Gutted precat headers

Started by Petrus, June 25, 2022, 11:18

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Petrus

Is there any hard data, as in méasured results, concerning gutting the precats?

The internet ´info´ is that gutting prevents piston ring damage/ oil consumption and results in extra powah.

The oil/ring myth I will leave aside as impossible to prove either way, but what about the powah?!

The theory is that gutting the chambers sabotages the design and should overal redúce the efficiency of the exhaust.
Apart from slightly more noise the result should be negligible but negative.


I have been looking for before end after dyno but ... :(

Have any of you found factual data?

Carolyn

Quote from: Petrus on June 25, 2022, 11:18Is there any hard data, as in méasured results, concerning gutting the precats?

The internet ´info´ is that gutting prevents piston ring damage/ oil consumption and results in extra powah.

The oil/ring myth I will leave aside as impossible to prove either way, but what about the powah?!

The theory is that gutting the chambers sabotages the design and should overal redúce the efficiency of the exhaust.
Apart from slightly more noise the result should be negligible but negative.


I have been looking for before end after dyno but ... :(

Have any of you found factual data?

My understanding is:

When the engine  lets oil by the pistons, due to gummed up and stuck oil control rings, and a lot of oil is getting though; that oil sits on the top of the precat and slowly cooks it until it starts to crumble.  Then the engine can suck back (due to VVTI version of exhaust gas recirculation) fine grit from the breakdown of the matrix.

So it's the oil consumption that comes first.  How often this has actually happened in he real world, it's hard to say, but there's sufficient documentation to assert that it can happen.

Gutting the pre-cats leaves a couple of empty cans acting as expanding gas containers and, if anything, slowing the exhaust gasses down  - a bit.

I can't imagine anybody wanting to spend the money to do the dyno experiment.

Perry Byrnes Memorial Award 2016, 2018.  Love this club. 
https://www.mr2roc.org/forum/index.php?topic=63866.0

Petrus

Quote from: Carolyn on June 25, 2022, 11:47So it's the oil consumption that comes first. 


and subsequent damage is basically bad maintenance/repair.
The crux is that the precats are not the cause of the problem.

QuoteGutting the pre-cats leaves a couple of empty cans acting as expanding gas containers and, if anything, slowing the exhaust gasses down  - a bit.

Exactly. So it shoúld result in a negligible but négative result.
The anecdotal internet ´info´ however is that mid range power will noticeably increase.

QuoteI can't imagine anybody wanting to spend the money to do the dyno experiment.

Yet there are a great many totally poíntless dyno experiments to be found. At least thís one does have a point.

So, best ask no?!




Dev

I have at one time gutted pre-cats long ago. I did not notice any positive or negative effect on performance and for its time no one did either. There was only a slight change in sound if you were very sensitive to change. If there was any performance decrease that was appreciable it would have been noted long ago as there was a large sample size of owners at time that gutted pre-cats.
 What actually made a difference was the downpipes. When the choke points were eliminated in the the flex section either by replacing the flex sections or by using a performance downpipe there was an immediate increase in power that was proven on a dyno. The restrictive downpipe is the bottle neck. Everything else is marginal gains and very expensive. Until you do an engine swap or turbo that is where all the real gains that are satisfying and appreciable.







shnazzle

I definitely noticed a choke at the top end with gutted pre-cats. My theory was the same as Carolyn's many moons ago. It doesn't make any sense to gut them, for the price of a cheap Toyo manifold off email without pre-cats.

Bearing in mind my car basically saw the rev limiter on every drive, the gutting of pre-cats left a bit of a flat spot once you were higher up in the rpms.

But, only at the top end. Nothing noticeable anywhere else.
To put it in perspective, Helen's car with stock cat and manifold WITH precats was just about as fast as my car with Zero mani and sports cat.

Although mine you had to wring its neck a bit more to get the power out because of the wider runners, whereas a stock manifold actually delivers better low-end torque but then quickly fell behind my car after about 3k rpm
...neutiquam erro.

Petrus

Quote from: shnazzle on June 25, 2022, 17:22I definitely noticed a choke at the top end with gutted pre-cats. My theory was the same as Carolyn's many moons ago. It doesn't make any sense to gut them, for the price of a cheap Toyo manifold off email without pre-cats.


The myth though is still going strong on the internet with first time owners being advised to gut to prevent possible catastrophic engine damage for early and late models alike. Not a single enquiry whether the thing uses oil or not.
Ah well.


Joesson

Iirc it was a Member, Oldman, on Spyderchat that was a leading advocate of de catting and the subsequent retention of the "high quality" empty manifold.
My recollection is that it was almost a question of not if but when the engine would self destruct due to pre cat failure, and there was more than one incident reported on here, one new member directly after buying from a dealer.
The collective experience from Forum members and the passing of time and mileage of our cars
has brought a better understanding of the situation, the problem cause most likely relating to irregular/ improper maintenance being something that few would readily admit to
likely did not help the diagnosis.
In my experience I did notice  a difference in the sound of the de catted OE manifold, something that was also recognised by a Mr T mechanic when he was servicing my car for the first few years of my ownership.
Performance wise I
don't recall an up or downside
and neither was that the object of that exercise.
Tubular aftermarket manifolds are however advertised as providing this.

shnazzle

What is worth mentioning is that pieces of pre-cat have been found lodged in the main cat on late-year cars. Possibly from shock damage. This is why there's the "get rid, for peace of mind" mentality.

I have never found anyone back up the myths of "oval bore". The closest I've come is @spit verifying that he once found what looked like pre-cat dust in the cam cover. But not in the bores.

So, we maintain the car best we can to ensure the rings don't get gummed up and cause issues and oil burning and move on with our lives. 
...neutiquam erro.

Ardent

Quote from: Petrus on June 25, 2022, 17:41The myth though is still going strong on the internet with first time owners being advised to gut to prevent possible catastrophic engine damage for early and late models alike. Not a single enquiry whether the thing uses oil or not.
Ah well.


One of those like and dislike at the same time posts.
Agree with what you say. But a sad face because of you what you say is so true.

Petrus

Maybe I, we, should spread the gravity theory on the subject.
As such going uphill there is NO WAY cat crap gets into the combustion chambers.
Downhill...well, don´t lift in gear.
Problem solved.
The proof of it is that we earthlings have an atmosphere. Despite the earth´s efforts to sling it off, gravity holds it down. Ergo gravitational force beats air force.