How easy it is to cause confusion with cam numbers?

[Stan Weiss]

I see people post I am running a cam with these specs. Or someone asks the question what cam should I run and gets different specs from a number of people. How is one to know which cam is correct. This post is not going to answer that question but confuse the issue somemore.

These are all flat tappet solid lifter cams: Which one is right for my engine.

cam a - lobe lift 0.425 - 0.012 lash
246.5 @ 0.050
217.5 @ 0.100
172.0 @ 0.200

cam b - lobe lift 0.429 - 0.018 lash
249.0 @ 0.050
220.0 @ 0.100
173.5 @ 0.200

cam c - lobe lift 0.433 - 0.024 lash
252.0 @ 0.050
222.0 @ 0.100
175.5 @ 0.200

cam d - lobe lift 0.437 - 0.030 lash
255.0 @ 0.050
224.0 @ 0.100
177.0 @ 0.200

While the physical lobe on the shaft is not the same. The actual design lobe minus the lash ramp is identical. If you take each of the lobe lift time 1.5:1 rocker arm ratio you get gross lift. If you subtract their lash from their gross lift all 4 have the same net / valve lift and your 0.050, 0.100 and 0.200 at the valve will be the same. My point begin that looking at numbers from the physical lobe does not tell the whole store. So duration at lifter raise does not always tell the story.

So when someone posts up numbers you better look a little deeper than just the raw numbers

Stan

[Stan Weiss]

227bhp said:

None of it makes any sense to me, we've been metric since 1971....

Does metric numbers help you any?

cam a - lobe lift 10.795 - 0.304 lash
246.5 @ 1.27
217.5 @ 2.54
172.0 @ 5.08

cam b - lobe lift 10.8955 - 0.4572 lash
249.0 @ 1.27
220.0 @ 2.54
173.5 @ 5.08

cam c - lobe lift 10.9982 - 0.6096 lash
252.0 @ 1.27
222.0 @ 2.54
175.5 @ 5.08

cam d - lobe lift 11.0998 - 0.762 lash
255.0 @ 1.27
224.0 @ 2.54
177.0 @ 5.08

Stan

[Stan Weiss]

Pumaracing said:

What were the valve clearances set to with the std and the high ratio rockers?

Dave,
That question bring up an interesting point. The lash point is set by the lobes lash ramp design. What that means is no matter what rocker arm ratio is used the lash at the valve should be adjusted based on that ratio. This means if calculated correctly the seat-to-seat duration will always be the same no matter what ratio rocker arm is used.

Stan

[Stan Weiss]

Pumaracing said:

The seat to seat duration yes. The effective (some decent flow is now taking place) duration at say 1mm valve lift changes of course.

But not by as much as one might think. The actual stock rocker ratio of A series rockers is about 1.24. The 1.5 rockers are therefore about 20% higher.

However this only increases the effective duration at 1mm net valve lift by about 4 degrees, PROVIDED the valve clearances are adjusted up from the nominal 16 thou on the stock rockers to 19/20 thou on the high lift ones. If this is not done then the effective extra duration at 1mm lift increases to 6 degrees and also risks the running clearance disappearing when the engine is hot leading to all sorts of nasty happenings. Burnt out valves etc and the cylinder not sealing properly.

So just getting the valve clearances wrong can account for 50% of the undesirable effects of fitting the high lift rockers in the first place.

To fully compensate for the high lift rockers the lobe separation angle should be increased by 2 degrees and the valve clearances increased by 20%. Not doing this increases the effective cam profile by about 1 point so from a rally cam to a race one or a race one to a drag one.

Dave,
Would you have any camshaft profile lift data that I can examine, even if it is from the stock cam.

Stan

[Stan Weiss]

Pumaracing said:

Do you mean specifically from an A series engine?

Dave,
Yes, since you talked about duration changes at the valve at different lifter raises because of changes in rocker arm ratio. I would like to get a better idea of the profile on which you are basing these statements.

Stan

[Stan Weiss]

Dave,
Thank you very much. What would the lash be with 1.5:1 rockers. That has a nice long constant velocity lash ramp. So getting the lash to tight would add a good number of seat-to-seat degrees of duration.

Stan

[Stan Weiss]

Pumaracing said:

The lash on that particular profile with 1.5 rockers would be 15 thou. Not quite as much as the MD310.

Dave,
Thanks. That is what I had guessed. The seat-to-seat duration of 320 degrees is what had me questioning myself.

Stan

[Stan Weiss]

Since I do not know what the ICL is I may have it off a little.

Rocker Arm Ratio = 1.500 - Valve Lash = 0.0150

VALVE___Lift______Opens___ClosesDuration
_______________Deg_BTDC__Deg_ABDC___________Area
_______0.00000__40.00_|_100.00_|_320.00_|__44.64
_______0.00600__34.29_|__86.67_|_300.95_|__44.54
_______0.01000__30.48_|__82.22_|_292.70_|__44.54
_______0.02000__26.04_|__76.19_|_282.23_|__44.30
_______0.04000__18.64_|__68.06_|_266.70_|__43.75
_______0.05000__16.17_|__65.28_|_261.45_|__43.75


Rocker Arm Ratio = 1.300 - Valve Lash = 0.0130

VALVE___Lift______Opens___ClosesDuration
_______________Deg_BTDC__Deg_ABDC___________Area
_______0.00000__40.00_|_100.00_|_320.00_|__38.69
_______0.00600__33.41_|__85.64_|_299.05_|__38.60
_______0.01000__29.57_|__80.51_|_290.08_|__38.58
_______0.02000__24.76_|__74.73_|_279.48_|__38.39
_______0.04000__17.12_|__66.35_|_263.47_|__37.92
_______0.05000__14.27_|__63.14_|_257.41_|__37.92

Stan

[Stan Weiss]

Pumaracing said:

Wotcha wanting Stan? Marks out of 10?

Dave,
Are you feeling OK? Your posted seem to be more barbed than normal.

Stan

[Stan Weiss]

Pumaracing said:

No barb intended Stan. Just wondering what exactly you were trying to demonstrate with the numbers you posted as you didn't explain any further. I thought perhaps it was homework you wanted me to check.

Dave,
Unlike this and some of my other posts that one was not addressed to you. I was showing everyone what I got, which also included what the areas were for the cam with and without the higher ratio rockers.

If you want to act like a teacher and grade me that is up to you.

Stan

[Stan Weiss]

Pumaracing said:

I'm sorry Stan, we're clearly at complete cross purposes here. I thought as it was my cam data and you asked if you could check my statement that the high lift rockers only made a very small number of degrees change to the effective duration that you "were" addressing it to me and that the "I may have it off a little" part was asking me to check something. I also didn't know what you meant by ICL.

But you used a different std rocker ratio than I did so I couldn't really tell if you were agreeing with me, disagreeing, asking me to check something or what.

Looking at it all again and allowing for me using 1.24 ratio and you using 1.3 we seem to agree that the effective duration change is small. Perhaps less than one might assume without doing all the maths I have to say.

Edited by Pumaracing on Friday 14th August 23:29

Dave,
You are correct, I did ask for the data to be posted based on what you posted. I also have a short term memory problem as I use Peter's Rocker arm ratio and not your.

Rocker Arm Ratio = 1.240 - Valve Lash = 0.0124

VALVE___Lift______Opens___ClosesDuration
_______________Deg_BTDC__Deg_ABDC___________Area
_______0.00000__40.00_|_100.00_|_320.00_|__36.90
_______0.00600__33.09_|__85.27_|_298.36_|__36.82
_______0.01000__29.33_|__79.95_|_289.29_|__36.62
_______0.02000__24.29_|__74.19_|_278.49_|__36.62
_______0.04000__16.57_|__65.73_|_262.30_|__36.17
_______0.05000__13.58_|__62.37_|_255.95_|__36.17

Here is your 1.24:1 and now there is third set of durations and areas to compare.

Stan