Ok,
So if you understand the title you probably understand why I’m asking.

I’m in the middle of an engine build and here’s the issue I’m finding. I have a 396ci (4.030”x3.875”) that has been 0-decked (9.000” deck height), and the pistons are flush (0.000” piston height). The pistons are flat-tops with 5cc valve reliefs. The heads are LT1 castings with slightly larger valve seats (and matching valves) which Lloyd ported to a bit over LE2 specs. The heads are 58cc I’m told though we haven’t confirmed just yet (boy I hope they’re larger now). The target was to hit 8.6 DCR using a IVC (ICP) of 79° ABDC which puts the static at 12.33:1… which is what I have.

The cam we have, and the reason we used an IVC of 79° in the calculations is the GM 847 cam (GM 12370847, Crane 109841)

Crane advertised specs for P/N 109841 (Grind #: HR-296-2S-12 IG)
Cam Timing: Tappet @ 0.004”
EVO: 85° BBDC
IVO: 37° BTDC
EVC: 39° ATDC
IVC: 79° ABDC

If I’ve done the math right these open/close points agree with the advertised durations (296/304) and the LSA (112) for this cam. So here’s my problem… Is this cam expected to be installed with 4° advance? If this cam was installed “straight-up” the ICL would be equal to the LSA (112)? Yet when I install it in an LT1 it will have that 4° advance already and it should have a 108 ICL as a result? So did Crane take that into account when posting these cam values?

• When calculating DCR do I use the IVC point listed by Crane?
• Or do I need to adjust it by 4°?
• Is my IVC on this engine 79° ABDC, 75° ABDC, or 83° ABDC?

I know this may be LT1-specific, and yes, I will be calling Crane when they open to figure this out, but I was hoping someone here can verify I may be in trouble (75° is WAY too soon… my DCR will be over 9:1).

 

Valve timing needs to checked after the cam install no matter what the cam card says. Most LT1 cams have 4 degrees of advance built in. In the absewnce of contrary info, I'd install it "dot to dot" and check the valve events. Adjust as necessary. In the case of the LT1, if you are using an Optispark, you need to keep in mind that installing the cam anywhere other than straight up will change your ignition timing. I find the easiest way to compensate for this is with tuning, though there are other ways.

BTW: it's very difficult to measure valve events to a tolerance any closer than +/- 1 degree. If you are that close, consider it done.

Rich

ps - this is not really advanced, fwiw. You might get more info in LT1 tech.

 

Most of the calculators I have seen use Advertised duration as .006" duration. Crane is the only company I see that doesn't give that spec, just .004". I'd say use 78 in your calculations just to see what happens and then of course degree the cam before you button everything up.

To find the Advance, use LSA - ICL

 

Called the Crane Tech Line and I was ummm.... unimpressed.

The guy on the back couldn't tell me if the 4* ground into the cam was accounted for in the cam spec or not. After some discussion it seems that it is and as rich said, if installed dot to dot that advance is already accounted for in the cam spec card.

Then this guy starts to tell me that DCR should be calculated using the 0.050" values and not the advertised ones as the lobe is at that height, but the valve itself closes at 0.050" lift due to lifter lash. Hense, he tells me to use the IVC@.050 value of 44 degrees instead of 79. I damn near crap my pants.

WTH?

 

That's because he probably uses the formula that adds 15 to the @.050" IVC value to "guesstimate" the advertised value. I'm not sure how accurate that would be since I wouldn't know how to guess how the IVC changes from .004" to .006". Could it be that rapid of a drop from 79 to 69? I doubt it, but who knows? Certainly not the guys at Crane, lol.

 

You always have to compare apples to apples. There is very little (essentially zero) valve movement, and hence very little flow, just as the lifter leaves the mid point of the base circle. Many people like to use 0.004 or 0.006" as the points to measure "adevertised" duration by because of this. Also, you can measure duration at 0.004 or 0.006" much easier than you can consistently measure just when the lifter starts to move. For lobes in the same family, comparing advertised durations may be useful, not so if the cams don't have very similar designs though. It's probably best to compare cam duration at multiple lift points 0.004 or 0.006, 0.050, and 0.200" are typically available). Similarly, for figuring DCR, using 0.004 or 0.006" makes more sense than using advertised duration. But you have to be consistent.

Sorry if this isn't too coherent, I just had a needle stuck in my eyeball (medical procedure, not entertainment or accident).

Rich

 

Steve- I know EXACTLY the dilema you're in becuase I used to pull my hair out over it all the time. ICL is something that depends who you ask and how they define it. It ain't exactly set in stone.

You want to know how they REALLY define it? They define it as the point of maximum lift. I know that's not what you've probably been told and you obviously have a good grasp of how to read a cam card and reverse-calculate the numbers for yourself so what I'm telling you is going to make your head explode, but that's the real answer.

AND the point of max lift is almost always about 4-5* advanced from the mid-point of the advertised valve event points. That's just how it works with anything approaching a modern lobe profile. They're not symmetrical- they always lift faster than they put down again. So max lift ends up happening sooner than the mathematical mid-point.

Want another noodle-twiser? OK. Even if your cam card doesn't list the .050 lift points of the cam they will again be 4-5* advanced from the mid-point of the advertised events. Just like the point of max lift is. Hence, if you calculate your ICL from the .050 lift points it WILL be at the mathematical mid point- the same as the point of max lift. It's like something funky goes on between the advertised lift numbers and the .050 numbers that thows everything off by 4-5*. And it's almost always 4-5* difference.

The more cam specs you look at the more you will see this same discrepancy over and over again. I've seen it so many times my brain has readjusted this to meaning "normal." In fact, I become instantly suspicious of cam specs that DON'T exhibit this quirk.

So my answer: You are calculating your ICL at 112* by looking at the advertised event points and finding the mid point. But it's not. It's 4-5* more advanced that that, as defined by all the usual cam companies. You're at ~108* ICL, just where it should be. That's the 4* of advance that's "ground into the cam from the factory." If you advance it another 4* at the chain you'll be at ~104* ICL.

 

I thought you might like an example, so here goes. I pulled these cam specs from Crane's website (becuase they usually list both advertised and .050 event points for their cams, which we need for this example to be meaningful). I have NEVER used this cam before, nor am I previously familiar with it's specs. I yanked it from a list at random. Here's the link:

http://www.cranecams.com/?show=brows...tType=camshaft

I'm only going to do the intake side since that's what you need for ICL stuff...

Advertised event points (.0045" lift):
IVO: 30* BTDC
IVC: 68* ABDC

.050" event points:
IVO: 6* BTDC
IVC: 36* ABDC

Point of maximum lift: 105* ATDC

So let's do some calculations. First off, mathematical mid-point using advertised event numbers (the way you are calculating ICL):

((30 + 68 + 180) / 2) - 30 = 109* ICL (that's 1*advanced against the cam's 110* LSA)

Now let's do it with the .050 numbers....

((6 + 36 + 180) / 2) - 6 = 105* ICL (that's 5* advanced against the cam's 110* LSA)

See how they're different by 4*? And always more advanced than the advertised numbers would indicate. Also, see how the ICL calculated using the .050 numbers exactly equals the factory spec point of max lift? That's what I'm talking about. You'll see this same basic difference repeated over and over again on just about any cam you could lay your hand to.

So when you hear about cams having "4-5* of advance ground into them from the factory", as you often do, this is what they're really talking about.

 

I say 75* by the math, as mentioned you need to degree it to know for sure.

 

Ok....

IF the cam is a 234/242 cam on a 112 +4

The specs are:

IVO 37°
IVC 74°
EVO 86°
EVC 34°

The same cam NOT advanced would be:

IVO 33°
IVC 78°
EVO 82°
EVC 38°

Your results would give you something like a 238° on a 110ICL

Bret

 

Ahh... I get it. This would explain the comment I recall the GM guys talking about in thier LS6 engine "launch party" a few years back. The article I read quoted the GM powertrain guy who delt with redesigning the LS6 cam's for more power and mentioned that while the "uptake ramps" were increased, they didn't see an improvement in closing the valve any faster... which would explain this whole "4* ground in from the factory"... it's simply a matter of easing the valve down slower than it opened... making the ramp profile unsymetrical... which results in the max lift actually occuring before the mathimatical center of the lobe duration, which I "was" equating with ICL... though it's clearly not.

And here I thought the cam companies were just cutting the dot in the cam 4* to the left.

Thanks guys... that helps a lot.