I use my car for track and am building my own stainless exhaust.

Having a read about online I was going to go with a 1.5" diameter pipe, decat and a single straight through rear box.

I have the standard 1.1spi sporting engine with a few mods, air filter, chip, larger tb.

Any hints or suggestions would be much appreciated.

Isn't it 1.75" standard? I'm sure my old Group N system was that sort of size.

Decat and straight through to glasspack rear silencer on that sort of size would make a very nice noise - not mega deep just raspy at the top end.

FWIW, I'm thinking a similar rig, but around 2" on my 1.4 16v Sei. Don't want to go too big as it'll deepen out too much but I want / need the extra bore to keep a decent exhaust flow rate for my future plans.

my standard exhaust is a tad over 1.5" but my new exhaust will have no cat, mandrel bends and a straight through rear silencer so would flow better anyway, even with an overall smaller bore.

i just dont want to go too big and we only have 1.5" and 2" sized pipe at work

Anyone have any idea?

Google it! 'Optimum Exhaust Size'
You'll end up finding some seriously complicated formulae which even if you know all the information such as cam duration etc. it will almost certainly come up with the answer that arseofbox has already given you 1.75"
It would be worthwhile you buying something like 'How to build Horsepower' by David Vizard. Always found his books make the dichotomies of engine building easier to understand.

I have googled it and came out with less than 1.5" in a few cases.

All I ever get on his forum is Google it. The forum wouldn't exist if everyone just googled every question that was asked.

I was after some real life experiences from people as calculations are only ok to a point.

dac69er said:

....All I ever get on his forum is Google it.

Click to expand...

That's because there is no definitive answer to your question - apart from the fact that a 2" pipe will almost certainly be too big (loss of scavenging etc).
Engines are mass produced not blueprinted & treated differently by previous owners so in the real world no two engines have exactly the same characteristics even if they have identical parts fitted.... therefore what size pipe someone has used on their engine may not be the best choice for yours.
Ether 1.5" or 1.75" will be fine, it depends on what you intend to do to the engine & how well all the constituent parts works together - as you have a 'cheap' source of 1.5" st/st pipe i would go for that .