LT5 First Look!

There’s a New King of the Hill

Hot Rod - - Contents - David Kim­ble Cour­tesy of Chevro­let Il­lus­tra­tions: David­vid Kim­ble

In 1990, reg­u­lar pro­duc­tion op­tion ZR1 was a spe­cial per­for­mance pack­age in­tended to trans­form a base Corvette sport coupe into the fastest pro­duc­tion car that could be sold in the U.S., and it was nick­named “King of the Hill.” Fed­eral ex­haust emis­sion reg­u­la­tions started horse­power num­bers fall­ing in 1971, and when they bot­tomed out in 1975, the Corvette’s base 350 was left with 165 hp be­fore slowly mak­ing a come­back, reach­ing 245 hp in 1989. But in 1990, power-hun­gry buy­ers had a choice. The ZR1 pack­age in­cluded the 375hp LT5 that would reach 405 hp in 1993, all out of the same 350ci dis­place­ment. At the time, how­ever, it was thought it took four camshafts, 16 fuel in­jec­tors, and 32 valves to stay within cur­rent fed­eral emis­sion stan­dards while mak­ing this kind of power. Ar­ti­cles on the ZR1 started ap­pear­ing in 1988 (an­tic­i­pat­ing a 1989 in­tro­duc­tion), but when its launch was pushed back to 1990, in­stead of los­ing in­ter­est, power-starved horse­power junkies like us just got more ex­cited, and for 10 years this au­thor owned the 39th ZR1 ever built.

Lo­tus in Hethel, Eng­land, was part of GM at the time, and the com­pany de­signed the LT5 based on its still­born Etna 4.0L V8 ar­chi­tec­ture, with the Chevro­let-Pon­ti­acCanada (CPC) Pow­er­train divi­sion de­vel­op­ing the in­duc­tion sys­tem in War­ren, Michi­gan. CPC Pow­er­train En­gi­neer­ing was also re­think­ing its 30-plus-year-old pushrod V8 to de­ter­mine what it was go­ing to take for it to pro­duce the DOHC LT5’s 405 or more horse­power. The com­pany started by re­design­ing most of its ma­jor com­po­nents. This brought the sec­ond-gen­er­a­tion LT1 350ci V8 up to 300 hp in 1992, with an op­tional 330hp ver­sion, the LT4, com­ing out in 1996. But to make fur­ther gains re­quired an en­tirely new ar­chi­tec­ture that all later gen­er­a­tions have been built on. The 1997 Gen III 345hp LS1 shared noth­ing with the past, ex­cept 4.40-inch cylin­der bore cen­ters. The LS1’s 90-de­gree alu­minum block had cast-in iron cylin­der lin­ers that re­duced its dis­place­ment to 346 ci, with four-bolt main bear­ing caps cross-bolted through deep side skirts. The heads were also cast alu­minum with equally spaced iden­ti­cal ports topped by a com­pos­ite in­take man­i­fold flanked by in­di­vid­ual ig­ni­tion coil packs over each cylin­der on cast-alu­minum rocker-arm cov­ers.

The LS1 was in­tro­duced with the fifth-gen­er­a­tion Corvette coupe, which was fol­lowed by a con­vert­ible in 1998 and a hard­top in 1999 that be­came a ded­i­cated ex­treme-per­for­mance model in 2001, which was named af­ter the 1963 St­ing Ray’s Z06 rac­ing pack­age. Un­like the 1963 Z06, this ver­sion in­cluded a hopped-up en­gine, the LS6, that eas­ily bested the 1990 dou­ble-over­head-cam, 32-valve

LT5 by 10 hp and matched its 1993-and-later 405 hp in 2002 with 4 fewer cu­bic inches, a sin­gle camshaft, 16 valves, and a pushrod val­ve­train. The in­no­va­tive ar­chi­tec­ture of the C5 Corvette’s chas­sis and the Gen III small­block V8 were an en­tirely new be­gin­ning, and af­ter two gen­er­a­tions of up­grades and re­fine­ments, their ba­sic lay­out re­mains in pro­duc­tion to­day. When we asked Dave Hill, the Corvette en­gi­neer­ing di­rec­tor from 1992 to 2005, about the sim­i­lar­ity of the C5 and C6 Corvette chas­sis, he re­sponded, “Why start over again when you can build on suc­cess?”

A forced-in­duc­tion en­gine first ap­peared on the Corvette’s op­tion list in 1987 with the Call­away twin-turbo pack­age avail­able through Chevro­let deal­ers, but not in­stalled by GM. It boosted the L98’s out­put from

240 to 345 hp. This sys­tem was ef­fec­tive, but bulky and hard to pack­age, while Ea­ton would later de­velop a more com­pact, belt­driven, roots-type su­per­charger with finned in­ter­cooler tubes over its ro­tors in the same hous­ing. This is the type of su­per­charger cur­rently avail­able on pro­duc­tion Corvettes, but the first su­per­charged en­gines to be in­stalled on the Corvette Bowl­ing Green assem­bly line were LC3 444hp North­star V8s in the two-seater Cadil­lac XLR-V in 2006. The Corvette got its first Ea­ton blower three years later on the Gen IV 376ci LS9, rated at 638 hp for the Gen VI in­car­na­tion of the ZR1. The sev­enth-gen­er­a­tion Corvette will even­tu­ally have two su­per­charged fifth­gen­er­a­tion small-blocks in its en­gine lineup: the 650hp LT4 in­tro­duced with the 2015

Z06 and the 755hp LT5 in the forth­com­ing 2019 ZR1.

HOT ROD Fea­ture Edi­tor Bran­dan Gil­lo­gly con­cluded his ar­ti­cle about the LT4 in the Dec. 2014 is­sue with this com­ment about the 650hp Z06 Corvette: “Chevro­let had ev­ery right to call this car the ZR1, yet they didn’t. What do you think the Corvette team is work­ing on now?” At the time this ar­ti­cle was writ­ten, Jor­dan Lee, chief en­gi­neer of the small-block en­gines’ team of en­gi­neers, had been work­ing on the sec­ond com­ing of the LT5 for more than a year, and the Corvette it was go­ing into would be the next ZR1. As­sis­tant Chief En­gi­neer John

Ry­dzewski led the team that squeezed an ad­di­tional 15 per­cent—or 105 hp—out of the al­ready su­per­charged LT4, which is no small feat with an emis­sion-con­trolled en­gine of the same dis­place­ment.

What has in­creased in dis­place­ment is the LT5’s su­per­charger. It grew from the LT4’s 1.7L to a newly de­vel­oped R2650 Twin Vor­tices 2.65L su­per­charger that was largely de­signed by GM’s Small-Block Group work­ing with Ea­ton’s en­gi­neers. Scott Hal­sall has been the su­per­charger de­sign re­lease en­gi­neer since the LT5 reached the gamma level two years ago, and the blower hous­ing that mounts di­rectly to the cylin­der heads was al­most en­tirely de­signed by GM’s Global Propul­sion Sys­tems En­gi­neer­ing Cen­ter in Pon­tiac, Michi­gan. The LT5’s four-lobe com­pres­sor ro­tors are larger in di­am­e­ter and longer with a tighter 170-de­gree he­li­cal twist than the LT4’s 160-de­gree ro­tors. The LT5 pro­duces 14 pounds per square inch of boost, com­pared to the LT4’s 9.4 psi. The pas­sen­ger-side ro­tor is driven by the crank­shaft pul­ley through an 11-rib belt—three ribs more than the LT4—with a pul­ley ra­tio of 2.4:1 for 15,860 rpm com­pared to the LT4’s ra­tio of 3.1:1. A wider pair of spur gears also drive the LT5’s sec­ond ro­tor.

To sell the C7 Z06 Corvette in Eu­rope, it had to meet strin­gent reg­u­la­tions. This meant tak­ing 3 inches off the height of the LT4’s su­per­charger, which was an un­ac­cept­able com­pro­mise for the new King of the Hill, so the 2019 ZR1 won’t be avail­able in Eu­rope, and the LT5’s mas­sive su­per­charger is 2.5 inches taller than the LT4’s. The

R2650 blower is un­der a car­bon-fiber dome mounted to its hous­ing cover that pro­trudes through the ZR1’s hood and vis­i­bly moves as torque re­ac­tion rocks the LT5 on its mo­tor mounts, mak­ing this the Corvette’s first “shaker” hood.

Air is drawn into the LT5’s vo­lu­mi­nous su­per­charger hous­ing through a newly tooled, elec­tron­i­cally con­trolled 95mm throt­tle-body— Chevy’s big­gest ever. The air in­creases in tem­per­a­ture as it is com­pressed go­ing through the ro­tors. The hot, high­pres­sure air goes up into a plenum cham­ber in the top of the su­per­charger hous­ing cover and then turns down­ward into a pair of stampeda­lu­minum plate­and-fin in­ter­cool­ers that are 30-per­cent larger than the LT4’s, with coolant run­ning through them in tubes and low­er­ing the tem­per­a­ture by 140 de­grees. The in­ter­cooler’s cool­ing sys­tem is sim­i­lar to the en­gine’s, with both its heat ex­changer and the ra­di­a­tor grouped to­gether near the front of the car, with pumps cir­cu­lat­ing their coolant through them and back to their heat

sources. The LT4 has a but­ter­fly by­pass valve con­trolled by a vacuum di­aphragm through a me­chan­i­cal link­age to re­duce ex­ces­sive boost pres­sure, while the

LT5’s by­pass valve is an elec­tron­i­cally con­trolled throt­tle-body sourced from the L5P Du­ra­max tur­bod­iesel. This ap­proach more pre­cisely man­ages the amount of pres­sur­ized air that reaches the in­take ports, par­tic­u­larly at idle and low en­gine speeds. This makes for bet­ter torque man­age­ment and im­proved throt­tle re­sponse.

The 1990 LT5 had 16 in­take valves with eight small pri­mary fuel in­jec­tors spray­ing into its pri­mary in­take ports and eight larger sec­ondary in­jec­tors for its sec­ondary ports that were only avail­able when the power or “valet” key was turned on. The 2019 LT5 has only eight in­take valves, but it also has 16 fuel in­jec­tors, eight large pri­mary cylin­der in­jec­tors that it shares with the LT4, and eight smaller in­jec­tors that spray fuel into the in­take ports from the base of the su­per­charger hous­ing. The en­gine nor­mally runs on the di­rect-in­jec­tion sys­tem with the port in­jec­tors pro­vid­ing ad­di­tional fuel only when the DI sys­tem can­not keep up with de­mand. The PFI sys­tem is sup­plied by the fuel tank’s elec­tric pumps at a pres­sure of 58 psi. For fuel in­jected di­rectly into the cylin­ders, de­liv­ery pres­sure is in­creased to 2,900 psi by a high-pres­sure pump. This me­chan­i­cal pump is driven by an ex­tra lobe on the camshaft in the same lo­ca­tion as the Gen I cam’s dis­trib­u­tor drive pin­ion and was first used on the 2014 nat­u­rally as­pi­rated LT1. This pump was bored and stroked to in­crease its ca­pac­ity for the su­per­charged LT4, and now the LT5. Dustin Gard­ner is the de­sign sys­tem’s en­gi­neer for the LT5 duel fuel sys­tem and most of the in­ter­nal com­po­nents be­low it, many of which carry over from the LT4. Nev­er­the­less, there are some nec­es­sary up­grades, start­ing with the crank­shaft and tor­sional damper. The crank is forged from higher-strength steel and its damper pul­ley has an ad­di­tional groove for the 11-rib su­per­charger belt with a nodu­lar cast-iron hub and a steel in­er­tia ring. It is driven by the crank through a stronger key to han­dle the higher loads. The crank­shaft runs in new tri-metal main bear­ings to sup­port the higher loads, and the LT4-sourced forged pow­dered steel con­nect­ing rods also have new coated bear­ings to with­stand the ad­di­tional heat and pres­sure. All Gen V V8s, ex­cept the LT5 and man­ual Ca­maro ZL1 with the LT4, have valve lifters that can be de­ac­ti­vated on half of their cylin­ders by an ac­tive fuel man­age­ment sys­tem, while the lighter con­ven­tional hy­draulic roller lifters used on the other cylin­ders ac­ti­vate all 16 of the LT5’s valves.

Like the LT4, the LT5 will be as­sem­bled by hand in GM’s Per­for­mance Build Cen­ter at the Corvette assem­bly plant in Bowl­ing Green, Ken­tucky, and both en­gines will be built on the same alu­minum cylin­der block. This Gen V block is also used for the LT1 and has 4.06-inch bore cast-iron cylin­der lin­ers that, with the 3.62-inch stroke crank, add up to 376 ci. The block fea­tures nodu­lar-iron six-bolt main bear­ing

caps and has a pro­vi­sion for oil-spray pis­ton cool­ing. The forged-alu­minum pis­tons are de­signed to with­stand the pres­sure of forced in­duc­tion, and their flat crowns have a re­cessed bowl and other to­pog­ra­phy to guide the di­rectly in­jected fuel and en­cour­age it to mix with the air for com­bus­tion. The pis­ton pins float in the rod bush­ings and pin bores, which re­duces stress, and both V8s have a 10:1 com­pres­sion ra­tio, which is un­usu­ally high for su­per­charged en­gines, thanks to the pre­cise fuel con­trol of di­rect in­jec­tion.

Dry-sump lu­bri­ca­tion has been a fea­ture of ul­ti­mate-high-per­for­mance Corvette en­gines since the 2006 427ci, nat­u­rally as­pi­rated, 505hp LS7, and both su­per­charged 2019 en­gines will be equipped with a sim­i­lar but more so­phis­ti­cated dry-sump sys­tem. In this setup, oil drain­ing back af­ter lu­bri­cat­ing the en­gine is scav­enged from the pan and re­turned to a re­mote sump tank that, for the LS7, was an 8-quart cylin­dri­cal tank mounted ver­ti­cally on the pas­sen­ger side of the fire­wall. The LS9-and-later oil tanks are in the same lo­ca­tion and hold 10.5 quarts, with a gero­tor scav­enge, and on Gen-V en­gines, a vari­able dis­place­ment vane pres­sure pump driven by the nose of the crank in the same hous­ing. Jets in the crank­case spray oil on the un­der­side of the pis­tons for cool­ing and on the sur­round­ing cylin­der walls, with the oil tem­per­a­ture kept within the op­ti­mal range by the en­gine’s cool­ing sys­tem through an ex­ter­nal cooler.

En­gine oil pres­sure also con­trols a cam phaser on the front of ev­ery Gen V camshaft, which changes its an­gu­lar re­la­tion­ship to the sprocket, al­low­ing the valve tim­ing to be ad­vanced for idle and re­tarded for max­i­mum power. Large, light­weight valves are used in the forced-in­duc­tion heads with 2.13-inch ti­ta­nium in­takes and 1.59-inch hol­low steel sodium-filled ex­haust valves. These valves, look­ing at them from the front, are in­clined in­ward 12 de­grees for ex­haust and 12.5 de­grees for in­take. Viewed from the side, in­take and ex­haust valves con­verge slightly to­ward the cen­ter­line of the cylin­der, form­ing a twisted-wedge 65.47cc com­bus­tion cham­ber, and by com­par­i­son, the nat­u­rally as­pi­rated LT1’s 59.02cc cham­bers give it a higher 11.5:1 com­pres­sion ra­tio. These heads are ro­to­cast with the mold ro­tated as the molten A356 alu­minum al­loy cools (a T6 hard­ness is spec’d) and are topped by the dis­tinc­tive Gen V rocker-arm cov­ers with their ig­ni­tion coils mounted be­tween in­ter­nally baf­fled domes. An in­te­grated pos­i­tive crank­case ven­ti­la­tion sys­tem is in­cor­po­rated into these cov­ers that sep­a­rates oil and air from the crank­case va­pors. The LT5 has a cen­ter-feed ver­sion with a re­vised oil-vent sys­tem that has ad­di­tional holes for a faster oil re­turn.

Six months is the av­er­age amount of time it takes for GM’s dy­namome­ter lab­o­ra­tory to de­velop a new ver­sion of a high-per­for­mance en­gine, but lead Dyno De­vel­op­ment En­gi­neer Gary Price Jr. and his team spent nearly a year get­ting the most out of the LT5. Dyno cell D116 took most of the beat­ing as test en­gines started pro­duc­ing enough power to over­heat its ex­haust sys­tem and take in more air than the com­bus­tion air han­dling sys­tem could

pro­vide, thus low­er­ing the baro­met­ric pres­sure in the cell. With these prob­lems over­come and spe­cial ul­tra-high-flow fuel carts, LT5s were run 16 hours a day for months—oc­ca­sion­ally, around the clock. This was to com­plete par­tic­u­lar tests tun­ing and cal­i­brat­ing ev­ery sys­tem with a lot of at­ten­tion given to spark and fuel. The pro­gram wrapped up with 105 hp over the LT4.

We asked Jor­dan Lee if, with GM spend­ing bil­lions on elec­tri­fi­ca­tion and au­ton­o­mous ve­hi­cles, the LT5’s 755 hp at 6,300 rpm and 715 ft-lb of torque at 4,400 rpm was the high wa­ter­mark for GM’s in­ter­nal- com­bus­tion en­gines. We were very re­lieved to hear that de­vel­op­ment of new com­bus­tion en­gines is far from over or even wind­ing down, and that the small-block pushrod V8 has a bright fu­ture.

This is what Jor­dan Lee’s boss, Dan Ni­chol­son, vice pres­i­dent of GM’s Global Propul­sion Sys­tems, has to say about the end re­sults of the Small Block team: “The LT5’s horse­power puts Chevro­let and our small-block over the 700hp thresh­old for the first time, but just as im­por­tant, that power is very drive­able in the ZR1. Pain­stak­ing en­gine in­te­gra­tion with a dy­nam­i­cally ca­pa­ble ve­hi­cle en­ables the use of all 755 hp. The sen­sa­tion be­hind the wheel of this dual fu­elin­jected, blown small-block is some­thing hard to find else­where in a life­time.”

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