Hey, I want to replace both my standard cats on my EL 5L soon, and I dont want to spend a mint (I know cats arent cheap) But I want some high flow ones that are better than standard. What do people suggest is my best bang for buck?

Cheers, Gab

from what i understand usually any cat with metallic substrate is "high flow" due to the low cell count

a quick look on ebay you can get 2 x magnaflow 2.5" metallic spun cats for $266 delivered

http://www.ebay.com.au/itm/Magnaflow-59 ... 4603e2c587

ok, we all know back pressure is important. On a set of Extractors, is there enough pressure before it hits the cat that a high flow cat and high flow exhaust does not effect the performance on a stock donk?

high flow cat wont make much more power, if any, however it makes sense to replace an ageing cat convertor when your doing the rest of the exhaust anyway, and a plus is different cats can change the exhuast note too. I had small magnaflow oval cats fitted to my fairmont and there was alot more "white noise" to the note than before

quantify that statement. Important to have or important to avoid? Keep in mind that backpressure is a result of exhaust gas volume overcoming the extractors ability to flow causing reversion (negative delat p), and choking up the cylinder with unscavenged dirty gas.

In the case of Catalytic converters, the more flow the merrier, just keep diameter as close to stock as possible on a stock engine. You want the gas to move quickly through the system. You do this by removing obstructions while keeping diameter (and thus pressure) where it should be to do the most efficient job at a certain rpm range.

You have to be careful when talking broadly in "back pressure" terms, it is all to commonly misunderstood and misused.



How long is a piece of string? What extractors for a start, not to mention the size of the Cat...

Extractors in general terms, with their different sized primaries and secondaries to stock headers will usually do a better job across a greater percentage of the rev range in reducing cylinder back pressure when under load. This in turn means higher exhaust gas velocities in the ranges the extractors a suited to. Tuned length extractors, like pacemaker comps, are tuned to use the pulse of the exhaust gas and the pressure drop between them (negative delta p) to provide greater scavenging of the cylinder post combustion. You then must ensure that there is adequate flow through the Cat/s, muffler/s and pipe/s so that you don't choke the system there. Simple rule: exhaust too small- pressure builds under load and chokes engine. Exhaust too big- flow speed (velocity) is diminished, the gas cools and becomes heavy and inert and requires more push to get it out, choking the system. You want hot and fast gas for an efficient system.

Actually, on a four stroke engine.. back pressure serves no useful purpose.



http://autospeed.com/cms/title_Franks-Exhaust-Part-1/A_108185/article.html

Terribly important on high performance 2 strokes though.

ok Grimketel, sorry to not quote your post but mine would then be a page in itself. It makes sense what you have said, I have always thought back pressure was relatively good and minimal increases in flow on a stock setup was the key. For example extractors reduce pressure a bit but the cat or the mufflers counter that so you do not loose too much. So do Mandrel bends reduce pressure or create optimum pressure? I am not having a go I honestly dont know and I think exhaust setup formula's are still a bit of a grey area in the modifying scene for the unqualified.
Question though, I remember a bloke who worked at Inghams in Sommerville (vic) had a Hq prem wagon, stock 202 but he had a 3.5" system all the way though instead of the stock. He lost power, so if back pressure is the enemy why did he loose power?
I always thought high flow cats on stock engines was just for the young blokes to show how little there d**k were, seeming like its me who's the d**k

Ask yourself why lots of doorslammer or other drag cars have very short straight out exhausts?

The tuned part of an exhaust is everything from the cat to the engine... Everything from that point should flow as free as you can make it.

Too big exhaust the velocity slows and heat dissipates faster..
Which means the exhaust has to be pushed out..
Extractors also work on pulses, the collector syphons/ pulls the following pulse and so on..
To a point equal length doesn't matter.. Although it does on some engines like formula Vw..
I guess every bit helps when you have 50 h.p... Aha...
A good cat keeps the heat in exhaust and sort of works like an afterburner...
Strange thing in my case having turbo V8 is steel cats melt !!!
I have switched to using ceramic cats in parallel..
Yes 4 of them...2 b legal 2 b legal...

ok Grimketel. I was going to ask a question but then EBXR8380 answered it. Now it makes sense. Thanks guys, that info will make my decisions in the future more informed.

I remember one of Fords exhaust engineers saying that all engines require a little bit of backpressure, but not enough to effect flow. Too much backpressure and you lose power, too little and you lose power again. You need to hit the sweetspot, from what i can understand.

Extractors don't reduce pressure per se, rather they optimise the velocity of the escaping gas to help create a pressure vaccuum behind the first pulse to help pull the gasses out on the following exhaust stroke. Think of it as a mild reverse supercharging effect. Mandrel bends are smoother than crush bends, which means flow is optimised. A crush point can slow velocity, and effect the speed of the gas. Get your garden hose, turn on the tap, and fold the hose at 90 degrees to create a small crush kink. You will notice the rate the water exits the hose is diminished. There is also a backlog of water trying to escape behind the crush bend. This is backpressure, as the water behind the bend builds up, and feeds back pressure behind it. Now if you make a rounded curve bend with no crush point in the hose for the same 90 degrees, you will notice the water flows much faster, and with no flow impediment, there is no pressure building up behind the bend. A pinhole on the tap (source) side of the hose will demonstrate the difference in pressure between the two types of bend.

As EBXR8 said, the size of his exhaust was not optimal for the amount of gasses his system was creating. If we reffer back to my origional novel,



The real signifyer in all of this is Velocity (since velocity is a product of flow and pressure). Thats the word you need to be talking about. Velocity is the key to power and efficiency in exhaust terms. Backpressure is merely a byproduct of too much velocity for the pipes to flow (too narrow), or not enough velocity because the pipes are too wide, and velocity is lost, resulting in cooler and denser gasses (inert/immobile) that require more of the engines power to push from the system. In other words backpressure is a secondary meassurement of the quality of an exhaust system.



High flow cats that are of too large a diameter on stock engines ARE wank, you are quite correct. That much is true- simply because the increased diameter results in a loss in velocity. The gas slows in the lower pressure area of the larger diameter section, and momentum is lost, creating a slow moving flow through the rest of the system, which then clogs it all up. For all intents and purposes the parts must be matched. A stock ford i6 manifold for instance won't flow freely enough at the top end due to its bottom end design to worry a stock catalytic converter. Change that to a set of tuned length extractors, and that may well change. The stock cat will cope to a point, but depending on your mods, you might find a lower CPSI high flow unit doesn't impede the increased flow you are now generating, allowing your cat back section to perform better. All things in balance an harmony. You arn't a d**k at all mate, these are things we have all asked dont worry about that!

What I like too is that you guys have explained it to me in a way thats not designed for a 7 year olds ears, I am now 10 so I get the big words, but seriously it pisses me off when people explain things to me like I am a retard. The way you guys have done it I understand fully as its been said in black and white, literally. Cheers again