porsche0nut

A design question for discussion:

On the 3.2 the passenger side bank crosses over to the driver side where the pipes collect prior to entering the cat.

This means that even if the lengths of each cylinder's headers are the same, the passenger side exhaust is considerably longer than the drivers side.

My question then, is how important, if at all, are the exhaust lengths from each cylinder on our cars? To the extent of my knowledge, the exhaust has a lot of potential for tuning the power/torque curves of an engine and its design can get quite complicated when you take into consideration back pressures and resonance at different overall lengths. Having different lengths from each cylinder adds a whole other dimension, but I'm sure Porsche did it for a reason?

Follow-up question: If one were to go with SSI headers that go straight back into a 2-in-1-out exhaust, where would they put the O2 sensor? Can it operate properly measuring from one side?

Looking forward to hearing your thoughts!

Ed Hughes

These cars respond quite nicely to headers. I'd think a 2-1 would defeat some of that gain, however. My headers had a bung on the LH bank for the 02, so it only ran on that one side.

Steve Weiner-Rennsport Systems

First,

The effects of exhaust tuning depends a lot on the camshaft profile's duration and lobe centers. With the wide LC and low duration smog cams in these engines, equal primary lengths are not as critical in stock tune. As Ed wrote, these things do respond nicely to a good set of headers or SSI heat exchangers but thats mostly due to eliminating the crossover and keeping both sides of the engine separate before the muffler. SSI's are a tad small for a 3.2, however they do provide more performance than the OEM system, especially with a good sport muffler like a Danske.

This is a very complex subject with a LOT of variables.

Second,

Since the O2 sensor is simply there for lambda info for the ECU, these are normally installed into the left-side pipe, 3"-4" behind the collector. SSI makes a specific version for these engines that has it.

Works perfect.

Hope this helps,

porsche0nut

Thanks Ed and Steve.

As usual, the answers spring more questions...

1) I've read that some put 964 cams in the 3.2 for improved performance... what other factors come into play here when doing such a mod? I'll do a search because I'm sure it's been discussed before but would be interested in hearing your views. Hasn't been something I've thought about.

2) "SSI's are a tad small for a 3.2" - can you please explain? Small in diameter such that they are still restricting exhaust flow? My car is a street car (maybe light track in the future) so my goal would be a bit extra grunt and an improved sound. What would the alternative be?

Thanks!

rusnak

Definitely headers. You might query Dr. Woot about his setup, which looks to be well sorted. And Ed had headers that retained heat, a major plus.

Ed Hughes

I ran B&B headers, and a Flowmaster muffler. They were plenty big.

Cams can add some power too, the stock injection system can handle the extra breathing. I'm sure Steve can tell you volumes on this, I can only relate my experience.

Steve Weiner-Rennsport Systems

964 cams improve mid-range power but require different software to adjust the Air-Fuel ratio so you are not too lean.

SSI's use 1.5" primary tubing which is slightly small for a mild 3.2. They do not offer a significant restriction unless one is tuning for power between 6K and 7K RPM. They do improve low-end torque and thats an advantage with the stock gear ratios. With a good sport muffler such as a Danske, its an excellent upgrade.

porsche0nut

Thank-you Steve, Ed, and rusnak!

Is there any recommended reading that I could do to learn more about the design, modification, and tuning of these engines? I'm not looking to jump into anything major (yet) but the engineer in me is interested in learning more.

Ed Hughes

Bruce Anderson's book is a good place to start, I hope this link will work:

rusnak

Porsche 911: Engine History & Development [Hardcover]
Tobias Aichele
Tobias Aichele (Author)
Hans Mezger (Foreword)

Ed Hughes

Let's try my link again:


rusnak-I've not seen that book before. Have you read it-the reviews look lukewarm?

Steve Weiner-Rennsport Systems

I have that book and the reviews are accurate. Nice pictures but no real technical information.

Paul Frere's, "911 Story" is FAR better in this regard.

rusnak

I agree 100%. The Tobias Aichele book is heavy on pictures, light on data. I do enjoy it as an adjunct to the Frere and Anderson books. There are illustrations and interviews that I had bever seen before, which the Porsche factory had never authorized before. I really enjoy the historical perspective of the Ludvigsen 3 volume set too. Books are best when they focus on something that no one has done, and these books do that.