Four-Cam Con­quest

Road­ster Shop and Mer­cury Rac­ing team up for an ex­otic, 750hp DOHC LS-based crate en­gine

Street Rodder - - Contents - By Peter Mur­phy Photography by Steven Rupp & Mer­cury Rac­ing ■ ■

Road­ster Shop and Mer­cury Rac­ing team up for an ex­otic, 750hp DOHC LS-based crate en­gine

More than 70 years ago, Zora Arkus-Dun­tov and his brother, Yura, de­vel­oped the hemi­spher­i­cal over­head­valve cylin­der head for the Ford Flat­head. Known as the ARDUN head, it rad­i­cally al­tered the per­for­mance ca­pa­bil­ity of the en­gine, of­fer­ing a dra­matic in­crease in horse­power that was due not only to greater air­flow, but an in­crease in the ef­fec­tive en­gine speed, which al­lowed it to make more power at high rpm.

It’s that same quest for high-rpm horse­power that pushed Mer­cury Rac­ing (a divi­sion of Mer­cury Ma­rine) to de­velop the SB4 7.0 en­gine; an au­to­mo­tive crate en­gine that, in a nut­shell, uses a GM LS foun­da­tion (the LS7 cylin­der block) with an all-new, cus­tom-de­signed set of four-valve, dual over­head camshaft cylin­der heads.

The re­sult is a nat­u­rally as­pi­rated LS-based en­gine that spins to 8,000 rpm and pro­duces 750 hp. Chevro­let

re­lies on a su­per­charger to get the same out­put for the LT5 en­gine in the Corvette ZR1. (Mer­cury Ma­rine was tasked with as­sem­bling the first ZR1, a DOHC V-8, in the early ’90s.)

“This en­gine rep­re­sents the best of ev­ery­thing in a hot rod crate en­gine,”

Phil Ger­ber says, owner of Road­ster Shop, the ex­clu­sive re­tailer for the SB4 7.0. “It takes LS-based per­for­mance to the next level, with a fa­mil­iar foun­da­tion that is al­ready sup­ported in the street rod­ding world when it comes to en­gine mounts, trans­mis­sions, and the like. It also of­fers dis­tinc­tive ap­pear­ance that’s un­like any­thing else.”

It does, in­deed. The ex­tra-wide cam cov­ers lend the en­gine a de­cid­edly high-tech, ex­otic look that’s com­ple­mented by a cus­tom in­take man­i­fold fea­tur­ing a pair of throt­tle bod­ies.

That Mer­cury Ma­rine de­vel­oped the heads should come as no sur­prise. Ma­rine en­gines spend much more of their time at high rpm than au­to­mo­tive en­gines, so it was only nat­u­ral that the com­pany would de­velop heads de­signed to live at and pro­duce big power at high en­gine speeds. In fact, the com­pany pro­duces a num­ber of DOHC en­gines, in­clud­ing a mon­ster 9.0L ver­sion.

The unique cylin­der heads, which are cast by Edel­brock, fea­ture four valves per cylin­der and a ro­bust, high-rpm val­ve­train. While not unique to DOHC de­signs, the heads are some­what unique to per­for­mance V-8 en­gines be­cause they em­ploy in­ter­nal cam-to­cam drive via straight-cut gears. One gear in each head is a scis­sors gear de­sign, which al­lows for zero ef­fec­tive back­lash and very quiet op­er­a­tion. It was de­signed that way to keep the heads—and the en­gine—as nar­row as pos­si­ble. With­out this de­sign fea­ture, on a nar­row head, the twin cam drive pul­leys would be con­sid­er­ably smaller so as not to touch each other. That would, in turn, lead to higher belt loads be­cause of the re­duced driv­ing ra­dius.

It’s a unique de­sign, for sure, and ef­fec­tive. Think of it this way: If it’s good enough for a power­boat, turn­ing 6,500 rpm or more across the lake, it will be more than durable enough for that high­way drive to the Street Rod Na­tion­als.

And, man, this en­gine likes to rev. From 2,500 to 6,000 rpm, the power ramps up on a 45-de­gree an­gle, cross­ing the 400hp mark by 4,300 rpm and 500 hp by 5,100 rpm. After 6,000 rpm, the power curve bub­bles up and races to 700 hp by 6,500 rpm and fi­nally tops out at 750 horses at a stel­lar 8,000 rpm.

It would be nat­u­ral, too, to as­sume all that high-rev horse­power comes at the ex­pense of torque. It’s true, the en­gine isn’t a big-block torque mon­ster, but it tops out around 570 lb-ft at

6,000 rpm, so it’s no slouch in the grunt depart­ment. Be­sides, how many pound-feet to you re­ally need to squirt your hot rod off the line?

As for Road­ster Shop’s in­volve­ment,

the com­pany part­nered with Mer­cury Rac­ing to bring the en­gine from the pro­duc­tion line to the mar­ket.

“We have made it our busi­ness to pro­vide cus­tomers with the lat­est in de­sign, chas­sis, and per­for­mance, all pack­aged to save time and money while help­ing en­sure trou­ble-free en­joy­ment,” Ger­ber says. “The SB4 7.0 of­fers a unique op­por­tu­nity for us to de­liver one of the in­dus­try’s most unique en­gines in a turnkey pack­age. We’ve al­ready used it in sev­eral of our ve­hi­cle builds and the re­sults have been spec­tac­u­lar.”

In­evitably, the ques­tion “Will it fit?” arises; and the an­swer is a de­fin­i­tive prob­a­bly. With those DOHC heads, ■ The LS7 block makes an ex­cel­lent foun­da­tion for the en­gine, as it fea­tures strong steel main caps and large 4.125-inch bores. Other 6.2L GM blocks fea­ture smaller, 4.065-inch bores. The LS7 block is also deck­plate-honed from the fac­tory for more pre­cise cylin­der seal­ing when the heads are clamped in place.

the SB4 7.0 is wide, no doubt about it. In fact, it stretches right around 30.3 inches (769 mm) at the front, mea­sured across the broad tim­ing cover, and 29 inches at the rear from head to thread. That’s roughly 6 inches wider than a con­ven­tional LS en­gine and about 10 inches wider than an old-school small­block Chevy. It’s also just about the same width as an early Hemi en­gine, so if you can make room for a 331, you can make room for this en­gine.

So, yes, it’s wide and not ex­actly in­ex­pen­sive, but the prospect of 750 nat­u­rally as­pi­rated horse­power with DOHC and an ex­haust note that will send a shiver up your spine are fac­tors that are dif­fi­cult to quan­tify. You could also build an ARDUN-headed Flat­head and top it with a blower for not much less, with out­put of around 400 horses.

When it comes to one-of-kind hot rod pow­er­plants, it is hard to get more ex­otic or pow­er­ful than this one.

■ The SB4 7.0 uses the Chevro­let LS7 cylin­der block for its foun­da­tion. Note the shaft in the camshaft po­si­tion here. It takes the full-length po­si­tion of the orig­i­nal camshaft to en­sure proper oil­ing through­out the en­gine but has no lobes. In­stead, it will mount gears to drive the camshafts.1



■ An in­ter­nally bal­anced, forged steel crank­shaft with a 4.000-inch stroke com­ple­ments the block’s 4.125-inch bores to give the en­gine its 7.0L (428ci) dis­place­ment.

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