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There are many ways to build horse­power, and each has its ad­van­tages and draw­backs. Power costs money, but there’s still a smart way to do it. You can push some com­po­nents, and oth­ers not so much. As a way of ex­plor­ing, we’ve run into many folks who’ve built th­ese 6.6Ls from mild to wild, and we de­cided to share our thoughts from 500 to 1,500 hp. En­joy!

500 RWHP 6.6L Duramax

When it comes to 6.6L Duramax en­gines, there are tons of des­ig­na­tions to choose from. There are LB7S, LLYS, LMMS, LMLS, and even the new L5P. While there have been a num­ber of up­grades avail­able for th­ese pow­er­plants over the years, the ba­sic ar­chi­tec­ture has re­mained the same, which means all mod­els re­spond to tun­ing and tweaks. In fac­tory form, the ear­lier LB7 (rated at 300 fly­wheel hp) will put down about 240 to 260 rear-wheel hp,


with the new LML (397 fly­wheel hp) at around 340 to 360 hp. The LLY, LBZ, and LMM will usu­ally fall some­where in be­tween those num­bers. For­tu­nately for horse­power junkies, a sim­ple tune is all it takes to get in the 450 to 500rwhp range; we’ve seen a bone-stock LB7 hit 477 rwhp on a stingy Mus­tang dyno with just a PPE Hot+2 pro­gram­mer. Shops across the na­tion also of­fer cus­tom EFILIVE tun­ing, which can be used to switch power lev­els on the fly, or dial an ex­act power or EGT level.

While the tur­bocharg­ers of all gen­er­a­tions gen­er­ally limit power to some ex­tent, the real weak point in a 500rwhp Duramax build is the trans­mis­sion’s hold­ing power. Es­pe­cially with the ear­lier five-speed Al­li­son, the higher gears are a weak point, and of­ten slip­page when stand­ing on it in Fifth is what will throw the trans­mis­sion into limp mode. It should be noted that limp mode (when the trans locks it­self in a sin­gle gear at a high line pres­sure) is not a death sen­tence. Back­ing down the tune just slightly can keep the trans­mis­sion go­ing for years un­til a re­build can be per­formed.

750 RWHP 6.6L Duramax

It may not seem like a 250-horse­power jump would re­quire a host of add-ons, but in terms of power pro­duc­tion, vir­tu­ally ev­ery­thing has to be changed to hit that 750 mark. The stock turbo peters out well be­fore 600 hp, so a larger tur­bocharger is a must, at min­i­mum some­thing in the 68-75mm in­ducer range. The fuel side will also need an up­grade, as the fac­tory in­jec­tors and CP3 will also conk out well be­fore 750. A sin­gle mod­i­fied CP3 or CP4 pump or twin pumps are a ne­ces­sity, as is a fuel sys­tem (lift pump) and a set of 60 to 100% over in­jec­tors.

The bot­tom end of the 6.6L also needs some help when it comes to mak­ing this type of power. A per­for­mance bal­ancer is a good idea, and an al­ter­nate-fire camshaft that takes the stress off the nose of the crank can also be a worth­while ad­di­tion. LBZ en­gines have pis­tons that are prone to crack­ing, so their pis­tons should be up­graded if the en­gine is re­built. Later LMM and LML own­ers mak­ing high-rpm power can get away with stock con­nect­ing rods, but for ear­lier en­gines up­graded con­nect­ing rods are a good idea. When ex­actly the stock rods will bend is sort of like pre­dict­ing the weather, but if you’re in the 650-750hp range, know that your en­gine is liv­ing on bor­rowed time.


For this type of power, the trans­mis­sion will also need a full re­build with a flex­plate and per­for­mance torque con­verter. The clutches in­side the trans­mis­sion should also be up­graded, and a deep trans­mis­sion pan or sec­ondary cooler is also a good idea. Up­grad­ing the shafts largely de­pends on the us­age of the truck, and whether it will be drag raced or used for sled pulling. It should be noted that the cost of this type of build isn’t cheap, and while there are short­cuts to mak­ing 700-plus horse­power (think stock en­gine with lots of ni­trous), reli­a­bil­ity will cer­tainly be a con­cern.

DURAMAX 1,000 RWHP 6.6L Duramax

Hit­ting the mag­i­cal 1K mark isn’t an easy task, but it’s one that can be per­formed with rel­a­tively few up­grades from the 750hp build. Tur­bocharg­ing is an area of added ex­pense, as a turbo in the 8088mm range is manda­tory, with many folks choos­ing to go with com­pound set­ups us­ing ei­ther twin or triple tur­bocharg­ers. In­ter­cooler up­grades, piping up­grades, up­graded Y-bridges, and a se­lec­tion of cus­tom parts are usu­ally part of bring­ing a 1,000hp build to life.

The fuel sys­tem will also need some help, as 200% over (or larger) in­jec­tors should be used, along with twin CP3 pumps. If the short block is al­ready beefed up it can han­dle 1,000 hp, al­though a crank­shaft and main caps and/or a gir­dle isn’t a bad idea. A larger camshaft and ported heads can also take some of the work­load off the com­pres­sors and help them spool faster, so if you’re plan­ning on a 1,000hp build, plan on tak­ing the en­gine com­pletely apart. A trans­mis­sion that can hold 750 rwhp should still be OK at one thou­sand, but if racing is in­volved, then a se­lec­tion of af­ter­mar­ket shafts and in­ter­nal parts are some­thing that should be in­cluded in the bud­get.

DURAMAX 1,500 RWHP 6.6L or 7.1L Duramax

There aren’t many GM diesels that can break 1,500hp at the wheels in this coun­try (or any coun­try for that mat­ter), as the time, ef­fort, and mon­e­tary out­lay that it takes to make this kind of power can be daunt­ing. For the en­gine it­self, you’ll be look­ing at an all-out racing long block, com­plete with a forged and in­ter­nally bal­anced crank, se­vere-duty rods, forged pis­tons, a gir­dle, filled block, bil­let caps, main studs, and high-dol­lar, high-strength head studs. Re­cently, com­peti­tors have even gone as far as to deck­plate th­ese en­gines, al­though that has mainly been a prac­tice among pullers.

Due to the com­plex­ity of th­ese types of builds, a num­ber of shops we talked to rec­om­mended go­ing with a full race crate en­gine, one that would have a spe­cific group­ing of parts set up with cer­tain tol­er­ances. Th­ese types of en­gines can be pur­chased from com­pa­nies like Dirty Hooker Diesel, In­dus­trial In­jec­tion, Socal Diesel, and Wagler Com­pe­ti­tion Prod­ucts. Af­ter se­cur­ing all the needed op­tions, ex­pect to pay a good $30,000 or more for the long block—and that’s not count­ing turbo or fuel up­grades. Speak­ing of tur­bos, it seems that “triples” are the pow­er­ad­der of choice at this rar­i­fied level. Even tur­bos as large as the 98mm Gar­rett and Pre­ci­sion tur­bocharg­ers are pushed to their max to make this power, so many folks go with three: two into one. For this setup you’d be look­ing at two 75mm or two 80mm tur­bos into a sin­gle 80mm, and to­tal boost in the 100psi range. The in­ter­cooler is most likely a cus­tom air-to-air or air-to-wa­ter unit, and con­nec­tions must be heav­ily re­in­forced. Fu­el­ing is an­other “all-out” sce­nario and 400 to 800% over in­jec­tors, twin mod­i­fied CP3 pumps, and twin lift pumps push­ing a to­tal of 300 gph or more are stan­dard fare.

Where Does It End?

At this point, the most pow­er­ful Duramax en­gines in the coun­try (that we know of) make some­where around 2,500 to 3,000hp at the fly­wheel. At this year’s UCC, the Tripp­ple­max en­try of Wade Min­ter cranked out 1,960 hp to the rear tires with a set of huge triple tur­bos and mul­ti­ple stages of ni­trous—with a fac­tory block. The en­gine en­dured through the drags, dyno, and sled pull with­out an is­sue, prov­ing that this type of power can still be some­what reli­able.

 Sin­gle tur­bocharg­ers can be used to make vir­tu­ally any amount of power, but for street trucks, com­pound tur­bos are a great mix of per­for­mance and re­sponse. They’re also the hot ticket for EGT re­duc­tion and tow­ing.

 Mod­i­fied CP3 (or CP4 in 2011 and later trucks) pumps are needed around 550 rwhp and up. Twin pumps are also an op­tion, and for high-rpm, high-horse­power ap­pli­ca­tions, twin mod­i­fied pumps are a good idea.

 There are many ex­haust man­i­fold op­tions to re­place the re­stric­tive fac­tory man­i­folds. From 50-state-le­gal high­flow man­i­folds to full race head­ers, high-flow man­i­folds are es­pe­cially help­ful when look­ing to lower ex­haust gas tem­per­a­tures.

 Duramax con­nect­ing rods are a weak point as power rises. It’s tough to know ex­actly when the rods will bend (a lot de­pends on tun­ing) but most ex­perts agree that 700-800 hp is push­ing it. For­tu­nately, com­pa­nies like Car­illo, Wagler, and R&R make...

 Thanks to their high-revving na­ture (stock Du­ra­maxes can spin to 4,000 rpm), larger sin­gle tur­bos work quite well on th­ese 6.6Ls, even on street trucks.

 Duramax en­gines are very ef­fi­cient and they don’t need much in­let pres­sure, so they don’t need quite as much lift pump as some pow­er­plants. Still, as power rises from 700-1,000hp lev­els, a good fuel sys­tem is manda­tory to keep com­mon-rail pump in­let...

 Fac­tory Duramax tur­bos were well de­signed, but you can only push them so far. For power lev­els past about 500 rwhp, it’s time for an up­grade. Tur­bocharg­ers in the 64-66mm range are good for around 600-700 hp, with 68-75mm tur­bos be­ing more suited to...

 Later six-speed Al­li­son trans­mis­sions can han­dle power a lit­tle bet­ter than the ear­lier five-speed mod­els, but all Al­lisons need some help when it comes to han­dling big power. Prices can range from $2,000 for a trans­mis­sion kit and con­verter to...

 Duramax en­gines are an ex­cel­lent plat­form when it comes to mak­ing power, as Tony Burkhart’s Pro Stock puller makes 1,779 hp (about 1,400 rwhp) on a sin­gle 3.6-inch (91mm) tur­bocharger. The en­gine has a deck­plated block, bil­let crank, alu­minum...

 Mod­i­fied CP3 (or CP4 in 2011 and later trucks) pumps are needed around 550 rwhp and up. Twin pumps are also an op­tion, and for high-rpm, high-horse­power ap­pli­ca­tions, twin mod­i­fied pumps are a good idea.

 At 700 horse­power, trac­tion can be­come a big prob­lem, and at 1,000 hp or more, it can even be a prob­lem in four-wheel drive.

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