Replacement in Trucks
The M-block engine was designed when first-generation pollution controls were already in place. Most Ford V8s required bulky and unsightly external tubing to feed Thermactor air into the exhaust manifolds and exhaust gas to the EGR valve below the carburetor, but this was all built in to the M-block engine. This made adapting the M-block to the second generation of emissions control equipment harder. One requirement of the second-generation equipment was an oxygen (O2) sensor in the exhaust, which had to be placed before the Thermactor air was added. Since Thermactor air was injected right into the block's exhaust ports in the M-block, there was nowhere for the O2 sensor to go.
It would have been possible to alter the M-block to work, but it would have required significant effort and cost. Ford decided to simply scrap the M-block engines and replace them with updated 351 Windsor engines at the small end, and a combination of the 6.9 L Navistar International diesel and the 460 at the top end. Sales of the M-block ended in 1982.
Read more about this topic: Ford 335 Engine
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