Peak lift has the least contribution to P/V clearance.

 

huh?

 

 

Zig, the valve events, deck height, gasket thickness, whether the heads have been milled, even the thickness of the valves themselves --- all are factors that affect the piston/valve clearance. With the right setup, you can easily run a cam with .600+" lift.

Do a search on some of Spin's earlier posts, such as the ones on dynamic compression or the L92 heads.

 

No vendor. Just looked at the comp lobe reference chart in the cam spreadsheet.

While on the vendor subject, I was not impressed with the 6 or so tuners / vendors I called with questions. Most kept trying to push their cam-of-the-month (typically the exec / cheater / type cams) despite my explicit indication of what mods I already had, plus my requirement that the cam behave down low (with tuning), produce no less torque than I have now (400 RW), and be reliable on a 400 mile road trip. In response to these goals, I had guys insisting the perfect cam was a 215/230 with .630+ on the intake side, another says the 222/222 .56x on a 113 is perfect, another equally insistent that their 228/232 on a 112 ot their 232/234 on a 112 was exactly what I needed. Tuning a 232/234 to run like stock? No problem according to the vendor of that cam . The exception to this is Cartek, whose owner Dave listened carefully to what I had to say and gave me some good info. Needless to say, the cam and definitely any heads I do will go through him.

Since I could not get a straight answer from the vendor sales guys, I ended up calling Comp to get the skinny on the XER / XFI lobe designs. Basically, the XER is a "fatter" lobe, meaning the ramp on it is very fast in the mid-lift area (sorry I know several of you know this well). It will therefore yield more area under the curve for the same maximum lift as the XFI lobes (which are an older design). So the XER lobes will make more power for a given max lift, while being easier on the springs.

On the other hand, the XFI lobes, for a given duration, have a more gradual ramp than the XERs. So they are easier on valves during the closing event since it does not slam into the seat as hard, and they provide a smidge more torque (less overlap), but they will make less power.

Given the recent issues with springs, and the fact that I have seen reports of problems with all spring brands at one time or another (Comp being the current loser), I will go with the profile that is easier on springs for a given power level (XER). Plus, one forum guru and several tuners have suggested the XER is better

Looks like it will be a 224/230 XER cam for me. To the vendors jamming one-cam-fits-all, stop being lazy and listen to the customer.

 

don't forget about the advance and or timing. more lift is good, provided the heads will flow. spin said peak lift, which is correct, peak usually won't get you into trouble. i was referring to lift in general, not just peak.

 

you know, you don't have to select one, they will grind one for you if you want.

 

Huh?

You're speaking in tongues again......

Advancing timing has absolutely nothing to do with p/v clearance. Perhaps you are referring to cam timing, which falls under the same category as valve events.

Spin did not say that peak lift would not get you into trouble, he said that it contributed very little to p/v clearance issues.

P/V clearance is tightest during valve overlap, especially on the exhaust side. As the exhaust valve closes, if the valve floats or bounces, it is likely to smack the top of the piston. Meanwhile, the intake valve is starting to open, but is under much more control. With a hydraulic cam, the lifter is also slightly compressed, giving perhaps .01-.02" extra clearance.

As Spin pointed out in one of his earlier threads, the thickness of the valve itself also comes into play. The thicker the valve, the less p/v clearance. Sometimes the machinist can cut the bottom side of the valve for extra clearance.

 

yes, i wasn't referring to ignition timing or advance but rather cam timing/advance. timing as in when the valves move.
and i said usuallly
as well as the shape or type of piston used.

i was simply referring to lift as a generic term meaning anytime the valve leaves the seat (it is lifted off the seat).

peak lift is good but you need to watch out for how much lift exists during overlap.

 

It will likely be a custom cam Zig, because none of the cams out there fit my parameters: less than 600 lift (knocks out GT11), wide LSA / powerband (knocks out most off-the-shelf grinds), and probably a fair bit of advance (as much as +4) to keep the DCR as high as possible.

Thought I was set on 224/230, but I came up last night with another combo that looks promising: less intake, more exhaust. Given my stock heads / FAST / CAI / Kooks 1.75, which of these will provide solid low-down torque with power ? Both are the same DCR (8.55 with the stock head gaskets).


224/230 .581/.592 114 +4 drivability -1
222/234 or 232exh .581/.598 115 +4 drivability -2 or -3

 

Here's some info that may help you decide which way to go.

Do you happen to have any additional specs. from the cam card?
i.e. intake centerline, the valve-opening and-closing events.

I grabbed the following from 'Chevy High Performance' online since they say it better than I.

Duration
224/230
222/234 or 232exh

Longer intake duration results in engine with poor low-and midrange response.
Increasing duration shifts an engine's torque curve higher in the rpm range.

Lift
.581/.592
.581/.598

An increase in lift almost always leads to an increase in duration,
because increasing lobe lift requires more distance to create the opening and closing ramps.

A camshaft's function can actually be reduced to four points:

intake opening (IO)
intake closing (IC)
exhaust opening (EO)
exhaust closing (EC).

An early IC improves low-speed torque.
A later IC limits low-end torque.

An early IO increases overlap and can lead to a sluggish engine.
A later IO reduces overlap, improves idle, increases low-speed torque.

An early EO can limit low-and midrange power but help high-rpm performance.

An early EC reduces overlap, improves idle but limits midrange power.
A later EC increases overlap, hurts idle but helps high-rpm power.

LSA
114
115

LSA has an effect on how and where an engine makes power.

Narrower LSA
Increased overlap
Increased low-rpm torque
Narrower powerband
Reduced idle quality
Increased cranking compression
Decreased p/v clearance

Wider LSA
Reduced overlap
Improved top-end power
Wider powerband
Improved idle quality
Reduced cranking compression
Increased p/v clearance

Asymmetrical cams have a high-acceleration opening ramp, while the closing side sets the valve down softly

http://www.chevyhiperformance.com/te...haft_profiling

 

Thanks Zig I had forgotten that article.