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Old 05-31-2013, 11:35 AM
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custom grind cam specs

These numbers look right for a blown 6-71 sbc? Follwed F-birds instructions but not sure of the readings. Suppose to be a custom grind but the numbers on the end don't tell me who made it. May be a custom grind from Comp Cams but haven't heard back from them.

LSA 115
Intake Centerline 112
IO 25
IC 82
Intake lift .336, max .504

Exhaust Centerline 118
EO 82
EC 67
Ex lift .341, max .512
Intake duration 287
Exhaust duration 329
overlap 92 degrees

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Old 05-31-2013, 01:04 PM
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Blower Drive Service, the Roots blower guys who have been in business for decades, has told us that the best power on gasoline will come from a 110 lobe separation angle. They say that a wider angle may work better on alcohol, but you need to tighten it up for gasoline.
Read through their recommendations here.....
http://www.blowerdriveservice.com/recommend.php
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Old 05-31-2013, 01:30 PM
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110 regardless of the rest of the combo? I would call a good cam grinder and talk to them they should set you straight. There is no magic lsa, it's all application dependent.
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Old 06-01-2013, 04:55 AM
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based on your timing numbers this cam is on a 108LSA. It is very retarded. 118in 97 ex c/L's based on the
valve timing.
Not a blower cam.

What is this? something you are reading? Something you measured?
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Old 06-01-2013, 05:04 AM
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The cams on the BDS site are Isky cams.
Call Isky. They will get you dialed in.
www.iskycams.com
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Old 06-01-2013, 06:02 AM
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Quote:
Originally Posted by F-BIRD'88 View Post
based on your timing numbers this cam is on a 108LSA. It is very retarded. 118in 97 ex c/L's based on the
valve timing.
Not a blower cam.

What is this? something you are reading? Something you measured?
I measured it using your methods. I done it till the numbers were duplicated, that's not to say that I didn't have something wrong. It's in the blown sbc I bought from a widow. I found some other questionable things in there too. The numbers didn't seem right, That's why I asked.
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Old 06-01-2013, 06:41 AM
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Measure the open and close timing points @ .050" lifter rise.
Be sure to find true TDC using a piston stop to set the degree wheel first.

Use a solid lifter when degreeing, or if using a hyd lifter set up the dial indicator on the edge of the lifter body, not the plunger.

Is this a flat tappet cam or a roller cam?
what are all the numbers stamped on the ends?

Last edited by F-BIRD'88; 06-01-2013 at 06:51 AM.
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Old 06-01-2013, 07:19 AM
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Quote:
Originally Posted by F-BIRD'88 View Post
Measure the open and close timing points @ .050" lifter rise.
Be sure to find true TDC using a piston stop to set the degree wheel first.

Use a solid lifter when degreeing, or if using a hyd lifter set up the dial indicator on the edge of the lifter body, not the plunger.

Is this a flat tappet cam or a roller cam?
what are all the numbers stamped on the ends?
Hyd lifters-flat tappet
Numbers are:
B4686, 5447 129 H34, H11464 and either a 3 or backwards E, kind of a squigglely thing.
Here's the method I used which I got from this site. Tech Inspector I think?
Note that the readings are at .006"


You have a short block there. Attach a degree wheel and go through the motions, just like degreeing it in the first place if you were building the motor.

I know you know how to do this, but I'm gonna outline the procedure for others who don't have the knowledge.....

Procure a piece of round steel, 1.000" O.D. by 12" long. Chuck it up in the lathe and turn the O.D. down to 0.841"/0.842, then cut it off about 8" long and finish both ends flat and smooth. This will be your checking tappet. Those of you who work on different makes of motors can make this checking tappet Chevy diameter on one end and another diameter on the other end for checking cams on a different motor. With the block tilted on the engine stand so that the bore of one bank is straight up and down, your checker tappet will follow the cam contour easily due to its weight. Set up your dial indicator on the other end of the checking tappet by using a magnetic base on the block deck.

First off, on a SBC, the front (forwardmost) tappet hole for #1 cylinder is the exhaust tappet. The intake tappet hole is second from the front of the block adjacent to the #1 cylinder.

Find top dead center on the #1 piston, either by using a positive stop bar across the bore (best way) or by using your dial indicator on the piston crown as you swing the piston through several degrees each way from what you perceive to be top dead center. With the piston at TDC, zero your degree wheel with your timing pointer (can be as simple as a piece of clothes hanger wire coiled up on one end to attach under a bolt head and sharpened to a point on the other end).

With the dial indicator on the intake lobe for #1 cylinder, roll the crank clockwise to the highest reading on your dial indicator. This will occur somewhere between 100 and 115 degrees after top dead center going clockwise and will be your intake centerline.

We'll say this point was 106 degrees ATDC. Write it down as "IC-106".

Roll the crank counter-clockwise until you are off the lobe of the cam and the dial indicator is no longer retreating. Zero the dial indicator. Now, come clockwise on the crank until you are showing 0.006" tappet rise on the indicator. Let's say this occurs at 38 degrees before top dead center on your degree wheel. This is the intake opening point at 0.006" (also called "advertised") tappet lift. Write it down as "IO-38".

Roll the crank clockwise through TDC, on around to BDC and a little past until the dial indicator again shows 0.006" tappet lift. This will be the intake closing point. Let's say it occurs at 70 degrees after bottom dead center. Write it down as "IC-70".

With the checking tappet on the heel of the lobe, zero your dial indicator. Roll the crank clockwise until you get the highest reading on the indicator. Let's say it's 0.375". Write it down as "Intake lobe lift-0.375". Multiply the lobe lift times your rocker arm ratio to find valve lift. (example 0.375" times 1.5 equals 0.5625")

Now, re-position your checking lifter into the exhaust tappet hole and set up your dial indicator. Run the crank through a cycle or two to get the highest reading on your indicator. Let's say this occurs at 114 degrees before top dead center. This is the exhaust centerline. Write it down as "EC-114".

We can now determine with this information, what the lobe separation angle of the camshaft is. Add the intake centerline (106) to the exhaust centerline (114) and divide by 2. We find that this cam is ground on a 110 degree lobe separation angle (LSA). Just to make it all the more confusing for you, the LSA is also called Lobe Displacement Angle or Lobe Center by some.

Turn the crank until there is no movement on the indicator and the checking tappet is on the heel of the exhaust lobe. Zero the indicator. Turn the crank clockwise until you observe 0.006" lift on the indicator. This will be the exhaust opening point. Let's say it occurred at 78 degrees before bottom dead center. Write it down as "EO-78".

Rotate the crank clockwise through max lift and on down to where the indicator is recording tappet lift of 0.006". This will be the exhaust closing point. Let's say that this occurs at 30 degrees after top dead center. Write it down as "EC-30".

With the checking tappet on the heel of the lobe, zero your dial indicator. Roll the crank clockwise until you get the highest reading on the indicator. Let's say it's 0.375". Write it down as "Exhaust lobe lift-0.375". Multiply the lobe lift times your rocker arm ratio to find valve lift. (example 0.375" times 1.5 equals 0.5625")

Now, you have all the information you need. On the intake, you add 38 and 70 and 180 and find that the advertised intake duration of this cam is 288 degrees.

On the exhaust, you add 78 and 30 and 180 and find that the advertised exhaust duration of this cam is also 288 degrees.

You know that the intake centerline is 106, the exhaust centerline is 114 and the lobe separation angle is 110 degrees.

You can add the intake opening to the exhaust closing and find that the overlap at advertised cam timing is 68 degrees.
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Old 06-01-2013, 03:14 PM
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when measuring @.006" the oening and closing side are very different and you are much more prone to a error.

Measure again but at .050" lifter rise. if your checking lifter does not have the very slight convex face you will get a slight error.
But the .050" numbers will show you what cam it is.
You can also repeat at .200"
from this you will find the cam lobes and cam LSA etc based on the .050" timing numbers,,, not the points of max lift.

On a modern non symertical cam lobe these will be different.

Again, the relevant specs are @ .050" and .200" lifter rise open/close.

Last edited by F-BIRD'88; 06-01-2013 at 03:37 PM.
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Old 06-01-2013, 03:23 PM
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Not all hyd cams are measured at .006" ... some are at .004" .0045" etc
solid lifter cams are typically speced at .015" or .020" advertized duration.

but the @ .050" open/close timing points are what you really need.

with the .050" and the @.200" rise timing numbers you will be able to tell one lobe from another
in a cam lobe library.

Never degree in a cam to determine its correct installed position or to adjust that position, using the seat timing spec.

use the .050" specs. Way more accurate.

Once you have found TDC to postion the degree wheel, only rotate the crank in one direction ( normal direction of rotation) to get the .050" open close timing numbers. Use a finger on the lifter to keep it riding on the cam lobe as required to get accuaracy

A hyd lifter can be used but you must read off the edge of the lifter body to avoid plunger error.

you can use cylinder #1 or #6

Last edited by F-BIRD'88; 06-01-2013 at 03:35 PM.
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Old 06-01-2013, 04:15 PM
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Quote:
Originally Posted by F-BIRD'88 View Post
when measuring @.006" the oening and closing side are very different and you are much more prone to a error.

Measure again but at .050" lifter rise. if your checking lifter does not have the very slight convex face you will get a slight error.
But the .050" numbers will show you what cam it is.
You can also repeat at .200"
from this you will find the cam lobes and cam LSA etc based on the .050" timing numbers,,, not the points of max lift.

On a modern non symertical cam lobe these will be different.

Again, the relevant specs are @ .050" and .200" lifter rise open/close.
Thanks for the info but I'm not clear on what the .200" reading is for.
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Old 06-01-2013, 05:42 PM
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Quote:
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Thanks for the info but I'm not clear on what the .200" reading is for.
Cam lobe libraries include a duration @.200" helps you define the cam lobe and
tell the difference between lobes with the same .050" duration and lobe lift.

with the .050" duration and .200" durations and the lobe lift you can find the cam lobes.
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Old 06-02-2013, 10:15 AM
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Originally Posted by F-BIRD'88 View Post
Cam lobe libraries include a duration @.200" helps you define the cam lobe and
tell the difference between lobes with the same .050" duration and lobe lift.

with the .050" duration and .200" durations and the lobe lift you can find the cam lobes.
Great big thank you coming your way! Been trying to find what cam I have for over a month. Your info did the trick.
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Old 06-03-2013, 06:28 AM
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So, what are the measured .050" and .200" cam timing specs, Now...?
if the cam or any of the 16 lifters are showing any wear at all,,do not reuse.
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Old 06-03-2013, 05:04 PM
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So, what are the measured .050" and .200" cam timing specs, Now...?
if the cam or any of the 16 lifters are showing any wear at all,,do not reuse.
Measurements, numbers on the end of the cam and the cam log link and a call to comp cams showed it to be a NX284H. (over kill? lol ) All the lopes n lifters look great. Now if I can get the block level I'll cc the cylinder. Ideas? It's on a engine stand. Here's some pix of the pistons. Any idea what they are and who made them? They're forged, didn't notice any manufacters ID when I had the block turned over to look at them. They are at TDC so an educated guess at cc would be great. Looking for 8-8.5 CR with 72 cc heads altho lower would be nice.
Please note that I'm handicapped and can't just roll the engine over. Many years ago when I was young and didn't need it I had an engine stand with a geared crank on it. Haven't been able to find another one.
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