Degreing a cam??

[Terry F.]

Hi, just a question about camshafts. When you degree a camshaft, is the purpose to double check the exact opening of the intake valves with the crank position? Thanks, Terry

[Jim R.]

Re: Degreing a cam??

some cams have 4 degree or so built in to them ,so what yor checking is for accuracy from the cam builder, they have been off a degree or two sometimes

[Mark #28455]

All depends on how you do it

If you use an 8" degree wheel and a paper clip bolted to a timing cover bolt, you're likely to have enough error in the measurement that the 1 or 2 degrees the cam is off won't even show up! In the old days, cams were frequently several degrees off, now with the newest computer controlled equipment used by the better manufacturers, the cams are usually right on.

If you're going to check, you may want to start with a degreed harmonic damper using a bolt-on timing tab. THEN, check the accuracy of what you have - check TDC on the damper using a piston stop. Now you can start checking the cam, but remember, some cams have asymmetrical lobes so you can't just split the difference and assume that's the lobe centerline. To be accurate, you need to really look at the 0.050" lift figures (now divide by the rocker arm ratio to find camshaft lift) and check it out. (Is your dial indicator in the same axis as the lifter)

Now that you discovered what a PIA it is to do it correctly, realize that if the cam is a degree or two off in a street motor with a full exhaust, you will only lose a few horsepower.

So, the best answer is to get your cams from one of the major cam manufacturers that measures their cams' accuracy in tenths of a degree and go from there. By the way, once you have a 1000 miles or more on the engine, your timing chain will stretch some and all your degreeing work will be retarded anyway! (why not set the cam up a degree advanced to start with?)

For a pro racing motor, it's important, for a street motor, it only gives you bragging rights.

Mark

[Duke W.]

Re: Degreing a cam??

A more descriptive phrase is "verifying camshaft indexing". Some cam vendors don't even list point(s) of maximum lift (POML) in their cam specs, but may say something esoteric like the cam is "ground with four degrees advance".

As a mininum, camshaft "specs" should consist of duration at a specified lifter rise above the base circle and at least two of the three following specifications:

1. Lobe separation angle, camshaft degrees
2. Inlet POML, cranshaft degrees ATDC
3. Exhust POML, crankshaft degrees BTDC

An easy formula to remember is that LSA is the arithmetic average of the two POMLs, so if LSA and one POML is specified, you can figure out the other POML, or if both POMLs are specified you can figure out LSA.

For example, the L-79 cam has inlet indexing of 110 degrees ATDC and the LSA is 114 degrees. Therfore the exhaust POML is 118 degrees BTDC. This is considered "4 degrees advanced" in aftermarket jive. If the L-79 cam is retarded 4 degrees the inlet POML would be 114 degrees ATDC, exhaust POML 114 degrees BTDC, and, of course the LSA remains 114 degrees. The aftermarket calls this "straight up".

All confusing gobbledygook, but they like to keep their customers ignorant so they can be snowed with marketing BS.

The aftermarket also talks about "centerlines", which is a misnomer because all '57-up OE camshafts, except the Duntov, have asymmetric lobes, so the POMLs does not correspond to the "centerlines".

Since I have all the detailed lobe design data for the OE cams and have analyzed their dynamics, I was interested in doing the same for some aftermarket lobes as part of my research to develop the "special 300 HP camshaft". When I asked for detailed lobe data, all I got was blank stares, so I selected a vendor that claimed to have exact duplicates of the two OE lobes I finally selected.

Checking lobe indexing can be a problem if you don't know the cam's asymmetry characterisitcs. It's best to check indexing by selecting two points on either side of the POML in the area of highest velocity, which is around half lobe lift, but this can get into the asymmetric portions of the lobe, which will yield inaccurate results.

All of the above are just a few reasons why I much prefer OE camshafts.

Duke

[Jim R.]

Re: Degreing a cam??

good point duke

[Terry F.]

Re: Degreing a cam??

Probably a stupid question but why do they build 4 degrees into the cam? thanks, Terry

[Terry F.]

GM 6272989 camshaft.....

Any chance you would know if this is the replacement cam for my 427 390hp engine?? Thanks, Terry

[Duke W.]

Re: GM 6272989 camshaft.....

6272989 is a "camshaft assembly" (consisting of the "assembly" of finished camshaft and indexing pin) that is listed in the Chevrolet Power Manuals as "Hi-perf. street or marine". Timing specs are listed at uspecified lifter rise, but they are very long, so I expect they reflect "gross duration", which is not very meaningful for hydraulic lifter cams. Neither POMLs nor LSA is listed and "net lifts" listed (at what I assume is a 1.7:1 rocker ratio) are greater than the OE 390/400 HP cam, so my assumption is that this is a completely different cam, but I have never seen enough data to meaningfully characterize it or compare it to other cams including OE.

The "camshaft assembly" is what is sold over the counter. The finished camshaft part number is cast into the blank. It is sometimes referred to as the "ID number" in the parts catalog. It is NOT the "casting number" as it is sometimes called. It is the fully machined camshaft - without the indexing pin, and the detailed lobe data is on an engineering drawing of the same number.

Finished camshaft and camshaft assembly numbers are sometimes sequential, but can deviate by up to about 50. It was just a matter of the sequence the various part number requests were processed on the day numbers were assigned.

There were two camshaft assemblies used for hydraulic lifter BB Corvette engines from 1966 to 1972 ('73-'74 was different, but I have never found any detailed engineering data on this cam. I suspect it may have more overlap to help meet NOx standards that went into effect in 1973.) The '66 cam has a grooved rear journal, and later versions do not, but only the grooved rear journal cam was listed in service parts because it is okay to used a grooved rear journal cam in a '67-up block, but NOT okay to use a non-grooved cam in a '65-66 block. (An analogous situation exists for the SHP mechanical lifter camshafts and this was discussed in detail in a thread about one to two weeks ago.)

The Corvette BB hydraulic lifter camshaft assembly and finished camshaft part numbers are as follows:

'66 OE and service for all '66-'72: Assembly 3883986, finished camshaft (ID no.) 3883944

'67-'72 OE only: Assembly 3904359, finished camshaft (ID no.) 3904364

Other than the rear journal groove/no rear journal groove these camshafts are absolutely identical including lobe indexing and lobe lift, which is listed on the engineering drawings to five decimal places every cam degree.

If you have a 6272389 camshaft assembly in your possession, I would like to know the cast in finished camshaft number, and any specs (and source of the specs) you might have on it.

Duke

[Terry F.]

Re: GM 6272989 camshaft.....

Thanks for the information! I will email you, Terry

[Duke W.]

Re: Degreing a cam??

...not a stupid question at all. I probably know what Jim is trying to say, but statements such as "building four degrees into the cam" are basically meaningless unless you understand aftermarket cam jive, which is not what engine design engineers use, and it is unintellible and confusing to the average vintage Corvette owner.

Since Jim made the statement, I'll let him explain it.

Duke

[Jim R.]

Re: Degreing a cam??

Terry like duke said this cam stuff can get confusing, but to try and keep it simple the reason to lets say advance a cam 4 degrees whether it is already in the cam when it is installed straight up or advanced straight up and then advanced 4 degrees using the timing gear is to pick up the torque numbers and the opposite will happen when you retard a cam you will sacrifice torque for increased hp .hope this helps, also if you have to advance or retard more than this to achieve your needs you need to choose a different camshaft.

[Terry F.]

Re: Degreing a cam??

Are you saying that if you install the cam as is, you sort of get the best of both worlds. But, if you choose, you can degree it as much as 4 degrees either direction based on your desire for performance?? Thanks for responding, Terry

[Terry F.]

Re: GM 6272989 camshaft.....

So the 67-72 big block cam 3904364 is the cam that was installed in all big block corvette engines with hydraulic lifters? That sounds like the one I want then... Thanks, Terry

[Duke W.]

Re: GM 6272989 camshaft.....

The only place you will find a 064 camshaft assembly is in an original '66-'72 Corvette hydraulic lifter BB. As I said, it were never available from service parts.

The camshaft assembly you want is 3883386, which has the exact same lobes and indexing along with the rear journal groove, and it's okay to use a camshaft with a rear journal groove is post '66 engines.

Duke

[Terry F.]

Re: GM 6272989 camshaft.....

I heard that about the rear journal groove. But, you still need to use the proper rear camshaft bearing or things get messed up, I believe. I think there are two sizes of oil holes for the rear cam bearing. Thanks, Terry