Pre cat removal

[Pierce]

I replaced the exhaust manifold and precats with a purpose built manifold which I wrapped with asbestos (or some such) tape to get as much heat as possible down the pipe. It is suggested elsewhere on this site that getting rid of the precats would compromise the air flow.Can this be true?

[Call the midlife!]

Short answer, yes, probably.
Longer answer is if you make use of the search function on the forum you'll find all sorts of different answers to the question and you'll be able to make your own mind up...

[Gaz mr-s]

I think there's a misunderstanding here.   The 'Toyo' & derivatives won't give poorer gas flow.

Members on here have been told by a very good exhaust maker that gutting the ceramic material out of the o/e manifold WILL cause poorer flow.  But it's not commonly noticed/reported.

[Iain]

Yep gutting the oem manifold isnt the best thing for flow, but wont cause any harm.

Replacing the manifold with an aftermarket unit is the best way to go.

[Dev]

After gutting the precats a very long time ago there was no perceivable difference and I am sure it would not register on a dyno because it would be in the margin of error.
 The real flow killer or bottle neck is the chokes in the OEM downpipe flex sections and then where the two pipes collect before the cat.

[Ardent]

the gutting is almost the reverse of restriction. The gas flow speed drops going into a big empty chamber.
Boyles law and all that. 

[Petrus]

the gutting is almost the reverse of restriction. The gas flow speed drops going into a big empty chamber.
Boyles law and all that. 

It will effectively eliminate any scavenging effect. Because of the most restrictive trottling bottle neck Dev mentions, it will all go unnoticed.
Same as with sports air filters, better flowing end mufflers. All marginal at best unless the weakest link is eliminated by fitting a decat/sports cat or a modified/derestricted flexi joint on the OEM cat.
With that bottle neck opened up though all mods to the flow wíll be noticeable for better of for worse.

[Zxrob]

I gutted mine and to be honest didnt notice a thing with regard to performance

Rob