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I've been wanting to remove all the cats and resevoirs from my exhaust for a while now. Basically straight pipe to a muffler. But I was informed yesterday that that would eliminate the back pressure and eventually f[:)][:)][:)][:)][:)][:)] up my motor.
Now this may not have anything to do with it, but I have Toms tune. Could he reflash some stuff to make it alright?
 

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lost me somewhere around resevoirs

Removing a CAT can be done, and Tom can tune for any differences.

You'll have to decide how you'll be handling the MIL for no CAT.
 

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Tom can rewrite your tune to remove the rear o2 sensor. A lot of people have removed their catalytic converters and used an Off Road Pipe instead. Also, a lot of aftermarket exhaust systems only have a muffler before the axle. All aftermarket exhausts I have seen have removed the rear resonator. But some also have a mid-mounted resonator in the exhaust tunnel before the muffler.
 

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Where do people get this back psi hurts engines or removing it hurts car engines

Yes I can make the needed changes , the loudness you will be tired of in the first day tho

Tom
 

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I ran ORP with no resonator with 2.5 piping back to a summit turbo muffler it was like 20 bucks and go alot of compliments now im back to stock and I love the factory SVT sound
 

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I'm not sure Tom haha thats just what people were warning me about and I have no idea what they're talking about
 

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Where do people get this back psi hurts engines or removing it hurts car engines
It's people who don't know what they're talking about parroting what they read somewhere on the internet combined with a game of telephone where the story gets changed a little bit each time it's retold. Eventually it exists in this form, where people believe you need backpressure on the pistons to dampen the stresses the rods feel as the piston changes direction or some other bs like that.

Google "open header dyno gain", there are plenty of results showing gains in both directions, both with and without a proper exhaust system.

It is possible to properly design an exhaust system (primary diameter, primary length, secondary diameter and length, catback diameter, muffler selection, etc.) that maximizes performance through proper scavenging and minimizes power loss from flow restriction.

Running an open header or straight pipe won't ruin your engine, but it will ruin your hearing and the peace and quiet for everyone you drive past. Don't be an [:)][:)][:)][:)][:)][:)][:)]. Gaining 4hp at 6000rpm isn't worth being that dick in the slow car with the loud exhaust constantly blatting around the neighborhood.
 

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. Don't be an [:)][:)][:)][:)][:)][:)][:)]. Gaining 4hp at 6000rpm isn't worth being that dick in the slow car with the loud exhaust constantly blatting around the neighborhood.

Well said.. Hahah love the automatic cavaliers with the cherry bomb hanging off the back..
 

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i was younger when I had mine...I got over it after about 2 years and I feel in love with the factory SVT sound...if you remove just the rear res it gives it crispier grawl
 

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'Where do people get this back psi hurts engines or removing it hurts car engines'

I think it comes from the old (REAL OLD) school Harley or biker guys back when engines often were run with virtually no pipe on them at all, maybe only a short length. Cold air ingestion with that short pipe leads to valve burning or even breaking off. The dirt racers ran into that too back when engine tech was still stone age. 12 inch long pipe don't cut it there.

I personally feel as Tom, having dragraced for many years. If you have a good exhaust then running it wide open will get you all the power the engine can make, back pressure is for the losers. Anybody thinking it helps is WAAAAAAAAAY behind the times.
 

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I personally feel as Tom, having dragraced for many years. If you have a good exhaust then running it wide open will get you all the power the engine can make, back pressure is for the losers. Anybody thinking it helps is WAAAAAAAAAY behind the times.
Tom never said that. He said that running without "back pressure" isn't harmful for the engine, and he said that he could make the changes to the tune necessary to run without mufflers.

But a properly designed, muffled exhaust system won't lose you any power vs. running a straight pipe.
 

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amc hit it with the Harley reference IMHO.

Those won't work right with plain straight pipes, you need a small baffle hidden in the straight pipes to be able to tune them to run well.
 

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What can I say?

Like everything else you can mess it up with aftermarket pieces or make it run better. Through the 90's at least some judicious aftermarket changes (including the ones Harley sold) could really wake them up from stock. Do it wrong & it gets worse.

Baffled straight pipes on a 70's Sporty I picked up didn't help at all with the stock intake setup, change that out to a used S&S carb/filter jetted right & it ran quite nicely - Mods. have to work together.

90's Buell worked nice, had a matched setup with muffler/air filter/jetting to make up a system.

Japanese/European Cycles were the same, hated seeing people buy a single performance part & throw it on - they were soon complaining about the "Junk" because it didn't work as well as before. Those were the shops I worked in, see the same thing with every brand.

Focus ECU adapts to SMALL changes, large ones need a "tune" in the same manner along with matched changes to really make a difference.

Agree on the proper exh. system, plain open & loud isn't a gain normally vs. a good one.
 

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Can someone define back-pressure and it's distinction from pressure?

It's a conundrum, isn't it?

Resistance to flow is obviously bad and no one will really argue that point. The part I argue is that the term itself is misunderstood in that it's referred to as a physical pressure. It is not. Back-pressure is a product of wave dynamics (harmonics). When you start reading up on how that works you should find diminishing returns from tuning size and that there is a dependance on CSA, EGT, length, volume, density ...

A simple blanket statement of back-pressure being bad and larger tubing making more power, or no tubing, or straight tubing is all based on nothing but lore.
 

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Can someone define back-pressure and it's distinction from pressure?

It's a conundrum, isn't it?

Resistance to flow is obviously bad and no one will really argue that point. The part I argue is that the term itself is misunderstood in that it's referred to as a physical pressure. It is not. Back-pressure is a product of wave dynamics (harmonics). When you start reading up on how that works you should find diminishing returns from tuning size and that there is a dependance on CSA, EGT, length, volume, density ...

A simple blanket statement of back-pressure being bad and larger tubing making more power, or no tubing, or straight tubing is all based on nothing but lore.
Exactly. Oversimplified explanations with just a tiny hint of technical magic in them are very popular and get repeated over and over again by people that don't understand how things actually work. That little bit of technical jargon makes dumb people believe the saying is credible and parrot it to all of their forum buddies.
 

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I was on the way to the Barrett-Jackson auction two nights ago with my son. Just as I made the parking lot, the SVTF got loud. I pulled over and looked under the car. The Exhaust seperated from the rear of the CFM cat. The exhaust was barely out of the way of the cat but not hanging so low that there would be damage. After the auction we drove home and found what 7500 RPM sounded like with a cat only for exhaust! Sounded pretty good! Of course it would be horrible day in and day out, but I believe it did allow for some more power. Tonight I had a newer Mercedes (2 seater, long hood, short deck) race me. He got the jump and I nailed it. I was pulling and in front of him at the top of second and that was the end of that spectacle and noise. No CEL either. I'll jack the car up tomorrow and figure out what went wrong.
An electric exhaust cutout would be ideal with the path of least resistance being the cutout inline with the forward exhaust and the rearward (muffled) section rerouted.
 

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I have been thinking about this idea for a while as well I've run into a header issue for the moment and then will be purchasing an orp (I think TruBendz) I thought about the straight pipe and id tell everyone id give Zero Foxtrots but I chopped the middle resonator from mine and it drones so bad on the highway im sick of it and its been almost a year. Don't get me wrong inside the cabin on some back roads with the radio low its a beautiful sound! but that was only one resonator a straight pipe the whole way would just be obnoxious ( and this is coming from some punk *** 20 year old kid. just my 2 cents but ive decided to build my own exhaust by ordering an orp a borla muffler or flowmaster and a magnaflow 14" res. the Flowmaster I feel like would "growl" a little more but once again we have foci not trucks. so im leaning towards the borla. I also wanted to go with 3" piping in the back but I think I might stick with the regular 2.5 since I'm pretty sure im stay N/A instead of FI. I would check out Trubendz theyre great guys and can answer all your questions along with all the foci experts on here. Obviously were on the same mind set when it comes to our exhausts so pm me when you've decided what to do. Hope it goes smoothly.
 

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No conundrum if you understand it. Ford describes it as the buildup of pressure in a running exhaust system and has nothing to do with harmonics at all. So who's right?

Lore indeed.

'Lore is a body of KNOWLEDGE (caps mine) or tradition that is passed down among members of a culture, usually orally.'

Let me know when you guys understand wave tuning enough to work on 2 cycle expansion chamber design, I was butchering them long ago and easily understand them. With cars you are only doing half the wave tuning. If that much. The exhaust length necessary to get to the back of a car severely hampers true hardcore wave tuning. Motorcycles are the only vehicles you can really wave tune hard.

'Back pressure refers to pressure opposed to the desired flow of a fluid in a confined place such as a pipe. It is often caused by obstructions or tight bends in the confinement vessel along which it is moving, such as piping or air vents.

Because it is really resistance, the term back pressure is misleading as the pressure remains and causes flow in the same direction, but the flow is reduced due to resistance. For example, an automotive exhaust muffler with a particularly high number of twists, bends, turns and right angles could be described as having particularly high back pressure.'

Long story short............the engine shoves out more gaseous matter than the volume of the exhaust system can handle to have it end up in a more compressed state. That is a function of flow vs. tube size and bends rather than wave tuning, it will show up even with no wave action there at all. You can easily read it with a pressure gauge in the pipe.

Understand wave tuning? Most of the book writers don't even understand it, I've yet to see one 'expert' detail the difference in exhaust vacuum tuning and true sound wave tuning, they are very close but NOT the same. Both can add big power if done right.
 

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We dont agree on a lot of things but on this I do agree with amc49 , there is such a combination of things that effect flow through the head , header , exh that is would take a volume of books to cover it
I learned so much when I started playing with the aqu-air headers that I made from square mandrel tubing that it would take me a day to type it all , I made a square tube header and full square exh system on a SVT and gained 12HP /16 TQ at 48-5000 to the wheels on a tuned to tuned SVT same car , same AF ratio

Stock SVT header with stock ZX3 Exh gained 4hp and 9tq I think it was which is saying a lot sense no other header has come close to making more HP then the SVT header does http://www.focusfanatics.com/forum/focus-power-tuning/195452-square-tube-focus-headers-pictures.html

What I found interesting was the psi tested ( back psi ) in 3 test locations was the same with both systems with in .25 of a psi

I also found interesting that when this Focus was raced it would tear the header to pieces from the harmonics , thicker piping and it did the same thing

On a 406 small block chevy making 456HP to the wheels with custom cooks headers I made a set of squ-air headers and gained 56hp and 71tq to the wheels at 6000 rpm , no tune change , with rejet of carb it jumped to 68hp and 83tq gain

There is sooooo much in the Exh side for performance its not believable , back psi isnt something any of the Focus engine have to worry about

Tom
 
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