C3 - L79 cam

[Jim Callan]

The machine shop that reconditioned my '71 350/270hp block(new cam bearings, plugs, +.030 with TQ plate hone) also supplied parts for reassembly - pistons, bearings, roller timing set, oil pump and screen, lifters AND cam. The cam is from Melling Performance #22200 which is the OEM L79 cam (151). He suggested that I install it +4 for more torque. It was suggested here, in a previous thread, to install at 0 (straight up). I have the TH400/3.08, A/C, PB, PS.
What would be the BEST way to go here ??

Thanks

Jim

[Mike Cobine]

Re: C3 - L79 cam

With an automatic, tuning for torque is a mistake often made by people who don't realize what a torque convertor is.

Basically, it gives 2x multiplication when starting off and slowly moves to 1:1, although it never locks up unless it is one of the newer locking type. As such, you have little need of extra low end torque. So while a T400 has a 2.48:1 first gear, you get much more response taking off than with a similarly geared manual transmission.

But you do need power in the 4500 rpm range in order to get the car to upshift in a full load situation, like passing. If your power peak is down around 3500 rpm, like many RV cams/improved torque schemes provide, you will not make enough power to continue to climb to the approximately 4500 rpm upshift point while in the passing gear mode.

[Duke W.]

Re: C3 - L79 cam

It's already effectively "advanced" four degrees. Consider the L-46/82 cam:

224/224 duration at .050" lifter rise, points of max lift 114/114

The L-79 cam is:

221/221, 110/118

The OE 929 cam in your engine is 194/292, 108/116.

As you can see, the L-79 cam is already advanced four degrees relative to the L-46/82 cam, and I would not recommend advancing it any further.

With a TH400 and 3.08 axle I am hesitant to recommend "more cam" than the OE "300HP cam" unless you plan on racing the car. Though L-82 was available with TH400, I think the tallest available axle was 3.36, and that was an option. The standard axle, depending on year was 3.55 or 3.70. You can peruse the NCRS spec guide for the details.

If you want more top end power, raise the compression to 10:1, massage the heads, rework the inlet air heat stove, and remap the ignition curve and fuel curve as required.

Duke

[Clem Z.]

Re: C3 - L79 cam

i would install a looser convertor with the L-82 cam because we did that with new L-82 corvettes because they were so slow out of the hole.

[Larry M.]

Re: C3 - L79 cam

Duke,

In 2002, Dale Pearman transported our 1972 LT-1 from New York to our new home in Virginia. He and I talked quite a bit about camshaft selection, as I told him that I had installed the L46/82 cam in our car and wasn't too happy with the bottom end power (3.70 rear ratio).

He told me that the L79 cam, advanced 4 degrees, would be a thrill.

In the case of a car with a 4-speed, instead of a TH400, would that change your recommendation? I have had, and continue to have, a strong desire to replace my current cam with something that gives me more lower rpm torque.

Dale's ride for the transport job was a big, white dual-wheel Dodge pickup, diesel if I remember, with a new Haulmark trailer, and it made quite a sight coming down our street. Dale was a great guy to talk with, and after reading his posts on the board for a number of years, I'm glad to have met him.

[Duke W.]

Re: C3 - L79 cam

Yes, I'm aware that Dale made that recommendation. According to Engine Analyzer simulations with a high compression 327, the L-46/82 cam makes nearly the same top end power as the LT-1 cam (but is limited to 6000 which limits average high rev power available), but is weaker on low end torque. The L-79 cam is marginally stronger on low end torque than the LT-1 cam and advancing the L-79 cam four degrees (106/122) increases the low end torque about five percent and losses about 5 percent at the top end and maybe 500 useable revs.

The best way to select a cam is to establish the bottom end revs where you want the 80 percent torque bandwidth to start, and then select a cam to achieve that goal. If top end power is a concern, you work the heads, inlet, and exhaust system until you achieve your goal or run out of money.

The later low compression LT-1s took a big hit in low end torque because the late closing inlet valve requires high compression to keep low end torque at acceptable levels. In our youth most of us craved high revving SHP engines, but for how most of us use the cars today the torque characteristics of the 300 HP cam better meets most of our needs. In your case it's a matter of whether the improvement in low end torque with a L-79 cam advanced four degrees would be worth the time, effort, and cost, and you won't know that until to do the job.

For a TH400/3.08 axle, I would still be reluctant to recommend anything but the base 300 HP cam. Many focus on top end power without regard to engine performance in the other 99.9 percent of the operating range, but the fact remains that most vintage Corvettes will spend 99.99 percent of their operating time below 4000 RPM, and that's the range where the 300 HP cam beats everything else.

Duke

[Duke W.]

Re: C3 - L79 cam PS

Since the 962 and 151 cams are withing three degrees duration, there will not be much difference between the two with the same indexing. Given this, rather than swapping cams, consider just advancing the 962 cam four or even eight degrees. At least that will save some work because you won't have to pull the inlet manifold.

Duke

[Duke W.]

Re: C3 - L79 cam PS 2

Since the '72 ignition map is tuned for emissions, you should notice better low end torque with ignition advance remapping. Being as how your engine is set up to run on 87 PON fuel, you would have no problem running the 327/365 ignition map with premium fuel, which would noticeably improve low end torque and fuel ecomony.

I would recommend pursuing this course of action first because it is both inexpensive and easy to accomplish. Install the lightest centrifugal springs you can find and use a VC1810 vacuum can signaled by a full manifold vacuum source, then go through the pre-emission idle speed/mixture adjustment procedure. Of course, this assumes that you are not required to have periodic emission testing

Duke

[Larry M.]

Re: C3 - L79 cam PS 2

Duke,

Thanks for all your information. Yes, the car does live below 4000rpm 99.99% of the time. The distributor does have a VC1810 installed, drawing vacuum from the port on the base plate of the carburetor that also provides a signal to the choke pull-off; is that a proper source? I'll install lighter springs as well. Is the adjustment procedure you refer to the one in the service manual?

Thanks for the suggestion to advance the cam eight degrees; it's certainly a cost and labor saver. I realize the 962 cam is designed to produce maximum torque and HP at higher rpms, but I would rather have those characteristics maxed out much lower in the rpm band.

Larry

[Jim Callan]

Re: C3 - L79 cam

In a previous thread about the L82 cam, you seemed to recommend the L79 over the L82 for my application. How much torque difference would I have if I used the L79 ? Should I keep the 270hp cam ?

Thanks

Jim

[Jim Callan]

Re: C3 - L79 cam

What are the specs for the 300hp cam ? The machine shop will exchange the L79. This sounds like the "better" cam to use for the type of driving this 'old' vette sees these days.

Thanks

Jim

[Duke W.]

Re: C3 - L79 cam PS 2

Tha idle mixture adjustment procedure is inb your CSM, but it would be an "emission setting adustment", which might include the "idle drop method".

A non-emission setting would be in older manuals for pre-emission controlled engines, but it's basically very simple.

Starting with the recommended initial idle mixture screw adjustment, which is usually specified as, for example, 1.0 turn out from the seat, turn the idle screws to achieve maximum idle speed/idle vacuum. If you haven't done this before, I would suggest turning the idle mixture screws in quarter turn increments in and out for a total of at least one-half turn to get a feel for their sensitivity. On most carbs in is leaner and out is richer. Then iteratively adjust the mixture and idle speed until you come up with what you think is the best combination.

Duke

[Clem Z.]

Re: C3 - L79 cam PS

there aftermarket cams out there that build cylinder pressure in these low compression engines and make the engine feel much stronger. if you are not conserned about the higher RPM performance these cams work well with tall rear gears but peter out around 4500/5000 RPMs

[Duke W.]

Re: C3 - L79 cam

Peak torque will be about the same, but with the base engine cam (3896929) it will occur at about 3000 instead of 3500 for the L-79 cam. The base cam will produce about 20 percent more torque at 2000 revs and be noticeably stronger up to about 3000-3500, but weaker than the L-79 cam above 4000.

The base engine cam specs are 194/202 duration at .050" lifter rise, 108/116 points of maximum lift, and lobe lift of .2600"/.2733".

For a TH400/3.08 axle I think this is the best cam assuming your primary use is "normal drving" such as cruising and touring.

You can achieve a 5-10 percent improvement in top end power and useable revs up to 5500 by pocket porting/port matching the heads and a multiangle valve job as written up is all the various "How to Hot Rod Your SB Chevy" type books.

Duke

[Larry M.]

Thanks again, Duke. *NM*