Big difference in cam specs for dynamic compression?

[MopaR&D]

I am looking at cam specs for calculating my dynamic compression ratio in my 360 LA with Magnum heads and noticed something that doesn't make sense... Dynamic compression is based off how late the intake valve closes after BDC, and the specs for "similar" cams between manufacturers seem to be vastly different. My current Lunati Voodoo 256/262* cam has an intake closing of 34.5* ABDC and gives me a dynamic compression of 8.7. I want to put in a longer-duration cam to bleed off some pressure at lower revs as my engine has 10.5:1 compression with the .060"-over KB flat tops and pings under certain circumstances (hot days, lower octane gas, etc.). The popular Voodoo 268/276* cam has intake valve closing at 39* ABDC and drops the compression only a little bit to around 8.55. However looking at the similar COMP XE268 cam the intake closing is a whopping 60* ABDC!! What the heck is going on here? Using that cam my dynamic compression would drop to the low 7's which would be lower than I need. Am I missing something here or are the cam profiles really that different across manufacturers? And would changing to the 268 Voodoo cam make any difference in reducing ping at lower RPMs?

 

[frosty_the_punk]

The lunati voodoo 268/276 and XE268 are are both cut on a 110LSA with 4 degrees advance and both have their IVC at 60.

Edit. Is it possible you are comparing the lunati 262/268 which is cut on a 112 and +4 to the XE268?

Lunati 256/262 = IVC @56
Lunati 262/268 = IVC @59
Lunati 268/276 = IVC@60
Comp XE268H = IVC@60

 

[MopaR&D]

How are you getting that, from calculation or another source? Please enlighten me I'm confused because the website says differently lol

 

[frosty_the_punk]

I calculated them using the seat-to-seat valve timing specs, lobe separation and intake centreline.

The numbers you calculated (34.5, 39 etc) are based on 0.050" lift values.

 

[MopaR&D]

Ah now I understand thanks... I guess that's the problem I didn't calculate the numbers I just read them off the cam card on their website. I also now see the dynamic CR calculator says to use the seat-to-seat timing values. Using that spec my current setup is at 7.8:1 does that sound right (I'm at 5500' elevation)? Then swapping to the 268* cam it drops down to 7.5:1 do you think that would make any difference in lower octane tolerance?

 

[frosty_the_punk]

No worries, that's what these forums are for

The lower dynamic compression ratio achieved with the 268 cam would help reduce the chance of pinging, but before you do that it's important to check that your ignition timing is setup properly as it is the single most important variable to eliminate pinging.

To begin with, try retarding the distributor a fraction or two at idle. If this makes the car run rough (even slightly) you've gone to far

Common practice is to find a shop that will change the ignition curve of your distributor to suit your camshaft. In my experience this usually costs about $100 and makes a world of difference to your engine's overall performance.

You can also experiment with different head gaskets to increase the combustion chamber volume. summit racing has a pretty large selection.

 

[MopaR&D]

I have the timing set to around 8-10* at idle and 32* mechanical; I recently bumped it down 2* from what it was before and it helped reduce the pinging noticeably. When I first got the motor done it ran fine on mid-grade gas with a little pinging but after having 10,000 or so miles put on it it's definitely more prone to ping; I'm assuming this is from carbon buildup in the chambers and piston faces. The 256/262 Voodoo cam in it now is definitely too small for 360 cubes and 10.5:1 compression.

I'm actually looking at a particular Howards cam, it's a single-pattern grind with 275* duration and .470" lift on a 108* LSA... With my 1.6:1 rockers the actual lift would be around .500" and the longer duration bumps my dynamic CR down to 7.3:1. I think this will complement my engine better as it currently has brutal bottom-end torque but it runs out of breath by 5000 RPM. I could stand to have the torque curve moved up the RPM band a bit and not be so incredibly 'flat'.

 

[MopaR&D]

Has anyone used that Howards cam #710991-08? Haven't heard much talk about their cams but they are available through most of the big suppliers (summit etc.)