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GMC V6 and V12 Engines Engine repair and rebuilding

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Old August 7th, 2018, 04:12 AM
1972RedNeck 1972RedNeck is offline
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Default Re: "H" or "X" Pipe On A V6?

Originally Posted by POWERSTROKE View Post
Whether it was a lowly little LoadStar with a 345 cid V-8, or a FleetStar with 478 V8, or a C65 with 366 V-8, or C70 with 427, anywhere from early/mid 1960's to late 1970's, they all had true dual exhausts. I think that was to manage heat in the engine and exhaust system. The duty cycle on trucks is amazing, hours on end wide open, exhaust manifolds glowing orange hot except for 3-4 hot spots that are bright yellow hot. An H pipe would add several more hot spots, and X pipe probably one big one that could cause problems. Not sure what current production trucks have to run for emission equipment. Big Block V-6's definitely none,

5 or 10 hp added to the 205 or 235 net hp of the 478 or 125 hp of the 305 "Might" be noticeable. True story, Buddy and I driving identical tractors, 320 net hp, first loads one day I was 400# heavier than his 70,000#. He walked away from me every 10 miles or so for 175 miles. Coming home I was lighter by 250-300#, I walked away from him 5-6 times in the 175 miles, maybe could have done more but I wasn't trying too hard.

I think lightening the truck by a couple hundred pounds would make more difference than the hp. Added hp usually costs burning more gas, lighter weight saves hp and gas.

I really think it's more than just a coincidence that a Chevy, GMC, Ford, and IH with big gas V-8 power had almost identical exhaust design.
Um, yes, this is all true (for the most part anyway), but what does it have to do with what I asked? I am building a 530+ cubic inch V6 that will be pumping out 500 ish horsepower in a 6000 pound pickup that will see 100% load for maybe 5% of the time.

I will be installing an "H" pipe unless, due the firing order, true duals would actually produce more power from better scavenging effect.

So back to the question: with the even firing order of the V6, would an "H" or "X" pipe be a detriment or a benefit to overall power output?
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Old August 7th, 2018, 10:15 PM
James James is offline
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Default Re: "H" or "X" Pipe On A V6?

If possible you can run it on a chassis dyno with just dual to see what happen. Then installed either the H pipe or the X pipe and run it again on a dyno to see if there is a difference.
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Old August 8th, 2018, 01:15 PM
massey478 massey478 is offline
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Default Re: "H" or "X" Pipe On A V6?

These connecting pipes make the most difference if you have mufflers that are restrictive. They allow both mufflers to handle the flow. My experience is that by enriching the fuel with bigger jet sizes to compensate for the alcohol content of today's fuels which drops power output and adding an MSD ignition system will make much more difference. Timing advanced above book recommendations also is an improvement. The alcohol in fuel allows you to do this. I never use a timing light except to mark where I get the best performance. At idle, no vacuum advance connected, I advance the distributor until it starts to miss, retard it to where it labors, then set it halfway between the two as a starting point. I set it just enough later that it does not try to kick back at the starter. Road tests then verify it is good, no spark knock. For the 478 in my Massey tractor I set it while running on the PTO dyno. I invariably find that if I check it with a light after setting it this way it is quicker than the factory setting. I have commented on the advantages of both jet size increase and MSD on other forums.
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