Over the years I have seen many on here use stock 6 exhaust headers on 2.7 - 3.2 engines. Seems to me they are a bit puny ( not enough i.d. to flow well ) on anything over a mild 2.4 Am I right on this ? Assuming so what options are there for the larger displacement / horsepower engines ?

It depends on exhaust port size.....you've covered a wide range. The header ID should be no smaller than the exhaust port....but if you want heat.........
The options with heat are hella expensive and some are not well made...from what I read.

I'm going to try and make my own clam shells to fit over my MSDS headers.

I'm going to do the same on the /4 headers I plan on installing next spring. Best of luck to you good sir.

I suppose it depends on what part of the rev range you want to optimize. You are correct that the stock 914-6 heater boxes are on the small side (1 3/8" dia. primaries), even for the stock 2.0 liter engine size, IF you're talking about race conditions and the 5000-6500 rpm part of the power curve. I had 1 3/4" headers on my 3.0 conversion for years, and can testify that it gave great results up near the redline, but was surprisingly gutless down where I used it 99+% of the time (2500-4500 rpm). As soon as I bolted on the stock parts, I noticed drive-ability and apparent power improved dramatically under daily driving conditions. I've also heard from others with sixes and six conversions that the stock exhaust parts are not only OK for engines up to about 3.2, but seem actually much better most of the time because they improve the mid-range power. No question that, if you got into a top end contest in a car like mine with someone in a similar car but larger diameter headers, you'd be at a disadvantage. What most people tend to overlook, however, is that you give up quite a lot in terms of mid-range power to get that top-end performance when you switch to larger headers, and you use the former much more often than the latter.

A little smaller is better than a little too big. Except for all out racing applications the flat 6's do best with a little back pressure.

You guys making heat exchangers out of mild steel headers had better make sure your life insurance is paid up, and your bucket list is checked off pretty well, as it's a dangerous proposition!!

There's a reason all Porsche heat exchangers since the mid 60's have tubing made of stainless, and it's because a few people got killed by exhaust gasses in the cabin. TRYING TO MAKE HEAT EXCHANGERS FROM MILD STEEL HEADERS IS POTENTIALLY LETHAL!!

I planned on making them out of Aluminum. Same story? I honestly didn't know mild steel was an issue.

If you're going to drive you car winters in the salt and crap with raw uncoated steel I can see it. In a summer car that sees limited mileage, with ceramic coated pipes I really doubt you would ever have a problem.

Cheap insurance is to run a battery operated CO2 alarm, if you can't afford $35 then a colourmetric CO2 detector is less than $10 and will last 2 seasons.

I've seen cracked an poorly welded SS pipes as well. All the commercially available aftermarket /6 exchangers have a weld inside the exchanger.

The exhaust gasses will help corrode the pipes from the inside out.

A CO detector is a great idea.

The factory 6 h/e's have no welds inside the heat collectors, nor do the B & B's. The only aftermarket ones I've seen with welds inside the heat collectors are the ones from England, so that probably limits them from a loss of life lawsuit. There's no way you can be sure a weld inside the heat collector won't fail, and of course you'll never see it.

No mild steel h/e's and no welds inside the heat collectors is the only way to go, but what do I know.

Post some pics of your wife or girlfriend though, as I'm sure there's plenty of lonely single guys in this group!