COLLAPSED PISTON

[1980_Cj7]

Yes, you got something wrong. You added 15 degrees to the actual intake valve closing point, not the closing point at .050 which is usually wrong anyway.

So when I'm using the Silvolite calculator, I should be using cam specs based on .050 events, right? So since the cam specs that AMX69Phatty posted for our Clevite cam say the intake valve closes at 34* ABC at .050" lift, I should use 34 + 15 or 49*, right?

Sounds like the stock cam specs aren't at .050 because of how AMX69Phatty deducted to get an assumed .050 event.

[82Waggy]

CJ7, You are correct. It should be the closing event degrees at .050 plus 15 - or 49 as you have indicated.

Your cam card should specify when the intake valve closes. Another way to determine when the valve closes is to take 1/2 Advertised Duration Plus the intake Lobe Centerline angle minus 180 degrees to come up with the closing angle After Bottom Dead Center. In this case you would not add the 15 degrees when using the calculator.

Note that adding 15 to the .050 closing event on any cam is a guess. The actual closing event may be more or less degrees away from the .050 event.

[AMX69PHATTY]

1980_Cj7, yes, for your Clevite Cam I used 34* + 15* = 49*
in the SilvoLite Calculator and it gave a Dynamic Compression of 8.63:1
with the Ross pistons you have, and a Static compression of 9.7:1

When I changed the value in the SilvoLite Calculator to 60 degrees,
which assumes a cam with a .050 Intake closing spec of 45 degrees ABDC
and then add the 15 degrees to that SilvoLite says to, and use 60 degrees
in their calculator, I then got a Dynamic Compression of 8.0:1 thereabouts.
And this was with the same Ross pistons and Static CR of 9.7:1

The KB 354 pistons come out more or less the same as the Ross units
because the KB's are a little taller but have a larger dish volume.

I'm amazed with all of this cause my 365 Motor had no detonation problems.
Per the SilvoLite calculator it had 9.53:1 Static and 7.93:1 Dynamic Compression,
and when I just checked, the cam had an Intake closing angle of 45 degrees ABDC @ .050 lift !
I did not do that on purpose when the 365 was built, but it looks like I was just dumb lucky
WOW ! I'm amazed .....

So it sounds like if you find another cam with an Intake closing angle
of 45 degrees or more ABDC @ .050 Lift you could be good to go with the parts you had.
Of course open up the bores on the pistons for more clearance.
It looks like the Clevite Cam was designed to create more Dynamic Compression
in a motor that was stock or built with LOW Static Compression.
Over the counter aftermarket replacement 401 cast pistons are for 8.5:1,
but your motor already had HIGH Static Compression with the Ross Pistons,
and therein is where the problem llies, ....... it seems ( escape clause )

[Blown7]

1980_Cj7,
but your motor already had HIGH Static Compression with the Ross Pistons,
and therein is where the problem llies, ....... it seems ( escape clause )

You don't need a escape clause because that was the major problem , and I will go so far to speculate that the cam was timed wrong ,and if it was installed in the retarded position it probably had even higher dynamic compression then the calculator shows.

Jeff

[82Waggy]

You meant advancing the cam would close the intake valve earlier.

Here is an interesting article on cam selection - probably more than you want to know. http://www.compcams.com/Community/Articles/

Scroll down to the article titled "Be The Camshaft Expert".

I'm going with a custom grind from Crower for my 401, similar to their part number 45241, http://www.crower.com . This cam has an actual intake closing point of approximately 65 degress ABDC and would work well with a static compression ratio in the 9.5:1 range.

[1980_Cj7]

We have a brand new Comp Cams camshaft sitting here (not the one that is currently in the engine). Here's the specs on it. Just when I thought I was getting a handle on some of this stuff, now I'm seeing new things like "valve timing at .006" and "cam installed at 106 intake clearance" and "duration at .006 tappet lift". What does that mean, and would this cam be any better? I'm thinking not. Seems like a little too much lift and duration, and the intake closes only 6* latter than the one we have in now (if that .006 is the right value to be looking at).

I was going to run the calculator, but I'm uncertain of some of the terms and what values I should use.

Camshaft Specification Table
Part Number 10-210-4
Engine 1966-1979 American Motors Co.
290ci-401ci
8cyl.
Grind Number A 292H-10
Description

--------------------------------------------------------------------------------

Intake Exhaust
Valve Adjustment 0 0
Gross Valve Lift 0.518 0.518
Duration At 0.006 Tappet Lift 292 292

--------------------------------------------------------------------------------

Valve Timing At 0.006
Open Close
Intake 40 72
Exhaust 80 32

--------------------------------------------------------------------------------

These Specs Are For The Cam Installed At 106 Intake CL
Intake Exhaust
Duration At 0.05 244 244
Lobe Lift 0.3235 0.3235
Lobe Separation 110

--------------------------------------------------------------------------------

Recommended Valve Springs 986-16

[82Waggy]

Way too much duration for what you are trying to do- you would have no bottom end power.

Read the article I listed above. It will clear up a lot of the questions you have on cam specs.

CL = Centerline, not clearance
The intake valve closing point is 72 deg ABDC
.006 refers to tappet lift and is used for the duration spec here because hydraulic lifters spring about that much before the valve begins to move.

[AMX69PHATTY]

Not sure if this cam would work for you or not.
Cam suppliers list working rpm ranges, but any one given cam
listed for an AMC also says it's for anything from a 290 to a 401,
so since yours is a 401 I would think the power band would be lower.
Do you have a Cloyes True Roller Timing Chain with the 4* Advance & Retard Keyways ?
If so, you could install this cam 4* Retarded, giving a 44* Intake closing @ .050 Lift.
Although Retarding the Cam does shift the power band RPM range upward.
It does have less duration, 228* IN / 235* EX @ .050, than the Crower.
It may be a little "radical", but putting it in a 401 would "tone it down" a little.
I think you can order a cam ground to whatever specs you want from any supplier.
They pretty much don't grind an AMC Cam until they get an order for one anyway.
Maybe try calling Crane, Crower, Iskerdian, Lunati, or whoever you prefer and ask'em.

Cam Suppliers give their specs in at least 2 different ways.
One is called "adverised" duration, which is usually at .004-.006 lift.
The other duration spec which is kinda "standardized" is at .050 lift.
The area on the cam lobes between these two durations is called the "ramp".
This is the area where the valve is opening and closing on the valve seat in the cylinder head.
During this time between .004 and .050 valve lift air flow is negligble.
The .050 lift is used as a standard so a person can compare one cam to another.
Cause if one suppliers specs are at .002 and anothers at .008 one can't compare'em.

[fuzz401]

that cam is close to what I have
Grind Number A 292H-10

[1980_Cj7]

Read the article I listed above. It will clear up a lot of the questions you have on cam specs.

Yes, I glanced at the article, but just didn't get chance to read it yet. Looks like a really good article with lots of great supporting illustrations. I'm looking forward to studying it. Thanks for the link.