Gutting cats increases back pressure.

In the fact vs. fiction article in the latest import tuner issue they dyno test a miata first with the cat, then they gutted it and it lost 2-3 whp over the intact cat. The theory is that the rough edges create turbulence that actually increases backpressure.

This is why you don't do stupid little mods like that, and actually go out and by a proper exhaust.

In the fact vs. fiction article in the latest import tuner issue they dyno test a miata first with the cat, then they gutted it and it lost 2-3 whp over the intact cat. The theory is that the rough edges create turbulence that actually increases backpressure.

 

I doubt that it had anything to do with the gutting. You could run the car on the dyno 5 times and see a 2-3whp variance or more.

 

Also, a Miata is not a LGT. Removing the cat in the Miata will have much less effect (if any without tuning) than removing the cat right after the turbo in the LGT.

I doubt that it had anything to do with the gutting. You could run the car on the dyno 5 times and see a 2-3whp variance or more.

 

Also, a Miata is not a LGT. Removing the cat in the Miata will have much less effect (if any without tuning) than removing the cat right after the turbo in the LGT.

 

Exactly lol, 2 to 3whp lol are you serious. I get that in between dyno runs with the same car on the same day changing nothing at all.

Stupidy also raise back pressure

 

Fact

 

Stupidy also raise back pressure

 

Fact

 

 

So does Stupidity.

Stupidy also raise back pressure

 

Fact

 

 

So maybe we should put prunes in the intake, to decrease backpressure?

baaa Gut them!

Like others have sort of said, in N/A cars you need to keep some back pressure to have a good power/tq curve. On a turbo car the turbo creates enough backpressure. This allows you to gut/remove the cats and get a good gain ( with a proper tune).

Ahhhh screw it, run open header FTW.

What about if I gut the cats using a sawzall?

 

Seriously, that was a stupid test.

Like others have sort of said, in N/A cars you need to keep some back pressure to have a good power/tq curve. On a turbo car the turbo creates enough backpressure. This allows you to gut/remove the cats and get a good gain ( with a proper tune).

Till you get boost spike

Yeah you guys are right, import tuner has no idea what they are talking about and im sure they didnt average the dyno runs to equal 2-3whp they just went click click boom and got two readings back to back. I mean they definetley have no experience runing a dyno right? Oh and p.s the only reason I posted this is so smart asses like you can stop telling people its ok to run stg 2 with a gutted cat. Oh and I am surely stupid because I cant spell the word stupidity correctly Oh wait that was you trying to call me stupid by molesting the english language, damn im wrong on all acounts.

Do you still think test results on a small displacement n/a engine are comparable to our engine?

 

I'm sure their test is perfectly valid... for a Miata.

Like others have sort of said, in N/A cars you need to keep some back pressure to have a good power/tq curve. On a turbo car the turbo creates enough backpressure. This allows you to gut/remove the cats and get a good gain ( with a proper tune).

 

Exactly my response. Quite honestly I'm a bit disappointed in that magazine for the reasoning they gave. If a true car person, who we all assume the writer is, gave that problem much thought they would have realized the above thinking. It's almost a face palm moment.

Well i guess they assumed it was universal for everyapplication that uneven course surfaces will created unwanted turbulence, the degree of which is obviosley variable, however I would expect it to be amplified with more pressure being thrown at it. But agian you guys are the experts not them, thats why they have a magazine and your posting on a forum;)

Well i guess they assumed it was universal for everyapplication that uneven course surfaces will created unwanted turbulence, the degree of which is obviosley variable, however I would expect it to be amplified with more pressure being thrown at it. But agian you guys are the experts not them, thats why they have a magazine and your posting on a forum;)

 

I've removed my cat and can say with certainty that there was a considerable power increase. My car has been logged extensively through stock, stage 1 and stage 2 configurations, and the most noticeable power increase was after removing the cat directly after the turbo. I did leave the final cat in place.

i thought on an N/A car you want backpressure. it makes sense that it lost power, because it lost backpressure.

 

http://br.answers.yahoo.com/question/index?qid=20070804195122AAwIiUT

 

not that yahoo answers is the bastion of truth and justice, but naturally aspirated engines work differently.

 

if its designed to work with some backpressure and you remove a component sometimes you get the undesired effect..

http://www.tercelreference.com/tercel_info/turbo_exhaust_theory/turbo_exhaust_theory.html

 

N/A cars: As most of you know, the design of turbo exhaust systems runs counter to exhaust design for n/a vehicles. N/A cars utilize exhaust velocity (not backpressure) in the collector to aid in scavenging other cylinders during the blowdown process. It just so happens that to get the appropriate velocity, you have to squeeze down the diameter of the discharge of the collector (aka the exhaust), which also induces backpressure. The backpressure is an undesirable byproduct of the desire to have a certain degree of exhaust velocity. Go too big, and you lose velocity and its associated beneficial scavenging effect. Too small and the backpressure skyrockets, more than offsetting any gain made by scavenging. There is a happy medium here.

 

For turbo cars, you throw all that out the window. You want the exhaust velocity to be high upstream of the turbine (i.e. in the header). You'll notice that primaries of turbo headers are smaller diameter than those of an n/a car of two-thirds the horsepower. The idea is to get the exhaust velocity up quickly, to get the turbo spooling as early as possible. Here, getting the boost up early is a much more effective way to torque than playing with tuned primary lengths and scavenging. The scavenging effects are small compared to what you'd get if you just got boost sooner instead. You have a turbo; you want boost. Just don't go so small on the header's primary diameter that you choke off the high end.

 

Downstream of the turbine (aka the turboback exhaust), you want the least backpressure possible. No ifs, ands, or buts. Stick a Hoover on the tailpipe if you can. The general rule of "larger is better" (to the point of diminishing returns) of turboback exhausts is valid. Here, the idea is to minimize the pressure downstream of the turbine in order to make the most effective use of the pressure that is being generated upstream of the turbine. Remember, a turbine operates via a pressure ratio. For a given turbine inlet pressure, you will get the highest pressure ratio across the turbine when you have the lowest possible discharge pressure. This means the turbine is able to do the most amount of work possible (i.e. drive the compressor and make boost) with the available inlet pressure.

 

*not sure how reputable it is*

 

But this makes a really good point for turbo charged cars. Having anything (ie cat) that will hender getting the exhaust gases out fast will have an affect on power, tq, and boost response.

So who here is going 4 inches?

 

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