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I just got back from the SEMA show in Vegas. The dominant car there was the mustang with the 4.6 You can make 600 hp with a 4.6 and pass California smog. No ford small block, that I ever herd of will do that . And you don't need a blower on the 4.6 to make 600 HP
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Bill, there is SEMA, then there is reality. I know a lot of folks that build 4.6L motors, a lot here at Haywood Performance in Raleigh, home of the fastest 4.6L no blower mod motor Stang in the country (one runs in the 7s). These fellows indicate you are not going to get 600+ hp out of a four valve 4.6L and pass smog without blower or without simply building an all out 281 inch race motor and it won't pass any smog restrictions.
If you think about it another way, the motor is only 4.6L or 281 inches. 100 hp per liter is a pretty high bench mark in the engine building world and would yield 460hp in the 4.6L.
600 hp naturally aspirated in the 4.6L would require 130 hp per liter, a really high specific output. Or, to look at it in inches, over 2hp per inch.
In contast the Ferrari Enzo, which I personally don't care for, has a specific output of around 110 hp per liter and is consdered quite high in that respect. Ford's own race 5.0L cammer available through Ford racing is rated at more than 400hp and less than 500hp - a far cry from 600hp and it has more displacement to boot. Specific output is less than 100hp per liter.
My personal opinion is once you guys get your mod motored cars up and running you'll find them in need of some power boost to offset the additional weight on your larger GT40s. Not knocking your cars, they are cool, but the mod motor is no torque monster and I'll bet you need to resort to a blower to keep some of your customers happy with the 4.6L motors. Don't want them getting scalded off the line by a Chebby.