VVT advance/retard limits to avoid piston/valve interference?


#1

I’m reading an article about retrofitting a Gen 4 LS engine (L77) to older vehicle (Hot Rod Magazine, 4.19). I came to this paragraph and got stopped in my tracks. What do these two sentences mean? Why does the cam phase get retarded with engine rpm? Wouldn’t the cam phase get advance with rpm?

As cams get bigger, you need to limit the amount of VVT advance to avoid piston to valve clearance issues. GM’s phaser scheme starts out at – depending on the model year – 7 or 17 degrees advanced from the cam profile’s as-ground position, then retards as much as 52 to 60 crankshaft degrees from there.


#2

They are talking about high performance cams, probably with higher lift and longer duration. Since these cams open the valves further, open them sooner and close them later you have to do something to stop the variable valve timing from advancing the cam with increased rpm as muck as it does with the stock cam in order to stop the valves from hitting the piston or each other.


#3

Yeah, I understand that part. What I don’t understand is why the cam phase would retard with increasing rpm. I think that’s what they mean by "phase starts out at 17 degrees advanced … then retards to 52 degrees from there. " Or am I misinterpreting what they are meaning to say?


#4

Don’t confuse ignition timing with cam timing. My thought is that the cams are retarded to match the dynamics of the airflow/valve system as it changes at higher rpms.


#5

Both @oldtimer_11 and @texases make valid statements. I was going to type a long response to describe this phenomenon but this article does a pretty good job… Good enough so that you can answer your own question by reading it.

https://www.hamotorsports.com/cam-gear-tuning.html


#6

way back when I had an aftermarket cam and adjustable sprocket on my 1981 Accord.
If I advanced the cam low end torque increased at the expense of high rpm output, and vice-versa.
Normally set to the base level: equal opening of in/ex valves at TDC overlap.
The adjustment was needed to compensate for the different deck height of the aftermarket head (non CVCC with bigger valves).


#7

GM’s newer pushrod V8’s achieve variable valve timing by literally rotating the the camshaft. The degrees retard and advance refers to how much rotation the camshaft experiences vs. it’s designated default position. That’s my understanding anyway.


#8

Good comments above. Albeit counterintuitive. Looks like I’ve got to do some studying up on this issue. Thanks to all.