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Discussion Starter · #1 · (Edited)
Back when I put the LT1 in my '91 OCC, the recommendation was to run 2.5" from the cats to the mufflers, then 2.25" the rest of the way back. Today it seems like most recommend 2.5" all the way back.

On roller rockers and a cam, the in vogue set up was a mixed set of 1.5 and 1.6 rockers, with the 1.6 only on the exhaust. Or was it the intake? Today, it is 1.6 on both. Actually, it sure is a lot easier to just buy one set and not have to buy two and sell what you don't need.

What was the reasoning behind that weirdness and why has it changed today?

On the exhaust, my wallet would be far happier if I just put the Pypes intermediate 2.5" with X http://www.summitracing.com/parts/PYE-XGI10/ in and left the 2.25" OTA with resonators deleted the rest of the way.

Am I just being a cheap ass?cwm3
 

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I'd like to say there's more empirical evidence, but something like that, it's really opinion, most people will say something different. Some people will say big pipes flow more, but also smaller pipes are hotter so exhaust velocity is higher. Blah blah blah run what you like.
 

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On the exhaust, my wallet would be far happier if I just put the Pypes intermediate 2.5" with X http://www.summitracing.com/parts/PYE-XGI10/ in and left the 2.25" OTA with resonators deleted the rest of the way.
Am I just being a cheap ass?cwm3
Nah, I think it's just being practical. If you're really worried about that 5 hp that may be left on the table, then you might as well run cutouts at the track and get ALL you can get out of it. On the street, you will never know the difference between 2.25 and 2.5 tails.
 

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FYI: The stock OTA pipes are only 2" not 2.25"

A possible explanation for a recommendation for smaller OTA pipes is that by then the exhaust gases have cooled significantly and therefore take up less volume. Also, smaller pipes will have slightly smaller bends to get over the axles. The point that is overlooked is that it is better to have the larger pipe when going through those bends. Many companies now offer 2.5" (and even 3") OTA pipes. 2.5" OTA pipes should be more than adequate for most setups.
 

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The split rocker set, is because you can have clearance issues between the valve stem seal, and the retainer on the stock iron heads. The ZZ4 cam has quite a bit of lift on the exhaust side, in comparison to some of the other baby cams. A split rocker set on that Cam is still the way to go, unless you do some machining for clearance.
 

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On the exhaust, my wallet would be far happier if I just put the Pypes intermediate 2.5" with X http://www.summitracing.com/parts/PYE-XGI10/ in and left the 2.25" OTA with resonators deleted the rest of the way.

Am I just being a cheap ass?cwm3
Not cheap, efficient. That configuration will yield a nice bump in torque for you. To me, the Pypes setup being available for an affordable price, and coming in stainless, is probably one of the best/biggest developments in the area of exhaust for cars with bolt-on mods.
 

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Discussion Starter · #7 ·
Yes, certainly efficient! I was really amazed at the price of that Pypes set up and, with the help of a friend who welds better than I can, we can install the system in my garage.

Interesting (to me, at least) that you should mention an increase in torque vs horsepower. My quarter mile time slips seem to show me that my low end torque is OK, but I am falling off on the big end, after 1000'. I would have thought that was horsepower, and engine breathing related. Here is the tread: http://www.impalassforum.com/vBulletin/showthread.php?t=250262

Thanks,
Greg
 

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Any advice regarding split rocker ratios is due to guys picking off the shelf cams that were not ground specifically with your engine/heads in mind...but they found they will work. Any cam's ground with the iron head LT1 in mind, will be ground for either 1.6 or 1.5 rocker arms, not a split.

Once in a while, people have experimented with that anyways though, taken a cam intended for 1.5's and put 1.6's on the exhaust side to get a little more lift to try to help say a nitrous system...on a stock cam, I would not hesitate to put better springs and 1.6 on all 16 valves...it's a nice bump in performance from the engine.
 
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