70 L46 Cam Specs?

[Mike F.]

Can someone give me the 70 L46 cam specs? I've been googling, but still can't find what I need. Valve lift/Cam lift/Duration/Centers?

My Coupe has the original engine, but during a rebuild (I've been told) the domed pistons were replaced with flat tops to lower compression for pump gas and a new cam was installed(see pic).

Trying to figure out what I have??? I'll be getting some dyno runs on it in the next couple of weeks to see where it sits.

How does this cam compare to the L46 cam? Any ideas on what compression may be?

TIA,
Mike

[Joe C.]

Re: 70 L46 Cam Specs?

Part number: 3896962
Durations @ 0.050: 222/222
Valve lift @ 1.5:1 ratio: .450/.460
Lobe Centerline: 114

The cam specs that you show are for a GM Crane "marine and off road" cam, GM part number 12353918. Intake valve closes @ 76 ABDC, which is the same point that the intake closes with the "151" (L79) cam. An engine equipped this this cam should have 11.0:1 static compression, minimum. The reason I say minimum, is that the ENORMOUS 74 degree difference between J604D and 0.050" durations, especially considering the extremely low lobe lift, points to the fact that considerable bleed off of cylinder pressure occurs between 0.050" valve lift and when the valve is fully seated (J604D). Although 11.0:1 SCR (static compression ratio) yields a theoretical dynamic compression ratio of 7.91:1 (which, in itself is barely adequate and 8.2:1 - 8.5:1 makes for a healthy torque curve), the excessive bleed off of cylinder pressure due to the exceedingly gentle cam flanks prevent the cylinders from developing adequate pressure and consequently, adequate torque.

[Mike F.]

Re: 70 L46 Cam Specs?

The cam specs that you show are for a GM Crane "marine and off road" cam, GM part number 12353918.

Would these cam specs make more torque or HP?

Thanks,
Mike

[Robert S.]

Re: 70 L46 Cam Specs?

Your engine has 11:1 compression according to the factory. I understand that 10.5:1 is probably closer to the truth. I have the same engine in my car and it runs fine on pump premium. I would recommend putting the stock pistons back in. Especially since its on your dime.

Bob

[Joe C.]

Re: 70 L46 Cam Specs?

Would these cam specs make more torque or HP?

Thanks,
Mike

The Crane/GM off road/marine cam is a slug! Too much cylinder pressure is going "out the door" because of its extremely lazy action. It needs more than actual (not "nominal") 11.0:1 SCR.

Read my edit to my first post. If your engine is as-built from the factory, and is nominal 11.0:1 SCR (which means that it can be anywhere from about 10:5:1 to maybe 10:8:1, depending on the cylinder being measured), then you would be much better off using the L46/L82 cam in it.

[Gene M.]

Re: 70 L46 Cam Specs?

Even the L82 and L79 cams are quite lazy. There are a number of options available that offer more.

The production grinds are just that, designed to cater to a very wide spectrum of drivers. Not specific to max horse power and torque as Joe pointed out.

[Duke W.]

Re: 70 L46 Cam Specs?

The actual .050" lifter rise durations that I measured right off the Chevrolet drawings are 224/224, (vs. 222/222 for the L-79 cam) and this cam is somewhat unique in that the inlet POML is indexed rather late at 114 deg. ATDC compared to the more typical 108-110 deg. ATDC for most OE cams.

Despite equal durations the inlet and exhaust lobes are different as can be seen from the slightly different maximum lifts. This cam is essentially the same as the L-79 cam, but indexed 4 degrees later, and it has some other advantages.

The L-46 lobe designs take advantage of the knowledge that Chevrolet engineering gained about valvetrain dynamics, which can primarily be seen in the jerk profiles, which are different on each side due to different valve masses, so it's easier on the valvetrain than the L-79 cam; and with proper setup of the OE springs will rev to 6500+, which takes good advantage of head massaging.

With massaged heads it makes very similar low end and peak torque as the LT-1 cam and almost as much top end power, but doesn't offer the extended power range "sweet spot" to 7000+ revs that the LT-1 cam offers. The higher power rating of the OE LT-1 engine was primarily due to the LT-1 inlet manifold, which is less restrictive than the L-46 cast iron manifold that was common to the base engine.

Despite the naysayers, it's an excellent cam, and I recommend it in place of the L-79 cam for L-79 rebuilds, but installed with an adjustable timing set to index the inlet POML four degrees earlier at 110 ATDC - same as the L-79 cam.

I recently recommended this cam to a member who is building a "cheater" L-79 that is stroked to 3.75" with a "design speed" (maximum) of 6000.

Although this cam was also used with the low compression L-82, it works best with high compression, and I recommend a not to exceed TRUE maximum CR of 10.5:1 with OE indexing and 10.3:1 if advanced four degrees, which should be easily achievable with typical OE machined blocks, OE domed pistons, and proper head gasket thickness selection.

The cam you referenced has less inlet duration and is indexed earlier, but has about the same effective overlap. It might have a slight low end torque advantage, but will likely not make as much top end power, however, if the L-46 cam is advanced four degrees the difference in performance curves will narrow.

Duke

[Joe C.]

Re: 70 L46 Cam Specs?

Even the L82 and L79 cams are quite lazy. There are a number of options available that offer more.

The production grinds are just that, designed to cater to a very wide spectrum of drivers. Not specific to max horse power and torque as Joe pointed out.

Gene....without getting "into the weeds", I restricted my comparison to another factory grind, and specifically the one that the OP seems to be interested in. The L82 cam has very slightly more duration and lift than the L79. Aftermarket grinds will, absolutely, extract more power from any given engine but, it must be remembered, at the expense of some durability. The only way around this little trade off is to use a hydraulic or solid roller.

The Speed Pro version of the "151" has a 68 degree difference between J604D and 0.050" durations. This cam is not as "lazy" as the Crane off road mentioned above, whose delta is larger at 74 degrees.

FYI: The larger this "delta", the lazier (gentler) the ramps and the less cylinder filling takes place on each cycle.

[Gene M.]

Re: 70 L46 Cam Specs?

Joe,
Right you are. Years back, the after market cam grinders would grind one (L79) "cheater cam" if requested. It would meet the max lift and duration at .050", had tighter centers, and I think less over all duration at .006" lift. Cutting back on the lazy ramps. With a bit of head porting performance was a big improvement.

I'll bet your "1965 327/365.
360 RWHP @ 6500 RPM..........all motor" does not have lazy ramps !!

[Joe C.]

Re: 70 L46 Cam Specs?

Joe,
Right you are. Years back, the after market cam grinders would grind one (L79) "cheater cam" if requested. It would meet the max lift and duration at .050", had tighter centers, and I think less over all duration at .006" lift. Cutting back on the lazy ramps. With a bit of head porting performance was a big improvement.

I'll bet your "1965 327/365.
360 RWHP @ 6500 RPM..........all motor" does not have lazy ramps !!

They are not.

[Mike F.]

Re: 70 L46 Cam Specs? UPDATED

I made some dynojet runs today!

70 Coupe, 350/350, M21, 3.70 gears. Engine, drivetrain, and exhuast are stock except for the above mentioned flat top pistons and cam.

93 oct. pump gas, Timing 16 degrees initial, 36 degrees @ 2500rpm.

(Graph 1) Air filter on, 213hp @ 4750, 276tq @ 2750rpm

(Graph 2) Air filter lid removed, 217hp @ 4750 282tq @ 2750rpm

Anyone out there have dyno numbers on a stock L46?