Too Hot of CAM?

[82Waggy]

cj7,

You had Ross forged pistons in there before, correct?

That cam has a lot of duration and valve overlap for Jeep motor. the actual intake closing point is considerably later than what you used in your calculation. I know KB says use the intake closing point at .050 lift plus 15 degees, but this is just a guess to help people use the cam card specs. The actual closing point with 294 duration at .006 tappet lift is 72 degrees - assuming the intake CL was set as specified.

To shorten up the discussion, I don't know what you finally arrived at for bore diameter after clean up but I would choose a piston that closes that quench distance to .045 (zero deck) and gets your static compression closer to 9:1 - coupled with a dish piston such as the KB 354 (28cc) you may also need to open up the head chambers to around 65cc to acheive this.

For a good torque motor in a Jeep, I would then choose a cam with a 112LSA and overlap of about 32-35 degrees. An intake closing point of 56 degrees ABDC should keep cylinder pressure in check with the above setup.

You are running stock unported heads, right? If so, an Engle 5016H cam or something similar would fit the bill.

PS: Those are some really slow lobe ramps on that Clevite cam = 294 duration @ .006 and 218 @ .050.
Compare that to an Engle 5020H = 270 dur @ .006 and 226 @ .050.

[1980_Cj7]

Yes, sorry, we had Ross forged pistons. I edited my post above.

And yes, stock unported heads.

[82Waggy]

OK.

I forgot to clarify in my previous post that closing the quench distance should improve detonation resistance - in theory if you have good compression (9.5:1 for example)and then lower it by increasing quench distance (piston to head clearance) it is possible that the engine would be more prone to detonation due to a more non-definate combustion event - which is kind of the situation you had.

[eight]

The only cam I find that matches your low overlap numbers is the Comp 252 which has 206/206 at .050, .433/.433 lift, 110 LCA, and 32 degrees overlap. That's a very small cam.

What do you think of the comp 260? It's 212/212 at .050, with .447/.447 lift, 110 LCA, and 40 degrees overlap. There is also a crane pt#863904 that is 260/272 total with 204/216 at .050, .456/.484 lift, 112 LCA, and 42 degrees overlap. And there's another comp I don't know the name of, think it's one of their 4x4 cams, but it's 256/268 total with 212/218 at .050, .447/.484 lift, 110 LCA, and 42 degrees overlap. These are the only cams I found with low overlap, most others with durations close to these are 50+. The little edelbrock has 63 and the summit 8600 is 53.

[Dusty]

Interesting mix of engines you have there Dusty. Your assessments would seem to coincide with the combination of cams and compression ratios used. Do you have a favorite?


Bottom line is if you are not going to increase compression ratio from stock, stay away from long duration cams.

Personally the 8600 was too small for the 401's i felt it really cut the motor off at its knees


i would say the 9.8:1 with the comp 262EX w/Performer a holley 670 or/ with a edelborck 750 was a really fun, worked well. but thats just because its most recent


Though Let me say that the Custom grind single pattern from crower 274 dur with 226@.050 and .512 lift on a 112 lobe sep that was in the 10.2:1 motor now has 58cc cast iron heads on it with mild Exh port and polish w/ gasket matching and cc's out to 9.72:1 with a performer and the Edelbrock 750 is near the top of my list. We put my buddys 670 on it and it picked up a little bottom end get up and go but the 750 is what i have sitting here so its what is on there now. I really like this combo very mild lope at idle.... idles at 650-700 rpm.




I'll say on stock compression that 292 dur 222@.050 dur .479 lift and 114 lobe seperation on the 8.5:1 motor worked really well

[82Waggy]

The only cam I find that matches your low overlap numbers is the Comp 252 which has 206/206 at .050, .433/.433 lift, 110 LCA, and 32 degrees overlap. That's a very small cam.

What do you think of the comp 260?

There are several cams with overlaps less than 40 degrees from Engle or Crower.

What compression ratio and heads are you running?

[82Waggy]

Here are several cams along the lines of what I have been talking about.

Note the fast ramps of the Engle's - lower overlap for low rpm torque and high .050 duration. This is one of the advantages of the AMC - its' large diameter lifters can handle faster lobe ramp rates.


CRW258H,Eng35018H,Eng5052H,Eng5016H,Eng5018H,Eng50 54H
ILCL, 109, 109, 109, 109, 109, 109
ELCL, 115, 115, 115, 115, 115, 115
LSA, 112, 112, 112, 112, 112, 112
Int@006 258, 248, 254, 256, 260, 260
Exh@006 264, 260, 254, 256, 260, 260
Int@050 204, 207, 209, 210, 216, 214
Exh@050 210, 216, 209, 210, 216, 214
Int Lift, 445, 466, 488, 472, 488, 501
Exh Lift, 448, 496, 488, 472, 488 501
IntOATDC,-20, -15, -18, -19, -21, -21
IntCABDC , 58, 53, 56, 57, 59, 59
ExhOBBDC, 67, 65, 62, 63, 65, 65
ExhCBTDC,-17, -15, -12, -13, -15, -15
Overlap, 37, 30, 30, 32, 36, 36

PS: dual plane intake manifolds, free flowing exhaust (headers and dual pipes), and higher displacement to intake valve size can handle more overlap. Higher compression ratios need to follow later intake closing points.