BEST OF BOOST

Chevy High Performance - - Front Page - TEXT: Evan Perkins | PHO­TOS: the Au­thor AND the Man­u­fac­tur­ers

10 tips to make your next en­gine build a suc­cess

10 tips to make your next en­gine build a suc­cess

Build­ing an en­gine for tur­bocharged duty is, for many, un­charted ter­ri­tory. Boost is a fa­mil­iar cor­nu­copia of man­i­fold-top­ping su­per­charg­ers, but the no­tion of hav­ing its source plumbed in­line with the ex­haust is for­eign. While the end re­sult of both—pos­i­tive in­take pres­sure and a smile-in­duc­ing horse­power surge—is the same, tur­bocharg­ing an en­gine does have some unique re­quire­ments that should be fac­tored into a build. It’s easy to think that boost af­fects parts just from the pis­tons down, but noth­ing could be fur­ther from the truth. An en­gine is an ecosys­tem, and chang­ing any one part af­fects the food chain from top to bot­tom. Here are 10 quick tips to make your next boosted build, whether small-block, big-block, LS, or LT, a fruit­ful one.

CON­NECT­ING RODS AND CRANKSHAFT

It could be said that any power-ad­der ap­pli­ca­tion could ben­e­fit from a bol­stered bot­tom end, and that would be true. But, with the power po­ten­tial of even the most cost-ef­fec­tive tur­bocharg­ers, a con­nect­ing rod up­grade is a wise move. When boost comes into the equa­tion, cylin­der pres­sures spike ex­po­nen­tially and most stock con­nect­ing rods, es­pe­cially those with high mileage, are not up to the task. Of­ten, it is not the con­nect­ing rod it­self that fails but the fas­ten­ers, which were never de­signed to deal with the cylin­der pres­sure or rpm a tur­bocharged ap­pli­ca­tion can of­fer.

For low boost ap­pli­ca­tions (6-8 psi), stock crankshafts—es­pe­cially in the LS fam­ily—have proven more than ad­e­quate. Tur­bocharg­ers are

by na­ture pro­gres­sive and typ­i­cally don’t have the power and torque spikes as­so­ci­ated with ni­trous and su­per­charg­ers. This trait spares the crankshaft shock-load­ing that could be po­ten­tially cat­a­strophic. That said, if your power goals are over

500 horse­power, or com­pe­ti­tion use is planned, in­vest­ing in a forged crankshaft for your ap­pli­ca­tion should be con­sid­ered a must.

CYLIN­DER HEADS

Boost is the great equal­izer. It can take a small port, lousy valve an­gle, and low-lift cam and blow gale-force winds into the cylin­der. While a bet­ter flow­ing cylin­der head will al­ways move more air—boost or no boost—it is im­por­tant to con­sider your power goals and bud­get. For ex­am­ple, Edel­brock’s Per­former RPM head flows a solid 253 cfm at 0.500-inch lift and is a per­for­mance bar­gain at around $730 each (as­sem­bled from Sum­mit Rac­ing). There are more ex­pen­sive, bet­ter-flow­ing heads on the mar­ket, but with a few psi of boost force­feed­ing the run­ners, bud­get heads can gen­er­ate se­ri­ous power. Rather than ul­ti­mate air­flow po­ten­tial, which is far more cru­cial in a nat­u­rally as­pi­rated ap­pli­ca­tion, con­sider price, ma­te­rial, and deck thick­ness, which is crit­i­cal to seal­ing high cylin­der pres­sure.

HEAD BOLTS, STUDS, AND MLS HEAD GASKETS

Boost is no good if you can’t keep it in the cylin­der where it be­longs. Per­for­mance head bolts, such as those from ARP, are a good start; head studs are bet­ter. Studs are not sub­jected to as much tor­sional (twist­ing) force as head bolts. Metal is ex­tremely strong in ten­sion, which al­lows head studs to gen­er­ate im­proved clamp­ing force over bolts. As boost goes up, studs be­come the pre­ferred op­tion.

Mod­ern af­ter­mar­ket head gaskets are light-years ahead of the gaskets of the mus­cle car era. But when it comes to boosted ap­pli­ca­tions there is no re­place­ment for a good MLS (multi-layer steel) piece. MLS gaskets use mul­ti­ple lay­ers of em­bossed steel to seal com­bus­tion in the cylin­der. Their de­sign, due to the spring rate of the com­pressed lay­ers, can ac­tu­ally com­pen­sate for a small amount of cylin­der head lift as com­bus­tion at­tempt­ing es­cape pushes the head away from the deck sur­face. One con­sid­er­a­tion with MLS gaskets is sur­face fin­ish. For proper seal­ing, both the block and the cylin­der head deck sur­faces must be ma­chined very smooth. Most ma­chine shops are ca­pa­ble of this, but is ab­so­lutely worth a di­a­logue with the ma­chin­ist to make sure.

ROCK­ERS, PUSHRODS, AND VALVESPRINGS

It’s easy to think of boost as only af­fect­ing the pis­tons down. In fact, the val­ve­train is equally af­fected. When the in­take valve opens and pres­sur­ized air rushes into the cylin­der, the back­side of the valve is also pres­sur­ized. As the valve be­gins to close, boost pushes against it, mak­ing the work of the valvespring more dif­fi­cult. For this rea­son, it is of­ten nec­es­sary to in­stall a stiffer valvespring ca­pa­ble of ef­fec­tively clos­ing the valve on sched­ule.

The ex­haust valve also sees ad­di­tional load. When the spark plug fires, pres­sure is cre­ated in the cylin­der, which drives the pis­ton down. But, be­fore the pis­ton reaches bot­tom dead cen­ter, the ex­haust valve opens. For ex­am­ple, a pop­u­lar LS cam grind has an ex­haust open­ing point of 83 de­grees be­fore bot­tom dead cen­ter (BBDC). That means the ex­haust valve is ac­tu­ally open­ing against com­bus­tion pres­sure, which acts on its face, at­tempt­ing to hold it closed.

That force trav­els up the valve stem, through the rocker arm, and into the pushrod. While this oc­curs in all en­gines, the higher cylin­der pres­sure as­so­ci­ated with tur­bocharged en­gines ex­erts ad­di­tional pres­sure on the val­ve­train that needs to be ac­counted for. A thicker pushrod is a good start and Billy God­bold of Comp Cams likes steel rock­ers for these ap­pli­ca­tions. Steel rocker arms are more fa­tigue re­sis­tant than alu­minum rock­ers and, for the price, are typ­i­cally stronger.

PIS­TONS AND RINGS

Cast pis­tons are the equiv­a­lent of time bombs when it comes to boost. It’s not that they don’t have the strength, as many fac­tory pis­tons are sur­pris­ingly strong, but their in­abil­ity to tol­er­ate det­o­na­tion—some­thing that will in­evitably oc­cur in an af­ter­mar­ket tur­bocharged en­gine. Late-model en­gines have knock sen­sors that are pre­ci­sion-tuned from the fac­tory. When knock/det­o­na­tion is de­tected, the ECU can re­tard tim­ing to lower cylin­der pres­sure, elim­i­nate the knock, and pro­tect the ro­tat­ing as­sem­bly. Few af­ter­mar­ket en­gines have that lux­ury. In­stead, mov­ing to a forged pis­ton, which is sig­nif­i­cantly stronger and more re­sis­tant to det­o­na­tion, should be con­sid­ered a must.

Proper pis­ton se­lec­tion is more in­volved than sim­ply low­er­ing the com­pres­sion ra­tio. A pis­ton de­signed for boost will have more ma­te­rial in key ar­eas. In boosted pis­tons, the top ring land is moved down on the pis­ton crown, which helps pro­tect it

from the heat of com­bus­tion as well as cre­ates more rigid­ity in the land it­self. Also, pis­ton ma­te­rial comes into play. Forged pis­tons are typ­i­cally made from two al­loys: 4032 and 2618. Forged 4032 pis­tons have more sil­i­con in their makeup and do not ex­pand as much as 2618 pis­tons, mak­ing them great for street en­gines, which ex­pe­ri­ence a wider tem­per­a­ture vari­ance and need to start cold. Forged 4032 pis­tons are per­fect for mid-level builds but lack the ul­ti­mate strength of­fered by 2618 pis­tons. Forged 2618 pis­tons are duc­tile and for­giv­ing in harsh, high-horse­power en­vi­ron­ments but are softer and wear quicker than 4032 units.

Pis­ton rings in turbo en­gines will need more end gap than in a com­pa­ra­ble nat­u­rally as­pi­rated en­gine. Be­cause forced in­duc­tion puts more air, and sub­se­quently more fuel, into the en­gine it will also gen­er­ate more heat, which causes the rings to ex­pand more. When choos­ing ring ma­te­rial, car­bon-steel rings, as op­posed to the gray iron of­ten found in cheaper and older, stock ring sets, is a pre­ferred op­tion. Car­bon-steel is much stronger, re­sists det­o­na­tion bet­ter, and does not need to be made as thick, which re­duces fric­tion against the cylin­der wall.

CAMSHAFT GRINDS

In a tur­bocharged en­gine, care­ful at­ten­tion to camshaft se­lec­tion can pay huge div­i­dends in power, torque, and drive­abil­ity. Be­cause pos­i­tive pres­sure in the in­take man­i­fold (boost) is force feed­ing air into the cylin­ders, a turbo cam can of­ten be very mild in com­par­i­son to a nat­u­rally as­pi­rated grind, need­ing less lift and du­ra­tion to ac­com­plish a sim­i­lar horse­power goal. Also, be­cause there is in­evitably back­pres­sure be­tween the ex­haust port and the turbo’s tur­bine wheel, care­ful at­ten­tion needs to be paid to valve over­lap. Too much over­lap for the ap­pli­ca­tion can cause ex­haust to back­flow into the cylin­der and heav­ily di­lute the air charge.

“Hon­estly, the boost to back­pres­sure is what we re­ally need to know to pick the camshaft,” said Comp Cam’s Billy God­bold. “A cam in the 270’s (de­grees du­ra­tion) at 0.050 with a 110 LSA might be right on a sys­tem with very lit­tle re­stric­tion, and very lit­tle back­pres­sure.”

Large turbo, low-back­pres­sure sys­tems found on, say, a high-

horse­power race car will be much more tol­er­ant of high-over­lap camshafts. This is why many tuners have found suc­cess with nearly stock cams in high-back­pres­sure street turbo ap­pli­ca­tions as they of­fer very wide lobe sep­a­ra­tion an­gles and very min­i­mal over­lap. High back­pres­sure may sound un­ap­peal­ing, but such a pres­sure ra­tio can be use­ful in cre­at­ing a turbo setup with ex­cel­lent throt­tle re­sponse and min­i­mal turbo lag— per­fect for a street car.

FUEL

Fu­el­ing a tur­bocharged en­gine al­ways re­quires more oc­tane than a com­pa­ra­ble nat­u­rally as­pi­rated en­gine. There is a mul­ti­tude of ways to ac­com­plish this. Pre­mium pump fuel when boost, ig­ni­tion tim­ing, and in­take air tem­per­a­ture are kept in safe ranges is the most con­ve­nient—but prob­a­bly the most power-lim­ited. E85 (ethanol­based) fuel, which is of­ten­times cheaper than gaso­line, though less read­ily avail­able, is an­other great al­ter­na­tive.

E85 has a higher la­tent heat of va­por­iza­tion than gaso­line, mean­ing it can help pull heat out of the air charge and has a 100-plus oc­tane rat­ing—though that can fluc­tu­ate slightly de­pend­ing on the mix, which is rarely 85 per­cent ethanol, 15 per­cent gaso­line as claimed.

E85 has a sto­i­chio­met­ric ra­tio of 9.75:1, which is lower than gaso­line (14.7:1) and means it will take a larger vol­ume to achieve the same horse­power level as gaso­line. E85 does have some cool­ing ben­e­fits that gaso­line does not. Also, when­ever boost is em­ployed, a vari­able-rate fuel pres­sure reg­u­la­tor will be re­quired to keep fuel pres­sure equal to boost pres­sure and avoid lean­ing out the tune as boost rises.

INTERCOOLING

Boost, whether de­liv­ered from a blower or turbo will in­evitably heat the in­take air as a byprod­uct of com­pres­sion. Hot air is less dense, which means less power, and is more prone to det­o­nat­ing. In or­der to quell the risk of det­o­na­tion and im­prove power, it is ideal to re­move the heat. This can be ac­com­plished a few ways. Wa­ter/meth in­jec­tion, such as kits sup­plied by Snow Per­for­mance, spray a fine mist of wa­ter and methanol mix into the in­take air stream. As the par­ti­cles of wa­ter

and methanol shift from liq­uid to gas (known in physics as a phase change) they ab­sorb en­ergy. This sucks heat out of the sur­round­ing air par­ti­cles and can rad­i­cally cool the in­take charge. More con­ven­tional forms of intercooling, such as air-to-air in­ter­cool­ers rely on air­flow over a bar-and-plate heat

ex­changer to pull heat away from the air charge.

Air-to-wa­ter in­ter­cool­ers are sim­i­lar to air-to-air ex­cept they em­ploy a liq­uid medium. In some cases, this is an ice bath, which is in­cred­i­bly ef­fec­tive at re­mov­ing heat but is im­prac­ti­cal for a street car due to space re­quire­ments and a con­stant need to re­plen­ish the rapidly melt­ing ice.

TIM­ING CON­SID­ER­A­TIONS

In car­bu­reted and af­ter­mar­ket fuel-in­jected ap­pli­ca­tions es­pe­cially, ig­ni­tion tim­ing is a ma­jor con­sid­er­a­tion. Dis­trib­u­tors are a great means of trans­fer­ring spark en­ergy to the cylin­der but they are quite dumb. No of­fense in­tended, but dis­trib­u­tors don’t re­ceive any feed­back from the en­gine—nor would they be equipped to deal with it if it did—and are ig­no­rant of any knock oc­cur­ring. For this rea­son, it is para­mount to have an in­tel­li­gent ig­ni­tion de­vice feed­ing a sig­nal to the dis­trib­u­tor that can de­tect boost and re­tard the ig­ni­tion ad­vance re­spec­tively. MSD’s pro­gram­mable 6AL, when cou­pled with a MAP sen­sor, does a great job of this. Most af­ter­mar­ket ECUs can ac­com­plish the same feat, and fac­tory ECUs, when paired with a MAP sen­sor ca­pa­ble of read­ing boost (2 bar and up) are also able to keep tim­ing in check.

FUEL IN­JEC­TION VS. BLOW-THROUGH CARB

This is the big-ticket item, and the one that fright­ens a lot of old-school car­bu­re­tor afi­ciona­dos: to in­ject or not to in­ject. It all comes down to con­trol. Blow-through carbs are not the black magic they once were.

They work well and have the added ben­e­fit of chem­i­cally intercooling the air charge. The low-pres­sure zone cre­ated by the ven­turi, along with the la­tent heat of va­por­iza­tion in­duced as gaso­line is at­om­ized at the top of the in­take plenum draws sig­nif­i­cant heat out of the in­take air charge. The down­side is that carbs are dumb. They of­ten don’t start well when the en­gine is cold and they aren’t par­tic­u­larly for­giv­ing of al­ti­tude and am­bi­ent tem­per­a­ture changes. Com­pa­ra­bly, fuel in­jec­tion is smart, it can adapt to chang­ing con­di­tions and al­ter fuel de­liv­ery ac­cord­ingly. In the big pic­ture, es­pe­cially when boost is in the mix, it is the bet­ter op­tion and yields su­pe­rior drive­abil­ity com­pared to even the best blow-through carbs, though it does of­ten come with a bit of a price pre­mium. CHP

The de­bate rages on be­tween the sim­plic­ity of a car­bu­re­tor and the ul­ti­mate con­trol of fuel in­jec­tion. The science and ex­pe­ri­ence is out there to make both work, and while fuel in­jec­tion of­fers tremen­dous drive­abil­ity ben­e­fits, the car­bu­re­tor’s ven­turi...

With the ex­cep­tion of en­gines run­ning ethanol, an in­ter­cooler of some type should be con­sid­ered manda­tory. While air-to-air in­ter­cool­ers (con­ven­tional bar-and plate-style) are the most com­mon. Wa­ter/ methanol in­jec­tion, such as this Snow Per­for­mance...

To­day’s driv­ers are af­forded more fuel op­tions than pre­vi­ous gen­er­a­tions. E85 is a fan­tas­tic, ethanol­based fuel that of­fers 100+ oc­tane (de­pend­ing on the blend) and ex­cel­lent cool­ing ef­fects. It does how­ever re­quire an up­sized fuel sys­tem due to the...

Tur­bocharged en­gines have spe­cific cam re­quire­ments that highly de­pend on ex­haust back­pres­sure. Over-cam­ming an en­gine and adding too much over­lap in high-back­pres­sure ap­pli­ca­tions can cause se­ri­ous ex­haust di­lu­tion prob­lems.

A durable pis­ton and rod pack­age is key to a long-last­ing turbo build.

A pis­ton de­signed for boost will over­all be thicker. No­tice how the ring land of the JE pis­ton on the right has been moved down sig­nif­i­cantly. This is to iso­late the ring from the dam­ag­ing heat of com­bus­tion. In a nat­u­rally as­pi­rated ap­pli­ca­tion (left)...

Fac­tory two-piece pushrods leave a lot to be de­sired in terms of strength. A one-piece pushrod with a formed end is sig­nif­i­cantly stronger. On the left is an OEM 5/16-inch pushrod while on the right is a 3/8-inch unit from Trend Per­for­mance. The wall...

This Comp Cams rocker arm is made from chro­moly steel. While heav­ier than an alu­minum rocker, it is much stronger and has a longer fa­tigue life, which is ben­e­fi­cial for an en­gine that will see a lot of street miles.

En­gines with boost will re­quire up­graded valvesprings. Note the spring on the left is taller and will have more pres­sure on the valve seat when in­stalled.

Boost is a game changer when it comes to head flow. For most ap­pli­ca­tions a bud­get cylin­der head, such as this Edel­brock Per­former RPM is an ex­cel­lent choice. Thick decks for strength and bet­ter cool­ing, and light­weight alu­minum make them a big up­grade...

Head studs and af­ter­mar­ket MLS gaskets are key play­ers in seal­ing boost in the cylin­der. The ex­tra clamp­ing load of studs and the re­sis­tance to head-lift of MLS gaskets is a sure­fire way to pre­vent troublesome head gasket fail­ure.

Cast com­po­nents have demon­strated an apt­ness for boost, es­pe­cially in the LS world, but that doesn’t equate to longevity. Forged con­nect­ing rods and crankshafts such as these K1 pieces should be con­sid­ered manda­tory for any boosted ap­pli­ca­tion.

Tur­bocharg­ers are now more ac­ces­si­ble than ever. No longer the clas­si­fied, mil­i­tary­grade tech­nol­ogy they once were, a plethora of af­ter­mar­ket com­pa­nies have them on the shelf ready to ship.

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