Exhaust diameter question

[Bergo]

My apologies if this has been covered already in another thread, but I wasn't able to find an answer with the search function.

I presently have a '74 911 with a '71 2.2T, and bad stock heat exchangers (exhaust leaks all over). I've disconnected my heat to avoid any issues with CO in the cab. I'm looking to replace my heat exchangers with a set of stainless headers, but I also hope to do a 3.2 swap down the road (this winter if I can find a core to rebuild), and would prefer to buy the exhaust only once if I can.

Would I run into issues buying a larger diameter exhaust (1-3/4") and using it on the 2.2 for a while?

[Bill Verburg]

Originally Posted by Bergo

My apologies if this has been covered already in another thread, but I wasn't able to find an answer with the search function.

I presently have a '74 911 with a '71 2.2T, and bad stock heat exchangers (exhaust leaks all over). I've disconnected my heat to avoid any issues with CO in the cab. I'm looking to replace my heat exchangers with a set of stainless headers, but I also hope to do a 3.2 swap down the road (this winter if I can find a core to rebuild), and would prefer to buy the exhaust only once if I can.

Would I run into issues buying a larger diameter exhaust (1-3/4") and using it on the 2.2 for a while?

SSI are perfect for what you have but a tad smal for the best performance on a 3.2, the thing is if it's a mostly street used 3.2 you'll never miss the last few ponies at the top of the rev range and you will enjoy a little better torque below the top.

[Bergo]

Also, if I buy a set with the 8mm flange, can I use them on a later engine, that stock, uses the 21.5mm flange? can I swap out the studs? AFAIK the spacing is the same for all the motors, only difference is the flange thickness, tube diameter, O2 sensor, and ducting position on heat exchangers.

[fred cook]

Originally Posted by Bergo

Also, if I buy a set with the 8mm flange, can I use them on a later engine, that stock, uses the 21.5mm flange? can I swap out the studs? AFAIK the spacing is the same for all the motors, only difference is the flange thickness, tube diameter, O2 sensor, and ducting position on heat exchangers.

Yes, you can change out the exhaust studs but you need to know what the process is. I had to change short studs on a set of Carrera heads a while back. I used a propane hand torch to heat the heads inside the exhaust port next to the stud. This does 2 things. First of all it will cause the aluminum head to expand slightly. Secondly it will melt the thread locker on the stud threads. Once the right amount of heat has been applied, the studs will back right out. I got all 12 out of my cylinder heads without breaking any of them. Good luck!

[1QuickS]

Get a digital pyrometer to help judge when enough heat has been applied. I have found that around 240F is the temp where the thread locker on Webers gives up the ghost. This is a LOT of heat from a propane torch so an oxygen/acetylene torch might be in order.

Just heat and check stud for willingness to unscrew, measure temp and try heating to a higher temp and retry. You will find a happy temp that seems to work well. You can double nut the studs, use Vise-grips or a stud removal socket (my preference) but only the double nut approach will conserve the studs...at least as far as I know.

[Bergo]

thanks for the tips! sounds like I can grab pretty much anything, fitment shouldn't be an issue, and swapping out the studs if I need to go with a larger or smaller flange shouldn't be an issue.

Performance wise, will I run into any serious issues using a large diameter (meant for a 3.2) exhaust on my 2.2T? street use only, so if I lose some low-end power, it's not a huge deal.

[Bill Verburg]

Originally Posted by Bergo

thanks for the tips! sounds like I can grab pretty much anything, fitment shouldn't be an issue, and swapping out the studs if I need to go with a larger or smaller flange shouldn't be an issue.

Performance wise, will I run into any serious issues using a large diameter (meant for a 3.2) exhaust on my 2.2T? street use only, so if I lose some low-end power, it's not a huge deal.

Just the opposite to what I described above happens when the exhaust is too big, The gas flow slows losing a little at the bottom but not really gaining more than a proper size at the top, more heat is lost to the surrounding areas due to slower flow.