70 L46 Forged Pistons, correct replacement?

[Robert M.]

Hello all,

I am in the process of planning the sorting out my motor and bring it back to what it should be, and wringing out some extra power with the components I have at hand. I am looking to get the compressiom back, as I now have some flat top pistons. In the Archives, Duke mentions the TRW L2104R-xx is the correct forged unit for the L46 / LT-1. I wasn't able to find that piston, but did find the TRW-L2304F30 which seems to have the same specs as Duke listed.

Am I correct in assuming the TRW-L2304F30 is the correct replacement piston?

The 186 Cylinder heads are also going to be massaged to bring out more potential in the motor. (Sorry, no Vortecs or aftermaket for this car!)

Lastly, if Duke is reading, Any more news on that Special 300hp cam? I've gone through the archives on that, and it sure sounds interesting, with that short exhaust duration.

Thanks all,
Bob

[Joe L.]

Re: 70 L46 Forged Pistons, correct replacement?

Bob-----

The Federal-Mogul/TRW/Speed-Pro L2304F is the closest thing you can get to the original piston used for 1969-70 L-46.

[Barry Chappell]

Re: 70 L46 Forged Pistons, correct replacement?

I rebuilt my original engine in 1996 using the L2304F .030. These looked virtually identical to the original pistons. Still working well.

Just an opinion on cams, I would think your original cam 3896962 (350/350)
would be a better choice than Duke's "special". With Duke's cam and 11:1 cr I would be concerned with effective cr being too high.

I'm still using an original LT-l cam with GM # 10105117 head gasket.(compressed thickness of .028).
This combo works exceptionally well with pump gas.

Barry

[Duke W.]

Re: 70 L46 Forged Pistons, correct replacement?

Joe is correct. The "R" should be an "F".

Dennis Clark has the first "Special 300 HP" engine running in his '62. He offered some very good SOTP impressions last fall, but wasn't able to get a chassis dyno test before winter set in. You should be able to find this thread with a search. I started the thread and Dennis chimed in with a post giving his impressions. John McRae is assembling his engine, and it might be installed back in the car before the snow melts!

The L-46 cam is very good. In fact, the Special 300 HP cam has nearly the same timing except the exhaust closing, which is much earlier.

The 300 HP cam may not be a good bet for your engine, since the idle quality is like the 300 HP engine - no lope as is typical with SHP cams.

The L-46 cam makes about the same top end power as the LT-1 cam all other things being equal. I think the increased rated power for the LT-1 engine is primarily the freer flowing inlet manifold, and the LT-1 cam offers an extended rev range due to the mechanical lifters.

I think the best way to restore a L-46 is to use the OE cam or the LT-1 cam, and with my clearance specs the mechanical lifters will be nearly undetectable. Of course, "massaging" the heads is the key to improving top end power regardless of the installed cam.

By the time you relieve the chamber overhang and gain a cc or two of head chamber volume you should be able to achieve my recommended not to exceed CR of 10.5:1 by selecting a proper thickness head gasket from the numerous offerings.

On a related subject, I finally came up with a "Special Mechanical Lifter Camshaft" that beats the LT-1 cam - primarily in the form of better low end torque, but idle quality will be closer to the 300 HP cam than LT-1 cam due to the low overlap. The design architecture follows the same pattern as the Special 300 HP cam with more inlet than exhaust duration, and, as with the Special 300 HP cam, is only suitable with massaged heads that achieve an approximate E/I flow ratio of about 0.80.

Alas, the combination of the long lobes and low overlap could not be ground. There is not enough material on the cam blank lobes to grind a 30-30 lobe on the inlet side and a Duntov lobe on the exhaust side at the very wide LSA of 121 degrees.

Duke

[Duke W.]

Re: 70 L46 Forged Pistons, correct replacement?

I agree. I call it the "Special 300 HP cam" because the low overlap yields the same 500 @ 18" OE base engine idle quality, so it is undetectable in Ops Check and PV while making substantially more top end power and useable revs.

Since the inlet valve closes quite late it needs fairly high CR with a nominal target of 10.25:1 and a NTE value of 10.5. This is easily achieveable with the KB 157 0.5 cc dome hypereutectic pistons on a 327 and a 20-30 thou head gasket with 61-63 cc finished chamber volume.

Finding a suitable piston that will achieve this target range on a 350 may be a little tricky, as my preliminary searches did not yield a good choice, but I'll wait until someone wants to restore a base 350 with the Special 300 HP cam to make a final judgment.

Duke

[Robert M.]

Duke, Regarding the L46 camshaft

I've seen that Clevite makes a cam that is a replacement, CEP 2291713, for the L-46, and GM still carries it under the 3896962 part number.

Is there any particular make of this replacement camshaft that you'd prefer over the other?

[Duke W.]

Re: Duke, Regarding the L46 camshaft

I prefer replacement parts from the big auto parts players like Dana Corp. and Federal Mogul. Clevite is a Dana brand and can be purchased at NAPA, which is a convenient outlet for many.

Among the aftermarket vendors, I prefer Crane. I believe they are the supplier to GMPP, and the two Special 300 HP cams they ground appear to be good quality - dimensionally correct with Parkerized lobes that aid break-in.

Crane does not show the 962 cam in their current catalog - possibly because they manufacture it for GM. They don't show the Duntov cam either, but they have the master lobes and can grind it or use the lobes for any custom cam.

Interestingly, the current GMPP specs for the 962 cam show 222 degrees duration at .050" lifter rise. Older specs like the Chev. Power Manual say 224 and that's what I measure from the lobe data on the GM drawing. Though both lobes have the same duration, they have slightly different lift and dynamics.

The 962 lobes are nearly identical to the L-79 cam lobe, and the difference is, perhaps, a result of what GM learned about valvetrain dynamics after the L-79 design was released. The primary overall difference is that the 962 is effectively retarded relative to the 151 with POMLs of 114/114 compared to 110/118 for the 151. Installing a 962 advanced 4 degrees in a 327 should be indistinguishable from the 151, and there may be an advantage in valvetrain dynamics.

The current GM 300 HP crate engine has the 962 cam along with a 8.5:1 specified CR - a combination that I don't consider adviseable - low compression and relatively high overlap. The previous version of this engine carried the 929 ('67-up base engine) cam that I think is a better combination for a low compression general purpose replacement crate engine.

Also of interest is that the 3896962 camshaft lags the 38969929 '67-up base engine by only 33 sequence numbers, so they may have been designed in the same time frame, but the 962 was not used in production until 1969.

Then again, it may just be a coincidence - a case where 3896962 was initially assigned to a part that was never released and went back into the part number pool for reassignement.

Duke

[Robert M.]

Re: Duke, Regarding the L46 camshaft

Duke,
I appreciate your responses, and willingness to share knowledge with a dope like me. I've many more questions on getting this motor up to potential, but I will refrain from asking at this moment. I've ordered the Vizard Cylinder head book, and will read it thoroughly to understand these heads. Hopefully many questions will be resolved with the book, and maybe then I'll be able to form a better question.

[Duke W.]

Re: Duke, Regarding the L46 camshaft

E-mail me and I'll send you the tech paper for the Special 300 HP engine. Some details differ for the L-46, but the basic concepts are the same.

The key to top end power is head flow. The "pink" rods used on the L-46 are vastly superior to the 327 rods, but by the time you Magnaflux, upgrade the bolts, and resize, you have spend nearly the equivalent of a new set of high strength aftermarket rods.

Other than the head work and maybe the rods, just do it "by the book" using OE or OE equivalent parts and middle of the production range clearances, and make all the necessary measurements so you can compute and manage the as-built CR.

Duke