Winds of Change

Dif­fer­ent Types of Forced In­duc­tion Ex­plained

Street Rodder - - Contents - By Evan Perkins Pho­tog­ra­phy by the Au­thor, Chuck Vranas, and the Man­u­fac­tur­ers

Dif­fer­ent types of forced

in­duc­tion ex­plained

There was a time when adding forced in­duc­tion to your en­gine meant a su­per­charger and noth­ing else. Even then, it wasn’t a pur­pose­built, high-per­for­mance unit, but a re-pur­posed scav­eng­ing pump bor­rowed from a GMC two-stroke diesel. Those days are now a half-cen­tury in the rearview mir­ror. To­day, when it comes to boost, you have op­tions. Be­low are the most pop­u­lar meth­ods to add un­nat­u­ral as­pi­ra­tion to your en­gine of choice as well as some of their unique ben­e­fits.


The old­est form of forced in­duc­tion, superchargers (also known as blow­ers) have come a long way in terms of de­sign, ef­fi­ciency, and ease of in­stal­la­tion. Su­per­charger is a fairly generic term that en­com­passes many dif­fer­ent sub-gen­res of boost builders. The uni­fy­ing char­ac­ter­is­tic be­tween all vari­a­tions of superchargers–or at least what the word has evolved to mean–is a me­chan­i­cally driven com­pres­sor.

This is ac­com­plished via a drive-belt and sys­tem of pul­leys 90 per­cent of the time, but gears and chain drives have been used his­tor­i­cally. Be­low are the most com­mon styles of superchargers in pro­duc­tion.

• Roots Type

Eas­ily the old­est and most tra­di­tional style of su­per­charger, roots-type set­ups are light-years ahead of their an­ces­tors. Their ba­sic ar­chi­tec­ture con­sists of an alu­minum hous­ing, a case, in­ter­nal drive gears, and two alu­minum ro­tors. This style is a pos­i­tive-dis­place­ment su­per­charger, mean­ing they move a fixed amount of air per rev­o­lu­tion, re­gard­less of how fast they are spun.

Think of them like a wheel­bar­row that can carry a spe­cific num­ber of bricks per trip. It doesn’t mat­ter how fast you push it, each trip it car­ries the same num­ber of bricks. Keep­ing with the ma­sonry anal­ogy, as the su­per­charger spins, it grabs bricks of air from the at­mos­phere (the size of which is dic­tated by the su­per­charger’s dis­place­ment) and stacks them in the in­take man­i­fold. Even­tu­ally, a point is reached where there is a higher pres­sure of air in the in­take man­i­fold than at­mo­spheric con­di­tions. This is known as boost. The amount of boost can then be in­flu­enced by spin­ning the blower faster or slower (more or less rev­o­lu­tions), which equals more bricks of air in the in­take.

One of the big­gest ben­e­fits of roots blow­ers is that they tend to have com­pa­ra­bly large dis­place­ments and can make in­stant boost at very low rpm. This makes them ex­cel­lent

at pro­vid­ing im­me­di­ate throt­tle re­sponse, ex­cep­tional torque, and a wide power­band. They do, how­ever, pro­duce a con­sid­er­able amount of heat in the in­com­ing air charge, mak­ing higher-oc­tane fuel and low­ered en­gine com­pres­sion ra­tio nearly manda­tory.

• Twin Screw

Twin-screw superchargers, which date back to the ’30s, use two ro­tors spun by the en­gine to move a set vol­ume of air per rev­o­lu­tion. If that sounds a lot like a roots blower, that’s be­cause it is. How­ever, un­like a roots blower, which has mir­ror im­age ro­tors, a twin-screw de­sign (also called a Lysholm su­per­charger) uti­lizes one male ro­tor and one fe­male ro­tor.

Much like a catch-and-re­lease fish­er­man, a roots blower snags air, reels it into the blower hous­ing, and re­leases it into the in­take man­i­fold. In com­par­i­son, a twin-screw blower pulls in air, com­presses it in­ter­nally, and ex­pels the al­ready-com­pressed air into the in­take man­i­fold. This is pos­si­ble be­cause the lobes of a twin-screw blower are slightly ta­pered and the vol­ume be­tween them shrinks from back-to-front as they ro­tate.

This “in­ter­nal pres­sure ra­tio” helps the twin-screw blower more ef­fi­ciently pres­sur­ize the in­take, im­proves ther­mal ef­fi­ciency, and makes for a very power-dense su­per­charger for a given dis­place­ment.

Mod­ern twin-screw blow­ers also con­tain an in­ter­nal by­pass valve (as

do some mod­ern roots blow­ers), which can open un­der low-load con­di­tions, al­low­ing air to by­pass the su­per­charger as­sem­bly and keep the en­gine out of boost when it’s not de­sired. This is ex­tremely ben­e­fi­cial for daily driven ve­hi­cles that still need ac­cept­able fuel econ­omy.

• Cen­trifu­gal

Of­ten con­fused with tur­bocharg­ers, cen­trifu­gal superchargers are any­thing but. The ba­sic anatomy of a cen­trifu­gal su­per­charger in­cludes hous­ing, an im­peller, and a sys­tem of in­ter­nal drive gears. This style of blower is not a pos­i­tive dis­place­ment style, mean­ing it re­quires rpm, and a lot of it, to pro­duce boost. A com­mon mis­con­cep­tion is that an en­gine needs to be revved ex­tremely high to make power with this type of blower. While that might have been true in very early set­ups, to­day’s cen­trifu­gal superchargers con­tain in­ter­nal trans­mis­sions, which en­able

them to spin at sig­nif­i­cant mul­ti­ples of en­gine rpm.

It’s not un­com­mon for a cen­trifu­gal su­per­charger to have an in­ter­nal trans­mis­sion ra­tio of 4.00:1 or larger. For ex­am­ple: if the en­gine were spin­ning at 4,000 rpm, the su­per­charger would be clock­ing at 16,000 rpm. And, while that may sound ex­ces­sive, mod­ern cen­trifu­gal superchargers are made with the cor­rect materials and en­gi­neer­ing fi­nesse to tol­er­ate such speeds.

While this style of blower can­not de­liver the im­me­di­ate boost of a much larger, pos­i­tive dis­place­ment setup, its strong suit is ex­treme ef­fi­ciency. Cen­trifu­gal blow­ers re­quire very min­i­mal en­gine power to drive them and also in­tro­duce min­i­mal heat into the air charge. They are also more mod­u­lar and adapt­able and can be de­signed

to snake around ob­sta­cles in an en­gine bay. Fur­ther­more, they can be plumbed into an air-to-air-in­ter­cooler, a high-flow finned heat ex­changer, which fur­ther cools the in­take charge; an im­pos­si­bil­ity with a roots blower.

Cen­trifu­gal blow­ers have en­joyed tremen­dous en­dorse­ment in the mod­ern drag-ra­dial rac­ing scene and with late-model mus­cle cars.

• Tur­bocharg­ers

With main­stream us­ing dat­ing back to World War II, tur­bocharg­ers are cer­tainly not new to the party. How­ever, to­day’s de­clas­si­fied units are more com­pact, durable, and af­ford­able than ever be­fore. The main dif­fer­ence be­tween a turbo and su­per­charger is where they de­rive ro­ta­tional en­ergy from. Un­like the afore­men­tioned superchargers, which all rely on a me­chan­i­cal link to the en­gine to spin them, a tur­bocharger re­cy­cles lost en­ergy from the en­gine’s ex­haust stream. As ex­haust ex­its the en­gine it’s fun­neled into the tur­bocharger’s tur­bine wheel, spin­ning it at rev­o­lu­tions that ex­ceed 100,000 rpm. The tur­bine wheel, via a com­mon shaft spins a com­pres­sor wheel, which com­presses air and di­rects it into the en­gine.

Be­cause it uses ex­haust en­ergy that is oth­er­wise wasted, tur­bocharg­ers are ex­cep­tion­ally ef­fi­cient at mak­ing horse­power. Per unit of boost pres­sure, they can usu­ally out­per­form a su­per­charger but at the trade-off of a far more com­plex in­stal­la­tion.

A tur­bocharged setup re­quires sig­nif­i­cantly more pip­ing and re­quires a sub-sys­tem to con­trol boost pres­sure. This con­sists of a waste­gate and blow-off valve. The waste­gate is plumbed in­line with the ex­haust that feeds the turbo. In essence, it’s

a di­verter valve that, via an in­ter­nal plunger and spring, can vent ex­haust around the turbo at spe­cific boost lev­els. When the waste­gate opens, the pres­sure spin­ning the turbo lessens and the turbo be­gins to spin slower, reg­u­lat­ing boost.

On the out­let of the turbo lives the blow-off valve. Be­cause the turbo is driven off ex­haust pres­sure and not di­rectly con­nected to the en­gine, it is less di­rectly re­ac­tive to throt­tle in­puts. For ex­am­ple, when the driver lets off the throt­tle, there’s a mo­men­tary pe­riod where the turbo pro­duces un­wanted boost. As the throt­tle shuts, this boost has nowhere to go and poses a risk of dam­age to the frag­ile com­pres­sor wheel. To coun­ter­act this, the blow-off valve opens and vents this ex­tra boost to the at­mos­phere.

The last, must-men­tion as­pect of tur­bocharg­ers is the con­cept of “lag.” Be­cause turbos are driven off of ex­haust sys­tem pres­sure, they re­quire a suf­fi­cient vol­ume of ex­haust flow to spool/spin them to the point that they be­gin to pro­duce boost. The lull be­tween no boost and boost is dubbed lag. While a tur­bocharger will never be as re­spon­sive as a me­chan­i­cally driven su­per­charger, it’s im­por­tant to note that a prop­erly sized turbo can still be very re­spon­sive.

■ A su­per­charger can make all the state­ment in the world or nearly none at all.This TorqS­torm su­per­charger on Clas­sic Trucks Edi­tor Ryan Man­son’s 265ci Chevy is hid­ing in plain sight. Be­neath the stealthy air cleaner beats a mod­ern 383ci heart with fuel in­jec­tion and, of course, boost.

■ The most tra­di­tional blower is the roots style. This Weiand 8-71 re­quires a ded­i­cated in­take man­i­fold but oth­er­wise is an easy in­stal­la­tion into most hot rods with a few brack­ets and ex­ter­nal belts.

■ Roots blow­ers are of­ten the most cost­ef­fec­tive so­lu­tions, es­pe­cially for car­bu­reted en­gines. This Weiand 142 blower has been around for decades and is still an ef­fec­tive boost maker–es­pe­cially on a tra­di­tional small­block Chevy.

■ Mod­ern roots blow­ers (left) uti­lize lobes with a slight twist (helix) twin-screw blow­ers have one fe­male ro­tor and one male ro­tor. They also ta­per so that the air is com­pressed as it trav­els, front to back, along the ro­tor face.

Cen­trifu­gal superchargers, such as these from Vortech and ProCharger use a geardriven im­peller to rapidly ac­cel­er­ate air into the in­take man­i­fold cre­at­ing boost. They are very ther­mally ef­fi­cient and ex­tremely uni­ver­sal, with only a set of unique brack­ets and pip­ing needed for each en­gine ap­pli­ca­tion.

■ Tur­bocharg­ers can typ­i­cally make the most power per unit of boost be­cause they do not re­quire any drive en­ergy from the en­gine. In­stead, they scav­enge lost en­ergy from the ex­haust sys­tem.

■ This ProCharger cut­away shows the in­ter­nal trans­mis­sion, which is needed to mul­ti­ply en­gine rpm to the high-rpm cen­trifu­gal superchargers re­quired to make boost.

■ An air-to-air in­ter­cooler such as this helps pres­sur­ized air “cool down” on its way into the en­gine. Cooler air is denser and con­tains more oxy­gen per unit, mean­ing more fuel can be added to max­i­mize horse­power.

■ A tur­bocharger uses ex­haust pres­sure to spin a tur­bine, which, via a com­mon shaft, spins a com­pres­sor wheel. The com­pres­sor wheel pipes pres­sur­ized air into the in­take man­i­fold cre­at­ing boost.

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