Vari­able Valve Tim­ing is an idea whose time has ar­rived

Chevy High Performance - - Con­tents - TEXT: Jeff Smith | PHO­TOS: Jeff Smith, Gen­eral Mo­tors, GM Me­dia Archive

Vari­able Valve Tim­ing is an idea whose time has ar­rived

Decades ago (OK, half a cen­tury or more), there was a prod­uct ad­ver­tised in many of the per­for­mance mag­a­zines called the Vari­cam. The idea was a set of springs at­tached to a cen­trifu­gal de­vice that would re­tard the cam with rpm. The idea was in­no­va­tive and right on tar­get, but the execution left much to be de­sired, and even­tu­ally it dis­ap­peared.

Now let’s fast for­ward to the 21st cen­tury. We’re not pi­lot­ing fly­ing cars, but we do have VVT (vari­able valve tim­ing). Start­ing with the Gen IV LS en­gines, GM engi­neers fig­ured out a way to direct en­gine oil pres­sure to swing the camshaft through an enor­mous range of move­ment. The early Gen IV en­gines could move the cam over a max­i­mum po­ten­tial swing of 62 crank­shaft de­grees. This is an idea whose time has come, but like most in­no­va­tions, it has been slow to be ac­cepted by the per­for­mance in­dus­try.

We thought it might be a good idea to do a quick drive-by, high­light­ing the ad­van­tages of VVT in the Gen IV and V en­gines. What we will re­veal is how well it works and why it’s best to take ad­van­tage of the power op­por­tu­ni­ties of­fered by a VVT-equipped en­gine rather than just per­form a VVT-delete be­fore you know what you’re dump­ing.

Be­fore we get into ex­actly how VVT works, we thought we should ex­plain why VVT is such a good idea for a num­ber of ex­cel­lent rea­sons. We sought out Ja­son Haines, who un­til re­cently was a devel­op­ment en­gi­neer at Lin­gen­fel­ter Per­for­mance En­gi­neer­ing (LPE) and has now started his own con­sult­ing firm—Prod­uct and Ser­vice So­lu­tions. Ja­son has a wealth of ex­pe­ri­ence with GM’s VVT and of­fered some in­ter­est­ing points.

Vari­able valve tim­ing of­fers some won­der­ful op­por­tu­ni­ties for the en­gine de­signer and builder. With a sin­gle camshaft/pushrod en­gine, mov­ing the camshaft si­mul­ta­ne­ously al­ters all four of the valve events: in­take open­ing (IO), in­take clos­ing (IC), ex­haust open­ing (EO), and ex­haust clos­ing (EC). While all four are essen­tial, the most im­por­tant valve event of the four is IC. This de­ter­mines cylin­der fill­ing through­out the en­gine’s rpm band.

If all we had to do was make power within a very nar­row 1,200-rpm band—as with a five-speed man­ual trans drag race com­bi­na­tion or a cabin cruiser boat en­gine that op­er­ates 98

per­cent of the time at a set cruis­ing rpm—then mov­ing the camshaft around is of lim­ited ben­e­fit. But street en­gines are ex­pected to de­liver ex­cel­lent power and throt­tle man­ners be­tween idle and 7,000 rpm. This is where VVT re­ally shines.

VVT is all about ad­vanc­ing or re­tard­ing the camshaft. To make this sim­ple, we can dis­cuss this us­ing the in­take cen­ter­line as our ref­er­ence point. Let’s use a mild street hy­draulic roller cam as an ex­am­ple. We will dis­cuss this in re­la­tion to the cam’s in­take clos­ing point as that’s what af­fects both power and, to a cer­tain ex­tent, street man­ners.

Ad­vanc­ing or re­tard­ing the camshaft means that we will be mov­ing the in­take clos­ing point in re­la­tion to the pis­ton. Ad­vanc­ing the cen­ter­line will shift all the valve events to open and close ear­lier. Specif­i­cally, ad­vanc­ing the cam four de­grees means the in­take clos­ing point will now oc­cur four de­grees ear­lier than its orig­i­nal po­si­tion. If we re­tarded the cam the same four de­grees from “straight up,” this would place in­take clos­ing four de­grees af­ter its orig­i­nal po­si­tion.

We’ve cre­ated a small chart that lists the ef­fects of ad­vanc­ing and re­tard­ing the camshaft that is worth study­ing. In a gen­eral sense, ad­vanc­ing the cam tends to im­prove low-speed torque while re­tard­ing the cam will im­prove high-speed power. For street en­gines with a fixed tim­ing po­si­tion, the en­gine builder is forced to choose a com­pro­mise po­si­tion mostly based on how the en­gine would be driven. If the en­gine was pre­dom­i­nantly street driven, this gen­er­ally meant the cam would be ad­vanced.

To this point, most street cams are ac­tu­ally ma­chined with a mi­nor amount of ad­vance built into the cam. You can quickly tell just by com­par­ing

All Gen V en­gines, even this Chevro­let Per­for­mance LT1 crate en­gine are built with VVT. Pro­duc­tion LT1s also come with Ac­tive Fuel Man­age­ment (AFM), but Chevro­let Per­for­mance crate en­gines dis­able this fea­ture, al­though the com­po­nents are still in place.

This David Kim­ble cut­away draw­ing of the Gen V LT1 re­veals the groove in the camshaft Num­ber Two jour­nal and also re­veals an in­side look at the cam phaser in front of the ac­tual cam gear. Note the amount of room avail­able for the phaser to move.

VVT re­quires a much larger vol­ume of oil and higher pres­sure to en­sure ac­cu­rate tim­ing con­trol. This is ac­com­plished with a vari­able vol­ume, slid­ing vane pump sim­i­lar in de­sign to au­to­matic trans­mis­sion pumps. The cen­ter of the pump can be moved to al­ter the out­put vol­ume.

This is a VVT camshaft from an LT1. The large gear on the front of the cam is the ac­tu­a­tor that con­trols the camshaft through its 31 de­grees of po­ten­tial move­ment. Also note the groove cut into the sec­ond jour­nal. This groove is what di­rects oil from the main gal­ley for­ward to the ac­tu­a­tor.

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