L46 camshaft recommendations

[Peter S.]

I need to replace the camshaft on my stock L46 due to three exhaust lobes showing excessive wear. The stock cam (3896962) is still available, but I wanted to ask the more experienced hands if there are any other recommendations.

Engine Specs: Stock L46

Cylinder heads: 3927186 cylinder heads - have been shaved 0.005" as machine shop trued them to 64ccs.
Carburetor: 7029207 quadrajet.
Pistons: Stock L46 domed pistons
Exhaust: Stock ramhorn manifolds, stock 2" dual exhaust
Distributor: 1111491 TI distributor
Transmission: Muncie M20
Rear Gears: 3.36:1.

Driving wants/needs: No track, very much a cruising car that I like to take off the line sharply every now an then. I do like a bit of rumpity in a camshaft. I would not describe the stock 962 as rumpity at all, very mild in my opinion.

I am concerned (may or may not be warranted) that the shaving of the heads just increased my compression, and I did have some detonation issues running on 93 octane (no ethanol) before tear down.

I would like to hear your opinions. If you have a specific grind recommendation, I'd love to hear about it as well.

[Peter S.]

Re: L46 camshaft recommendations

Here are the specifications for the 3896962 camshaft:

222/222 duration @ 0.050" lift
0.450"/0.460" lift
114 LSA

Peter

[Duke W.]

Re: L46 camshaft recommendations

Duration at .050" lifter rise is actually 224 degrees on both sides, but the exhaust and inlet lobes are different.

This is an excellent camshaft for a high performance road engine. It doesn't have excessive overlap, which is why it doesn't have much rumpety-rump, but that means it makes decent low end torque. Higher overlap will just kill low end torque, which is not what most guys will be happy with unless they're seriously racing.

I also recommend this cam to replace the L-79 cam, but installed with an adjustable timing set that allows it to be advanced 4 degrees to bring the IPOML to L-79 spec 110 deg. ATDC. As ground the L-46 cam IPOML is 114 ATDC.

Even though GM still offers the L-46 cam, Federal Mogul offers an exact duplicate CS1095R. Use the standard Sealed Power lifters and VS677 valve springs, which are identical to the OE springs.

Another option is massage the heads and the engine will make about 10 percent more peak power and extend the useable power rev range to lifter pump up speed of 6500+.

The actual as built CR of the engine is likely no more than about 10.5:1 with the OE .018" gasket. If it's assembled with a typical .035" composition gasket you will actually lose about half a point.

If the heads are still off, measure deck clearance and compute the prior CR with the originally installed gasket. Was it the OE .018" shim gasket.

The OE piston dome volume is 2.4 cc.

The detonation may be caused by the emission oriented spark advance map including ported vacuum advance. Full time vacuum advance will reduce the tendency to detonate and will likely allow a more aggressive centrifugal curve.

You really need to provide more information on the history and configuration of this engine along with its current assembly status.

Duke

[Peter S.]

[Duke W.]

Re: L46 camshaft recommendations

If you really mean the bores are worn five thousanths and not five ten-thousanths, then it needs to be bored to the next oversize, which is not unusual for that mileage.

The OE 201 15 VAC does not meet the Two-Inch Rule with typical L-46 idle of 750 @ 14". I'm surprised Lars missed that. It needs a 12" B26 when you convert to full time vacuum advance.

You should have measured deck clearance upon disassembly so you could have accurately computed the existing compression ratio. Ask your engine shop if they have tools to measure block deck height - all eight bores - then you can compute the prior CR, and drop it a bit with a thicker gasket, but I need more details on detonation conditions to make an educated guess on how much that should be. For reference a .010" thicker gasket will knock the CR down about 0.3.

The durations I quoted for the L-46 cam are listed as such in the Chevrolet Power Manuals. In addition, I verified them to be correct using cam lobe data (lift in inches to five decimal places every cam degree) from the GM drawing.

You can't believe everything you read. Published errors and intenet blabber are like politicians' lies. If you read it/hear it enough times most will actually believe it's true and not question.

Narrowing the LSA a couple of degrees will lose a few percent on the low end and gain nothing on the top end. That's a great cam for your application. Just replace it.

If you want a lumpier idle you can always lean out the idle mixture until it idles as rough as you want, or install a LT-1 mechanical lifter cam, which has more idle lope. But the L-46 cam is best for your application because it's stonger on the bottom end, and the LT-1 cam doesn't really make more power until you get beyond 5000, and even then it's marginal. Most of the LT-1 configuration's greater power than the L-46 is the inlet manifold design, plus the mechanical lifters extend the peak power range another 500 revs due to higher valve train limiting speed.

It's best to get someone like me involved in your engine restoration project BEFORE you start tearing down the engine, but at the point you're at you haven't done much harm.

BTW, can you inspect the head gaskets and see if you can find a part number? That gasket was not available in service after about 1963. The service gasket after than time frame was .026", because it was assumed that if the heads came off for valve work they would be milled five to ten thou, and Chevrolet didn't want to take a chance that the slightly higher compression would get the engine into detonation and make the customer unhappy.

I don't know of any aftermarket .018 shim gasket, so if you find one of these on your engine (or you can just measure it at the corners of the block head interface) you can pretty well bet that it was installed at Flint when the engine was assembled.

Since your engine is a '71 vintage CE with that gasket, I assume it was a warranty replacement long block.

Duke

[Tom B.]

Re: L46 camshaft recommendations

I never found a .018 shim gasket when I built my last engine. I used this one instead.

Head Gasket Set .020 compressed thickness steel shim Victor HS-1178-BJ

Tom

[Peter S.]

Re: L46 camshaft recommendations

Duke,

I very much appreciate the help. I will get the gasket info tonight.

Lars did not rebuild the distributor, just the carburetor. The MS-201 was on the 1111491 distributor when I purchased the car. Joe Lucia and Michael Zamora have both previously confirmed that this was correct.

I confirmed with the machine shop that the engine wear is five thousandths of an inch. He still did not recommend an overbore. Vizard's book, which I'm following, also says anything less than seven thousandths of an inch does not need an overbore.

The machine shop will measure the block deck height today.

The 962 cam does sound ideal for my build. I was interested to know if there were any modern grinds that have any improved performance in my operating range.

[Duke W.]

Re: L46 camshaft recommendations

The 201 VAC is "correct" as it is the OE VAC, but it doesn't meet the Two-Inch Rule for full time vacuum advance. With ported vacuum advance, which is for emission control the Two-Inch Rule doesn't apply.

I would get some other opinions on whether the engine needs boring with .005" cylinder wear. Maybe someone else here can chime in. When I rebuilt my 340 HP SWC engine it had six worn .003" and two at .005". I bored it .030", which was the next oversized for the forged SHP pistons.

There are hundreds of different camshaft designs out there and many think that there is something better than "stock", but most have more overlap, which just roughens up the idle and kills low end torque without much effect on top end power. They also often have more aggressive lobe dynamics, which requires higher rate springs that increases valve train loading, and this can lead to valve train durability issues. The OE valve train with OE lobes is dead-on reliable and trouble free. Why mess with a proven great design? I think when most guys restore a Corvette engines they want to retain original characteristics like idle behavior, but often want "more power".

Unfortunately, more power from a camshaft alone reduces low end torque, often below what I consider to be a minimum spec for a responsive road engine - 80 percent of peak torque at no more than 2000. Less than this will often result in a soggy, torque shy engine, that needs a short axle ratio that means the engine is screaming at freeway speed. I don't think most guys want that. OE SHP cams other than the 30-30 meet the requirement.

The sure fire way to get more top end power without screwing up the low end is to massage the heads. The increase begins about 3500 and grows with increasing revs in addition to offering more useable revs at the top end.

The L-46 cam design objectives were to achieve L-79 characteristics, but the details of the lobe design are based on what Chevrolet learned about valve train dynamics in the early to mid-sixties, so it's easier on the valve train and will rev higher.

A few years ago I designed a couple of camshafts for base engine Corvettes, and the story is reported in the Fall 2010 Corvette Restorer. I used OE lobes to preserve proven OE valve train dynamics, and the low end torque design requirement was 90 percent of peak at no less than 2000 to make it user-friendly for both manual and automatic transmissions with OE stall speed torque converters. The second design offered the best broad range performance, and a few other engines have been built of are in work with the "McCagh Special" camshaft. It was specifically designed for massaged heads and relatively high compression and is very low overlap - same as the OE 3896929 "300 HP" camshaft to maintain the smooth 500 @ 18" idle behavior of base engines and stump pulling low end torque, but with more top end power and higher useable revs. It makes 300 HP type low end torque and L-79 top end power and revs.

Given that success I tried improving on SHP cams, but found that the only way to increase power across the range was to reduce overlap (increase LSA) to the point where original idle behavior was not maintained, so I gave up on that project, and I now recommend the L-46 cam for all small blocks were the owner wants a SHP-type hydraulic lifter cam and the LT-1 cam for all 327/350 mechanical lifter engines.

BTW, do not let the machine shop touch the crank other than checking for dimensional conformity and straightness. Turning the crank will remove the Tufftride surface treatment, which reduces crankshaft durability. Unfortunately most machine shops just cut metal without measuring anything, inclduding block decking, and main bearing align boring.

Duke

[Peter S.]

Re: L46 camshaft recommendations

Thanks again Duke. I did not have any issues with the 962, so I think I will stick with it. I prefer to have good driving performance over a lopey idle quality.

I understand you now regarding the vacuum advance. I moved from ported vacuum to full vacuum and did not change out the unit. I will make the switch - much easier with the distributor out of the car.

[Duke W.]

Re: L46 camshaft recommendations

The following text is from the 1963 Corvette Shop Manual:

"If the cylinders were found to have less than .005" taper or wear they can be conditioned with a hone and fitted with the high limit standard pistons. A cylinder bore of less than .005" wear or taper may not entirely clean up when fitted to a high limit piston. If it is desired to entirely clean up the bore in these cases, it will be necessary to rebore for an oversize piston. If more than .005" taper or wear, they should be bored and honed to the smallest oversize that will permit complete resurfacing of all cylinders."

I'm sure the '69 CSM or COM has similar words.

I'm surprised the machine shop is willing to just hone and reuse the original pistons with .005" bore wear because the outcome may be a crapshoot - poor ring seal, excess piston slap due to excess skirt clearance, and excess skirt clearance can also cause skirts to break.

I'm also surprised no on else has commented on your plan to reuse the original pistons with this much bore wear.

The Federal Mogul OE replacement forged pistons are expensive, but something like a Keith Black hypereutectic with similar or a little less dome volume is perfectly acceptable IMO, and they are a lot less expensive.

Duke

[Jack H.]

Re: L46 camshaft recommendations

I'll say that I wouldn't want my Corvette engine rebuilt with .005" cylinder bore wear. If you really want to use standard pistons I'd at least get file-fit rings which are a few thousands oversize so you can at least get the end gaps closer to factory spec, but that would not be my choice personally.

Also, I know the OE replacement forged pistons are available in .020" oversize for the 327 pistons (L2165F+20 for flat top, L2166F+20 for raised dome), but I'm not sure on the 350 pistons since I'm not as familiar with those. So you don't always have to jump to .030" oversize, although those are more common. .040" oversize also uncommon, but available. So it's not just a path of standard->.030->.060, i.e. std->.020->.030->.040->.060. Again, speaking from 327 experience with SpeedPro/FederalMogul/TRW, and not specific to 350.

[Gene M.]

[Duke W.]

Re: L46 camshaft recommendations

That cam is hardly "modern tech". It's an old school "truck/RV" cam designed for low compression with that early indexed inlet event. Modern LS engine camshafts have similar inlet duration, but the POMLs are indexed about 10-12 degrees later. The McCagh Special inlet event is almost exactly the same as the LS2 and LS3 cams, albeit with less lift because the much stiffer LS valve train will handle more aggressive dyanmics.

Modern tech is about lobe indexing, which is very different than the sixties. Lobe dynamics is limited by the basic design architecture, so installing an aggressive lobe cam in a Gen I small block with the required stiffer valve springs is just asking for trouble.

Simulations show the Edelbrock 2101 slightly stronger below 4000 than the L-46 cam everything else equal including high CR, but then it falls ever further behind, and it's down over 50 net HP at 6000. Also the earlier closing inlet valve won't tolerate as much compression without detonation as the L-46 cam, which has a much later inlet closing point, and lower compression will just about eliminate the low end torque advantage.

If one wants to detune a L-46, massaged heads and a McCagh Special cam would walk all over it from off idle to 6000, but the McCagh Special will idle like a 300 HP engine. It will also make as much low end torque as a 350/300 and as much power and revs as the L-46 cam with unmassaged heads, so you get the best of both worlds.

I know there are several guys who like this cam, as it has been mentioned before, but I think it's a lousy design for a Corvette.

BTW, Peter, in the not too recent past there have been dyno tests results posted for L-46s, but IIRC they may not have all used the OE cam and may have had both OE and massaged heads, so you should do a little searching in the archives.

The L-46 is basically a stoked L-79 with some refinements like the cam lobe design, and I've never run across anyone who was any less than completely satisfied with a dialed in L-79 or L-46 even with the modest OE head flow, and massaging the heads makes these engines much stronger on the top end without loss of low end torque or altering the the idle behavior, which has just a little lope - enough for those nearby to know it's not a run of the mill passenger car engine, but not enough to make it too soggy down low.

L-79 and L-46 are probably the best all-around Corvette small blocks because they are about midway between the very tame base engines and the somewhat torque-shy and peaky mechanical lifter engines.

Duke

[Joe L.]

Re: L46 camshaft recommendations

That cam is hardly "modern tech". It's an old school "truck/RV" cam designed for low compression with that early indexed inlet event. Modern LS engine camshafts have similar inlet duration, but the POMLs are indexed about 10-12 degrees later. The McCagh Special inlet event is almost exactly the same as the LS2 and LS3 cams, albeit with less lift because the much stiffer LS valve train will handle more aggressive dyanmics.

Modern tech is about lobe indexing, which is very different than the sixties. Lobe dynamics is limited by the basic design architecture, so installing an aggressive lobe cam in a Gen I small block with the required stiffer valve springs is just asking for trouble.

Simulations show the Edelbrock 2101 slightly stronger below 4000 than the L-46 cam everything else equal including high CR, but then it falls ever further behind, and it's down over 50 net HP at 6000. Also the earlier closing inlet valve won't tolerate as much compression without detonation as the L-46 cam, which has a much later inlet closing point, and lower compression will just about eliminate the low end torque advantage.

If one wants to detune a L-46, massaged heads and a McCagh Special cam would walk all over it from off idle to 6000, but the McCagh Special will idle like a 300 HP engine. It will also make as much low end torque as a 350/300 and as much power and revs as the L-46 cam with unmassaged heads, so you get the best of both worlds.

I know there are several guys who like this cam, as it has been mentioned before, but I think it's a lousy design for a Corvette.

BTW, Peter, in the not too recent past there have been dyno tests results posted for L-46s, but IIRC they may not have all used the OE cam and may have had both OE and massaged heads, so you should do a little searching in the archives.

The L-46 is basically a stoked L-79 with some refinements like the came lobe design, and I've never run across anyone who was any less than completely satisfied with a dialed in L-79 or L-46 even even with the modest OE head flow, and massaging the heads makes these engines much stronger on the top end without loss of low end torque or altering the the idle behavior, which has just a little lope - enough for those nearby to know it's not a run of the mill passenger car engine, but not enough to make it too soggy down low.

L-79 and L-46 are probably the best all-around Corvette small blocks because they are about midway between the very tame base engines and the somewhat torque-shy and peaky mechanical lifter engines.

Duke

Duke-----


The L-46/L-82 cam is about the best overall cam ever for a small block street engine. It's still available today from GM after being continuously available for well over 40 years. It's also still used in some GM SERVICE engine assemblies. This cam may still be manufactured by GM at the Toluca, MX engine plant or the Bay City, MI engine components plant.

[Joe L.]

Re: L46 camshaft recommendations

The following text is from the 1963 Corvette Shop Manual:

"If the cylinders were found to have less than .005" taper or wear they can be conditioned with a hone and fitted with the high limit standard pistons. A cylinder bore of less than .005" wear or taper may not entirely clean up when fitted to a high limit piston. If it is desired to entirely clean up the bore in these cases, it will be necessary to rebore for an oversize piston. If more than .005" taper or wear, they should be bored and honed to the smallest oversize that will permit complete resurfacing of all cylinders."

I'm sure the '69 CSM or COM has similar words.

I'm surprised the machine shop is willing to just hone and reuse the original pistons with .005" bore wear because the outcome may be a crapshoot - poor ring seal, excess piston slap due to excess skirt clearance, and excess skirt clearance can also cause skirts to break.

I'm also surprised no on else has commented on your plan to reuse the original pistons with this much bore wear.

The Federal Mogul OE replacement forged pistons are expensive, but something like a Keith Black hypereutectic with similar or a little less dome volume is perfectly acceptable IMO, and they are a lot less expensive.

Duke

Duke------


If the taper is actually 0.005", I agree that's too much. A re-bore is in order. I recommend 0.020" overbore, NOT the 0.030" that most machine shops want to do.

Personally, I NEVER recommend the re-use of pistons when an engine is completely rebuilt. I would NEVER do so. The KB or Sealed Power hypereutectic pistons are the way to go for virtually all street engines, including those originally fitted with forged pistons.