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NC Wayne
03-30-2006 19:54:00
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Re: detune 6-71 detroit in reply to BSer, 03-30-2006 15:48:06
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You got me curious awhile ago so I went out to the truck and got my book. According to the Detroit Field Service Data manual, a typical 4-71N (nonturbo) 2 valve industrial engine at 1800 RPM has a HP rating anywhere from 106 to 121, with all depending on the injector and the timing. With a 4 valve head the 4-71N industrial is rated around 120 at 1800 RPM, again all depending on the injector and timing. The closest I see to that range for a 6-71N, two valve head, at 1800RPM, is 160+ and if you go to a 4 valve head it jumps to 175+. All of the truck and automotive rated engines are speced at 2100 RPM only and they all have HP ratings of 240+. It doesn't actually say in so many words but I'd say all of these ratings on the NA engines are gonna be with a standard 1:1 blower drive. Based on what I can see the minumum amount of extra HP your gonna be able to achieve is 30+ over what a typical 4-71 is capable of, and that's gonna be on an engine without an overdriven blower. I'm not sure how having the overdrive would effect the engine at the slower speed other than to give a higher airbox pressure because you can't get but so much scavaging air to flow through the cylinder at the slower speed. This higher pressure in the air box would in turn raise the crankcase pressure which would then lead to more oil slobbering than normal out the draft tube. Just my opinion here, but by the time you go through all the trouble buying the parts, gasket kits, etc to change the blower drive to get it slowed down, buy 6 new injectors (in the neighborhood of $300+), modifying everything on the machine to accept the larger engine, and then dealing with any future problems caused by all the extra HP it's still gonna have, to me, it just isn't worth all the trouble and expense. You'd be better off to keep an eye on Ebay or try checking some of the government surplus sights, etc and finding a good 4-71 to go back in it. Or if you can do the work yourself it's not that expensive to rebuild the one you've got. Typically for us to pull one from the machine, tear it down, have the block and head work done, build it and put it back in would run in the $4000-$6000 range, and that's with all labor included. Take away the labor costs and it's really not that bad, especially in this case when there's the potential for future damage and future expenses from a larger engine if everything isn't "just right". Just my .02 for what it's worth...Good luck....
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