BEST FOR BOOST?

LS 706 vs. 317 head test

Chevy High Performance - - Contents - TEXT & PHO­TOS: Richard Hold­ener

What hap­pens when you make a post in any of the many LS per­for­mance fo­rums and ask the fol­low­ing ques­tion? What is the best fac­tory LS head to use on a turbo en­gine? The ob­vi­ous an­swer for any ques­tion re­gard­ing fac­tory cathe­dral port heads is never the stock 706 heads, and al­ways the le­gendary LS6 heads, right? I mean, the 243 (or later 799) is the go-to head for any ap­pli­ca­tion, but the 317 usu­ally isn’t far be­hind. Af­ter all, the 317 truck heads were blessed with the same ports as the 243 and 799 heads, but were sad­dled with much larger com­bus­tion cham­bers. For turbo (and blower) guys, the drop in com­pres­sion is of­ten seen as a pos­i­tive, as boost brings all that back, right? In truth, the drop in com­pres­sion can be ben­e­fi­cial for keep­ing det­o­na­tion at bay, as­sum­ing the cham­ber de­sign of the 317 heads is less prone to det­o­na­tion than the 706 heads we tested them against. That is a dif­fer­ent de­bate for a dif­fer­ent day, but right now we plan on find­ing out just how much power the drop in com­pres­sion is worth when you re­place the stock 706 heads with a set of 317s on a turbo 5.3L.

To fully demon­strate the change in power of­fered by the head swap, we ran the 5.3L test en­gine both nat­u­rally as­pi­rated and tur­bocharged with each pair of cylin­der heads.

This way we could demon­strate that the power gains of­fered in nat­u­rally as­pi­rated trim car­ried over un­der boost. The 5.3L test en­gine used to il­lus­trate this was a high-mileage LM7 yanked from a lo­cal LKQ Pick Your Part. In prepa­ra­tion for the test, the

5.3L re­ceived a cam up­grade, a fresh set of Fel-Pro MLS head gas­kets, and ARP head studs. Rather than rely on the stock truck in­take, we in­stalled an LS6 in­take and man­ual throt­tle body. Both pairs of heads were run with stock rock­ers, hard­ened pushrods

(of the same length), and long-tube headers. The fuel sup­plied by the Hol­ley 120-pound in­jec­tors and the tim­ing val­ues were con­trolled by a Hol­ley ECU. Run with the 317 heads, the mod­i­fied 5.3L pro­duced 448 hp at 6,800 rpm and 398 lb-ft of torque at 5,000 rpm. Af­ter in­stal­la­tion of the 706 heads, the power jumped to 468 hp at 6,800 rpm and 413 lb-ft of torque at 5,300 rpm. The 706 heads con­sis­tently im­proved the power out­put of the nat­u­rally as­pi­rated

5.3L by 20 hp. But how would they com­pare un­der boost?

To il­lus­trate the gains of­fered un­der boost, we set up the 5.3L to

ac­cept a sin­gle turbo sys­tem. The kit fea­tured a cus­tom Y-pipe de­signed to work with a pair of stain­less steel turbo headers. The headers fea­tured V-band clamps to con­nect to the Y-pipe cross­over tube. The cross­over tube in­cluded not only the nec­es­sary T4 turbo flange but also a pair of waste­gate flanges de­signed to ac­cept Tur­bosmart Hyper-Gate45 waste­gates. The use of two waste­gates en­sured proper boost con­trol, as our Pre­ci­sion Turbo 7675 tur­bocharger was ca­pa­ble of sup­port­ing over 1,100 hp if left

unchecked. Boost from the Pre­ci­sion turbo was chan­neled through an air-to-wa­ter in­ter­cooler sup­plied by ProCharger. Like the turbo, the in­ter­cooler was de­signed for 1,000+ hp ap­pli­ca­tions so it had no trou­ble on our 5.3L run­ning just 7 psi. The in­ter­cooler was fed 85-de­gree dyno wa­ter. Ob­vi­ously, ice wa­ter would work best, but this test wasn’t about max­i­miz­ing the com­bi­na­tion. Also part of the turbo sys­tem was a 4.0inch ex­haust sys­tem and a Race Port blow-off valve.

As with the nat­u­rally as­pi­rated test­ing, we ran the tur­bocharged 5.3L with both the 317 and 706 heads. The dis­charge tube from the in­ter­cooler was plumbed to the throt­tle body.

Run with 7-psi waste­gate springs, the 317 heads pro­duced peak num­bers of 691 hp at 6,700 rpm and 612 lb-ft of torque at 5,000 rpm. Run­ning just 7 psi, the turbo sys­tem im­proved the power out­put by 243 hp. But the real ques­tion is, would the 706 heads still make more power un­der boost? Af­ter in­stal­la­tion of the 706 heads, we got our an­swer.

Run with the same air/fuel ra­tio, tim­ing, and boost, the turbo 5.3L pro­duced (iron­i­cally enough) 706 hp

and 641 lb-ft of torque. De­spite run­ning just on the waste­gate springs, the boost with the 706 heads was down slightly (by 0.5 psi) at the power peak, but even with that dif­fer­ence in boost, the 706 head still of­fered al­most 20 hp and

nearly 30 lb-ft of torque where the boost dif­fer­en­tial was only 0.2 psi. Given that ev­ery pound of boost was worth 32.66 hp, the ex­tra 0.5 psi might be worth as much as 16 hp. Ei­ther way, the ex­tra power of­fered in nat­u­rally as­pi­rated trim trans­lated di­rectly into power un­der boost. Af­ter this test, it looks like LS guys might be dig­ging out all those old 5.3L heads! CHP

16 | Though we lim­ited boost pres­sure to just 7 psi, we ran the boost through this air-to-wa­ter in­ter­cooler from ProCharger.

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01 | The only way to find out which fac­tory heads were best for boost was to run them on the dyno, with a Pre­ci­sion turbo. (Ed note-The TEA head was used as a photo mockup only.)

06 | Rather than waste our time with the least pow­er­ful of all the fac­tory cams, we re­placed the LM7 grind with a more per­for­mance-ori­ented unit. The hy­draulic roller cam of­fered a 0.614/0.624-inch lift split, a 227/243-de­gree du­ra­tion split, and a 113-de­gree LSA.

04 | Used on the 6.0L truck ap­pli­ca­tions, the 317 heads of­fered more flow and larger valves than the 706 heads, but they also fea­tured larger com­bus­tion cham­bers (by 10 cc). This dropped the static com­pres­sion ra­tio by over 1.2 points.

02 | Though it was hardly nec­es­sary at the low boost level we were run­ning, we up­graded the high-mileage 5.3L LM7 with a set of Fel-Pro MLS head gas­kets and ARP head studs.

05 | Prior to dyno test­ing, both heads were run on the flow bench. The 317 heads flowed as much as 19 cfm more than the 706 heads, but the ex­tra flow was not enough to over­come the dif­fer­ence in com­pres­sion.

03 | The stock 706 heads are of­ten dis­carded due to the smaller valve sizes and per­ceived lack of flow, but this test shows they worked well on a turbo 5.3L.

07 | Both heads were topped off with this fac­tory LS6 in­take and man­ual throt­tle body.

11 | First up on the dyno in nat­u­rally as­pi­rated trim were the 317 heads bolted to the 5.3L. Af­ter tun­ing, the mod­i­fied 5.3L pro­duced 448 hp at 6,800 rpm and 398 lb-ft of torque at 5,000 rpm.

09 | Con­trol­ling the tim­ing and fuel was crit­i­cal on the turbo ap­pli­ca­tion so we stepped up to this Hol­ley HP ECU.

08 | To en­sure ad­e­quate fuel de­liv­ery un­der boost, we in­stalled a set of 120pound Hol­ley in­jec­tors. Hol­ley also sup­plied this 2-bar MAP sen­sor.

10 | To work with the cam up­grade, each of the fac­tory heads re­ceived a valvespring up­grade from Brian Too­ley Rac­ing.

12 | Af­ter test­ing, we per­formed a head swap to re­place the 317 heads with the 706 heads.

13 | Af­ter in­stal­la­tion of the 706 heads, the power out­put of the mod­i­fied 5.3L jumped to 468 hp at 6,800 rpm and 413 lb-ft of torque at 5,300 rpm.

14 | Af­ter the NA test­ing, it was time to du­pli­cate the com­par­i­son un­der boost. In this case, boost was sup­plied by a sin­gle Pre­ci­sion Turbo 7675 tur­bocharger.

15 | Boost was con­trolled by a pair of Tur­bosmart Hyper-Gate45 waste­gates.

19 | Af­ter in­stal­la­tion of the 706 heads, the peak num­bers un­der boost jumped to 706 hp and 641 lb-ft of torque. The gains of­fered in nat­u­rally as­pi­rated trim con­tin­ued and were slightly mul­ti­plied un­der boost.

17 The turbo sys­tem con­sisted of a pair of stain­less headers feed­ing a cus­tom (3-inch) Y-pipe. The cross­over pipe fea­tured a trio of flanges, one for the T4 turbo and a pair for the dual waste­gates.

18 | Run on the dyno with the 317 heads and sin­gle turbo, the 5.3L pro­duced 691 hp and 612 lb-ft of torque.

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