151 valve timing question

[King M.]

I am trying to degree a comp cams 12-106-3 replacement cam for the l79 151. Engine is set on top dead center on degree wheel and timing chain marks are timed accordingly. Dot match dot at tdc. I get a 222 duration for both intake and exhaust and a 114 lobe seperationand .447 valve lift. My question is the card shows:
.001 int 61 btdc open and 101 abdc close. I am showing 45 open and 72 closed

does anyone have the .050 valve time for the 151? I am showing 0 opening and 138 closing.

any help is appreciated

thanks
king

[King M.]

Re: 151 valve timing question

Would I be 4 degrees retarded like installing i oem factory cam straight up?

[Duke W.]

Re: 151 valve timing question

The approximate .050" lifter rise timing is 1-41, 49-(7). The POMLs are 110 deg. ATC/118 deg BTC, which yields a LSA of 114 deg. Isn't that data on the cam card? The data you posted matches the factory print, but I've always been suspicious of Comp Cams products that claim they are identical to OE. I suspect that the lobe dynamics are more aggressive, so durations at small values like .004" may be shorter than OE. Your "138" is reading has no context. It needs to be stated in degrees ABC, which is probably 42 (180-138), which checks out within the accuracy that this data can be measued with a degree wheel and dial indicator.

The duration at .004" lifter rise is 290, but you can't use this data to compute the POMLs because the lobe is asymmetrical below about .25" lifter rise. On the opening side it takes 29 degrees to go from .004" to .050" lifter rise, but on the closing the .050" to .004" angle is 39 degrees. The POMLs should be determined using the mid point of measured angles at no less than .25" lobe lift.

Why didn't you buy a Sealed Power CS-179R? They are ground to the L-79 cam OE print.

Is the cam Parkerized? That would be indicated by the lobes having a mottled grey finish and the journals shiny silver.

Also, for L-79s I recommend installing the 3996962 (L-46/82, 224 degrees .050" duration, Sealed Power CS1095R) camshaft, but it needs to be installed with an adjustable timing set to advance the as-ground 114 deg. ATC inlet POML back to 110, which is the same as the 3863151 inlet POML.

Why? It's a later design with better lobe dynamics.

Duke

[Kenn S.]

Re: 151 valve timing question

Not to hyjack the thread, but I'd planned on using the CS-179R in my '80 L-48 (with flat pistons, hardened rod bolts, and pushrod guides, so much like the L-82). I'd used the 151 cam in a base '70 with excellent results.

But now you have me wondering if I shouldn't use the L-82 cam (which I just picked up as well, in case...whichever I don't use, I'll save for another car).

So the question would be going the other way, the 3996962 is the better cam grind for the L-48, for the same reasons I would assume?

[Duke W.]

Re: 151 valve timing question

The ...151 or ...962 with low compression is a bad combination. CR should be raised to at least 10:1 with domed pistons, not to exceed 10.5. The 151/962 cams will shift the torque curve up the rev scale, so you get more top end power and useable revs, but lose power on the bottom end. The crossover point is 3500-4000. If you have a manual trans and relatively short axle ratio, that may be okay, but it's a poor choice for an automatic with a tall axle ratio.

One of the reasons the L-82 was a poor performaner relative to the L-46 after 1974 is the single catalyst exhaust. Exhaust pumping power eats up most of the indicated power increase. Dave McClellan stated in his book that the single catalyst exhaust increased exhaust back pressure by "half an atmosphere". That's over 7 psi above the 3 psi of the full dual exhaust system on a high output small block, and that 10-plus psi backpressure is HUGE, which just kills output.

There's little point in trying to get "more power" from the single catalyst exhaust models. It's much better to design the engine to achieve very high torque at low to medium revs, where exhaust backpressure is relatively low. It increases approximately with the square of WOT engine revs.

Duke

[Kenn S.]

Re: 151 valve timing question

Gotcha. I'll save the 151 for a future car. In the '70, the 10:1 base engine purred like a kitten at idle but roared down the road. Was amazing.

I can neither confirm nor deny that this car will be equipped with true dual exhaust, for off-road use only, of course, if it were. If it were, it would be 2.5" pipes to low-restriction mufflers, using headers.

[Duke W.]

Re: 151 valve timing question

Headers won't do much. Even the modest backpressure from a true dual exhaust will negate the wave dynamics, then there are the other headers issue like excess radiant head transfer to the engine comparement, burned spark plug wires... If you still have the manifolds and related hardware, use them.

Duke

[King M.]

Re: 151 valve timing question

image.jpgDuke this is the card that came with the cam. I used this cam simply because it was given to me a while ago and was sitting on the shelf. I will go back and check the 138 ABDC to verify. Thanks for your response, I appreciate your input.

[Duke W.]

Re: 151 valve timing question

I'm surprised the cam card doesn't list the .050" lifter rise duration. From the OE lobe data I measured 342 degrees duration at the lash point, so it looks like what you have matches OE specs or is very close.

I'm still curious if the cam is Parkerized as indicated by a gray mottled finish on the lobes, but the journals would be silvery. What do you observe?

Duke

[King M.]

Re: 151 valve timing question

Duke, thanks for your opinions. The cam does not have the grey parkerizing. It is blackened on the cast ares between and on the sides of the lobes. The coating has a fine texture to it and by fine I mean using your fingernail it has a roughness to it. Unlike bluing on metal. So, in your opinion, the cam's valve timing is correct?

[Duke W.]

Re: 151 valve timing question

As long as you can covert the "138" degree reading to 42 ABC as I suggested in my first response, then it looks like it's ground to the GM print. I have no way to know for sure because I don't know the context of you "138" degree reading.

Inlet valve timing is usually specified as opening at degrees BTC and closing at degrees ABC. If you measure duration at the lash point or some low lift these data are usually positive numbers, and duration is the sum of the two plus 180.

On the exhaust side opening is specified as degrees BBC and closing as degrees ATC, and if measured at the lash point or some low lift value the numbers will be positive.

However, at .050" lifter rise inlet opening can be after TDC and exhaust closing before TDC. In this case the numbers would be negative and are indicated as such by being enclosed in parenthesis as done in financial accounting.

This is the case with exhaust closing with the L-79 cam at the as designed indexing of 110 deg ATC inlet POML. So again the duration is the arithmetic average plus 180: 49 -7 +180 = 222.

It's not clear to me what surface finish this cam has. Do the lobe faces appear to have the same or different finish than the journals? Usually the rest of the cam casting is sort of a dark brown. If Parkerized the cam will be grey except the journal, because the Parkerizing is polished off as part of the finishing procedure, so the journals appear to be clean polished cast iron which is silvery.

Duke

[King M.]

Re: 151 valve timing question

Duke, Spoke to a tech at comp cam today and he told me that the 0.050 reading should be (3)-45 intake and 45-(3) exhaust. Mine is degree wheel is showing 0-42 for intake, 49-(7) for exhaust. What is your thoughts on the variance of the readings? I also asked him what coating was applied to the cam. He informed me that it was a black parkerized finish that they used. I can forward you a picture of it if need be.

[Gene M.]

[Duke W.]

Re: 151 valve timing question

Duke, Spoke to a tech at comp cam today and he told me that the 0.050 reading should be (3)-45 intake and 45-(3) exhaust. Mine is degree wheel is showing 0-42 for intake, 49-(7) for exhaust. What is your thoughts on the variance of the readings? I also asked him what coating was applied to the cam. He informed me that it was a black parkerized finish that they used. I can forward you a picture of it if need be.

Your readings are within measurement tolerances of the specs I listed in a previous post. How did I come up with them? I calculated .050" timing points to the nearest whole degree from the engineering data on the GM drawing, which is lobe lift, in inches, to five decimal places.

As you might have surmised, I'm not a big Comp Cams fan. They have lots of failures and their "tech line" is some ten-buck an hour guys reading off a script. The .050" timing data they gave you yields 222 deg. duration on both side, but those timing numbers would be for the cam advanced 3 - 4 degrees from OE indexing, which I do not recommend.

Take a photo of the some lobes and at least one journal and post it here. It's been my understanding in the past that they did not Parkerize cams, but now it is "optional at extra cost".

Duke

[Donald O.]

Re: 151 valve timing question

Your readings are within measurement tolerances of the specs I listed in a previous post. How did I come up with them? I calculated .050" timing points to the nearest whole degree from the engineering data on the GM drawing, which is lobe lift, in inches, to five decimal places.

As you might have surmised, I'm not a big Comp Cams fan. They have lots of failures and their "tech line" is some ten-buck an hour guys reading off a script. The .050" timing data they gave you yields 222 deg. duration on both side, but those timing numbers would be for the cam advanced 3 - 4 degrees from OE indexing, which I do not recommend.

Take a photo of the some lobes and at least one journal and post it here. It's been my understanding in the past that they did not Parkerize cams, but now it is "optional at extra cost".

Duke

What about the lifters? Should there be any special treatments if new with new cam?

Doesn't advancing the cam 4* produce more lower rpm torque and retarding give it more at the top rpm? I could have this reversed though.

DonO