Negative overlap cam

Negative overlap cam

Joined: December 11th, 2010, 3:10 pm

September 8th, 2011, 1:45 am #1

To shorten the story some, I looked at the Lunati "factory Replacement" cam for D2ZZ-6250-A. According to the cam card it has -5 degrees overlap. So the exhaust actually closes before the intake opens. So the negative pressure of the exhaust can not aid the intake charge. The same cam appears in the Elgin Pro stock listing. How does this work? Not well is probably the answer. Please chime in. Thanks, Chuck
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Joined: October 1st, 2004, 8:05 pm

September 8th, 2011, 2:16 am #2

Chuck

what you've gotten your hands on are the Cobra Jet camshaft timing event specs @ 0.050" tappet lift. Cam vendors who list timing events @ 0.050" tappet lift are full of bovine excrement. When we are considering camshaft timing the timing event data must represent actual valve movement, when it comes to hydraulic camshafts we normally use the advertised duration data and call it "close enough".

The Cobra Jet camshaft actually has 46 degreees of overlap. It makes about 30 BHP over the M code camshaft, and operates over a very wide powerband. Peak horsepower will be in the range of 5600 to 5800 rpm, depending upon the combination. This camshaft was used in the 390 GT and 428 Cobra Jet too. Its a very well liked camshaft for a daily driver type motor.

-G
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Joined: December 11th, 2010, 3:10 pm

September 8th, 2011, 6:42 pm #3

for the information. So how do you compute the actual overlap of that cam? Thanks, Chuck
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Joined: December 16th, 2003, 1:59 am

September 8th, 2011, 6:54 pm #4

simply add the time in degrees that the Exhaust is open After TDC and the time in degrees that the Intake is open Before TDC

the Elgin E-806-S D2ZZ-6250-B cam is listed at 63* overlap on my sheet

but yeah Elgin has a bunch of .050" specs with -numbers in the valve timing events, just to avoid changing ATDC to BTDC ?

there'll be phantoms, there'll be fires on the road... and the white man dancing
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Joined: October 1st, 2004, 8:05 pm

September 8th, 2011, 7:30 pm #5

for the information. So how do you compute the actual overlap of that cam? Thanks, Chuck
just add the intake valve opening spec (degrees BTDC) plus the exhaust valve closing spec (degrees ATDC).

-G
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If you use a 351C 4V powered vehicle for a grocery getter ... the eggs aren't going to make it home!
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Joined: October 1st, 2004, 1:55 pm

September 8th, 2011, 7:39 pm #6

for the information. So how do you compute the actual overlap of that cam? Thanks, Chuck
The overlap can be setermines a couple of ways. The easiest way is to add the intake opening times to the exhaust closing time. So, for example, if you know the Intake opening time is 30 and the Exhaust closing time 30 you just add them together and then the overlap is 60.

If you don't know the valve timing events then you can calculate overlap by using the advertised duration numbers and the Lobe Separation Angle (LSA), but do NOT use the duration numbers @ .050 lift. Here is the calculation:

1. Add the intake and exhaust advertised durations
2. Divide the results by 4
3. Subtract the LSA
4. Multiply the results by 2

So if you had a 280/280 degree cam intake and exhaust with a 110 LSA the calculations would be:

1. 280 + 280 = 560
2. 560 ÷ 4 = 140
3. 140 - 110 = 30
3. 30 x 2 = 60

The Lunati cam you mentioned has an overlap of 57 degrees. Let's see why.

1. 280 + 290 = 570
2. 570 ÷ 4 = 142.5
3. 142.5 - 114 = 28.5
4. 28.5 x 2 = 57

The overlap is 57. I hope this helps.

Edited for clarity.
Last edited by Luc69stang on September 8th, 2011, 7:41 pm, edited 1 time in total.
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Joined: October 1st, 2004, 8:05 pm

September 9th, 2011, 6:46 am #7

for the information. So how do you compute the actual overlap of that cam? Thanks, Chuck
the Lunati D2ZZ-A-FOMOCO camshaft is not a copy of the Cobra Jet cam, its a modified version of the D1ZZ-BX camshaft, the 290/290 - 0.505"/0.505" cam.

I saw D2ZZ-A and just assumed. That was wrong. Since Lunati has modified the specs, I can't be specific about its characteristics like I can the original.

-G
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If you use a 351C 4V powered vehicle for a grocery getter ... the eggs aren't going to make it home!
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Joined: December 11th, 2010, 3:10 pm

September 9th, 2011, 11:58 am #8

George, I got this from Lunati.

Chuck,



The events are done at .050" of lift. If you figure the events at .006" of lift this is what they are. Hope this helps.



Thanks,



Shane



Duration IN.
280
Duration EX.
290
Lobe Sep.
114
Adv.
5
Open
Close
Intake
31
69
Exhaust
84
26
IN. Center Line
109
EX. Center Line
119
Over Lap
57

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