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I had my newly built, mild 383 run on a dyno. It is going in my 64 Impala, so good torque was important as was keeping the idle civilized enough for power brakes and air conditioning. Given those constraints and the stock block, heads and intake manifold tow work with, it came out rather well. It is worth noting that I took a long time to find best flowing chevy exhaust manifolds..
One of the things I wanted to test was the headers vs stock rams horns exhaust manifold topic that seems to come up from time to time. Since this engine is made of stock castings, overall breathing ability is rather limited. So the headers never really get a chance to show much advantage at the extreme top end. The rams horns are reproduction 2.5 inch manifolds that have the correct casting numbers on them. They were mildly ported and the anti-reversion bumps removed.
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I have to say, that in my test conditions, using the rams horns is a no brainer. Why deal the disadvantages of headers for such a small gain?
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What made a bigger impact on performance was adding the 2.5 to 2 inch reducers to the pipes, to simulate what actually happens at the muffler on the stock exhaust system on the 64 Impala. It also points out that an all 2.5 inch exhaust system would be an advantage.
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And just for fun I did a run with the stock air cleaner and filter in place, along with the 2.5 to 2 inch reducers on the exhaust (trying to simulate how it will be once installed in the Impala). On a 64 Impala the air cleaner horn has a remarkably small opening and I was expecting it to really cause a hit on the performance.
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Surprisingly, the air cleaner/filter had little impact. This is probably highlighting how restricted the intake manifold is. I am thinking it might be worth porting the intake a bit to pick up some more power higher up the RPM range. I don’t think a mild port would have much if any effect on idle quality, since it is a 383 breathing through a 327’s intake.
 
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