I've having trouble with an exhaust leak on my '77 351M passenger side exhaust manifold. I replaced the exhaust gasket, but it continues to leak. Is this manifold bad about cracking? Anyone had any problems with exhaust leaks on this side of their 351M? Thanks.

 

The passenger side manifold on many different engine families is bad about cracking because it houses the heat riser for the hot-air intake system. It traps more heat around it than the driver side manifold. It's also possible the mounting surface itself is warped.

 

How necessary is the heat riser? I've been thinking about taking it off - it's rusted out on the bottom - and replacing the stock air cleaner assembly with an open element type. As for the exhaust manifold, I've thought about getting one from a salvage yard, but I don't want another bad one. New ones are high. The cost of a new manifold would go a long way toward headers.

 

I have a 77 F150 with the 351M and my heat riser was rotted on the bottom as well so i replaced the exhaust manifolds because they too were leaking and replaced with aftermarket Summit Racing Headers but to answer your question, they are not necessary but be sure you go ahead and retune your Carb's Choke to compensate. My truck does not have the pull choke, its the heat activated one and just had to advance it a notch or two. Hope this helps

 

It's good to have because it prevents the carburetor from icing up on cold yet humid days. Drawing cold, most air through tiny passeges in the carburetor forms ice which makes the engine run really rough, or stall out. If you install headers or an open air cleaner, there really isn't any way to retain it. I run headers on mine and ditched it, but luckily here with the Texas weather I can get away with it. I'm not totally sold on open air cleaners because while they do have more room to breathe, all they take in is warm under-hood air, as opposed to cooler outside air that the stock air cleaner takes in.

 

I haven't looked - does anyone offer a replacement heat riser?

 

I had the same issue and ended up going with headers. All of the exshaust manifolds I found in the JY were cracked as well, and a new one ran ~$100. I had to redo my exhaust anyway, so I ordered a set of Hooker headers from Summit for ~$150.

If your old manifold does not appear to be cracked it could just be warped like fmc400 said. You can either have it machined, although your head may not be straight either, or use a thick gasket used to make up for gaps. I used a carbon fiber gasket from Remflex that is made for a 351C 2V head, which has the same exhaust port size as a 351M/400, and is made to compress up to 50%.

I would also advise against an open air filter, especialy if you go with headers, since these engines tend to have ping issues and it is my understanding that hotter air increases the chance for it.

 

What headers do you like, and for a street engine, stock or near stock, wouldn't you go with 1 5/8-in primaries? I've had Blackjacks and Hedman's in the past. I've noticed that there are two different styles available, a standard and a "low-profile". The standard looks really long, and the "low-profile" looks more like the headers that I considered standard. I don't want them to hang down, so I'm leaning toward the "low-profile".

 

To keep low end torque smaller primaries and long tubes are better. Not sure about the standard or low profile differences. I know that there are shorty headers by Sanderson's that are built for the 351C, but the tubes are short and 1 3/4 inch so it kills bottom end from what I have read. My headers do not hang down but some manufactures do a better job than others at fit/clearance issues. <?xml:namespace prefix = o ns = "urn:schemas-microsoft-comfficeffice" /><o></o>
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Might do a search in this forum and the engine forum to see what people think of the different brands available and how they fit. Make sure you specify if it is 2wd or 4wd. I know there seemed to be a lot fewer options for my 2wd truck- had to go with 1 3/4 primaries but they fit well.<o></o>
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Also look at the thickness of the mounting flange, thicker is less likely to leak, and tube gauge/coating for longevity. <o></o>
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