I've got the impression from several posts that 2.0 stainless steel heat exchangers (SSI or other?) are a performance improvement over the stock 2.0 heat exchangers. Have I got that right and if so what about the stainless design leads to the performance improvement?

I've got a set of original 2.0 HEs that look OK from a metal standpoint but could use a serious refurbish. I'm wondering if it's worth the effort to do the referb or try and find a used set of stainless replacements.

It's my understanding that the 2.0 stainless are the same interior diameter as the oem giving no enhanced performance. However, the 1.7 ssi's are bigger than the oem steel giving an increased performance.

My understanding there is no performance improvement, just no issues with rust. The SSI do have their own set of problems that you need to make sure are not a problem before you buy them. The pipe inside the exchanger can crack over time and can allow exhaust fumes to get into you cabin.

I do a simple water test to check for cracks. Put the exchangers on a couple of saw horses and raise the muffler end an inch or two. Fill both pipes with water until water comes out the muffler end. Let them sit for 30+ min or so. If you see no water drops coming out of the shield area then chances are they do not have any cracks.

Click on this link ---> BAD SSI Exchangers for pictures of what can happen.

Depends on how you define 'performance'. The factory HE's are quieter than SSI's if that is important to you.

The SSIs are the same diameter for both the 1.7/1.8 version and the 2.0 version. That diameter is larger than that of the stock 2.0 exchangers. In theory, that would result in more power. In practice, it doesn't seem to.

The stock HEs do a much better job of sound damping, as Jeff stated. The SSIs are noticeably louder. The mounting ears can also be more problematic on the SSIs than the stockers, though that's not to say the stock ones never have problems.

I got them because they won't rust. Period.

--DD

The "performance" improvement is in rust resistance, as stated by the others, & not much else.

Also ditto on the SS HE's mounting ears tending to crack, but they're easy to TIG weld back on (SS needs TIG, MIG won't work well).

You should also test your OE mild steel HEs inner pipes for leaks too - &/or any other stock ones - cuz that exhaust leaking from the inner exhaust tubes into the out air channel of the HEs ~ & therefore into the cabin with heat on ~ is a malady of all types of HEs on the old aircooled cars - Porsche, VW, Tatra, etc.!

That said - you can now again buy SSI HEs now that Dansk has restarted production (+/- $1000-1200 per side new IIRC), but an NOS set of stock ones are like Hen's Teeth!!

The SSI's can be painted or ceramic coated to a grey close to the stock steel exhaust color too, if you're worried about originality or that look.

We sent our 2.0 heater boxes to Extrude Hone for them to get the i.d. opened up. Then off to the metallic ceramic coating shop.
Result was a real decent set of original heater boxes, better exhaust flow and no rust issues. Going on 10 years use (street+track) and they look brand new.

A light weight 914 - all else equal - will accelerate faster, turn quicker, and stop shorter.
The SSI HEs weigh quite a bit less than OEM

Thanks for the feedback folks. If I'm understanding right, the stock HEs and the stainless HEs share the same basic flow characteristics so the only real benefit is the rust resistance for the stainless versions.

Thanks Garold for this insight!... My 73 1.7 has SSI's and no matter what I clean, refurbish or replace the exhaust is coming into the cabin...

I'll do the water test this weekend. THANKS!

Older 2.0 liter stainless or plain steel heat exchangers are a big improvement over later 75/76 heat exchangers.