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Author Topic: How do I determine valve timing events from cam specs?  (Read 3606 times)
danford1
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« on: October 19, 2014, 09:01:04 pm »

What is the formula(s) for determining valve events from given cam specs?
Lets take a mild street cam, a Comp XE264HR.
The data is 264/270 advertised, 212/218 @ 0.050", 114 LSA. ICL 108.
Yes I can go to Comp's site and look up the answers, but I want to know how to figure out the answers.
How do you figure all 4 valve events for 0.050" lift?
How do you figure all 4 valve events for 0.006" advertised lift?
Thanks
Danford1
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Joel5.0
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« Reply #1 on: October 20, 2014, 08:11:09 am »

Here you go......

Here are the formulas used:

CAMSHAFT SPECS FORMULAS
1.Intake Duration  (ID)IO +180 + IC degrees
2.Exhaust Duration  (ED)EO + 180 + ECdegrees
3.Intake Center Line   (ICL) ID/2 - IO degrees ATDC
4.Exhaust Center Line  (ECL)  ED/2 - EC   degrees BTDC
5.Lobe Separation Angle (LSA)    ICL/2 + ECL/2degrees
6.Lobe Separation Angle (LSA)    (ICL + ECL)/2degrees
7.Valve overlap IO + ECdegrees
8.Valve overlap (ID + ED)/2 - LSAdegrees
9.Intake Opening  (IO)ID/2 - ICLBTDC
10.Intake Closing  (IC)ID - 180 - IOABDC
11.Exhaust Closing (EC)ED/2 - ECLATDC
12.Exhaust Opening (EO)ED - 180 - ECBBDC

For your example above:
ICL = LSA - degrees advanced = 109 - 2 = 107
ECL = LSA + degrees advanced = 109 + 2 = 111

For the intake events:

IO = ID/2 - ICL = 250/2 - 107 = 18° BTDC
IC = 250 - 180 - 18 = 52° ABDC
To verify: ID = IO + 180 + IC = 18 + 180 + 52 = 250°

For the exhaust events:

EC = ED/2 - ECL = 260/2 - 111 = 19° ATDC
EO = ED - 180 - EC = 260 - 180 - 19 = 61° BBDC
To verify: ED = EO + 180 + EC = 61 + 180 + 19 = 260°

If you get a (-) result for the IO or EC events, it means it is on the other side. For example, in the exhaust for a X-303 112LSA +5:
EC = 224/2 - 117 = 112 - 117 = -5° ATDC which is 5° BTDC


For your case:

IO = (212/2) - ICL = 106 - 108 = -2° BTDC or 2° ATDC
IC = 212 - 180 - (-2) = 34° ABDC
To confirm..... ID = 34 + (-2) + 180 = 212°

ECL = (LSA - ICL) + LSA = (114 - 108) + 114 = 120°

EC = (218/2) - ECL = 109 - 120 = -11° ATDC = 11° BTDC
EO = 218 - 180 - EC = 38 - (-11) = 49° BBDC
To confirm...... ED = 49 + 180 + (-11) = 218°

Repeat the above steps using the Advertised specs.
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danford1
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« Reply #2 on: October 20, 2014, 08:56:55 am »

 Thank you Joel5.0. That is exactly what I was looking for.
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Danford1
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Those who say it can't be done should not interrupt those that are busy doing it.


Dart 376 SHP block, 4.155" bore, 3.47" stroke, Woody custom cam, TFS 11R 205, ported Vic Jr, 750 mechanical carb. 1 5/8" Hedman headers. 517 Flywheel hp, 498 Flywheel tq.
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