Here's what I did for cheap and it worked.
1. Removed all bolts, manifolds, crossover. Bolts into heads were stuck but more at the manifold than in the threads. Hitting them on the heads did better loosening than a week of spraying chemicals for a week.
2. Had heads resurfaced at NAPA.
3. Got new FelPro "heat shield" gaskets. These are a stamped steel gasket with flanges between the ports that protect plug wires from heat.
4. Reinstalled with Never-Seez nickel compound on all four surfaces of head, gasket and manifold.
5. With new Bolts. This part violates the rules but I chose stainless bolts with a flat then a lock washer on each.
6. Torqued to only enough that the stamped gaskets stopped crushing, about 25-lb. Similar feel to installing a sparkplug with a new gasket that has to crush.
7. Replaced 2-1/2" kinky OEM pipe and Midas muffler with a Cat-Back kit. Ours is a FlowMaster, I think 70 series muffler. NOTE: Ours had no cat converter from OEM. Adapted 2-12" crossover to 3" (adapter furnised in kit), extended to muffler location with 3" pipe (again in kit but intended for extended wheelbase).
Better power, a little louder but none from the engine compartment and manifold areas.
I check the SS bolts once in awhile. Took about 1/2 turn after first few hundred miles, no tightening required after several thousand miles more.
I think this worked because the manifold-to-head interface can slide with thermal expansion and not side load the bolts. I also think the smooth 3" mandrel bent exhaust disposes of heat so much better than OEM that the manifolds don't get as hot as before.
This is in a C class with 460 carbed engine. I tried headers on a Dodge van we had and the doghouse got so hot we had to wear boots to drive it. Luckily I'd bought a full kit from Sears with mufflers and duals and returned the whole smash "not satisfied." BTW - It was unbearably loud and fuel mileage dropped. It's possible to get carried away on exhaust flow.
bill h wrote: Since few of us have a dyno, or the inclination to pay for dyno time, I wonder what correlation back-pressure measurement and comparisons would have.
Dave, did you do any back-pressure measurements on your muffler changes?
No, I did try to rig up something when I was having so many issues with the 70 series Flowmasters, but it did not give me any real comparison. What really is most interesting in the specs I posted above, is that the straight through pipe, with no muffler actually put out less power. That would suggest that the engine was designed for some small amount of back pressure.
I would suggest anyone who embarks on a project like this should do a couple of benchmark run with a stop watch, that way you have some real true comparisons.
Just for info, that 1400 degree temperature in the Ford documentation that I mentioned wasn't EGT, it was the external surface temperature of the manifold.
Rumor has it that the final series of carb-equipped 460s were deliberately jetted rich so that there was enough unburned fuel getting into the manifolds for the air injection system to work to maximum effect. In essence, it worked like an afterburner to get rid of the smoggy stuff. It might explain the 6 mpg at sea level and 4.5 in the mountains on a 26' Class C.
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