Cam and Piston Info Needed for Rebuild

[Gregory G.]

Need info on a Doug Hubert solid lift cam, 500 lift, and hypereutectic pistons.I was told they are between a Forged and Cast type piston.
Looking to rebuild a '70LT1 engine with better driveability and to run efficiently with todays pump gas 93-94 octane.I will reduce compression to 9:0:1 with moly rings,new rod,main,cam bearings,new high volume pump,HD drive and oil pickup.A new LT1 crank, magged and polished.The block will be bored .30 over using the correct 186 heads 2.02 x 1.60 valves stainless steel or hardened exhaust seats,new solid lifters,valve springs,keepers,retainers rockers,screw in studs and guide plates.
I expect HP to be close to stock 370 with better torque.Engine would be balanced and all other LT1 engine parts will remain the same,TI Ignition System,Holley 780,Aluminum Intake,etc.
Any other help on this rebuild would be appreciated.

Greg

[Ted S.]

Re: Cam and Piston Info Needed for Rebuild

Before you jump in and change the pistons to lower the compression ratio and swap cams you need to figure out the Dynamic Compressiion Ratio (DCR) on the existing combination. You very well may be able to run the stock components on pump gas (especially 93-94 octane)with appropriate timing. The DCR takes into account the intake closing event to determine the compression ratio the engine actually "sees" which is less than the static compression ratio (i.e. moving from 11:1 to 9:1). The early high performance cams had such a late intake closing event that it bled off a lot of the compression. If you lower the compression and put in a large cam you'll really end up with a dog of an engine. The target DCR for an iron headed engine is 8:1 with a .040 quench. This can be accomplished with an 11:1 static compression and appropriate cam selection

As far as Hypereutectic pistons they're fine for normal street usage.

Maybe Duke will chime in on this also as he's responed to alot of these in the past. Also do a search on dynamic compression ratio there's lots of postings on this topic.

[Duke W.]

Re: Cam and Piston Info Needed for Rebuild

In '70 GM lowered the CR to 9.0:1 so the engine would operate on UNLEADED REGULAR. Is THAT what you want to do?

I have never found a cam that produces equal or better peak power than the LT-1 cam with equal or better torque bandwidth. Most aftermarket cams that make near the same top end power will have poorer idle quality and less low end torque because they have too much overlap.

Hypereutectic pistons are okay, but so are the OE forged type. If you shoot for an actual CR of no more than 10.5:1 the engine should operate detonation free on pump premium, and with typical production machined deck height and a compostion gasket, 10.5 is about what you get. Of course, you should make all the measurements and select correct parts - like head gasket thickness to ensure that you achieve you target CR. This begins with measuring piston to deck clearance BEFORE the block is disassembled. You also have the option of going with a thin gasket to minimize quench height, and then shaving off the domes to achieve your target CR. On a recently refreshed L-79 - all stock parts except for substitution of the LT-1 cam and a standard thickness compostion head gaskets, the static CR computed at 10.35:1 and the Engine Analyzer simulation program computed a dynamic CR of 7.48. As far as I know it is operating detonation free on pumnp premium

The late closing inlet valve on the LT-1 results is relatively low dynamic CR. At most you might have to juggle the igntion timing map a bit.

The LT-1 was an exceptionally well engineered engine and represents the peak of SB development before emissions became the priority. The various parts all work very synergystically, and I would advise all LT-1 rebuilders to go with OE spec parts throughout the engine. You just cam make a stock displacement street SB any better than the LT-1!

The one area that can be improved is cylinder head flow. Pocket porting, port matching, and a mulitangle valve job will increase top end power about 7 percent without adverse affect to other areas of the operating range including idle and low load operation.

What are the "driveability" issues you refer too?

Duke

[John H.]

Re: Cam and Piston Info Needed for Rebuild

You don't need a high-volume oil pump, and I'd think twice about aftermarket cams with high-pressure springs; the LT-1 cam and stock springs/rockers is a proven-reliable combination. The LT-1 cam (and similar aftermarket cams) need compression to work, and have enough overlap to bleed off cylinder pressure so they generally work fine on pump premium, with a properly-tailored advance map.

[Gregory G.]

Re: Cam and Piston Info Needed for Rebuild

Would any of you gentlemen care to make a running list of internal parts that can be used comprable to the factory original GM parts that were originally used.Mostly all are discontinued,especially the LT1 cam.Of course keeping in mind todays octane ratimg.

Greg

[Ted S.]

Re: Cam and Piston Info Needed for Rebuild

The TRW forged equivalents I believe are L2304F30. Summit Racing shows them at 10.63:1 with 64cc heads. I've never been able to figure out how TRW comes up with their numbers because if I plug the specs into a calculator I usually come up higher than their number. Comp cams makes a cam that is a replacement for the GM 3849346 which is their part number 12-107-3. That cam shows the intake closing at 103*ABDC which would bleed off a lot of compression. The piston is over 1/2 way back up the cylinder before the intake valve closes. DCR using the TRW pistons and the Comp Cam is at 6.1 which is pretty low and should easily run on pump gas (even 87 octane). I'm sure crane and other cam manufactures also make an equivalent. It looks like if you pulled the intake closing event back to around 79* abdc you'd get pretty close to the desired 8:1 DCR. Hope this helps.

[John H.]

Re: Cam and Piston Info Needed for Rebuild

The -346 is the "30-30", which is not a good choice for the street. The LT-1 cam is still available through GMPP as a kit with lifters under P/N 12364054, and uses stock pushrods, springs, and rockers.

[Dave McDufford]

Re: Cam and Piston Info Needed for Rebuild

Speed-Pro makes the original LT-1 cam as CS1145R. I used it in my 327 rebuild and I like it. Dyno'ed at an honest 360HP and was still pulling strong at 6500rpm with a stock valve train. It is very streetable but idles with an attitude. Pistons are 10.35 compression forged TRW's - no problem with Sunoco 94.

Listen to John and Duke, they know what they are talking about - I did.

Dave

[Dick W.]

Re: Cam and Piston Info Needed for Rebuild

The 30-30 was not the best choice for the street when it came out. Absolutely no low end torque. Use of this cam ensured that you would become profficent at installing clutches

[Ted S.]

Re: Cam and Piston Info Needed for Rebuild

John, Is GM grinding the cam or is another manufacturer making it for them? Also do you know the specs (i.e. intake closing) for the cam? With the 30 - 30 cam closing at 106*ABDC it's no wonder you'd end up without any lower end. Even with 11:1 static it calcs to about 6:1 dynamic which is way too low. Thanks.

[Duke W.]

Re: Cam and Piston Info Needed for Rebuild

The CR quoted by Federal Mogul Speed Pro for L2304F-30 is with all nominal dimensions - 9.025" deck height, 5.700" rod, 4.031" bore, 64 cc, AND A .038" composition gasket instead of the .015" OE steel shim gasket.

Even if you use the OE thin shim gasket you will probably not get 11:1 because the OE deck heights are typically machined high.

If everything is nominal, the following should be the deck - piston crown clearance.

9.025" -(1.740" + 5.700" + 1.560") = .025".

This is why you want to measure the piston-deck clearance BEFORE you disassemble the block, so you can compute the actual CR based on how your engine decks are machined. You will likely find that deck clearance is greater than .025".

The 1.740" is the crank throw radius - half the stroke: 1.560" is the nominal compression height - the distance between the center of the pin and the machined crown of the piston, which is lower than the top of the dome.

Go with the GMPP or Federal Mogul LT-1 cam clone, the F-M (formerly TRW) OE replacement forged pistons, and buy Federal Mogul OE equivalent parts for EVERYTHING ELSE YOU NEED - bearings, valves, valvegear - everything (except rod bolts). If you properly manage the machine work and assembly, you will have a great engine, especially if you work the heads.

DO NOT BUY AND INSTALL ANY "HOTROD PARTS" FROM ANY HOT ROD PARTS VENDOR to include oil pumps rocker arms, pushrods, valves, rockers, etc. (rod bolts are ONE exception)

LT-1s have the "pink" OE connecting rods, and if they pass Magnaflux, they are perfectly okay for a 6500 RPM street engine. At most you can install higher quality bolts and have the rods resized. These rods were heat treated to a higher harness, magnafluxed, and shotpeened on the nut and bolt seats. They are a good rod - vastly superior to the 327 rods, especially the early 327 rods, which I alway recommend be replaced with Crower Sportsmans on a mechanical lifter 327. That's 500 bucks you DON'T have to spend on an LT-1 rebuild. You can put that into head massaging.

BTW with properly massaged heads - like the prep work that Dave McDufford's machinist accomplished, a LT-1 should approach 400 gross horsepower at 6000. Dave McDufford's 327 would probably be close to 400 gross too (at 6500), but the 327 SHP inlet manifold is holding it back. The LT-1 inlet manifold is excellent, and I don't think any aftermarket 180 degree manifold is materially better.

The LT-1 is a fabulous engine! Absolutely FABULOUS!!! Rebuild it to "blueprint stock" using OE or OE equivalent parts. You just can get any better than that!

Duke

[Duke W.]

Re: Cam and Piston Info Needed for Rebuild

The 30-30 specs are based on a clearance ramp height of .017" and a rocker ratio of 1.5:1 with .025" lash. Being as how the actual rocker ratio at the lash point is about 1.37:1 you would have to lash the valves at .023" to have the valve touch the seat that late. Using Cherolet's specifications, the duration of the 30-30 is 346 degrees on both sides, and the LT-1 cam is 317 on the inlet side and the same on the exhaust side since the LT-1 exhaust lobe is the 30-30 lobe, advanced four degrees.

Using the same timing scheme as the 30-30 cam the LT-1 (top of inlet ramp .012" above the base circle) inlet closes at 90 ABDC, and it will take both cams about 20 crankshaft degrees to move the valve the last .010" of travel to the seat.

It's more relevent to compare these two cams timing number at .050" lifter rise ABOVE THE TOPS OF THE CLEARANCE RAMPS. These can be compared to typical hydraulic lifter .050" lifter rise timing numbers.

LT-1: 3.5-44.5/61-(-2), 228/239
30-30: 9-50/57-2, 239/239

Timing numbers must always have CONTEXT - timing points at some lobe or valve lift, and if a valve lift is specified, a rocker ratio must also be specified. For SBs the rocker ratio is usually specified as 1.5:1, but the OE rockers are NOT 1.5:1, and they aren't even constant. Chevrolet's timing data context was unique to them. Back then everyone seemed to have a different context.

Modern aftermarket cams' "advertised duration" is usually taken at .006" valve lift, but this also requires an implied rocker ratio.

The best specs to compare are the .050" lifter rise, HOWEVER, mechanical lifter cams CANNOT be compared to hydraulic cams using this standard, because a good part of that .050" lifter rise is clearance ramp. What IS COMPARABLE is the .050" lifter rise ABOVE THE TOP OF THE CLEARANCE RAMP, which in the case of the 30-30 cam is .067" lifter rise and .062"/.067" for the LT-1 cam.

Being as how the timing numbers I quoted are from the GM drawing using total lifter rise above the base circle as specified above, you can use these number with confidence to compare to typical hydraulic lifter .050" lifter rise specs.

Duke

[Kenny Hancock]

Re: Cam and Piston Info Needed for Rebuild

I have read many posts regarding the excellent performacne of the LT-1 cam. Are there any hydraulic cams that approach the LT-1 perfomance? How does the L-79 cam compare?

[Duke W.]

Re: Cam and Piston Info Needed for Rebuild

The L-79 cam produces a little more low end torque and about the same peak torque, but less top end power. It's also limited to about 6000 RPM due to lifter pump up, which is 500 revs less than the 6500 RPM that the LT-1 cam will reliably rev to.

Duke

[Kenny Hancock]

Re: Cam and Piston Info Needed for Rebuild

Thanks for your input Duke. How good will LT-1 and L79 work in larger engines such as a 383? I assume at least the LT-1 will work fine in a 355 cid engine. Would the L79 work in a 355 also.