4/8 and 5/7 Exaust Note - Impala Tech
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post #1 of 5 (permalink) Old 05-07-2016, 11:22 PM Thread Starter
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4/8 and 5/7 Exaust Note

Haven't posted in 2 years and wanted to sell the 68 BBC 4spd convertible but decided to keep the ol girl. Any ideas/opinions on toning down the pop especially on the 5/7. Other than long tube's an X or H pipe.
This is for a 402 running true 2.5" duals on Thrush turbo mufflers. Thanks all!
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post #2 of 5 (permalink) Old 05-08-2016, 01:17 PM
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The purpose of a long tube header (longer than eighteen inches from port to collector merge) is to prevent the high pressure gasses in the exhaust from backing up and entering an adjacent cylinder (such as 5-7) as they fire. This high pressure area that builds as the exhaust opens on seven can rush back into cylinder five if you have a long duration (late closing valve) cam and cast iron manifolds or shorty headers. This wasted gas that rushes back into cylinder five dilutes the incoming charge and makes the cylinder burn lean (acts like an EGR valve). That is probably the source of the noise you hear.


Freer flowing exhaust prevents back pressure build up in the headers, but it allows more noise to get out as well. The factory used a two and half inch dual exhaust on all of their high performance cars. Larger diameter pipe was available back when the cars were designed but the engineers at r the time though that a five hundred horse big block running wide open wouldn't be impeded by the smaller pipe and intentionally chose that size. Chrysler used the same size on their 440 and 426 Hemi cars as did Ford with their 428-460 engines. On the street you can not tell any difference in performance. It you want to race you uncap and remove the extra weight of mufflers and tail pipes.


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post #3 of 5 (permalink) Old 05-08-2016, 04:18 PM Thread Starter
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Thanks Dave. It's noticeable and consistent. Others don't really notice it but its like a small dent in a fender on a perfectly painted car that your eye see's every time. But a little gun shy because I discovered a flat #5 intake lobe on my last cam. I have a set of painted long tubes just hanging. I was going to throw them on with an X or H. I wanted to get some opinion's first. If that doesn't do it, it looks like I might go with a roller setup or at least check for a flat cam.
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post #4 of 5 (permalink) Old 05-08-2016, 04:56 PM
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The sooner you check the better. As that cam wears down the lobe goes from being a cast iron part to a pile of iron filings that you really do not want floating around in your oil;. Many are so small they go through the filter to embed themselves in the bearings. This turns bearings into sand paper to score and wear your crank journals needlessly.


All it takes to check is a dial indicator mounted at each valve and compare valve lift across the cam as you turn it over by hand.


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post #5 of 5 (permalink) Old 05-08-2016, 05:20 PM Thread Starter
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True that!
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