Equal length exhaust without SSI!!

[tbitz]

I've been searching for some pre-74 exchangers to give my '78SC some extra power.
Since everyone else is also looking for the same I'm not having much luck. Forget new SSI, I simply can't afford it.

I thought if the problem with the current exhaust is that the pipes are not equal length, then why not make a custom cat bypass pipe that adds a small loop to the left exchanger making the exhaust length equal when the two pipes merge. This shouldn't be too hard to make for someone who has equipment to bend exhaust pipes. An off the shelf cat bypass pipe could be butchered to make a prototype unit.

Any thoughts why this shouldn't increase performance?

Think how simple this custom pipe would be to install. Think of all the headaches going to SSI, new muffler, new heat exchangers, broken exhaust studs, new oil lines, etc....



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Tony
'78 911SC

[911pcars]

The benefits of an equal length exhaust system lie in the length of the individual exhaust pipes of each cylinder leading to a common collection point. Merely equalizing the overall length using the collector pipe in an otherwise unequal length system does not accomplish the desired tuning effect.

Sherwood Lee
http://members.rennlist.org/911pcars

[tbitz]

I assumed that the post 74 exchangers had equal length pipes until the point where the three pipes merge.

I though the problem was the "cross over" pipe which increased the length of the right 3 cylinders versus the left 3 cylinders. This is why I figured add the length of the "cross over" pipe to the left 3 cylinders at the cat bypass pipe.

Has anyone removed the shroud from a post 74 exchanger to see if the pipes are equal length?


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Tony
'78 911SC

[This message has been edited by tbitz (edited 06-06-2001).]

[Early_S_Man]

Tony,

I think you have oversimplified the design of a tuned exhaust system. The part you missed was the 'required' length of the exhaust runners, which typically fall in the range of 24" to 36" ... depending on the rpm range of the torque peak of the engine, which needs to match the camshaft and intake tuning!

If you have Bruce Anderson's Porsche 911 Performance Handbook, take a look at the factory racing headers available around 1980-83 for the SC on p. 144. The problem with the factory heat exchanger tubes is that they are far too short to be effective, even if equal length. If you want to build your own alternatives to SSI or early '67-'74 heat exchangers, they would need to come very close to those factory headers for the individual runner lengths and tube diameter.
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Warren Hall
1973 911S Targa

[This message has been edited by Early_S_Man (edited 06-06-2001).]