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So I have a hollowed out cat on my car, really made an improvement in sound and torque. Now I have done extensive reading on this and tend to agree with the statement I am about to make: That a straight pipe is better than a hollowed out cat because there is no turblance in the pipe as opposed to the cat.
So I get this and understand the theory behind it, now for the questions.
1) How much horsepower and torque are not being produced bcause of this situation?

2) Has anyone dynoed this yet to check it out?

3) Wouldn't the cat work similar to a crossover pipe?

4) With knowing that some back pressure is good for low end torque wouldn't this be benificial to have the gutted cat insted of a complete straight pipe so the low end is there and it doesnt move your torque curve higher in the rpm range? (that is if you want that low end torque and not the high rpm stuff)

5) Say i replaced the cat with a single straight pipe in it's place when both pipes dump into that one pipe will it cause anything benificial to happen besides the sound and no turblance?

6) How close does that straight pipe come to acting like an "H" pipe or "X" pipe?



Thank you for chcking this out and any and all opinions are appreciated.



P.S. If I get the chance and am able to I wll build a test pipe to run on my second run on the dyno in Crawfordsville on the 10th. That way I will have proof as to what the cat-vs-pipe actually does with dyno numbers
 

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..................................?

4) With knowing that some back pressure is good for low end torque ...............................
Backpressure does nothing beneficial except for sound muffling.
 

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I'd tend to disagree with the above statement just a little. When I went to the 3" true duals on my 87 without an H-pipe or cats, the low end went to **** in a basket:(

I replaced the H-pipe, hollowed out the cats and replaced them, and got a good bit of that low end grunt back:thumbsup:

What happens is, you increase the exhaust without increasing the intake. IT actually breathes too well:laughing: but only on the exhaust.

Think of it as being able to expell all the air in your lungs while running, but only being able to intake 1/2 the amount to replace it.
 

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I'd tend to disagree with the above statement just a little. When I went to the 3" true duals on my 87 without an H-pipe or cats, the low end went to **** in a basket:(

.........................................................................

What happens is, you increase the exhaust without increasing the intake. IT actually breathes too well:laughing: but only on the exhaust.

Think of it as being able to expell all the air in your lungs while running, but only being able to intake 1/2 the amount to replace it.
No disrespect intended, but that doesn't make sense. I'm an engineer. I gotta have things explained by physics. If you increase the scavenging ability of the exhaust, it reduces the amount (in both psi and mass) of residual exhaust remaining in the cylinder at the end of the exhaust stroke. This reduction causes less intake dilution (a good thing) and also allows an increase in (productive) VE as there is less cylinder volume taken up by non-productive exhaust in the intake stroke. This increased amount of pure air and fuel allows more heat and combustion pressure to be produced. Torque is a product of cylinder pressure on the piston (and the crank arm length). More cylinder pressure equates to more torque.
My choice is an exhaust system with as little backpressure as possible while still allowing me to drive around without going deaf in the process.
 

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I know it doesn't make sense. But so many have reported it, it has to go beyond the placebo effect.

Basically the large pipes decrease exhaust velocity which decreases port velocity, so you lose scavenging ability and lose VE. It's not a steady state flow, so the fluid dynamics you expect to apply dont'. You need to use the momentum of moving air to pull stationary air into the system.

I'd take small pipes (dual 2.25" or 2.5") anyday before backpressure from exhaust restrictions. There's no excuse to be running dual 3" on a near stock 350, especially not a low revving TPI.
 

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No disrespect intended, but that doesn't make sense. I'm an engineer. I gotta have things explained by physics. If you increase the scavenging ability of the exhaust, it reduces the amount (in both psi and mass) of residual exhaust remaining in the cylinder at the end of the exhaust stroke. This reduction causes less intake dilution (a good thing) and also allows an increase in (productive) VE as there is less cylinder volume taken up by non-productive exhaust in the intake stroke. This increased amount of pure air and fuel allows more heat and combustion pressure to be produced. Torque is a product of cylinder pressure on the piston (and the crank arm length). More cylinder pressure equates to more torque.
My choice is an exhaust system with as little backpressure as possible while still allowing me to drive around without going deaf in the process.
No disrespect taken:thumbsup: and as CC stated, it's just something that happens.

I'm not an engineer and there's no way I could explain the physics behind it, but I know what I felt and how it performed.

It was my exhaust guy, a real old timer (probably 70) who'd been doing custom exhausts for rodders and racers of decades who suggested it.
When we added back some back pressure, things became "fun" again, just like he said they would:thumbsup:

Like CC also said, it's been reported so many times, there has to be something to it.

CC, I used the 3" because I got a smokin' deal...thats the only reason:laughing:
 

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Here's an article from David Vizard on the subject. He knows more than anyone on here about it:

http://www.superchevy.com/technical/engines_drivetrain/exhaust/0505phr_exh/index.html
I read the article. Very interesting, informative write-up, and told in a way that we average gearheads can understand.
After looking at this, I see nothing that I said that contradicts what Mr. Vizard wrote. (I won't be so presumptious to imply that he agrees with me, as he has done far more experimentation than I have, and well before I have.) He mentions that cylinder scavenging via correct header size, pressure wave tuning, and minimizing backpressure are the keys to maximizing performance. I mentioned improved scavenging (correct header pipe sizing) and minimizing backpressure. Pressure wave tuning is a method I admit as something I have been trying to understand better. On the other stuff, I stand by my statements.
 

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Your last post makes it seem like you're in a trivia contest, which is unnecessary. We're all here to learn. There's no need to defend what we think to be correct since fluid dynamics isn't a matter of opinion nor did any of us invent it.

I think in your first post you were saying there'd be no torque loss from large pipes, because you assumed the larger pipes would scavenge better because the are less restrictive. This is not true.

Smaller pipes do become a restriction at higher rpms and add backpressure. But Vizard is saying they increase power/torque at lower rpms because they actually reduce backpressure at low rpms. That is, they are pulling a vacuum on the cylinder.

The short answer is backpressure never ever helps. According to Vizard, sticking a muffler on a 3" system will not increase low end torque. At best it will have no effect (if it's sized properly).

Backpressure does not increase torque, properly sized exhaust pipes do. Others on here obviously don't agree with that. I don't have a personal experience with that as I've never had mufflers on my car.

My senior project was related to wave tuning / Helmholtz resonators, so I'm familiar with the theory behind it and how it's supposed to work. But there are so many other factors in the real world that makes the results unpredictable. This is why the theory more often than not goes out the window and is replaced by rule of thumb and empirical data.

People like Vizard that understand the theory and have done thorough testing first hand are a rare breed!
 

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Your last post makes it seem like you're in a trivia contest, which is unnecessary. We're all here to learn. There's no need to defend what we think to be correct since fluid dynamics isn't a matter of opinion nor did any of us invent it. Not trying to play a trivia game here. I'm an engineer. I only know how to explain my perspective in dull engineering terms.
I think in your first post you were saying there'd be no torque loss from large pipes, because you assumed the larger pipes would scavenge better because the are less restrictive. This is not true. In my first post, I said only that backpressure does nothing beneficial. I said nothing about pipe size.
Smaller pipes do become a restriction at higher rpms and add backpressure. But Vizard is saying they increase power/torque at lower rpms because they actually reduce backpressure at low rpms. That is, they are pulling a vacuum on the cylinder. I am familiar with the benefits of correct header sizing, hence my comments about proper scavenging and the reduction in residual exhaust gas left in the cylinder at TDC exhaust. I see no disagreement between us in this regard, or any error in my original statement.
The short answer is backpressure never ever helps. That was my position in the first post. According to Vizard, sticking a muffler on a 3" system will not increase low end torque. At best it will have no effect (if it's sized properly).

Backpressure does not increase torque, properly sized exhaust pipes do. We are once again in agreement. Others on here obviously don't agree with that. I don't have a personal experience with that as I've never had mufflers on my car.

My senior project was related to wave tuning / Helmholtz resonators, so I'm familiar with the theory behind it and how it's supposed to work. But there are so many other factors in the real world that makes the results unpredictable. This is why the theory more often than not goes out the window and is replaced by rule of thumb and empirical data.

People like Vizard that understand the theory and have done thorough testing first hand are a rare breed!
Again we agree.
 

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There's a lot of fantastic info in here guys, that we can all agree on.

:thumbsup:

Often times I've read about this issue in regards to header sizes on the L98 headers 1 7/8" vs 1 5/8". The 7/8 is recommend for higher rpm gains where as the 5/8 is recommended for stock or mild engines to maintain high torque levels at lower RPMs.

All I know is, I'm glad I went the route I did with my exhaust mods.
 

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There's a lot of fantastic info in here guys, that we can all agree on.

:thumbsup:

Often times I've read about this issue in regards to header sizes on the L98 headers 1 7/8" vs 1 5/8". The 7/8 is recommend for higher rpm gains where as the 5/8 is recommended for stock or mild engines to maintain high torque levels at lower RPMs.

All I know is, I'm glad I went the route I did with my exhaust mods.
Dan, What exhaust are you running from the headers to the tips? Pipe sizes etc? I know that you are running a cat.

-Also, what gains or losses have you noticed with your system?

Thanks,
Yosi
 

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I know it doesn't make sense. But so many have reported it, it has to go beyond the placebo effect.

Basically the large pipes decrease exhaust velocity which decreases port velocity, so you lose scavenging ability and lose VE. It's not a steady state flow, so the fluid dynamics you expect to apply dont'. You need to use the momentum of moving air to pull stationary air into the system.

I'd take small pipes (dual 2.25" or 2.5") anyday before backpressure from exhaust restrictions. There's no excuse to be running dual 3" on a near stock 350, especially not a low revving TPI.
But does it have the agreement of a dyno?

AFAIK, straight pipes or headers dumping into atmosphere are great for cars if you want to drag race. We have seen it on drag cars. However, has anyone shown on a dyno that they won't skew the curve to hell if you use straight pipes? IOW, you get good results at one point only to lose it everywhere else?
 

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........................

Best average HP between the shift points, that's what you need.
:agree:


It's amazing how rarely this important concept is mentioned or understood.
 

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If by dyno you mean peak power, then the dyno is meaningless.

Best average HP between the shift points, that's what you need.
I agree. Peak power is good for bragging at the bar. I suppose it is also good if you are drag racing where you are at WOT most of the time and can set the shift points to match the engine.
 

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Dan, What exhaust are you running from the headers to the tips? Pipe sizes etc? I know that you are running a cat.

-Also, what gains or losses have you noticed with your system?
Stock manifolds - 2.5" y-pipe - 3" catco converter - Borla 3"-2.5" rear y-pipe

Basically a couple inches before and after the cat are 3". I did this to try and give optimum flow through the cat. Straight pipes are 2.5" that end with 3" ends.

No dyno results to show exact gains, but I know it's BIG because of the 0-60mph times I consistently see and the runs I make to 75+. Unknowing, my stock cat was severely clogged. Not to mention the wimpy stock rear piping and mufflers. Used to be at least 7 seconds to 60mph, now it's easy to hit 6 flat and better. Overall power off the line and from a roll, are truly up! By leaving the cat on, I got rid of the resonance, got the tune I wanted and stayed legal. My wife thinks it's a tad loud, but it's fine at speed. We can hear the radio and talk normally at all speeds.

My whole system was piece bought, rigged up myself. Saved BIG money, but took more time than I wanted. Getting to a welder took me the longest time, but shoot once we got a hold of one, it went in quick! I went with stainless steel, simply so I didn't have to replace this for a loooooooong time.
 

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Very nice Dan seems like a good set up.
Im looking to do something like this plus an upgraded manifold with long tubes on there this may. These 84's have a tough time breathing:whip:
 

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I agree. Peak power is good for bragging at the bar. I suppose it is also good if you are drag racing where you are at WOT most of the time and can set the shift points to match the engine.
I don't go to bars much anymore. But I do beat plenty of LS1s in at the local 1/8th mile.

I think big pipes are overrated, especially from the cat back. Remember that your exhaust cools and shrinks. So it can fit through a smaller pipe.

My exhaust setup: Stock manifolds with the ports dremeled out and smoothed out, a high flow 3" cat which really helps keep the noise down, a 2.5" rear Y with a crossover (useless), and 2.25" muffler elims. I've never been on a dyno though.
 

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So I have a hollowed out cat on my car, really made an improvement in sound and torque. Now I have done extensive reading on this and tend to agree with the statement I am about to make: That a straight pipe is better than a hollowed out cat because there is no turblance in the pipe as opposed to the cat.
So I get this and understand the theory behind it, now for the questions.
1) How much horsepower and torque are not being produced bcause of this situation?

2) Has anyone dynoed this yet to check it out?

3) Wouldn't the cat work similar to a crossover pipe?

4) With knowing that some back pressure is good for low end torque wouldn't this be benificial to have the gutted cat insted of a complete straight pipe so the low end is there and it doesnt move your torque curve higher in the rpm range? (that is if you want that low end torque and not the high rpm stuff)

5) Say i replaced the cat with a single straight pipe in it's place when both pipes dump into that one pipe will it cause anything benificial to happen besides the sound and no turblance?

6) How close does that straight pipe come to acting like an "H" pipe or "X" pipe?



Thank you for chcking this out and any and all opinions are appreciated.



P.S. If I get the chance and am able to I wll build a test pipe to run on my second run on the dyno in Crawfordsville on the 10th. That way I will have proof as to what the cat-vs-pipe actually does with dyno numbers

if you do get the chance to test the difference, please post the results, im interested to see them.
what method did you use to gut the cat?
:thumbsup:
 
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