Newbie question: why do SSIs give more power?

Ok, this may sound like a really dumb newbie question but...
Why does a free flowing exhaust give more power?
With my limited knowledge of these things I understand that SSIs provide a better escape route for the exhaust gases. But I thought power was generated on the down stroke after the mixture got ignited.(thats the limit of my rather sketchy knowledge of these thing's
TIA, Alex

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911 2.7S Targa
email:alex@cascade.f9.co.uk

they breathe better. Bruce Anderson says you can see this easily if you look at them (off the car). The OE heat exchangers provide a more restricted passage for exhaust gasses.

No doubt SSIs are tuned to give proper back pressure on the heads, yet allowing exhaust to easily get away from the engine.

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'83 SC

The SSI's are copies, or clones, albeit made of a better looking and lasting material, of the original factory 'tuned' equal-length header design heat exchangers used from 1964 thru 1974. The reason the tuning provides more power is that for rpm near the torque peak, pulses from adjacent cylinders pull, or 'extract' exhaust gases from the next cylinder whose exhaust valve opens, emptying the cylinder a little more efficiently than if using a non-tuned exhaust, so there is less waste combustion products left in the cylinder to dilute the intake charge. In addition, coupled with the more aggressive cam profiles with more overlap, the extracted exhaust charge actually pulls in more intake charge than would normally occur with out the tuned exhaust!

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Warren Hall
1973 911S Targa

Cool, thanks Warren & Superman. I think I understand now.

[This message has been edited by Alex Counsell (edited 05-12-2000).]

Um, another point to make is that while one piston is firing, another is exhausting waste gases through the exhaust system. if the piston that is firing has less work to do, it's more efficient, and therefore more power goes into the crankshaft and out of the engine.

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Mark Szabo
1986 911 Targa 3.2

OK, the SSI’s sound good to me. However Bruce Anderson says there is nothing we can do in California between the block and the muffler, with an 80’s car, and still pass smog. My understanding is that it will pass all the sniff tests but won’t pass the visual inspection because it’s not stock. Has anyone had any experience with this?
Thanks Andy
84 “arrest me red” cab

It really all depends. It's alot like front window limo-tint in California. It's "technically" illegal but MOST people do it anyway, and SOME get cited for it. (usually only if the cop was pulling you over for another reason, rarely have I heard of anyone being pulled over for that sole reason) Its the same with those kinds of smog laws. Before moving on to Porsche's I did alot of import/domestic hot-rodding. And being that practically every Law in cali prohibits any vehical modification (unless that product comes off the shelf with a C.A.R.B. number) those guys were breaking every law in the book! But they also know what you can and can't get away with, come smog time. First, you MUST pass the sniff test, period. From there, everyone was passing visual inspection with any aftermarket exhaust, and intake. And most of the time they'd pass headers. ALSO, as long as you passed emissions, you could get away with practically ANYTHING if you could make it look stock! (from there it was a matter of hoping that your smog guy didn't know much about your particular car. Basically, in Cali, almost everything's illegal, but people do it anyway, and generally, they get away with the minor things. And I've seen some incredible things pass (like turbo's! one of the biggest no-no's) because they looked stock!

The real problem here in CA is finding someone to smog the car who knows what they are doing. Now with SMOG3 it gets even harder. There was a real problem with some year Corvettes (oh that word) when they went by 'the book' and 'the book' said there was supposed to be a smog pump, when the Corvette did not need it. People had a real hard time gatting them passed. As an earlier post in this thread said, if it looks like the originall, then you stand a good chance at passing. Some places will just look and say YUP! looks good to me and then do the sniff. One of my pet peeves has always been 'so what if it does not look originall, as long as it passes the sniff' but alas, the price you pay to live in paradise.

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John Birkett
72'T Targa

I just had my 1979 911 smogged for the first time since I've owned it. I live in the Bay Area and I purchased the 911 knowing it did not have the EGR valve or any of it's 5 vacuum hoses installed. The smog station did not have a clue as to what was what and the car passed with flying colors. The smog report stated that the "visual" showed that all components where there, including the EGR valve. (I've since installed the EGR valve - for peace of mind) Have a pre-inspection that way if it doesn't pass you aren't plugged into the system as a "gross polluter" than you can get the car squared away and have it re-smogged.

I just installed a downpipe for my four cylinder Porsche 944, the factory headers are equal length for all cylinders, however the downpipe (meaning the straight through instead of cat) does not. This is only the 2-1 part of my exaust, so the 4 into 2 part, the one that bolts on to the head is stock (equal length).

After reading Warren's post, I wonder if this is going to do any good at all, should I have just welded a straight pipe where my cat was? Any ideas? How important is having all the pipes equal length?
Thanks