a good cat in line, specially if they have a 02 sensor after the cat. it sends the readings off the scale and the puter wont adjust for it, hope this helps.

 

For this to be true the earth would need to be flat as well, same set of physics.. Stock exhaust is not tuned in any way or any more than a toilet drain..
Dave

 

had I not done this for a living I would agree with you, why dont you provide some tech info, again not talkin bout 'stock exhaust, which I actually agree with you about , he wasnt asking bout 'stock exhaust', and neither was I. I actually find 'blanket ' coments such as yours that dont provide any real life info, somewhat sensless and un productive.

 

Engine is an air pump. Air in, air out.
How is it going to be more efficient when it has pressure working against itself to get air out?

anyway theres nothing wrong with a good high flow cat you wont notice any s.o.t.p. gain, up to you.

 

I respect that you folks are interested in helping, but I do know what I'm talking about.

 

You do not need backpressure. The EGR does not need backpressure; the exhaust is drawn into the engine through the EGR by vacuum in the intake.

At part throttle there is no backpressure in a stock exhaust system, since you are moving small amounts of exhaust. When you go to WOT you will have some backpressure in a stock exhaust system and it does not help performance. A fuel mileage drop cannot be attributed to too low of backpressure since even a stock exhaust has no backpressure at cruising conditions.

 

I race cars and bikes for a living have for 35 years on pro cars. I have designed cylinder heads intake manifolds and computer systems along with tuning on a dyno for the same 35 years. It does not matter if 2 1/4 or 3 inch it is not tuned at the length to get to the rear of the car. Anything useful was done in the first couple of feet and the rest is ballast you drag around to put the exhaust where it needs to be. If a restricted exhaust ever helps anything the camshaft is wildly wrong on the exhaust lobe design and i would sincerly doubt anyone sells one that bad. If the conversation was about header primary tubes and collector design then there is a lot after the collector it only needs to be large enough to not add any back pressure. In a real race3 car we have used tuned headers dumping into a much larger pipe to route the exhaust and still have it look like open atmosphere..

I am unsubscribed from this thread..
Dave

 

race cars and bone stock daily drivers are two different animals, I was really hoping for some tech info. I've read some of your other posts and am always interested in learning, even at this age.

 

I appreciate ALL of the responses. I surely didn't mean to start anything between posters. I'm just looking for ways to get a little extra from my 89. Again, thanks for the info!

 

we havent had our coffee yet

 

Ugh. I give up.


Why is there a vaccum line attached to the EGR, hmm.

 

Coffee is getting going right now...better hammer this out before then... haha

I've run and dyno'd about 6 different setups on my car. Short and sweet, the lower the restriction (aka no cat, bigger mufflers etc) the better peak and average power. However you can run a smaller more restrictive muffler and a better scavenging system (x-pipe) and out perform larger mufflers without the better scavenging system.

The stock exhaust (similar to an X but with a long center section composed of two y-pipes) does mimic the effects of an X-pipe though not as efficiently.

The cats/mufflers do not contribute to power via backpressure, this is because backpressure does not contribute to power in any positive sense. Increased scavenging (due actually to the y-pipe merge than to the cat itself) increases power as well as exhaust secondary length tuning (if the pipes/cat are appropriate sizes that they could act as termination boxes). However if back pressure was good, I could take my wide-open race exhaust and throw restrictor plates in it and make more power. I can tell you regardless of placement and diameter this would not happen, it would lose average and peak power.

That said, you can run two hi-flow cats to minimize backpressure and maximize flow and these will way out perform running the exhaust all through 1 cat.

Unfortunately exhaust tuning is far above what most of us have the ability ($, dyno access) and time to do but if you want to get a better guess at what you need you can check out PipeMax and see what it suggests.

 

The "the engine needs the back pressure" was an old hand me down generational concept that was pretty much embraced by a huge number of people up until just a few decades ago....it seems not everybody got the message....but this concept has been dismissed with modern day testing - the fact is you dont need back pressure - as others have already mentioned in this thread. Do your own internet seach for more inforamation on the topic.

The exaust will pretty much have a "stink" smell without the converter in place. There a quite a few direct fit high flow catalytic converters available. Check your local laws but I vote you ditch the precats and run a single high flow main cat.

 

This is not true, look at how the system on our corvettes works. Intake manifold vacuum (low pressure zone) is used to open the EGR valve and then the low pressure in the intake manifold causes the exhaust to flow into the intake manifold. No backpressure is needed and very little is present at cruising conditions.

 

excellent, respectfully, bob

 

Having build a few exhaust systems when I was working for a shop and doing some research at the time...

Needing back-pressure is a myth. The key is exhaust gas velocity. Too small a pipe and you create back-pressure and you are choking the engine. Too large and the gas cools and slows down as you are trying to get the gas out. As stated above the header does a lot of the scavenging of exhaust gases and can actually help pull gasses out of the combustion chamber as when the exhaust valve is open. On the flip side a poorly designed header can create a reversion and hold exhaust gas in the combustion chamber.

The hot exhaust gas is moving in pulses through the pipe. If it's properly designed the vacuum created behind one pulse will help pull another behind it. Even the best flowing catalytic converter will add an obstruction to the exhaust and slow things down. It's also a matter of what works best all around, a 3" pipe might work great for wide open throttle between 5k and 7k rpm and be great on a track car, but if you have a street car, you may be better with 2.5" to keep drive-ability down low and in the mid-range.

The early comment about Cats being needed because of oxygen sensors is because of how the ECU handles adjusting fuel trims to meet emissions requirements. If the computer doesn't think the catalytic converter is functioning properly it will throw a Check Engine Light. The engine doesn't need the Cat, the computer does (and the environment prefers it, haha).

 

not sayin back pressure in every instance, but its where stock engines get low end torque, among other things, you will gain power by opening up a restrictive exhuast, but when you get to the point where the intake wont keep up with it, you start losing your bottom end.