Why Are Most Street En­gines Lim­ited to 6,000 RPM?

Hot Rod - - Pitstop - Mar­lan Davis Wes Al­li­son and Mar­lan Davis

Q:

Why is it that the up­per rpm limit on most street en­gines seems to be in the 6,000-rpm range? Why isn’t it 5,000 or 7,000, or even 9,000 like a NASCAR mo­tor? Is it ma­te­rial strength, air­flow due to the 14.7 psi of am­bi­ent air, or some­thing else?

A:

For typ­i­cal over­head-valve (OHV), gaso­line-fu­eled, street en­gines used in cars and light trucks, a 5,500- to 6,000-rpm red­line seems to be the tar­get speed for de­liv­er­ing a sat­is­fac­tory dai­ly­driv­ing ex­pe­ri­ence at a cost the av­er­age con­sumer can af­ford. Gen­er­ally, a typ­i­cal street en­gine has about a 1,500- to 1,800-rpm power­band—de­fined here as the rpm range of sep­a­ra­tion be­tween the torque peak and the power peak. With the usual street gear­ing, this yields a ve­hi­cle well suited for daily driv­ing. Re­ally heavy, diesel-pow­ered trucks and farm equip­ment may lower rpm to max­i­mize down­stairs grunt, of­ten de­vel­op­ing max­i­mum torque be­low 2,000 rpm and peak power be­low 4,000 rpm.

The point to re­mem­ber is that rpm is not an end unto it­self; it is a means to ac­com­plish­ing an end. For any en­gine combo—be it race or street—the goal is to make power and torque in an rpm range con­ducive to ac­com­plish­ing the task at hand: plough­ing a field, spin­ning an air­foil (aka “an air­plane pro­pel­ler”), com­mut­ing to work, or go­ing 200 mph for 500 miles at Tal­ladega.

Com­pare the needs of a typ­i­cal street car ver­sus a race car. On the street, en­gi­neers have to be con­cerned about idle qual­ity and low-rpm, part­throt­tle drive­abil­ity. A street-driven car spends a rel­a­tively low per­cent­age of its duty cy­cle at full throt­tle and max rpm, but lots of time at low-rpm, part-throt­tle cruise. From a masspro­duc­tion stand­point, the com­po­nents must be rea­son­ably priced, last for tens of thou­sands (if not hun­dreds of thou­sands) of miles with min­i­mal main­te­nance, have de­cent idle qual­ity, min­i­mize vi­bra­tion and noise, and (on a late model) meet fuel mileage and emis­sions stan­dards. The cam and heads are sized to re­flect this, de­liv­er­ing good low- and midrange per­for­mance, but at high rpm, the head ports be­come sat­u­rated and the cam runs out of breath.

In rac­ing, the goal is to make the most power pos­si­ble within the sanc­tion­ing body’s rules con­straints. A race car spends lit­tle time at low rpm or part throt­tle (ex­cept un­der a cau­tion flag). Par­tic­u­larly on a nor­mally as­pi­rated mo­tor, you gain power pri­mar­ily by in­creas­ing cylin­der-head air­flow and in­stalling ever-larger camshafts. But there’s no free lunch: The trade-off is a big cam and high-flow heads shift the torque and power peaks to a higher rpm, in­vari­ably move the two peaks closer to each other, and trade low-rpm per­for­mance to get there. The en­gine is pretty much

DOA un­der 4,000 rpm; in ex­treme cases, the torque curve may even still be ris­ing to­ward peak above the 5,252-rpm torque/horse­power cross­over point. Idle qual­ity? Fuhget­taboutit. Part-throt­tle tip-in? Yuh­got­tabekid­din’.

Parts qual­ity and dura­bil­ity must in­crease com­men­su­rate with any rise in rpm. Af­ter all, the en­gine must last the race! Parts ca­pa­ble of sur­viv­ing 9,000 rpm for 500 miles are su­per­ex­pen­sive and beyond the bud­get of even many Sports­man rac­ers. In fact, the tech­nol­ogy that per­mits run­ning 9,000 rpm over an ex­tended time with a tra­di­tional Amer­i­can OHV, pushrod en­gine is a rel­a­tively re­cent devel­op­ment, but it’s one rea­son power keeps go­ing up even as the sanc­tion­ing bod­ies try to re­strict per­for­mance with ever-tighter rules.

If the tech­nol­ogy ex­ists to al­low an en­gine to live at such a strato­spheric rpm, then cam and cylin­der-head de­sign­ers can up their games ac­cord­ingly. It’s got­ten to the point where even higher-than-9,000 rpm is pos­si­ble for an ex­tended time if cost is not a fac­tor, but a new trend is ban­ning cer­tain parts made from ex­otic ma­te­ri­als (like ti­ta­nium rods) in an ef­fort to keep rac­ing af­ford­able. To a large ex­tent, tech­nol­ogy is no longer the lim­it­ing fac­tor, it’s the cost of that tech­nol­ogy! The ar­gu­ment for rules re­stric­tions is that if costs were not reigned in, at some point there’d be only a few teams left to com­pete.

Noth­ing is harder on the ro­tat­ing assem­bly than high rpm. If you can make the power and torque needed to ac­com­plish your per­for­mance goal at a lower rpm range, the en­gine will last longer and be more durable. In the nor­mally as­pi­rated world, there’s no re­place­ment for dis­place­ment, which is why big-blocks (where they’re al­lowed) are still fa­vored by many be­cause they can make the power and torque needed at a lower rpm. Size mat­ters.

As to your ref­er­ence to am­bi­ent air pres­sure, it’s only at 14.7 psi un­der the­o­ret­i­cal “per­fect-day” con­di­tions: sea level, 60 de­grees Fahren­heit, 29.92 in-Hg baro­met­ric pres­sure, and 0-per­cent hu­mid­ity. These pa­ram­e­ters are cod­i­fied by SAE J607, the cor­rec­tion fac­tor used by most hot rod en­gine dynos, but such a per­fect day rarely (if ever) ex­ists in the real world. But re­gard­less of den­sity al­ti­tude, the pre­ced­ing com­ments ap­ply, with the ad­di­tional caveat that rac­ing en­gines can be tuned to a ra­zor’s edge for the track and at­mo­spheric con­di­tions en­coun­tered on a par­tic­u­lar race day, whereas a street en­gine must be able to op­er­ate ac­cept­ably un­der a wide range of at­mo­spheric con­di­tions. Even with mod­ern cli­mate-con­trolled test cells that can sim­u­late just about any sort of cli­mate, one of the stages of OE new-car devel­op­ment is to wring

out pro­to­types in the sum­mer and win­ter, from the high Rock­ies of Colorado to Cal­i­for­nia’s be­low-sea-level Death Val­ley. One of the rea­sons to­day’s cars run so well com­pared to the clas­sic cars of yes­ter­year are their elec­tronic en­gine-man­age­ment sys­tems that self-tune for vary­ing cli­matic con­di­tions.

I ex­pect as govern­ment fuel-econ­omy man­dates con­tinue to in­crease, we will see across-the-board use of cylin­der de­ac­ti­va­tion as well as re­duced to­tal dis­place­ment. The torque and power-peak rpm points may be raised as well to main­tain per­for­mance. When the higher-rpm, smaller en­gines inevitably be­come peakier with nar­rower power­bands, en­gi­neers will com­pen­sate with so­phis­ti­cated staged tur­bocharg­ers to pre­vent per­for­mance from de­clin­ing as it once did dur­ing the dis­mal 1970s. An­other way to com­pen­sate is by

in­creas­ing the num­ber of trans­mis­sion gears or even by per­fect­ing the con­tin­u­ally vari­able trans­mis­sion. Al­ready, we see 10-speed au­to­mat­ics ap­pear­ing on some GM of­fer­ings. Which brings us around to: It’s the to­tal com­bi­na­tion, not just the en­gine combo in iso­la­tion, but that’s a dis­cus­sion for an­other time!

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