Dynamic Compression Question
 

Does rocker arm ratio affect valve timing when calculating Dynamic compression?

I would say yes, it does. the bigger the rocker arm ratio, the larger the lift and cam duration at the valve. Having bigger rockers would increase dynamic compression as long as the intake valve still closes before BDC. If the added duration caused the valve to close after BDC, it would shorten your dynamic compression. Same goes with smaller rockers. If they shorten your valve duration enough, you can increase your dynamic compression, but it can be shortened too if they take away too much duration. Now, depending on the cam duration, if your intake valve closes after BDC, adding larger rockers will always decrease your dynamic compression. Running smaller rocker arms in the same situation will always increase your dynamic compression as long as the lost duration doesn't put you back into the "BBDC".

Is there any way to calculate the change? I’m running a CC305 camshaft (part # 07-305-8) and I’m going from a 1.5 to a 1.7 ratio rocker arm. I assume by looking at the Dynamic Compression Calculators that I need the change for the Intake valve timing (ABDC), please correct me if I’m wrong. The cam specs are listed below.

Duration:
Intake: 276 Exhaust: 290

Duration @ .050” lift:
Intake: 220 Exhaust: 230

Valve Lift:
Intake & Exhaust: .51 w/ 1.5 rocker arms
.578 w/ 1.7 rocker arms

Lobe Center Angle: 114

Intake Center Line: 110

Valve Timing @ .006” lift
Open Close
Intake: 28 BTDC 68ABDC
Exhaust: 83 BBDC 27ATDC

First, we have to through some numbers around. looking at the cam card on cc's page, lift with 1.7 is .581". now, what we really have to look at is openings and closings of both valves. the card says intake opens at 28* BTDC E and closes at 68* ABDC C and for the exhaust, 83* BBDC P and 27* ATDC I W/1.5 rockers. Although this is for .006 lift, it's the most accurate when calculating Dynamic Compression (DC). (TTL VLV DUR)*(2/3)= LOBE DUR * 1.7= (NEW VLV DUR)-(OLD VLV DUR)/2= added duration at opening and closing. Lamens terms: 276/3*2= 184*1.7=312.8-276=36.8/2=18.4+28= Intake opening at 46.4 degrees BTDC E, closing (18.4+68=) 86.4 ABDC C. You can do the exhaust numbers. I'll figure out exactly how much this changes your DC later tonight. I have no engine i can physically measure right now, so i have to do it the long way:lol:

Thanks for your help cusz28!! I’ve been searching the internet and reading for the last week and didn’t even come close to figuring it out. The only thing that I found is that the 1.7's would give me a couple more degrees of duration.

not a problem. i also found a really cool way to figure everything out on the computer. i made a 2d sketch in alibre and made a line from the center origin 3.48/2 long, then added a 5.7'' line to the end of that and added a constraint to the center vertical axis of the origin. for better sense, i made a stick figure crankshaft and connecting rod that followed the actual path of a crank and rod. now, i can plug in an angular dimension to the "crank throw" and have the computer actually tell me the difference from where the end of the 5.7" line is (this would be where the piston pin is). say, i plug in your original intake opening as an angle of the crank throw, take a measurement, and move it tdc. no bulls**t trig to sort out, just pure numbers. glorious.... i'll have all the numbers for you tomorrow afternoon.:D

Seat to seat duration also known as advertised does not change - there will be a very minuscule change in the .050" numbers as the higher ratio rocker will accelerate the valve open to that point faster.

Think about that before you get too involved as the compression ratio change will be almost nil as it's due to how much air can flow in that short .050" to dropped on the seat.

seat to seat duration does change with bigger rockers. your increasing both your duration and lift. the point is to find what the dynamic compression is, valve flow is moot in this subject. in order to accurately calculate dynamic compression, actual lift has to be used, not .050. if you just look over the math i did ealier again, you can see that changing from 1.5 to 1.7 rockers has a HUGE affect.

So you're trying to tell me that putting on a 1.7 ratio rocker all of a sudden magically changes the cam lobe and makes the valve open earlier :think:

Why don't you stew on that for a while before answering ;)

^^^^^what he said. Think about it this way. Does changing the rocker change the point on the cam where the lobe just starts to push up on the lifter?? No.

After some more searching I came across an article that clears things up.

http://www.pontiacstreetperformance....ockerArms.html.

Thanks for the help.

ok, i see. i stand corrected about adding duration. but, you still wanted to figure out your dynamic compression? i finished the numbers for piston lift. these will help anyone with a 3.48 stroke and 5.7 rod. I measured piston lift for every degree of revolution from BDC to 90*. these values will be the same for 90*BBDC-BDC and BDC-90* ABDC. The numbers are measured from crankshaft center to piston pin center. subtract 3.960 for the actual lift value per 1* rotation. Ab 90 is absolute 90* as you have to count zero and you end up with an extra number. I will crunch the numbers from 90* to TDC tomorrow as they are different. Enjoy!
sorry if this is long.

ab 90 5.428
90 5.398
89 5.368
87 5.338
86 5.308
85 5.278
84 5.249
83 5.220
82 5.191
81 5.163
80 5.134
79 5.106
78 5.078
77 5.051
76 5.023
75 4.996
74 4.969
73 4.943
72 4.917
71 4.891
70 4.865
69 4.840
68 4.815
67 4.790
66 4.766
65 4.742
64 4.718
63 4.695
62 4.672
61 4.650
60 4.627
59 4.605
58 4.584
57 4.562
56 4.541
55 4.521
54 4.501
53 4.481
52 4.461
51 4.442
50 4.424
49 4.405
48 4.387
47 4.369
46 4.352
45 4.335
44 4.319
43 4.303
42 4.287
41 4.271
40 4.256
39 4.242
38 4.227
37 4.213
36 4.200
35 4.187
34 4.174
33 4.161
32 4.149
31 4.138
30 4.126
29 4.115
28 4.105
27 4.095
26 4.085
25 4.075
24 4.066
23 4.058
22 4.049
21 4.041
20 4.034
19 4.027
18 4.020
17 4.013
16 4.007
15 4.001
14 3.996
13 3.991
12 3.987
11 3.982
10 3.978
9 3.975
8 3.972
7 3.969
6 3.967
5 3.965
4 3.963
3 3.962
2 3.961
1 3.960
0-180 3.960

Changing RR does not change the calculated DCR as it does not change the nominal valve opening point. However, it makes the motor act (when it's running) as if the cam were larger (longer duration) and thus slightly decrease octane requirement and low end torque, etc., similar to a lower DCR.

Rich