Advance a cam or change the LC

[DKK Ted]

Question for the day, what is the primary difference of advancing the cam, via offset washers, or having the cam ground on a smaller LC. The same or is there another difference? Maybe a dumb question or maybe not. 

Ted

[DKK Ted]

Come on guys, there's gotta be some cam guru's that know the answer! 

Ted

[Taylor]

Ummm, not sure if this is what your looking for but they are two separate events. Advancing or retarding changes when the intake valve closes and raises or lowers your powerband in the rpm. Going to a narrower or wider lc changes the overlap between intake and exhaust.  Don't think of it as a lobe center, instead a lobe separation angle or LSA otherwise you get confused between LSA and intake lobe centerline (which is where you install it.) 

For example; you have an engle fk-89 with a 108* LSA but instal it 2* advanced,  the intake valve is now on a 106* installed centerline.

[John Maher]

... what is the primary difference of advancing the cam, via offset washers, or having the cam ground on a smaller LC. The same or is there another difference?

Ted

There's a big difference between the two....

Advancing the Cam
Advancing (or retarding) the cam has zero effect on the lobe centre (LC) relationship between intake and exhaust - also known as Lobe Centre Angle (LCA) or Lobe Separation Angle (LSA)..
e.g. advance the cam 3°.... opening and closing events on both valves take place 3° earlier.
LCA (or LSA) is ground into the cam - you can't do anything to change it, other than change the cam.

Advanced cam timing tends to improve performance at lower rpm, in main due to earlier intake valve closing but will rob some peak power.
Most cam manufacturers have already ground the lobes with around three degrees of advance, i.e. dial in your cam as per the cam card and you're already advanced.

Check your cam card.
If the cam has been designed with zero advance, intake open and ex close numbers will match. Same goes for intake close and ex open.
e.g. Engle FK87: int open 32, ex close 28. Int close: 64, ex open 68.
Take the difference and divide by 2 to determine amount of advance ground into the cam...
(32 - 28)/2 = 2° adv.
or... (68 - 64)/2 = 2
i.e. dial in cam as per the cam card and your cam is installed 2° advanced.
Retard the FK87 by 2° compared to cam card and your figures will be 30/30 and 66/66 (true zero advance).
This calculation applies to cams with identical intake and exhaust lobes. Split duration cams are a little more complicated.

Smaller Lobe Centre Angle
Assuming same design lobes, grinding the cam on smaller lobe centres increases overlap and reduces the time (in crank degrees) between exhaust valve open and intake valve close.

Smaller lobe centres increase valve overlap, which can benefit performance at high rpm but will generally result in rougher idle and loss of torque at lower rpm due to a portion of the intake charge getting blown straight out the exhaust port - one of the reasons many turbo specific cams use wider lobe centres and less duration than your average n/a cam .

[Zach Gomulka]

Most cam manufacturers have already ground the lobes with around three degrees of advance, i.e. dial in your cam as per the cam card and you're already advanced.

Why is that?

[John Maher]

Most cam manufacturers have already ground the lobes with around three degrees of advance, i.e. dial in your cam as per the cam card and you're already advanced.

Why is that?

In most cases it will give a very mild improvement in performance at low and mid-range rpm - the range in which your typical street engine spends a lot of its time.
It's easier for them to grind the advance right into the cam - most customers don't want to deal with 'complicated' dial-in instructions.
They're catering for their core market 

[Zach Gomulka]

Makes sense, I suppose

[Jim Ratto]

Hey John,

I noticed in your post on the thread about venturi diameter vs single cylinder cc /bore x stroke comparisons/rod angle the example you posted had ILC @ 100ATDC. Have you run cams set up that advanced in a VW motor, or ground with ILC @ 100 vs 104 or 106 or 108?
(I typically set street engines up @ 103-105ATDC)
My current engine, I didn't advance to 104, and I regret it now. Same engine, very similar cam (now 86C, then Pauter R6E8), same heads, same venturi, etc etc, lost its hard-edged "urgent" throttle response it had with 3.5 deg cam advance. I can't honestly say it's what softened its tune, but with all else being same....
Any thoughts?

Thanks

[John Maher]

Hey John,

I noticed in your post on the thread about venturi diameter vs single cylinder cc /bore x stroke comparisons/rod angle the example you posted had ILC @ 100ATDC. Have you run cams set up that advanced in a VW motor, or ground with ILC @ 100 vs 104 or 106 or 108?
(I typically set street engines up @ 103-105ATDC)
My current engine, I didn't advance to 104, and I regret it now. Same engine, very similar cam (now 86C, then Pauter R6E8), same heads, same venturi, etc etc, lost its hard-edged "urgent" throttle response it had with 3.5 deg cam advance. I can't honestly say it's what softened its tune, but with all else being same....
Any thoughts?

Thanks

Nope, not tried advancing as far as 100° ILC. That was a quirk of the program I took the screenshots from.

Your experience with cam advance tallies with my comments as to why many cam manufacturers grind a few degrees advance into their cams as standard.
Assuming your cam has 108° lobe centres, you're installing it 4° advanced (104 ICL), which probably accounts for the improved throotle response and urgency you're referring to.
If your priority was top end power (drag racing), you might pick up some bhp by dialling in the cam with zero advance (108ILC on 108°LSA cam) but trade off some of that low rpm 'snap' you're referring to.
It's a 'rob peter to pay paul' kind of thing.