Hi All,

What size bore would you recommend for a catback system for a Octavia Mk2 105PD.

I went to exhaustsUK and they quoted £285 for one and it would be a stright pipe from the cat to the new backbox. If I remeber correctly he suggested either a 3" or 3.5" bore, but most prebuilt kits like miltek seem to have 2.5", the last thing I want is the loose power.

The car has a panel filter and is remapped.

What are your suggestions and recommendations?

Regards

Steve

I was thinking 2" myself. It's not like back pressure is really a problem with a turbo-diesel anyway, because the turbocharger absorbs most of the energy of the exhaust gas (and uses it to compress the intake air).

I would have thought for a turbo diesel car, the bigger bore the better.

Why? Some back pressure is actually desirable, since it stops the turbo running down too quick, and helps support the oil seals.

Just spoken to JBS and they quoted around £550 fitted for Downpipe and catback 2.5".

Just asked Sarah at Awsome for a quote as they are doing free fitting etc, main problem is there isn't a kit out for the Octavia PD105 so they will have to experiment with my car :s

yes we are on it

Sarah

Thanks Sarah! Cant wait to hear from you!

You can get a miltek system off of fleabay for around £350 if iirc.

Yes, but the Octavia 1.9 Miltek exhausts are made for the Mk1 not the Mk2.

Ok then thats me corrected then hehehe.

Why? Some back pressure is actually desirable, since it stops the turbo running down too quick, and helps support the oil seals.

I wasn't meaning the biggest possible diameter you could fit under a car, but it has been proven many times before tht TD engines do not require the same amount of back pressure as petrol due to the back pressure being made up in the turbo. So bigger bore means quicker gasses escape, more gasses to be produced, etc etc.

But to be more specific about my answer to the original thread I really only meant go for a 3.5 bore.

I phoned ExhaustsUK today and I must have missed heard them before as I asked what size bore he would use and he said 2" 1/4 or 2" 1/2.

That makes sense to me. Selecting the sizes to get numbers I can do in my head, a 2" ID system has a 3.14sq inch section, and a 3" has about a 7sq inch section. This clearly means that the exhaust gas velocity is more than halved, which is good for noise provided that any silencers, turbos etc in the system remain constant (other than increased inlet and outlet bores downstream of the turbo exhaust nozzle).

I'm less clear about the effects on back pressure, but I am clear that if the research guys at Garrett, IHI and KKK all say that some back pressure on a turbocharged engine is a good thing they're likely to be right!