L46 cam

[William F.]

Do L46 and L79 use same cam or not?

[Joe L.]

Re: L46 cam

Do L46 and L79 use same cam or not?

William-------


They do not. The L-79 cam was GM #3863151. The L-46 cam is GM #3896962. The latter was also used for all L-82. The 3896962 is still available from GM, 50 years after it was originally released. In fact, it's still used in some GM crate engines with flat tappet cams.

[Mark E.]

Re: L46 cam

The book Corvette by the Numbers is a good reference for PNs by year.

[Duke W.]

Re: L46 cam

I recommend the L-46/82 cam to replace the L-79 cam. It has two degrees more .050" duration, which is noise level and the same 114 deg, LSA. The other difference is it is manufactured with an inlet POML of 114 deg. ATC, which is okay for long stroke engines, but it should be installed four degrees advanced with an adjustable set on 283s and 327s to bring the inlet POML back to 110 deg. ATC, same as the L-79 cam. The primary reason I favor it is better lobe dynamics as it was designed about 3 years after the L-79 cam and benefited from Chevrolet's better understanding of valve train dynamics that occurred as the sixties progressed.

[William F.]

Re: L46 cam

Thanks, all

[Joshua C.]

Re: L46 cam

Duke,

I read your article, "A tail of two camshaft". Very interesting and well written. Your recommended camshaft seemed very close in spec to the 350/350hp 3896962 for a 327 (larger POML # than typical). Does your reco below of advancing the cam 4 degrees to bring the intake centerline back to 110 change if your recommended head work is done on a 327 300/340hp motor? I believe the overlap of the later 962 cam is significantly less than the 151 cam which should also help with low end torque, correct? I am very interested as I have a 62 340hp motor with an aftermarket cam that has no low end torque even with 4.11 gears. Going to change the cam this winter to either Duntov 97, 151 or 962. Not interested in Erving the motor much over 6 grand so not really interested in LT-1 cam.

Thanks, Josh

[Joe L.]

Re: L46 cam

Duke,

I read your article, "A tail of two camshaft". Very interesting and well written. Your recommended camshaft seemed very close in spec to the 350/350hp 3896962 for a 327 (larger POML # than typical). Does your reco below of advancing the cam 4 degrees to bring the intake centerline back to 110 change if your recommended head work is done on a 327 300/340hp motor? I believe the overlap of the later 962 cam is significantly less than the 151 cam which should also help with low end torque, correct? I am very interested as I have a 62 340hp motor with an aftermarket cam that has no low end torque even with 4.11 gears. Going to change the cam this winter to either Duntov 97, 151 or 962. Not interested in Erving the motor much over 6 grand so not really interested in LT-1 cam.

Thanks, Josh

Josh-----


I'll let Duke answer for himself. However, I'll add this: the GM #3896962 is one of the best small block, hydraulic lifter, performance camshafts GM ever developed. First used in 1969, this camshaft was used for all Corvette L-46 and L-82 engines. It's still used today in some of GM's crate engines and it's still available from GM almost 50 years after its first use. To live that long of a life, it's got to be something special.

[Duke W.]

Re: L46 cam

The ...151 and ...962 have nearly the same duration and the same LSA, so effective overlap measured in square-in-deg is about the same, and they have the same idle behavior, about 14-15" @ 750 with a slight lope.

As Joe said this is the best high performance hydraulic lifter cam of the era, and would be a good choice for an original mechanical lifter engine where you want to go with a high performance hydraulic cam.

As I said previously the primary difference is the IPOML, which is 114 deg. ATDC for the ...962 versus 110 for the ...151. So on short stroke engines like 327s and 283s, the ...962 should be installed with an adjustable timing set to bring the IPOML back to 110. (I recommend the NAPA OE replacement truck chain, which costs about 25 bucks and is made by Cloyes.) Either the ...151 or ...962 (advanced four deg. on 283/327s) should make more low end torque than the Duntov or LT-1 cam, but you must make sure that the spark advance map is appropriate. The OE '62 centrifugal curve is 24 @ 4600, which is very lazy. Low end torque will be noticeably improved by bringing full advance in quicker and running total WOT advance as high in the 36-40 deg. range as the engine will tolerate without detonation. You should work on the spark advance map before considering a camshaft change. (See my 2012 San Diego National Convention Tuning Seminar, which is an easy Web search.)

Of the two camshaft designs reported in the Tale of Two Camshafts article, the McCaw Special was the superior design based on dyno test results. It was designed specifically for base engines, and one criterion was to maintain the smooth 18" @ 500 idle behavior. It was also specifically designed for the flow characteristics of massaged heads, which is an absolute must. DO NOT USE THE MCCAW SPECIAL UNLESS THE HEADS ARE MASSAGAED PER THE DISCUSSION IN THE ARTICLE. Compression ratio should also be managed to achieve at least 10:1.

As explained in the article the McCaw Special is simply the '67-up 300 HP cam with swapped lobes and the same LSA with the IPOML considerably retarded compared to the 300 HP cam, and the inlet timing is nearly identical to the LS2/3 cams.

With proper head massaging the McCaw Special will make about the same low end torque as a base engine and about the same top end power as the ...151 or ...962 with UNMASSAGED heads, so you get the best of both worlds.

The McCaw Special is worth considering for a SHP engine if you are willing to put in the time (or expense if done professionally) to have the heads properly massaged, and are willing to sacrifice SHP idle behavior, however, it's possible to "simulate" SHP idle behavior by leaning out the idle mixture to create a little roughness, but this might make it tougher to get going from a dead stop without a bit of stumble.

Duke

[Gene M.]

Re: L46 cam

William, if you are dealing with a 327 the L79 is correct choice as it is advanced correctly so the off ide power level is as intended.

More cubic inches is less affected by the retarded L46 cam off idle power.

Setting up the advance with a degree wheel if you have one was not the procedure GM used in building engines.

Assembly tolerances could yield more or less advance but not put you out of the ball park. Retarded timing in the 327 will hamper off line power.

[Joshua C.]

Re: L46 cam

Duke,

I understand your advice on advancing a 3896962 cam for a 327 to get the ICL back to 110 degrees. My only question is this will also put the exhaust centerline at 110 as well (correct or did I miss something)? Is a 110 ECL ok?

Would it be worth getting Crane to custom grind me a 3896962 on the 3863151 centerlines (110/118)? My application is a 62 340hp motor. I do not care about idle vacuum as I do not have power brakes. Would it make sense to tighten the LSA to 112 (108/116)? Thoughts? Am I over thinking this and should just use the 3863151 327/350HP cam?

Thanks,
Josh

[Joshua C.]

Re: L46 cam

Duke,

I did some more reading and now realize that advancing the 3896962 cam will give it exactly the same intake and exhaust centerline as the 3863151 cam (110/118) because ICL is measured ATDC and ECL is measured BTDC. Now it makes sense. I thought I read in one of your responses that you have Duntov 97 dyno results in a 327 and it was more like 290HP verses 340hp? Just curious as I have thought about putting the correct cam in my car. I have worked on the spark advance map in my car. It improved low end torque slightly but I really do not like having to slip the clutch on a car with 4.11 gears and a 2.54 first gear (car has the lower 1st gear T-10 gears).

Thanks,
Josh

[Duke W.]

Re: L46 cam

(IPOML + EPOML)/2 = LSA, so if you have any two of the data points you can compute the third as you figured out.

For 327s I recommend the LT-1 cam to replace the Duntov and 30-30. The LT-1 cam has about the same effective overlap as the Duntov for essentially identical idle behavior, makes about the same low end torque, but more top end power. (The 30-30 took duration and overlap to a ridiculous extreme and makes for a very soggy engine below 3000.)

You can search the TDB for a thread stated by me about ten years ago "327 LT-1", and there is a full report and extensive discussion on the CF in posts started by "Dave McDufford" and "ghostrider20". You should research those. The L-79 cam, L-46 cam advanced four degrees, or the modified Duntov cam with IPOMLs of 110/118. have less effective overlap, so better idle quality, more low end torque and similar top end power. Crane has Duntov lobe masters, so they can grind the "modified Duntov" as specified above. I don't think they have the ...962 lobes, but it's cheaper to buy on off the shelf Federal Mogul replacement and install it four degrees advanced with the adjustable version of truck OE replacement roller chain.

You really shouldn't have much problem getting a Duntov cam engine started from a dead stop with a 4:11 axle and 2:54 low WR trans without excessive clutch slippage or stumble. You should tweak the idle mixture and see if you can get some improvement Increasing idle speed may help, and I recommend 900. I have no idea what you did to your spark advance map, but you should follow the guidelines in my tuning seminar, get the centrifugal in as soon as possible and set total WOT advance as high in the 36-40 range as the engine will tolerate without detonation.

Yes I have a lab dyno test report, STD correction of a "stock rebuilt" 327/340 and it made 295 GHP @ 5500, which is probably about right. The advertised gross power ratings of the sixties were typically 10-15 percent overrated. Massaging the heads will get close to the advertised GHP rating, all other things equal.

Head massaging is also discussed in my "Tale of Two Camshafts" article.

Duke

[Joshua C.]

Re: L46 cam

Duke,

Thank you for responding to my post. I think my engine has a 30-30 cam in it currently. It is definitely not the 97 cam (I know this from setting the valve lash, cam is not on base circle unless I rotate the crank in qtr increments).
I will search and read the LT-1 thread as you suggested.

Thanks,
Josh

[Joshua C.]

Re: L46 cam

Duke,

What lifters do you recommend for the 962/151 Sealed Power camshafts? Is going with Crane and using their recommended anti-pump up lifters worth the $$?

Thanks,
Josh

[Duke W.]

Re: L46 cam

Go with a Sealed Power cam/lifter kit, and VS677 OE replacement valve springs. With proper installed height thy valvetrain is good to 6500+ before lifter pump-up.

Duke