<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>Originally posted by Chris Liokos:
I personaly would find it difficult to understand how a multicam engine, in concept at least, can not have far more potential than a pushrod.
Also, there was a question asked on an earlier post on this thread that I am still curious about... What is fundamentally wrong with the quad-cam 4.6 that makes it unreliable, and why can't it be rectified with aftermarket parts?<HR></BLOCKQUOTE>
To respond to your first question, and in regards to the Ford modular 4.6 in particular, its biggest limitation is bore spacing. The modular engines were originally designed for wrong-wheel drive cars, and the bore spacing was minimized for packaging reasons. This means that in order to get good power, they have to turn high revs, which causes reliability problems, or they have to have an absurd bore/stroke ration to make better displacement, which causes yet another set of reliability concerns.
Regarding specific reliability issues of the 4.6 DOHC, and what can be done to overcome them, here are a few I'm aware of:
<UL TYPE=SQUARE><LI> The hydraulic chain tensioner over-extends at high rpm, and actually advances cam timing. SHM has a simple yet absurdly expensive fix for this. Interestingly, the tensioner also tensions the wrong side of the timing chain.
<LI> DOHCs are prone to oil starvation. One fix is to use seven quarts in the pan along with an Accusump; I wouldn't open track a DOHC without an Accusump. Another fix is to drill out the oil return passages in the heads because at high rpm oil can't getr out of the heads fast enough. There are also windage issues with the DOHC. Finally, anyone who has an investment in a DOHC would be well advised to upgrade the pot metal oil pump gears and the crank position senor to billet. SHM and Accufab offer these.
<LI> DOHCs are prone to oil consumption. The only solution I know of is to have the heads rebuilt and use high-quality valve guides and seals.
<LI> Ford uses cheap fasteners, in the mechanical sense, and error-prone fasteners in the human sense. The 4.6 DOHC is hand-built at Ford's Romeo plant, but I'm not sure that's such a good thing. My second DOHC developed a coolant leak, which I eventually traced to the fact that two of the head bolts were only finger tight from the factory.
My first DOHC spun the cam sprocket and grenaded the engine, and several others have experienced this exact failure. I'm not sure if the fastener was bad or if it was improperly torqued, or maybe both. Ford extensively uses crappy torque-to-yield fasteners, and I've been told that tolerances can change drastically depending on applied torque value. The solution is to use ARP fasteners throughout the engine and build it yourself.
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Regards,
Mark