Quote:
Originally Posted by dannichols1474
mb911 thank you for all that info.
I went through the link to the earlier forum thread and saw all of the dyno plots and the table of various exhaust configurations versus hp at 3000 and 6000rpm.
I also read several other earlier posts about header tube sizing. One thread from 2004 mentions ID versus OD. I just measured a primary tube OD on the 1.75" headers currently on the car and it measures 1.75" so when we talk about 1.50" or 1.625" headers we are talking OD, right?
That same thread also mentions camshaft specs being a factor in addition to engine displacement for determining the correct header primary tube size. The idea was that engines with "bigger" (more duration and lift) cams would benefit from larger headers.
So my now 2808cc 2.7L motor (92mm vs stock 90mm bore) with 69-73 911S MFI cams (duration: 264/236, lift .450/.402) versus stock CIS cams (220/200, .410/.348) still should have 1.50" headers versus 1.625" headers? I have also boosted the CR from the stock 8.5:1 to 9.8:1 - does CR have any effect on the optimal choice of header size?
Thank you.
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Yes so be careful with OD vs ID . OD is how ALL tubing is measured so when someone refers to 1-3/4 it is usually .0625 wall and that equals 1.625 ID and down the line
I would also suggest looking at Dyno runs on a 3.2 vs 3.0 with SSIs which up until recently were all 1.5” OD and 1.375” ID again .0625 wall is common wall thickness. Yes cam and CR have an effect but you were at such a far extreme that you had no problem up top but low and mid range suffered (think track applications when running mostly higher RPMs) when street driving a car you want the best of both worlds but keep in mind you spend 75% of the time in the lower rpm ranges. You will experience better performance overall stepping down to 1.625 OD (again how all tubing and headers are measured across the industry) but you may leave some TQ down low on the table by not going 1.5”. I made a decision on my car which is a hot 2.4 to use 1.625 (993 exchangers) and sleeved them to 1.5” to maintain as much low end TQ as possible. It works amazing on my setup. Primaries length and collector design play a factor to in exhaust and ideally a 31-32” primary should be used for a Porsche 6 if low end TQ (street driving) is to be optimized. Typed this on my phone hope it makes sense