Ukraine Train

it turns out GM doesn't release the timing specs of their cams. does anyone know the intake centerline of the hot cam? in an earlier post someone said they tested the cam and these are the specs they got:

112.3 lobe seperation angle
108.7 intake centerline
219.1 intake duration
229.3 exhaust duration

using these specs, dyno 2000 gives me these specs:
.9 IVO BTDC
50.6 EVO BBDC
38.3 IVC ABDC
-1.3 EVC ATDC

Shouldn't IVO be more like 4°?

Using published specs (112 lsa, 218/220 duration, i used 112 for intake center line, it might be wrong) I get an IVO of -3 BTDC which should be the same as 3 ATDC because the 3 is negative. I know that can't be right, I've never heard of a retarded cam.

So how the hell am I supposed to degree my cam?? should i just use 4°BTDC for IVO?

RB83L69

There's the intake lobe centerline. Using a different value such as 112° is not consistent with reality.

It's a GM cam, not a performance cam. You're not supposed to degree it, they don't supply enough specs, and they don't "degree" their cams themselves anyway, they just put the motor together. You're just supposed to stab it in there and take what you get, that's how they designed it. After all they're not a performance manufacturer, you know.

The absolute value of the IVO is not directly correlated with "advance" or "retard". That is, just because the intake valve opens ATDC, does not mean in and of itself that the cam is "retarded".

Although, GM changed their timing sets in the late 60s, such that the cam was retarded 4° compared to using the same cam (929 for example, used in 283s, 307s, 350s, 400s, etc.) with the early-modle timing set. So, in that sense, all GM cams (or anyone else's) are retarded if used with GM's retarded timing set.

Why do you think the IVO is supposed to be 4°? And in the bigger picture, what are you really trying to do?

Ukraine Train

well after i put the cam in, the intake valves were hitting the pistons and unless i didnt line the dots up right on the timing gears, which i'll check tonight but i'm 99% sure they're right, then my cam timing is too far advanced so i want to degree the cam.

my understanding is that most performance cams are advanced 4°, so isn't that what the IVO number is supposed to be?

RB83L69

OK. Now it makes more sense.

No, the IVO is the point at which the intake valve opens. Whether a cam is "advanced" or "retarded" is a function of the intake centerline: that is, if the intake centerline occurs before or after some arbitrary standard. The intake lobe centerline is the base point from which all other measurements are made; it represents how many degrees after TDC the peak of the lobe is designed to occur; LSA is the number of degrees between that point and the exhaust lobe's peak.

It isn't really meaningful to talk in a vacuum about a cam being "advanced" because you would have to specify what is is advanced with respect to. It is however meaningful to talk about the intake lobe centerline's absolute value, 108.7° ATDC or whatever.

Most performance cams are "advanced" 4°, but that's an attribute of the timing set, not the cam. But, you have a GM cam not a performance one, so who knows? On the other hand you do have someone's measurement of their cam's profile, which shows the intake centerline at 108.7°, with whatever timing components they happened to have (it would be real nice to know what they were); you could measure your cam and make sure it's within a degree or 2 of that, and be relatively assured that it's either OK or not.

As far as the event timing is concerned and what those numbers are actually telling you, think of it this way: imagine you have a cam lobe with its center at 112° ATDC, and a duration of 224°, and that the lobe is perfectly symmetrical (i.e. the same on the leading edge as it is on the trailing edge). In this case, ½ of the lobe is 112° wide; so since it starts 112° before its center, which is at 112° ATDC, then it will have its opening event at exactly TDC. Take a cam with its intake lobe at 106° ATDC (much more typical of a performance cam) and the same duration, its opening point will be at 6° BTDC. Take a cam with 202° of duration and a centerline of 106°, its opening point will be at 5° ATDC. What this means is that you can't equate the intake opening point with "advance" or "retard" without knowing more complete specs. One of the specs that no cam mfr will typically tell you is whether their lobes are in fact symmetrical; quite a few of them (especially the more modern aggressive ones) are not. That's one of the cam mfrs' most closely guarded secrets: that's how they get the real fast ramps without making the valve bounce on the seat when it slams shut.

Now look at a head. Look how far the valve is from the deck surface when it's at rest. This means that in order for that valve to interfere with the piston, the valve has to be open at least that far with the piston at TDC for there to be interference. In most heads this would require probably at least .250" of lift with the piston at TDC. But, as you can see, its "opening" event is within a couple of degrees of TDC, so the intake valve just begins to open as the piston reaches TDC, therefore the possibility of there being interference is extremely remote.

If your valves are in fact hitting the pistons, the only ways this can happen is for the cam to be drastically out of time (not likely if the motor ran anywhere near right), or for the valves not to be following the cam profile. My guess in your case is that your valve springs are inadequate, and at whatever RPM you've run the motor at, the valves were floating.

What springs do you have? Have you measured their seat & open pressures? I believe that would be a great place to start. Have you ever run the engine at high RPMs (like above 6k)?

Ukraine Train

i changed springs when i put the cam in, they are at about 350# at full lift. the engine has only run a total of about 20 minutes with the new cam and no more than 3500 or 4000 rpm.

at this point i've pretty much exhausted every option for the valves hitting except the valve timing being off. is it safe to use the afore mentioned timing specs from dyno2k to degree my cam?

F-BIRD'88

Don't degree your cam in your motor based on someone
else's degreeing results.

First find true top dead center. You can do this with the heads on
with a piston stop and a degree wheel. Most cams like your Hot CAM will run best when advanced 4-5 degrees from straight
up. GM publishes .050" duration and LSA specs so use these
to time your cam. If you have a "matching set" of timing gears
your's should check out within 1 deg either way.
I once replaced only the cam gear while installing a new cam and
found the cam to be 15 degrees advanced from straight up
after degreeing it. ( the crank gear rarely wears because it's hardened). Now I always replace them in matched sets.

Your cam does not have enough lift at or near top dead center
(overlap) to run into valve/piston interference. (normally)

If your valves are hitting the pistons then you have a serious timing mismatch or an alignment mismatch. The head may not be located properly on the block or the piston valve reliefs are
not in the right spot on the pistons.
If the valves did hit the pistons, they are probabily bent now.

The .050" timing numbers for a cam with 218/228 duration@.050
ground on 112deg LSA when advanced 4 deg, would be

intake opens 1* BTC intake closes 37* ABC

exhaust opens 50* BBC exhaust closes (-2) ATC **{note}**

**{note}** actually closes @ 2deg before top center resulting in a negative timing number.
The .050" timing midpoints are 108*in and 116*ex .
These may not be the points of maximum lift points if the lobes are aysemetrical.

Ukraine Train

i found my problem last night. when i put the new timing set on, i aligned the cam gear dot to the 0 right above the keyway on the crank gear instead of the 0 that's three teeth in the counter clockwise direction. so i fixed that and put the driver's head back on w/clay on the piston, i now get .252" of clearance between valves and piston. thanks for everyone's help.

Vader

U-Train,

What timing set are you using? Is it one of those three-way sets? Just curious...

wasp

just a thought cuz im no cam wiz but i know i have an slp cam with 4 degrees built into it so it must be installed straight up from what they tell me..........

a73camaro

Great you found the problem! Just a little curious...

You gonna fire cfabe

cfabe

He's using a 3 keyway timing set, I think it was a dynagear. The markings were really faint and we just didnt see the other 0 degrees marking, so we lined it up with the tooth above the 0 degree keyway. It made sense at the time. I guess this is one of those things you just learn from experience. I'm really surprised it ran as well as it did 4 teeth off.

Anyway, if he can find a balancer isntaller that isnt missing the big bolt then she should be running within a few hours.

Agian thanks for all the help guys.

cfabe

Yea I installed the thing but he thought it was right as well.

I just hope I did it right on my engnie, I can't recall now, it was like 6 weeks ago. I used a comp cams set that had good instructinos and I followed them to the letter, so I think I'll be ok.

a73camaro

Just a friendly little ribbin' is all.

I've done plenty off OOPS's in my lifetime. And I know I'm not finished.

Shows good character when one can admit that they made a mistake.

Ukraine Train

cfabe and i got her running tonight, definately sounds a lot better without parts crashing into each other. thanks for everyone's help.