When I did my -6 conversion a couple years ago, I did some wheeling and dealing on parts and ended up with a set of 1-3/4" headers which I am currently using on the bone stock 2.4T CIS motor. I have been fussing with it on and off the last couple years trying to make it run right from idle to 4500....it stumbles and seems genuinely unhappy until it hits the magical 4500 RPM mark, when it takes off and runs smooth as silk. After messing with timing, monitoring A/F ratio, checking for vacuum leaks, and much general head scratching, I got to wondering about the header size. I know the 1-3/4 that I have on there is rated for 3.0 and up displacement, but I didn't think primary size would have that big an effect??

So my questions are:
1- Will running a too large diameter header primary cause the symptoms I described (rough running from idle to 4500, smooth 4500-up)?

and
2- What size would be more appropriate for a 2.4T with CIS? 1-1/2? 1-5/8?

Jason, 1 3/4" is IMHO too big for a 2.4. I'd go with 1 1/2" or smaller. Stock 914-6 heat exchangers are like 1 3/8" and they work well for anything up to and including a 3.0. I'd also make sure that your timing is dead on the nuts. Good luck!

Perry-

I learned that motor is VERY sensitive to timing, so yes, it is indeed spot on.


Do you think the headers could actually cause the symptoms I described, or might there be some other damn issue?

Exhaust that is too big will cause exactly the symptoms that you are describing. Too much flow and you loose some down low but gain some up high. Too little flow and it will run great at low RPM's but crap out at 5000.
Those headers are too big. I don't think anything under a 3.0 could handle 1 3/4" headers. Probably not anything less than a wild 3.0. If you look at Pelican Parts' engine rebuilding forum and maybe their 911 forum you can get firsthand experience.

Hey Jason, I haven't heard from you in forever!

Just to add some extra input beyone what you see above, I think I saw a rough header sizing chart in either Bruce Anderson's "911 Performance Handbook" or Wayne Dempsey's "How to Rebuild and Modify Porsche 911 Engines 1965-1989". I can't remember which. I have both at home and will try to post later tonight with what they say.

Richard!!! It has been a while! I have been following along your build on your website....excellent work!!!

I have those books at home too, I'll dig into 'em tonight. I feel kinda stupid about this deal, I really didn't think the header size would have THAT big of an impact....
Wanna buy a set of wrapped headers?? haha!

I updated the website day before yesterday. I need to update my progress thread here.

The charts in Wayne's book. For those who don't have the book (it a great book and fun to read even if you don't plan to do a rebuild) here is the chart...

2.0 - 2.4 1 1/4 - 1 5/8
2.5 - 2.8 1 5/8
3.0 - 3.2 1 5/8 - 1 1/2
3.4 - 3.5 1 3/4
3.6 - 3.8 1 3/4 - 1 7/8

As he mentions in the book, those are rough starting points for a stock engine and what header you pick depends upon a lot of factors (such as mild vs. wild). He also says, if in doubt, it's better to go too small than too large.

I would take you up on the header, but if I go by the chart above, it may be too big for what I expect will be a mostly stock 3.2 in my car. Looks like it might be a good fit for a modifed 3.2 or someone doing a 3.6 conversion.