Hello. Had the engine in my 1980 L-82 Vette rebuilt: bored 30 over, heads milled, new valves (1.94 intake/1.50 exhaust), everything else is stock (q-jet, auto tranny, 3.07 rear.) The shop that rebuilt it put in a "mild cam" with these numbers: 214-224 443-465. Has anyone else used this cam set up? Is it appropriate for this car? What did I have originally? I'm getting a lot of detonation under load, and have to turn the timing back to 8* (from 12*) just to minimize it. Could this cam be the problem? A novice to these things, but I'm learning! Thanks for any help. JG700

 

I'll 'assume' (and I hate using that word..... ) those numbers refer to duration @ .050" valve-lift (214* intake and 224* exhaust ), while the 2nd set of numbers are the amount of valve-lift with the stock rocker arms (.443" intake, and .465" exhaust ):
I am not-sure of the specs for an '80 L-82 cam, but it isn't much bigger or longer (if-ANY ) than this cam, I wouldn't imagine.....


I don't think that cam would cause pre-detonation by-itself..... when the motor was over-bored, what pistons did the shop use (flat-tops, pop-ups, dished? ), and/or how-much did they whack-off of the cylinder-heads?

 

Flat top pistons with 4 "eyebrows." Heads milled .010. Does this help?

 

These are the specs for the L-82 cam:

Intake duration @ .050- 222
Exhaust Duration @ .050- 222
Intake Lift- .450"
Exhaust Lift- .450"
Lobe Centerline- 114

 

You have an L-82 with 1.94/1.50 valves? The correct valve size for an L-82 is 2.02/1.60. Sumthin's wrong.

 

Would this cause my detonation problem?

 

I am not sure why it would. This engine uses the same cam and has the same valves as what you have:

http://paceperformance.com/index.asp...D&ProdID=84482

 

Did you try disconnecting and plug the vacum advance to the distributor yet and see how it runs? This still sounds like timing/distributor issue to me.

What is the intake center line on that cam, (point of max lift on the intake, 106 or 107 degrees? Also, what does it tell you the exhaust closing at .004" is? Next question, did the shop install it straight up or even bother to degree it when installed.

Good comment on the valve sizes too, did they swap out your heads or were they changed at one time, basically what chamber size do you have?

 

Disconnected and plugged vacuum. It ran better (fewer knocks) but not completely eliminated under load. I even tried it after turning the distributor back to 8*, but still had detonation under load. I'll check on the other stuff you asked. Thanks for the help everyone!

 

Cam centerline is 112* (107/117). Does this help?

 

You have increased compression and reduced the cam duration. You need 100 octane to run it without detonation or change the cam as the L82 was originally designed to run premium. The L82 cam also had a late intake valve closing to help prevent detonation. I would change the cam to something with around 224 int duration and 230 exhaust duration, Dial your timing in good and run premium fuel. You will destroy your motor if you don't resolve the detonation problem.

 

First off, make sure the balancer hasn't slipped. Should be new after a rebuild but you never know...

The cam is mild, about what I'd pick for a stock rebuild... not too crazy, should have really good manners on the road...

Check your timing and vacuum advance again, the engine should run fine anywhere between 4 and 12 degrees advanced at idle (vacuum hose plugged).... you should see around 30-36 degrees total timing at 2500-3000rpm.

Check one more time if the plug wires are connected correctly.

 

I used to run flat tops (didn't even know, the p.o. installed them), 64cc aluminum heads with a edelbrock Performer cam (204-214)..... no detonation problems with 93octane fuel but low end power like a truck...LOL

Yes, with low grade fuel you'll have detonation, especially with iron heads....

 

I like the XE268H From Comp cams for your application. It has 224 int and 230 ex duration. Intake lift is .477 and exhaust lift is .480. It comes with a 110 LSA. A 112 LSA would be a better match for you. a custom grind cost a couple bucks extra and adds 2 days to your delivery time but will give you a better idle and a little better power under the curve with your high gear rear end. Your motor would love a 3.55 gear in the back. That is a lot of the reason for the detonation. Your lugging the motor a little with the 3.07 rear. You probably wouldn't have an issue with aluminum heads with your combination but between the small duration cam, high rear gear, iron heads and the compression bumped up they all combine to cause your detonation problem.

 

I did about the same to the LM-1 in my '79 Z28 a few years after it was new, adding 64cc iron heads, and a cam with 218* @ .050" with a 114* LSA, and never had pinging on good-grade of gas (higher-octane SUNOCO or GULF )

 

As already stated by 63mako, you do not have a matched build. That is an early closing inlet cam with 7 degrees of advance ground into it over what your stock cam had. IMHO, these types of cams are made for 8:1motors. Not knowing what the rest of your build is, that cam is suspect for causing too much dynamic compression. Myself, I would yank it out and install a new L-82 replacement from Crane or Sealed Power. With it's late closing inlet valve, it should resolve your detonation problems with any pump gas.

 

72 cc heads, if that is important.

 

I'm sorry I'm suspect of a shop that increases the SCR then puts in a milder cam. Why would you put in a milder cam than the original with a rebuild?

If they can't even properly match a cam, I'd be affraid of what else they did.

 

Why is it milder? I don't really understand the numbers.

 

It has a bit lower lift and a shorter intake duaration. This cam has 214 degrees of intake and 224 degrees of duration while the L-82 cam has 224 degrees of intake and 224 exhaust so it is not terrible different in those regards but it is ground advanced as indicated by the Intake Center line points, 107 for this cam and 114 for the L-82 cam. For reference, your new cam's intake valve closes at 63 degrees ABDC as opposed to the L-82 cam's closing point of 79 degrees ABDC, it closes 16 degrees sooner creating more compression.

With 72 cc heads and flat tops it is pretty hard to believe that you have too much compression. I ran a quick SCR/DCR calculation using www.empirenet.com , assuming you did not mill the block with a standard deck height of .025", and you have true 72 cc heads, flat tops with a piston volume of -7 CCs for the eyebrows, and you used a standard thickness head gasket of .039", you are just shy of 9:1 static compression (SCR) which is about the same as your original L-82. The resultant DCR from this program shows you at 7.2:1 so in theory you should be OK compression wise ASSUMING that nothing is way out of bed here.

Did anyone bother to degree that cam when they installed it. Many an aftermarket cam requires some adjustment during install. Did the the cam get installed straight up or advanced a few degrees which would be hurting you.

You are going to have to investigate the timing and the distributor and possibly back off your timing more.

You need to pull a plug and see if it is the correct tan color, if not address any carb issues such as possibly installing bigger main jets.

You need to mark off your balancer either with a degree tape or measure it out. Hookup a timing light with the vacum advance hooked up, and see how much total advance you are running.

If you haven't read Barry's sticky on timing and carb set up, please do so, it explains this in detail.