olysan

It's time to start planning for potential offseason projects, and High on the list is a possible cam change. I'd like to get some opinions on potential gains, any issues with idle, reversion potential etc.

Here's what I have:

It's a 406 running stock vortec heads with ~9.3:1 compression the cam is a comp cams extreme marine 262H-12. Lobe separation - 112, Duration - 262I/268E advertised, Installed at 110 intake centerline. Exhaust is from a 350 Mag, merc manifolds, 4" risers and pipes, with silent choice. I'm considering stepping up 1 maybe two sizes in the extreme marine series:

Cam Link

Up 1 size = 112, 270/286
Up 2 sizes = 112, 278/292

I idle at 650 now, will these cams kill my idle? What kind of gain if any will I see?

rmbuilder

Because two of your primary concerns are idle quality and reversion the CS XM 270H-12 with I 226 @.050 and E 236 @ .050 and the CS XM 278H-12 w/ I 234 @.050 and E 244 @.050 are marginal on those counts. As duration and overlap increase, idle quality suffers and reversion becomes an increased possibility. A better alternative, (if your willing to go the additional expense) that would make substantially more power, would be the switch to a hyd roller setup in place of the flat tappet. The CS XM 270 HR-12 (I .495 E .503) I [email protected] E [email protected] would work great on a 406 and have good manners around the dock.. I would say you could expect a 20 - 30 HP gain over your current setup without any reliability issues
Hope this helps,
Bob

rmbuilder

One side note. The Vortec heads provide excellent flow numbers up to .500 valve lift but any cam you choose with over .470 will require machining the guides to provide proper retainer clearance.
Bob

220BR

You might consider Isky's 281-12 Marine cam: 221/232@050; .465/.485; 112 LSA.

The XM 262 has -3* overlap at 050; the Isky has +6.5*, the XM270 has +7* and the XM 278 has +15*.

I have chart comparing these cams and others. If you send me your address, I'll email it to you.

olysan

Thanks 220br. My address is [email protected].

220BR

Done

rmbuilder

Actual overlap cannot accurately be calculated using .050 figures.

http://members.uia.net/pkelley2/Overlap.html

Using seat to seat (.006 SAE standard) the

Flat Tappet
XM 262 H-12 41 deg overlap
XM 270 H-12 54 deg overlap
XM 278 H-12 61 deg overlap
Hyd Roller
XM 264 HR-12 43 deg overlap
XM 270 HR-12 49 deg overlap

Isky 281-12 51 deg overlap

The XM 270 HR-12 has Int .030 and Ex .018 more lift than the Isky with 2 deg less overlap. Isky does calculate their advertised duration figures @.020 rather than .006 like Comp Cams so in apples to apples the XM 270 HR-12 could be closer to 4 deg less overlap.
Hope this helps,
Bob

220BR

Can't argue with that! 006 is better than 050, 004 is better than 006 and 002 is better than 004. My problem is I can usually on get 050 numbers. The best measure is the area under the curve in degree-inches, but you still have to pick a starting point (eg 006").

Your point about ramp profile is well taken. Good link.

220BR

And just to show that its really about degree-inches (ie ramp profile or area under the curve) and not just degrees at a particular lift, consider the stock merc 5.7L cam and Comp's XM262

Merc 5.7 : +44* at .006 and -14* at .050

XM262: +41* at 0.006 and -3* at .050

Overlap, no matter how you measure it, it just one of many cam varialbles. You just have to keep apples with apples and oranges with oranges.