L81 Factory Cam Spec or P/N

[Vader]

I've done the search, both on the sites and www. No one seems to list definitive specs or a part number for the factory cam grind of the '81 L81. Any ideas?

 

[tom81]

3896962 Hydraulic Flat Tappet
This hydraulic flat tappet is used on the 68-81 L-46 and L-82 Corvette. The duration at lash point in degrees (intake/exhaust) is 312/312; duration at .050" tappet lift (intake/exhaust) is 222/222 and maximum lift with 1.5:1 rocker ratio (intake/exhaust) is 450/460. Valve lash is zero/zero and lobe centerline is 114 degrees


http://www.gmgoodwrench.com/gmgoodwrenchjsp/perfparts/partlist.jsp?cat=9387&section=ep

 

[Vader]

Tom,

Thank you sincerely for your information. I've been striking out in my searches, and could not find a part/cast number.

I found a similar description but thought it may have had typographical errors. The advertised duration seemed excessively long for the relatively average 222° duration @ 0.050. I'm guessing they must have ground some really long pre-lash ramps (even for a flat tappet) in the profile. Maybe the theory was an attempt to scavenge a little better to compensate for the pathetic static compression pressure ( Just guessing here). I'm more accustomed to seeing the longer ramps on solid flats.

Just one clarification. Is the 114° intake lobe centerline for "straight-up" installation, or the lobe centerline angle?

Again, thanks for the details.

 

[tom81]

Vader,

If you click on the link you can tell I just cut and pasted the info.
From the description given I would assume this is the cam they
used in 81 vette. However I don't know for certain that this is the exact cam used, I was just going on the information given on the GM website, which one would think would be correct.

The last line in the paragraph states the lobe centerline angle is
114° which seams a little much, I would have expected the
lobe centerline angle to be more like 110° or 112° at the most.

I did some more looking around and found another link using the
part number from GM

http://www.geocities.com/MotorCity/Downs/3000/cmspec.html

From what I have found the correct GM part number is 3896962.

Perhaps there is someone out there that can shed a little more light on this subject.

 

[Vader]

Thanks again. I'm getting conflicting information. I've got specs from .410/.423 - 222°/222° 114° to the 3896962 you listed.

I guess I'll throw the dial indicator on the deck when the heads come off and see if I can bolt on the new heads or go shopping for a lumpity stick....

Obviously, I'm hoping you're right.

 

[GerryLP]

Tom, Vader et al,

Was the L82 cam and L81 cam the same specs?

 

[Tom Bryant]

Tom, Vader et al,

Was the L82 cam and L81 cam the same specs?

I can say almost positively no. Cam 3896962 is the L82 cam used from '73 to '80. The L48 did not use this cam. That listing is in error. The L48 used the the standard small block cam part number 3896929. This was the base 350 cam that was used in everything that wasn't labeled hi performance or special high performance.

I don't have any books new enough to tell what the L81 used but they don't lope like the L82 and I don't think they used the 929 either.

This may take some further investigation.

Tom

 

[Jack]

In a related thread ... http://corvetteactioncenter.com/forums/showthread.php?s=&postid=227894#post227894 ... GerryLP posts some lobe lift specs from L81 manual that seem quite similar to L48's 3663152 .390'/.410" valve lift. I agree w/ Tom ... L81 sure don't lope like an L82. Additionally, neither does an L48. My guess is that L81 cam is either same as L48 or quite similar.

BTW ... I bought new wolverine cam that is oe-type replacement for L82 ... I measured it ... it truly does have a rather wide 114* lsa & .450"/.460" valve lift.
JACK:gap

 

[Tom Bryant]

I think that the L81 cam was different than either the L48 or L82: something designed for the computer controlled engine. It may have been used in other engines too.

I wonder if Burglar can help with this?

Tom

 

[Vader]

Maybe I'd better pull a cover and check lifts. There isn't much room for a degree wheel, so I don't know if I'll get duration and timings until I tear into it. Good thing I have a spare (already run-in) 3863151 and lifters packed away in the tube...

Thanks for the information. Keep it coming. Nothing I've found on any sites has been very helpful, only more confusing and speculative.

 

[Tom Bryant]

Here's what I got from Dave (BURGLAR):

Tom - GM part# for L81 cam is 14060651 (new or current # 14088839). Glad to help...Dave

I'm thinking this cam was also used in other early computer small blocks.

Tom

 

[Vader]

Thanks again. I expected something lower than .450/.460. As I mentioned, when the heads are ready, I'll be tearing in and measuring lifts.

 

[Tom Bryant]

Dave also says that after '81 that cam only showed up in light truck applications so it must be a torque grind.

 

[GerryLP]

Now...if one feels adventureous, then one could change the rockers to stamped-roller-tipped rockers with a 1.6 ratio, and one could watch the torque numbers improve...specially if they are CompCams or something quality like that. I recommend against GM stamped rockers.

Its easy...well lets say do-able

First, mark the #1 cylinder ignition electrode on the rotor cap to the distributor body.

Second, disconnect "battery" lead at distributor rotor cap

Third, remove distributor rotor cap

Fourth, turn motor over with the aid of the starter, and watch for the mark on dampener approach the timing placard. Short starter impulses works great, for this allows you to go and look at the dampener mark and see how close it is to the timing reference on the timing cover. A remote starter switch work wonderfully on this. Is the distributor rotor getting close to mark that you made in first step? if so then you are approaching TDC, if the rotor is pointing downstream of mark, then the #6 cylinder piston is approaching TDC.

Fifth, if the engine's #1 cylinder piston is at TDC, then start removing the valve covers ( I know...its a bit tedious)

Sixth, with the #1 cylinder at TDC the following valve rockers can be replaced:

Exhaust Valve rocker--- [cylinder #] 1, 3, 4, 8
Intake Valve rocker --- [cylinder #] 1, 2, 5, 7

What this means is that those exhaust and intake valve lifters are on the heel side (base circle) of the cam. Also, remember the sequence in which the rockers are laid out on each engine head. For example, the corresponding rocker for the #1 cylinder EXHAUST VALVE is to the left (as one looks at the head from the left side of the engine). The one to the right is the intake valve; however, on cylinder #3 (the one behind #1) the valve to the left is an intake valve and the one to the right is an exhaust valve. Follow the sequence and you'll keep it straight. Its best to use the exhaust manifold as a reference to remind you which cylinder starts with an exhaust valve.

Coat the rocker balls, rollers, and pushrod tips with the break-in lube sold by any of the cam companies. I like using the CompCam version.

Don't worry about the additional lift that a 1.6 rocker will give you. 1.6 rocker will add approximately 0.020" of lift (depending on lobe lift) to the valve. However, if these are the better rocker brands (not GM stamped rockers), then the lift will be accurate throughout the cam profile -this equals added power. The valve springs will easily take this added lift.

Replace the rocker arm nuts if they seem loose. They're not expensive and you'll have peace of mind.

Ensure that the pushrods have not fallen out of the lifters by lightly pressing down. One should feel a slight bounce on hydraulic lifters.

This short paragraph is an editorial for step seventh. The GM L81 shop manual calls for adjusting the valves to zero lash plus one full turn of the rocker nuts. I believe that this is too much. My factory cam went flat at 38,000 miles, and I feel that this was the reason. Most cam manufacturers recommend zero lash plus 1/2 turn of the nut.

Seventh, adjust those same rockers mentioned in step sixth above to zero lash plus one full turn (if stock L81). See editorial above. The zero lash is determined by turning the pushrod between your fingers while turning the rocker nut. When the push rod won't turn with normal pressure (some of you strong-corn-fed football players heed this...lol), then this is zero lash. Its easy to losen the nut slightly and try a second time. If the zero lash is reached, then turn that particular rocker in the sequence mentioned in step sixth to one full turn (again, I prefer the 1/2 turn recommended by Crane and CompCam).

Eight, once all the rockers identified in step sixth have been replaced and adjusted, then rotate the engine over until the dampener completes one full rotation. With the dampener mark aligned with timing reference on timing cover the #6 cylinder piston is at TDC.

Ninth, repeat step sixth above BUT USE THE FOLLOWING SEQUENCE:

Exhaust Valve rocker--- [cylinder #] 2, 5, 6, 7
Intake Valve rocker --- [cylinder #] 3, 4, 6, 8

adjust the valve rockers as outlined in step seventh, but use the sequence outlined in THIS STEP.

Tenth, re-install covers with new gaskets onto clean surfaces and re-install the distributor rotor cap, wires, and the "battery" lead to the rotor cap. Start engine and check for leaks.

Comments, suggestions, and discussion are welcomed. Did I forget something (without getting too technical, I mean)???

 

[Tom Bryant]

Gerry.
I have heard that the drippers cast into the inside of the valve covers will interfere with roller rockers. I can't remember if the L81 valve covers have the drippers like the LT1 covers or not. I thought that they were the same casting. Of course the body of the roller tip rockers are about the same as standard stamped rockers so this is probably not a problem.

What's your feelings on this.

Tom

 

[GerryLP]

Tom,

I agree with you 100%!!! The full roller rocker would require the tall valve covers, and if one elected this option, then one would have to get rid of the A/C bracket on the right side of the engine (or manufacture a complete custom bracket arrangement to hold the A/C compressor).

I used stamped roller-tip rockers (CompCam), and these cleared the original L81 valve covers. ChevyHiperformance Magazine did a good article on rockers, and definetely GM rockers are not accurate when compared to CompCam's and Crane's.

In my last cam change (XE274), I noticed how the original valve cover finish was peeling-off the covers, so I bought some "cheapee" aluminum covers with flat black finish and exposed aluminum grooves lenghtwise. But I used the L81 covers first, and they worked fine.

This discussion also reminds me of another point I would like to bring to everyone. I had the opportunity to run my engine with the L81 computer, rochester, and distributor together with my Edelbrock Aluminum heads (p/n 6090), my first cam (HE268), headers, and bored-over 1968 short block. My car ran strong, and it behaved like an L82 (240 Hp or so); however, I didn't have any problems with the computer's check engine light or with codes. I even checked for codes, and I found no fault. The only check I DID NOT perform was to have the vehicle smog checked (by the way, those Edelbrock heads are 50-state legal). Nevertheless, the computer will not be able to extract all the potential performance out of these aftermarket high performance components. This would require a custom computer chip from someone like Jet Performance or ditch the computer; however, one can do small mods that would improve one's L81 engine greatly.

Therefore, some of these small hidden modifications are worth your while. Getting a little more lift on the L81 valves will give more room to the excaping gases and incoming mixture to do their thing and improve performance. It will not screw-up the computer.

However, I still remember how funny Bud was when he vocalized what "Rare" would say if it was a person -"but you used to like me the way I was?!?!"...:L

 

[Vader]

O.K. HEre's where I'm at. Mostly disassembled. Intake openings are so poorly gasket matched that they actually dead-ended into the slightly less-poorly-matched openings in the head (gotta love that core shift). After getting that close, accessories out of the way, and heads off (soon to be recast into something useful, like a park bench perhaps), I stuck a magnetic base on the deck and measured lobe lift. I got about .253-.256" on average on the six lobes I checked (both intake and exhaust).

I could peer through the drain hole in the front all and see a nylon-toothed cam sprocket still there. So, the water pump came off. Balancer was pulled, Timing cover off, and oil pan dropped a bit. The radiator was removed and condenser only loosened from its mountings. The rear transmission mount was loosened and the trans raised to provide a little nose-down attitude on the engine. At this point, the cam was removed with just a wee bit of clearance at the condenser.

So, for the first answer, Yes, the cam can be swapped with the engine in the vehicle.

Once out, I double checked with the 2" mic. Yup, 0.253" on the intakes and 0.256" on the exhausts. Considering the average 1.5:1 ratio of the factory rubber rockers, that works out to 0.383/0.388". I didn't even bother to throw it in 'V' blocks and check duration. I didn't see the cam number behind the fuel pump lobe either, but anything this lame has to be a factory grind. I'm guessing by the lift specs, it's a pretty short duration, too. I may get a wheel and indicator on it and check it later if I'm bored, since it surely isn't going back in. Would make a nice can crusher, though.

Therefore, for the second answer, 0.383/0.388" lifts. It figures. It goes along with the 13cc dished pistons and 76cc chambers with smallish valves, poor valve bowls, and really sharp short-turn radii and intrusive guide bosses. Those '416s should be really nice on that pathetic setup. The xE-262H should be just fine for a stock 2.73 gear set, too.

I'll have more tomorrow, and probably some photos of the cam clearance at the condenser, and a few others of interest.

 

[Vader]

As promised:

I'm still looking for a factory cam with that lify spec. I think it was a .384"/.390" before wear.

 

[Vader]

Well, I got a little bored and dropped the cam in 'V' blocks with a degree wheel. Some of the lobes were worn (not surprised) and the best lobes I could measure yielded these numbers:

(SPEC-INT-EXH) Lobe Lift .253" / .266", Gross Lift .381" / .402", Total Duration 306° / 323° (crank degrees), Duration @ .050 196° / 204°, LSA 113°.

Also, after a trip through the parts washer, the cam does have some numbers "GM 54 6930" I couldn't find a reference to that anywhere.

The next time the question arises, hopefully someone who searches will be able to find this thread.

 

[Vader]

It seems that GerrLP is the Grand Prize winner! That original cam was indeed the infamous 929 (3896929) 0.390"/0.410", 195°/202°, 112° LSA, extremely common and useless POS. That casting number checks out for that profile. I'm afraind that even 1.7:1 FMS rockers wouldn't help that thing. No matter, the new cam takes care of all of that with lobe lift alone. PVS 1.5:1 long-slot roller tips fit nicely under the stock covers.

And thanks for the "primer" on cold lash setting, but I had that handled. I'll relash hot once the break-in molykote is drained with all the rest of the junk that I couldn't clean out of there. And incidentally, I prefer ¼-turn hot lash preload for any lobe profile with valve accelkeration rates close to 0.004"/°, mainly to prevent lifter pump up at higher RPM. 0.030" of plunger travel has proven to be plenty for a cam that's been run in. The replacement springs and lightweight retainers will handle it, so I need to be looking at that sort of thing now. BTW - stock length push rods were oddly the best choice, since the base circle was still 1.240" on the replacement lumpity stick. Wierd, huh? We should be roadworthy tomorrow, following an afternoon of drywall taping on another "project".