I'm pretty sure that
"Valve lift is calculated with zero lash and rocker ratio of 1.75"
is a typo - should've been 1.5 (.290" x 1.5 = .435").

Also, when Isky says "Lobe Center: 108*" that's their spec for LSA (according to an old catalog).

http://www.iskycams.com/timingchart.php?product_number=111262

Here are my points of confusion:

I was trying to determine the intake center line, and LSA from the info given in the top section of the timing chart.

It looks to me like the ICL should be
(262/2)- 23, or
131 - 23 = 108 ATDC, which = 72 BBDC

And the exhaust center line would be
(262/2)- 59, or
131 - 59 = 72 ABDC

which leads me to conclude that the LSA is 144*, since (72 BBDC + 72 ABDC) = 144*

This can't be right. Every LSA spec I've seen for these engines is somewhere between 107 and 112.

Am I doing something wrong, or is it possible that the intake and exhaust lobes are asymmetrical (making the highest point of lift not dead center of the total duration)?

That was the first question, here's the second.

In the middle section of the link they list some specs at .050" lifter rise:
...............Intake..........Exhaust
Duration...208................208
Open.....-4BTDC...........32BBDC
Close....32ABDC...........-4ATDC

What's the deal with the minus signs?

At first I ignored them and thought the
open + close figures + 180 would = duration, or
4 + 32 + 180 = 208 That's wrong, it = 216

But, when the minus sign is used
-4 + 32 = 180 = 208
which checks out with the specs

Do the minus signs indicate asymmetrical lobes?
I don't get it.

Are both the ICL and LSA for this cam 108*?

Some Crane cams have minus signs in the specs too.

Glen,

The "tappet" lift @ TDC on that lobe should be approx .042.So that means that at TDC the tappet has not yet reached .050...So that @ -4 BTDC ( witch actually means 4 ATDC is where .050 " Tappet " lift would be.Hence the term -BTDC....I agree it's stupid but they print the same timing card for all grinds and it's only the small cams that useually have " Tappet " lifts under .050 @ TDC.

That deal of diving the duration by 2 would suggest a symetrical lobe...It wont work correctly with an Asymetrical lobe since each side looks differently .

When I talked with Ron when I ordered up my BBC cam We batted around different LSA's ... he wanted 110 and I wanted 108...So we setteled on 110 as I figured he knows his lobes better than I do....Any way he told me mine is ground advanced relitive to the pin...I located mine on a 107 ICL...3 ahead... worked great.

As another example as I recall from decades ago my old 097 solid was ground on a 110 with 108+112 = 220/2 = 110.Installed it on a 106 .. two ahead.

The old pre 929 hyd I "think " was ground on a 107 and both in on 107's.... 107+107=214/2 = 107.I dont know if the 262 Isky is all 108 but it is very possible.

Look at the "Lobe Center" (108º) and the "cam advance" (0º). This tells you that the LSA is 108º with no advance; the ICL is 108º

I suggest ignoring the .050" timing values. They are totally useless to man or machine. The -4BTDC is actually 4ºATDC, the same with the exhaust (-4ATDC is 4ºBTDC). The seat timings at the top of the card are important and useful.

The error occured in (262/3)-59=72. The correct numbers for exhaust are (262/2)-23=108, the same as the intake. This cam is symetrical and 'straight-up'.

UDHarold

Al,
Thanks for the explanation. I think I thought it might be something like that, and thought "no, they wouldn't do that - they'd call it 4ATDC". I guess they do.

I don't know if you noticed, but this 262 is for the 60* V6. Something in your answer made me curious enough to go back to Isky's site and check specs on the 262 grind for different engines. The V6 and sbc 262 are identical - the bbc is almost the same, it has more lift.

It also looks like the V6 lifters are the same as for the sbc - Crane's solid lifters for the V6 are also a p/n for the sbc. This is good news to me.

I've checked several cam sites and all but Isky have only dual-pattern cams, and Crane is the only one I found with a solid cam. They have two, and I don't like the looks of them.

I don't see any reason Isky couldn't make a Z-25 or Z-30 for the V6 on a special order basis, do you? I thought I was going to be stuck with a hydraulic, but maybe not.

Pat and Harold,
Maybe I don't understand the definitions of ICL and LSA.

I thought ICL was the center line of the intake lobe in degrees going clockwise from TDC.
That would be:
(262/2) - 23 = 108 ATDC
That much I understand - and advancing or retarding the cam will alter the ICL, but not the LSA since the lobes stay put on the shaft.

Isky calls LSA "Lobe Center".

I can't figure out where the "Lobe Center (LSA) = 108*" comes from.

I thought LSA was the amount of degrees going clockwise from the center of the intake lobe to the center of the exhaust lobe.

So, if the ICL is the center line of the intake lobe, the amount of degrees clockwise from there to the ECL would be the LSA.

Am I wrong?

The ECL would be (262/2)- 59 = 72 ABDC

Since it's 180* from TDC to BDC, won't the ICL of 108 ATDC also be the same as 72 BBDC (180 - 108)?

It looks to me that the LSA will be the amount of degrees between 72 BBDC (ICL) and 72 ABDC (ECL), or 144.

Harold, you said that the ECL would be
(262/2)-23=108

Wouldn't that 108 be in terms of BTDC?

And wouldn't ECL of 108 BTDC = 72 ABDC?

I'm lost - maybe I misunderstand the concept of LSA altogether.

Glen,

Cam mfgs are at the mercy of costs....It cost them money to test and evaluate various lobes...it also costs alot of money to print and distribute catalogs.

They have to make a decision as to what cams to list under all the different engines that they offer cams for.Since SBC , BBC and SBF's are the most popular they have alot more cams available for them than say a V6...Most V6's owners never even think of changing cams...But V8's guys to it all the time.

Yes it is entirely possible to have a V6 cam ground to some other spec.....This is almost embarrassing but the lobe I have currently is listed a Mopar Slant 6 cam.... Very small mechanical cam that fits my needs perfectly.My last 327 had a Crane mechanical that they also showed as a Slant 6 lobe....No problem haveing them ground on a Chevy core.All ya have to do is ask them and see if it's possible to do what you are thinking about.
They often dont offer these different cams for non popular engines because they figure they are not going to sell many anyway so they use that money to print cam listings for more popular engines.

One of the problems here is that of terminology. There are different words that mean the same thing.

Lobe Seperation Angle.
Lobe Displacement Angle.
Lobe Centers.
These all mean the same thing.And different mfgs call them different things.This is ground into the cam and cant be changed....except by regrinding that core....

Glenn1018,

Your confusion comes fro assuming the intake valve opens first; It does not.
The exhaust valve opens first, then the intake. LSA, or Lobe Seperation Angle, is measured from the exhaust center line, wherever it is, to the intake centerline, wherever it is. This angle, the LSA, is ground into the camshaft by the manufacturer, and cannot be changed without regrinding the cam.
The Intake CenterLine, ICL, can be put wherever you wish, and the Exhaust CenterLine follows along, keeping the LSA constant.
LSA is figured in CAM degrees, and intake or exhaust centerlines are in CRANK degrees, 2 crank degrees for every cam degree.
The Isky had 108 Exh CL, and 108 Int CL, for 216 crank degrees, or 108 CAM degrees.
If the Isky had been advanced 4°, the Int CL would be 104, and the Exh CL would be 112, for 216 crank degrees, or 108 CAM degrees.
I hope this sheds some light on the 2 different 'lobe centers'.....

UDHarold

Al and Harold,
Thank you both for the help.

After my last post I went to the junkyard (no luck) and came home and thought over this LSA issue a while, and fell asleep watching the race (never fails).

I figured that I was measuring the distance between lobe centers the wrong way. I was looking at the cam timing illustration, and, starting at TDC going CW to the intake CL, and then going CW from there to the ECL in hopes of obtaining the LSA.

Doing that is backwards, as Harold just said. That method would make cause the overlap to decrease as the LSA is made smaller. That's not right - that's backwards.

Looking at the picture threw me off because I just knew (no, I didn't know) there was no way the LSA was found by going from ECL to ICL since it's plain to see it would be over 180*.

I was measuring the angles from BDC instead of TDC.

I was also forgetting to divide by 2. graemlins/clonk.gif

Once again, thank you very much.