Need advice on 67 300 HP cam swap (Duke?)

[Joe R.]

Hellow All:

As some of you may recall, I have been chasing a poor idle condition on my 67 327/300 Powerglide car for a couple years now. The engine idles poorly in Drive. After going through the fuel and ignition systems, suspicion fell upon the cam.

This past weekend I degreed the cam and found what Duke had predicted I might find. A previous owner had installed the L79 350 HP cam. I measured 222 degrees duration at .050 lift, and .298 lobe lift. I'm pretty sure this is the 350 HP cam.

Due to the fact that the entire A/C system, including the condenser, is out of the car right now, it seems like a convenient time to replace the cam (the old "while I'm in there" syndrome). However, I'm prepping my car for the National in July and I have a lot of other stuff to get done by then, so I don't have a lot of time right now for extra-curricular activities. I can only work on the car a few hours per weekend.

I have on file most of the extensive postings over the last few years on camshaft selection, so I'm not starting from scratch here. However, the postings I have on file and all the pros and cons of the different cam options have left me almost paralyzed with indecision. Based on a review ofthe previous postings, I'm considering the following four options:

1) Just swap in a new '929 300 HP cam and lifters (194/202 duration, .390/.410 lift). This will fix the idle problem and improve low end torque, but the power peak will be around 4500 RPM. Even though I dislike the idle quality of the present L79 cam when the Powerglide is in Drive, this cam does pull pretty hard from 3000 to 5500 RPM.

2) Install an aftermarket hydraulic tappet cam that is sort of midway between the 300 HP and 350 HP GM cams. For example, the Competetion Cams 12-230-2 has duration of 206/212 degrees and lift of .432/.444. I'm thinking that something like this might be a suitable compromise cam.

3) Install the 929 cam but also pull the heads and have them pocket ported as Duke recommends. This is more work and expense, but it promises to extend the power peak to 5000 RPM or beyond.

4) Install an aftermarket hydraulic roller cam retrofit kit, staying on the mild side to keep idle quality for the Powerglide. This could be combined with pocket porting in an all-out approach. The main benefits of the roller cam are reduced concern about low-zinc oils (manageable right now with Shell Rotella) and the broader power curve that the roller cam ramps reportedly allow. All this sounds nice but I start getting bogged down with related issues such as cam buttons and possible valve train changes.

I would appreciate hearing from Duke and others to help me settle on a strategy. As I said, I have limited time to spend on this right now, but if I don't do it now I will have to repeat a lot of disassembly that has already been done at this point. At a minimum, I think I will implement Option 1 (simple swap to the '929 cam)

By the way, the rear axle in this car is a 3.70, and the long term replacement for the Powerglide will be a 700R4 automatic.

[Tracy C.]

Re: Need advice on 67 300 HP cam swap (Duke?)

Joe,

Given your time constraints and a desire to be done before the National Convention, I would go with option 1 as well. Save the serious head work and rebuild stuff for a day when you can do it right.

Duke will tell you that anything less than a complete "systems engineering" approach on the whole engine package is a waste of piece parts, time and money.

The General picked the stock cam for a reason. Go with it.

tc

[Duke W.]

Re: Need advice on 67 300 HP cam swap (Duke?)

Your measured numbers definitely say L-79 cam - about 99.99 percent probability!

Given your time constraints, I recommend you install a 929 cam for now. If you want to recover some top end power and revs, you can do the heads at a later time, and the only redundant task is R&I of the inlet manifold, which is not much of the total job.

Given an auto trans, and especially a PG with its 1.82:1 first gear, you need a low overlap cam to allow it to idle with any degree of quality/stability and all the low end torque you can muster, even if a 3.70 axle was retrofitted (3.36 was the shortest OE axle ratio with the base engine regardless of transmission).

The head work with the 929 cam will increase top end power and useable revs to near OE L-79 levels. Retarding the 929 cam four degrees will bump top end power and revs more, but there will be some low end torque loss, so I am hesistant to recommend retarding the 929 cam with PG.

A final option is to install the "special 300 HP cam" that I designed (along with the other required mods for this engine configuration including massaged head and at least 10:1 true CR), and by the time you get around to doing the heads, we should have test results for the two prototypes that are currently in work and can make a determination whether it provides sufficient low end torque for PG.

This cam would be excellent with a 700R4 because the idle characteristics will be the same as the 929 and a 3.06 first gear will mask the loss of low end torque relative to the 929 cam.

A "special 300 HP" configuration with a 700R4 will give the car better than SHP performance with auto trans friendly idle charatertistics.

I highly recommend you stay away from the Comp cam you mentioned. Since the LSA is 110 degrees versus 116 for the 929 and 114 for the L-79 its EFFECTIVE overlap in sq-in-degs is closer to the L-79 that the 929, so you will end up with the same idle quality problems and LESS top end power. Not a good choice.

Given the literally hundreds of cams available from SB Chevies, what does one do?

My research is still ongoing, but I've basically boiled it down to three cams for SB restoration projects depending on what original engine option is installed, drivetrain combination, and what tradeoff you want to make between low end torque and top end power and revs.

My three recommendations are:

1. 3896929 '67-up OE "300 HP" cam
2. 151-929 "special 300 HP" cam (REQUIRES pocket ported heads)
3. 3972178 "LT-1" cam

The clever among you might surmise that the "special 300 HP" cam is a combination of lobes from two different OE cams, and you would be correct, but for now the POML indexing and LSA are "classified".

BTW, one of the "special 300 HP" cams manufactured by Crane has gone through a QA check and it appears to be a quality part. It's straight and the lobes are Parkerized. Lobe checks indicate that lobe dimensions are within OE spec and indexing is within reasonable tolerance of my specs.

As far as repro cams are concerned, I think Crane is an acceptable source along with Federal Mogul, but they the only two cam vendors I recommend at this time.

Duke

[Duke W.]

VERY GOOD!!! *NM*

[Mark H.]

Re: Need advice on 67 300 HP cam swap (Duke?)

Duke,

Is the "special 300hp cam" available to the masses yet?

[William C.]

Re: Need advice on 67 300 HP cam swap (Duke?)

Also keep in mind that the Air conditioning produces an added complication as a camshaft change from the 929 will also likely produce stalling when the A/C kicks on at idle unless you install a throttle "Kicker" to open the throttle slightly in concert with the A/C clutch application.

[Duke W.]

Re: Need advice on 67 300 HP cam swap (Duke?)

"Is the "special 300hp cam" available to the masses yet?"

No - have to wait until the prototypes are built and tested to see what the next step is.

Duke

[Duke W.]

One other thought

As with any engine configuration, the vacuum advance control must be matched to the engine's idle characteristics - it must provide full advance at not less than 1-2" Hg. less than typical idle vacuum.

All '67 300 HP have a 12" VAC (IIRC the ID is 16 355), which is okay for PG with the OE cam, but with a L-79 cam idle vacuum is likely less than 12" so total idle timing will vary, which can lead to idle instability. Installing a 8" VAC might improve idle quality as a "quick fix", but in the long run a more auto trans friendly cam is the solution.

I ran across an interesting situation last year - a '62 with a 700R4 and 3.55 axle. Idle quality was poor, and it overheated at idle and low speed driving. Idle at about 600 in drive yielded only about 8" manifold vacuum and it had a "B1" VAC, so it effectively had "ported" vacuum advance since the B1 VAC doesn't even start to add advance until 8". Installing a NAPA VC 1810 8" VAC considerably mitigated the overheating. Idle quality was still marginal, but more stable.

A check of valve clearance showed about .032", and the suspicion was that it had a 30-30 cam. A chassis dyno test showed about 270 lb-ft SAE corrected rear wheel torque at 3500, but only about 200 RWHP @5000.

Upon my recommendation the owner bought one of those 14008829 cams for ten bucks that Sallee Chevrolet was pulling out of crate motors. The 839 is the current GM replacement for the 929 and is virtually identical.

The installed cam was, indeed, a 30-30, and though the engine has less top end power with the 839 the owner is much happier with the combination since his primary driving application is weekend cruising. A NAPA VC1802 12" VAC was installed along with the new 839 cam.

Duke

[Joe R.]

Re: One other thought

Thanks for all your input, Duke. I'm still pondering what to do.

Regarding the vacuum advance can, you and others talked me through that last year. Indeed, my engine had insufficient idle vacuum to keep the advance pulled in at idle.

A switch to the L79 vacuum can (all in at 8 inches, as I recall) made a BIG improvement in idle quality. It's a lot better now than it was, but idle quality in Drive is still rather poor, especially when the engine is cold. I don't think the present behavior is something that I want to keep in the long run, so some sort of cam change will be necessary.

[Joe R.]

Re: Need advice on 67 300 HP cam swap (Duke?)

I agree, Bill. Hopefully the A/C will be fully functional when I finish rebuilding it as part of the work that is presently underway. The combination of Powerglide and A/C will certainly stress the idle quality in Drive. Right now the idle quality is marginal even without the A/C.

[Duke W.]

Re: One other thought

If you install a 929 cam, reinstall the OE 355 VAC or buy a NAPA VC1765 or VC1808 or equivalent. (Both have the same specs as the OE 355.)

The 8" VAC is too aggressive for the 929 cam and may cause detonation under part throttle acceleration.

Duke