gmgod

I searched through old posts on the topic and came across one by rb83l69. He stated that the new timing sets from gm do not have 4 degrees advance built in like aftermarket sets do. My question is did gm grind this cam with this in mind? Would running an aftermarket timing set with 4 degrees advance make it run worse?

gmgod

ttt

ede

WAG i'd suspect gm ground the advance into it and intended for it to be installed straight up. advancing it would get you more torque and a reduction in HP

Damon

Aftermarket timing sets do not have any advance built into them. They may have 3 keyways so you can CHOOSE to install it 4* advanced, straight-up or 4* retarded, but that's about it. If you choose the "straight-up" keyway it's really straight up- they didn't "hide" 4* more advance in it somewhere without telling you.

Low quality timing sets may not be perfectly accurate (frankly, even high quality ones can be off) but they don't intentially build in advance without telling you.

I've laid a LOT of factory and aftermarket timing sets up against eachother and believe me- you could actually SEE 4* of difference with your eye at the keyway. In fact, even on engines that I don't actually degree the cam in I ALWAYS lay the new timing chain set up against an old "known accurate" set I keep in my bag of tricks for just such purposes. I've found a few that were way out (dot stamped in the wrong place, keyways mis-marked, etc.) but they were flukes. I would return them for a like replacement and the new one would be jsut fine.

Unless you know exactly what you're doing or you are going to go through the process of actually degreeing the cam in properly you're almost always going to be better off installing factory cams in the "straight-up" position. No advance or retard at the chain.

RB83L69

The "0°" setting on an aftermarket timing set is equal to "4° advanced" compared to a late-model stock timing set.

Stock timing sets made since about 1971 install the cam with the intake lobe at 110° ATDC. Older (pre-emissions) ones installed it at 106° ATDC, which is what aftermarket timing sets still call "0°", "straight up", etc. I know of no replacement timing sets whatsoever that match the OEM ICL for non-roller cams; I don't know for sure whether there are any or not for the factory's roller setup.

I'd suggest using a good quality replacement timing set and installing it at the "0°" point, which will be 106° ATDC.

If you simply degree the cam, and use the actual measurement numbers to describe where you want the cam to end up (which I would suggest a target of 106° ATDC), it's alot more accurate and less prone to the buzzword confusion that you end up with by calling it "advanced", etc.

TunedPort 335

I degreed my cam and made sure everything was "in spec". After that I installed it straight up after doing some research. I would probably do the same in your situation

Damon

RB- I'll have to respectfully say that from my experience and observation that I haven't found that to be the case. I have 2 GM flat tappet timing chains in my hands right now, both out of early 80s 305s that I beleive to have been original engines, never rebuilt. I also have a couple of aftermarket 3-keyway chains. They're as alike as 2 peas in a pod. Crank keyways in identical postions on the bottom gear and cam dowel holes in identical positions on the cam gear. If the crank key was off by 4* it would be obvious. If the cam gear dowel hole was off by 2* (4* at the crank) it would still be obvious. They're not off- they're identical.

Also, when installing aftermarket cams with aftermarket chain sets when I degree them in they're usually +-1* of the cam specs when using the "straight up" keyway. If the aftermarket chains were off by 4* then the cam would usually be off by about 4*. I suppose it's possible that aftermarket cam companies still use the "old style" timing marks like you describe when they grind the cams so that it matches aftermarket chain sets, but that's really starting to get funky, if it's true. Please note that many aftermarket cams do have some advance ground into them but this is definitely in the cam, and not dependent on the chain. Like I said, I almost always do aftermarket cams (Crane, Comp, Lunati) and they almost always line up dead-bang with the cam card specs when using an aftermarket chain on the "straight up" keyway.

I have not had the opportunity to degree in a factory GM cam with either a factoy or aftermarket chain set so I can't comment about that.

I'm going to go down to my local speed shop on Monday, get a real GM replacement timing set and compare it with the aftermarket ones I have laying around. Then we'll know for sure.

Damon

Besides, we're talking about a ZZ-4 roller cam here, not a flat tappet. Since all factory roller cams were 87-up you would think that the aftermarket and GM would all be doing it to the same spec, being a completely different design chain set and all.

RB83L69

This is exactly what I mean about arguing over whether it's "advanced" or not.

It is a tempest in a teapot. Or as someone far more gifted with words than I will ever be, "like a tale told by an idiot: full of sound and fury but signfying nothing".

And in the end, the engine doesn't give a rat's hindquarters for all of that; all it cares about, is where the valve events occur. It does not dissect the etymology of the timing set and decide how it's going to run based on whether a particular set alls itself "advanced" or "2° retarded" or whatever; all it knows about is when the valves open and close, in relation to piston motioin and position.

Install the cam at the appropriate intake centerline by measurement, and don't worry about whether it's "advanced" or "stock" or whatever. Just put it where it belongs and be done with it.

gmgod

Does anyone know the valve timing events for the zz4?

RB83L69

208°/221°, .474"/.510", 112° lobe separation (i.e. the exhaust lobe peak is 112° of cam rotation before the intake lobe peak)

Damon

You need the cam card. It'll give you the .050 lift event points (intake open, intake close, exhaust open, exhaust close). When you degree it in and your actual MEASURED .050 events happen exactly when the cam card says they SHOULD happen, it's degreed in properly.

Some cams (namely Comp Cams) instead recommend that you simply find the point of maxium lift and make that equal to the recommended ICA number.

RB83L69

I could be wrong; but I expect if you walk up to a GM parts counter and ask for a "cam card", they'll look at you like you've got 3 heads, and then ask "What's that?" and when you tell them they'll say something like "Naaah, we don't have anything like that, must be something they have to give out with that cheap aftermarket s***" That's just from my limited knowledge of how these things work, having had 3 brothers that have worked work at various GM and other dealerships, and one of whom is the parts mgr at a Mitsu dealer right now.

GM doesn't give out the full specs on their cams, mostly because the only place they even might exist in that format, is on the drawings for the masters. Their approach to these things is, you buy part # 12345678 cam, and part # 87654321 timing gear, and part # 43215678 timing chain, etc.; and bolt it together, and then if you did it right, it is what it is, whatever that is. They don't support customizing one's engine. They are mass-producers.

Best you can do is to use the specs I gave you; install the intake lobe at 106 crank degrees ATDC, and then check and make sure that the exhaust lobe peak is at 112 cam degrees ahead of it; which would be 224 crank degrees before the intake peak, which would put it at 118 crank degrees BTDC.

I have no clue whether GM grinds asymmetrical lobes, with different opening and closing rates. That's yet another variable in trying to guess valve event timing from a cam card or by any other method. For example, the ZZ4 cam's intake lobe is 274 "advertised" duration, whatever that means to GM (.001"? .004"? .006"? lobe lift or valve lift? even they can't tell you), and 208" @ .050" of lobe (not valve) lift; so, is there (274-208)/2 of duration in between those events on both the opening and closing ramps, or is the opening ramp shorter (steeper) and the closing ramp longer (gentler), like modern state-of-the-art designs? I have no clue, nor have I ever found anyone who does, about GM cams. That kind of information simply doesn't exist outside of their engineering dept.

Justin 87 GTA

iTrader: (3)

back to life.....
Okay, I dgreed my cam, the centerline should be 108, but i got 107, whcih I know is "okay".
But, IN THEORY:
If it should be 108, but it was actually 107, then the cam is advanced right? I know that most would think it is retarded, but from the reading I've been doing, if it was 109, it would be retarded. (maybe i'm just retarded).
Anyway, an "advanced cam on a 107 centerline that is supposed to be 108, will make MORE bottom power/torque,...right? I know advanced cams make more low end....but the real question is at 107, is it advanced?

RB83L69

If the cam was "designed" to be installed at 108°, but your installation placed it at 107°, then yes, it is 1° "advanced" compared to its design. Whether that's "advanced" in some more general sense, is anybody's guess.

That 1° will probably disappear within the first 5 seconds of the engine running, as the timing chain's surfaces micropolish each other.

Even if it doesn't change, I doubt that 1° will make as much as 1% difference in the engine's output at any RPM. In other words, it's negligible. You have other, far larger potential "errors" built into your motor that will affect how "perfectly" everything lines up, that you CAN'T control; everything from deck clearance, to crank journal indexing (are they all 4 exactly 90° apart? Is the keyway exactly lined up to #1 journal like it's supposed to be?), to variation from one rocker to another, to lifter bore location accuracy (lifter bores aren't always in a perfectly straight line, and that affects valve timing, even if the cam is perfect; are all 8 intake valves really at the same centerline compared to their pistons?), and the list just goes on....

Sounds like what you've got there is good to go, to me; and you're close enough that you wouldn't be able to tell the difference if you had an infinitely adjustable cam gear, and you could move the cam that one degree. I don't think you'll notice any discrepancy in the motor's "bottom end" power or anything else that can be attributed to the cam timing.

Justin 87 GTA

iTrader: (3)

well atleast it's on the side of the spectrum that adds bottom end if anything, cause I have a low compression blower pistons, and a big cam. So, I think I amy be lacking in bottom end a little. I figure that if it's on that side of it, then it's fine. If it was at 109. then, I would advance it 2 degress to get it on the bottom end power part of the rpm band. this thing is gonna run ppretty strong, as long as i have enough bottom end.

Justin 87 GTA

iTrader: (3)

But the question I have is:
Why is it called advanced?
Is this because the cam should be on centerline at 108 degrees, and the cam should be on centerline when the motor gets to 108?But it is actually on centerline later in the rotation (at 106-107 degrees)? I just don't understand completely. Also, why does this give you more low end power? It opens the intake valve later right? Please help me understand completely.

RB83L69

107° is more "advanced" than 108°, because those numbers are degrees after TDC of the intake "centerline". In other words, the lower the number, the sooner the intake events occur after the TDC where the exhaust closes and the intake opens, then the more "advanced" - close to TDC - the intake centerline is.

Advancing the cam shifts the power band downward because it closes the intake valve sooner. Remember that cams almost always close the intake after the compression stroke has already started. At low speeds, this means that a certain amount of intake charge "reverts" back into the intake as compression begins. However, at high speeds, the speed of sound is too slow for the air molecules at the top of the cylinder at the valve to "know" that the piston is starting to come up, until relatively late in the compression stroke; so because they have inertia from flowing down the intake tract, they continue to flow, and fill up the cyl more. Exactly how much low end you "gain" and how much high end you "lose" depends on the relative strength of the 2 opposing effects, of reversion and inertia-driven continued cyl fill.

Keep in mind also, most modern cams (especially the more "intense" ones, like Comp's Xtreme or Crane's Powermax or Lunati's Voodoo or nearly any race roller) have lobes that are "asymmetrical"; i.e. the closing ramp is longer and slower than the opening ramp. This means that there's more duration on the closing side of the peak lift, than there is on the opening side. Which means that it's even harder to characterize "advanced" and "retarded" cams. Specifically, where's the point you're supposed to be measuring? Is it peak lift? Is it the "centerline" between the .050" duration points, or the "advertised" duration points, or the .200" lift duration points.... since all 4 of those events could occur at different spots? Which one did you use for your measurement? Is it the same one that the cam mfr used?

Justin 87 GTA

iTrader: (3)

I see your point with the different ramp angles. It makes perfect sense. But on a regular cam, theoretically, how would this apply to my setup CONSIDERING it has a supercharger. It is interesting to me what you mention about airflow at low speeds, reverting back up the intake tract. But if I understand what you are saying about high speed flow, then it becomes less of a factor cause the air is almost being forced in. So you would get more driveablity and usable low torque at regular speeds, but not as much more low torque depending on that ratio at WOT.
Now getting back to my setup with the supercharger, that would mean that advancing the cam would have little to no effect on low end power, because you are cramming the air in, and the one effect overpowers the other. So having a blower cam is better than a regular cam, but it doesn't matter what the advance is. My theory was that adding a blower to a motor makes the cam much less signifigant anyway. I'm sure that blower cams are designed to get rid of exhaust faster and the lobe separation angle is different pertaining to the duration, less air passing through and wasted....and whatnot, BUT I would have to guess that it doesn't matter all that much if it is advanced/retarded. Right? does this make sense?

Damon

The amount of advance or retard is measured by comparing the Lobe Separation Angle to the Intake Centerline.

If it's a 112* LSA cam and it is installed on a 108* Intake Centerline then it is 4* advanced.

112* LSA installed on a 112* ICL is straight up

112* LSA on a 116* ICL is 4* retarded.

MANY small block Chevy cams are ground with advance built into the cam itself from the factory (meaning if the timing chain is installed "straight up" the cam will still show some amount of advance when you actually measure the ICL). 4* advance is very typical in a SBC performance cam.

That's why you almost always installed cams with the timing chain "straight up" unless you REALLY know what you are doing and WHY you are changing things.

Agreed, 1* of additional advance over spec is nothing. That'll go away as soon as you run the motor for the first 30 minutes from timing chain stretch.

The cam definitely DOES matter even with a blower. All the same basic rules still apply. I certainly would not advance that cam any more than it already is. You MIGHT consider retarding it slightly if you don't mind trading some low end torque for some top end power (how much traction do you have coming off the line??). But again, you better REALLY know why you are making the change.

RB83L69

That is not corrrect.

LSA (lobe separation angle) is the angle between the nominal exhaust centerline and the nominal intake centerline, measured in cam rotation degrees. It's the only cam spec commonly given in cam degrees. All the others are given in crank degrees.

ICL (intake center line) is the number of crank degrees after TDC that the intake reaches its nominal centerline. Since the exhaust opens [before the intake, larger LSA numbers mean earlier exhaust events.

112° LSA on a 112° ICL means that the exhaust nominal centerline is 112° of cam rotation, which is 224° of crank rotation, ahead of the intake nominal centerline. That example means the exhaust centerline is at 224° - 112°, or 112, crank degrees, before TDC; keeping in mind that the exhaust reaches its maximum opening as the piston reaches somewhere near halfway up its exhaust stroke on the way to that instance of TDC.

You could take that same cam and install it (twiddle the chain) at 106° intake center line. 112° LSA on a 106° ICL means that now you've taken the same cam, twisted the timing set around 6°, and now you still have 112 cam degrees between the exhaust and the intake (LSA) but now you've moved the whole cam with respect to the crank. In that case, the intake centerline would be at 106° ATDC, and the exhaust would be 112 cam degrees ahead of it, or 224 crank degrees, which means that the exhaust CL is at 224 - 106 crank degrees, or 118, BTDC.

The ONLY spec that has to do with "advance" and "retard" is the intake centerline. That (ICL) is what you move around by playing with the timing chain. LSA is ground into the cam, and CANNOT be changed in the field, and DOES NOT have anything to do with "advance" or "retard".

Damon

RB, with all due respect, think it through to the end. You just said exactly what I said.

Any time you "advance" or "retard" the cam you are doing it to BOTH the intake and exhaust cenertline at the same time.

We're in what I like to call "violent agreement."

Advancing or retarding the cam can not change the LSA. True. As you "advance" the cam all the intake events happen sooner. So do all the exhaust events. And vice versa.

An advanced intake centerline give you a lower ICL number but a higher Exhaust Centerline (ECL) number. Add the two numbers together and divide by 2 and it will ALWAYS be your cam's LSA. An advanced intake event will happen closer to TDC (a lower ICL number). An advanced exhaust centerline number will be closer to BDC, and so the ECL number will be higher since it's still also meased from TDC.

112* LSA on a 112* ICL will also give you a 112* Exhaust Centerline. That's "straight up."

112* LSA on a 108* ICL will also advance the exhaust to 116* ECL. That's 4* advanced.

112* LSA on a 116* ICL will also retard the exhaust to 108* ECL. That's 4* retarded.

Now the cam MAY (often does) have advance ground right into it from the factory but that does not mean the cam is "straight up" just becuase you put the timing chain at zero. It just means that the cam designer WANTED the cam to run advanced or retarded from true straight up. True straight up is always when ICL = LSA.

I think you know I've done this kinda stuff more than just once or twice. I know my way aroud a cam card. I wouldn't post it if it wasn't right.

RB83L69

No, that's not right. All that it means when that happens, is that the intake and exhaust "centerlines" (whatever those might consist of) are equally offset from TDC. Not all cams are designed or ground that way; not even factory cams. For example, the "843" L69 cam has a lobe separation of 114.5°, but is installed at 110°.

ICL = the angle between the crank and the intake lobe and the crank (determined at install time)

LSA = the angle between the intake lobe and the exhaust lobe (determined at grind time)

"Advance" and "retard" are measured on the basis of where the ICL is installed. The exhaust follows along, separated by the lobe separation. When talking about "advance" and "retard", there's no need to throw the LSA into the mix.

The ICL and the LSA are not connected in any manner way shape or form. One cannot be calculated from the other, and neither one affects the other.

Most often, when the cam designer wants the ICL to be at some specific angle ATDC (ICL), he'll grind the cam such that when installed with a "straight up" timing chain, the cam ends up where he wants it.

Don't worry, I'm not calling you a fool or anything. I also know my way around cams. I got pretty familiar with them when I was doing contract engineering work at Comp some years back (didn't have anything directly to do with cams; but I got a helluva cam education out of the deal).

REALPOWER

If you are using nitrous, advanceing the cam is good.

Justin 87 GTA

iTrader: (3)

well Damon:
I'd like to think that I will be okay with advancing it slightly. It is actually installed at 106.5, and the centerline is 108. This time around I built a top end motor, with a good amount of duration and a supercharger too boot. But it has low compression blower pistons. Unlike my last motor, which made a ton of low end power, I think I could use all the low end I can get. So considering the cam is a custom blower grind specific to my setup and the compression is low, I think 106.5 is perfect, so if anything I get the lost torque back. Worst case, the blower will do most of the work. I'd rather have it advanced on MY setup than retarded considering my low compression and high power band.

Damon

OK, let's try it this way....

Post a link to any cam card on the internet. I'll dissect it for you. LSA and ICL are INTIMATELY related. LSA, we agree, is the spread in crankshaft degrees between the ICL and ECL. Therefore they are not just related but are, in fact, what DEFINES LSA. Sort of like the relationship between HP and torque. Inseparable.

Using your example of the 843 L-69 cam you say it has an LSA of 114.5* and an ICL of 110*. Fine. I can tell you what the ECL will be for that cam with NO further information. LSA - ICL = adv/ret amount ground into the cam. 114.5 - 110= 4.5* advance ground into the cam. That same advance on the exhaust side would therefore ADD 4.5* to the ECL (reverse of what it did to the ICL). 114.5 + 4.5 = 119* ECL. This is not an estimate, but exactly what the ECL will be assuming the cam is ground accurately to spec. It is true by definition. In short, it must be that number. If it was any different, the LSA would be different.

I know you stated that the LSA is the one spec that is measured in camshaft degrees (true) but if you think about it, it's really a statement of crankshaft degrees on EITHER SIDE OF TDC, too. And the advance or retard amount pushes both events a little one way or the other simultaneously. Subtracting from ICL and adding to ECL if advanced. Adding to ICL and subtracting from ECL if retarded.

Justin- If putting the chain on the "straight up" position dropped your cam in at 106.5 vs. a recommended 108 spec you are just fine. Like RB and I said, it'll stretch about 1 degree shortly after startup. That would put you at 107.5 which is plenty close to spec for a street motor. I certainly would not artificially advance it any further at the chain. If you want to trade a little bottom end torque for some more up top later on you can move the chain back to 4* retarded position (which will put the cam at true "straight up" on a 112* ICL) and get what you're after.

Justin 87 GTA

iTrader: (3)

Damon,
I actually want the extra low -end torque.

Abubaca

iTrader: (3)

I've been following this, and while I'm no expert, I think you both are saying the same thing.

Damon

Yep, that's my point about us being in "violent agreement." I know that RB is very knowlegable, but I know my way around a cam, too- we're just coming at this from different perspectives. I beleive he's just not buying my math (yet) but it's right. Like I said, I would never have gone this far with my posts if I wasn't certain the information I was putting out was correct. We're here to HELP eachother. Posting erroneous information is not a help to anyone and I wouldn't knowingly do it.

pasky

Believe you got that backwards ede. Retarding brings more torque lower hp, advancing loses torque gains hp.

Justin 87 GTA

iTrader: (3)

no pasky you are wrong. advanceing gives more low end torque and retarding gives more high end torque, althought they both give torque, it just changes where it is in the powerband

REALPOWER

No, Im pretty sure that advancing the cam will make more high end and vice versa.

ljnowell

You retard the cam to push the power band higher.

REALPOWER

OOOHHH ya your right. I was thinking about the piston movement being more advanced, therefore the valve timing would be more retarded.

Streetiron85

He's right