Anyone running The GSCA Pop Mech cam in thier 455?

Thanks, Jim.
That's exactly the kind of real-world info I needed! I suspected it wasn't possible, but the numbers said it was.
I was most likely going to go with a dished piston, just to be safe, but it never hurts to ask.
Here are the numbers I ran:

 

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Jim,
I ran the numbers on that combination (assuming a 22cc dish, 10 out of the hole, 65cc heads and the TA294-98F cam) and found the dynamic compression to be in the 8.15 range, without any advance on the cam. With 4 degrees advance on the cam, you would be looking at 8.45 DCR. This would explain the detonation, as it's on the high side for pump gas.

 

The issue is simple..

Cylinder pressure..

You either need to close the intake sooner, shorten the overlap, or bump the compression, to achieve optimum cylinder pressure to make the HP..

The program is trying to simulate this number.. but the variable here is cylinder filling. And that's all about cylinder heads and intake, along with the cam, headers ect..

Try this one..

20 CC dish pistons
0 deck
67 CC heads
4.400 gasket bore
4.350 cylinder bore
3.9 stroke

Cam - 238/238 @.050 That 290@.006 .. 110 LC. cam installed at 106*.

See what you come up with.

JW

 

Actual specs for that motor are..

4.350 bore
3.9 stroke
.040 gskt (4.400 bore size on gasket)
65 CC heads
.002 in the hole.
20 CC dish in the piston

Aluminum heads, which generally speaking, can withstand .8 more compression, to operate on the same fuel.. due to thermal heat loss.

JW

 

What is the advertised intake and exhaust duration on that cam?
I understand what you're saying about the cylinder pressure- how intake duration and overlap and all that affect it. That's why I can't understand why I couldn't run that GS112 cam with 320 322 and 97 deg. overlap, since it's bleeding off so much cylinder pressure (that is, if I can trust Poston's specs. That seems like a MONSTER of a cam for what it was intended to be used in). When I calculate the DCR, it is way on the low side with a normal compression ratio like 10:1.
Cylinder filling and intake velocity is something of a mystery to me still. I have been trying to get more information to get a better understanding, but there just isn't much available to read (at least on the net). I would venture a guess that one of the reasons why these way-too-big cams are working so well in an unported head motor is that they are ground in such a way that the scavaging is being helped, and the velocity is being kept up... there is definitely something going on there. There is no way that a low compression motor should work with a cam like this, but it does work for some reason. Any thoughts on this?
I trust what you're saying... just trying to get a better understanding of what is really going on.

 

A word of warning...... make absolutely sure that the cam you buy is actually ground to the specs advertised. There are some issues with some of the cam grinders ( not going to badmouth anyone), but it has been an issue with my buildup. All i am saying is make sure you know what you are actually installing, or you may not get the desired result. Then you won't have any doubts about it. Brian :Smarty:

 

ALWAYS degree your cam before installation. Like Brian said, there seems to be an awful lot of cams out there that are not ground to their specified numbers. Sometimes they're so far off that they are unusable for the intended application.

 

why don't you let us know who it is, so we will know? It's not bad mouthing- you're just sharing your experience with what you got from them.

 

Jim,
What pistons did you have that had a 20CC dish? Are they still available?

 

Port velocity is what allows it. With higher port velocity, the port flow stalls later in crankshaft degrees than an engine with large, high-volume ports. I would assume that a set of track eliminator heads would idle worse and have less low-end torque than a set of iron heads with the same long-duration cam. I don't have the personal experience to back it up, but I'm sure others that have swapped will know.

 

Adam,
That's kind of what I thought was going on. I am almost afraid to touch my heads with this type combo, because it might screw the whole thing up by reducing velocity if I port them.
Anyway, I know it works from what I've seen in real life, so I'm going to duplicate that combination (iron heads, 107).
If I listened to desktop dyno, it would say that it makes 400 hp max, but I know that it's not possible. 12.20's with a 4100 lb. car equates to more like 495 hp. Most likely it's not that much. I'm sure a lot is due to the torque it's making (1.68 60' time). Desktop dyno also says it shouldn't make much torque at all... go figure.
Maybe someone should look at these old designs and figure out why they work so well!

 

Yes. It has to do with the maximum lifter velocity (in inches per cam degree) achieved. Cam lobe profiles are determined/designed by acceleration, velocity and position (sometimes jerk and yank if you're actually concerned with minor details like avoiding breaking valvetrain components due to high impact loading). What limits velocity or rate is lifter diameter, if the velocity is too high, the lobe/lifter contact point will not be on the face of the lifter (that's bad). The max velocity that can be used by a certain lifter diameter is: vel = (diameter/2) / 57.3 . You want to have some margin, usually 0.025" and can be as little as 0.015" . For our .842" lifters, the max is .0073"/cam deg (zero margin).

My guess with an agressive cam that won't rev is that the acceleration is just too high, the lifter can't follow the closing deceleration right after max lift.

My guess with an aggresive profile that breaks stuff is that the change in acceleration (jerk) is too high, resulting in high impact loads.

 

Be careful with advertised numbers, they're just that, advertising. At which point are those actually measured? Depends on the grinder. Comp cams uses 0.006" tappet lift. Crane uses 0.0045" lift. GM used used something different, an SAE J-something standard. If you're going to compare DCR numbers, it's best to use the Comp standard for advertised, since the majority of the Chevy guys use those, and the DCR "knowledge base" is most familiar with DCR numbers obtained from those cams.

 

I guess I should rephrase my question:
How can I tell if a camp is a fast ramp design? Is there any way to tell how "fast" the lobes of a certain grind are? From the manufacturer's specs, there is nothing I can see that will clue you in.
I was suspicious of the manufacturer's advertised duration on the GS112... enough so that I wouldn't trust my DCR calculations and build a motor by it. I just don't have the money to waste doing it a second time.
Thanks for the info.

 

If that's a symmetrical pattern (opening and closing) cam, I get IC = 71, IO = 39, EO = 79, EC = 31

 

You're right, you can't tell from the spec's. You (or someone else)would have to either use a cam doctor, or simply degree the cam in. But while degreeing, you would take more measurements, say every 0.050" or every 10 deg, then plot your results on a graph and estimate velocity from that.

You could also find someone that has used that cam, measure the wear on the lifter to get the margin from the edge, and estimate veocity from that.

But, the most accurate way would be to ask the grinder.

 

Yep... I get the same thing.
The static compression is 10.79:1 and the DCR is 8.15:1