L79 Cam Replacement

[Ron Fratoni]

I just had my original 1966 L79 rebuilt with the instructions to the rebuilder that I wanted it to be as close to original as possible. That meant idle and sound included. Since installation, I have not been able to get it timed right with the distributor in the proper location. It also has a sharp rapping sound from the side exhaust. It's too annoying to drive it that way. He installed a Comp Cam 270H. He is willing to install a different cam of my choice. Any suggestions to make this feel and sound like it used to?

[Clem Z.]

Re: L79 Cam Replacement

did your engine builder block the heat riser crossover in the intake because that will cause the sharp rapping sound in the side pipes because this happens when you split the exhaust into a 2 4 cylinder engine system.

[Joe L.]

Re: L79 Cam Replacement

Ron-----

I don't think that the cam that you have currently installed is the source of your problem. However, since the rebuilder has offered to replace the cam, I'd just go to a stock replacement. The GM #3863151 camshaft is still available from GM for about $175, list.

A Comp Cams 270 advertised duration cam, depending upon which one it is, has been used in a lot of small blocks with GREAT success and driver satisfaction. So, as I say, I don't think that this is the source of your problem. But, since you want as much originality as possible, I think that installing the '151' is the way to go for you.

[Ron Fratoni]

Re: L79 Cam Replacement

Joe,

I fooled with the distributor at length and no matter where I put it, I cannot get rid of the sharp sound. I installed a rebuilt Chevy distributor just to ensure that it's not my distributor. Also replaced the carb to eliminate that consideration. He thinks it sounds right, but with the side exhaust it is noticably louder. It sounds like it is still firing with the exhaust port open. He agree that might be the case because it has additional duration.

He says he uses the 270H as a regular replacement for SB with no complaints. If I was getting some exceptional performance, I would consider it a trade off. But I don't feel any gain - even over the original motor with 160k on it. Something doesn't add up. He expected that I would get serious improvement. He feels that putting in the original cam is a real step backwards, but is willing to do whatever I want.

Any other ideas why the distributor(s) will time correctly when dropped in correctly. This seems to me to be a clue that something is wrong. I measured TDC on compression and it is right on the money.

[Ron Fratoni]

Re: L79 Cam Replacement

Clem,

I will check with him on that.

Thanks.

[Patrick H.]

Re: L79 Cam Replacement

Ron,

If he's willing to replace it, do so.

In addition, I'd advance the cam 4 degrees for better low end torque. You'll like it when it's done.

I have the 265 DEH in my 71 and like it, but agree the 270 may not be what you'd like. You maybe having a problem with vacuum too (cam doesn't make enough) and tht may also affect your idle.

Patrick

[Ron Fratoni]

Re: L79 Cam Replacement

Patrick,

Is yours a Comp Cam?

Ron

[Duke W.]

Re: L79 Cam Replacement

"I just had my original 1966 L79 rebuilt with the instructions to the rebuilder that I wanted it to be as close to original as possible."

You give him these instructions, and he installs an aftermarket cam!!! Okay, I won't get into that rant again.

Install a correct 151 cam and new lifters. Check that the heat riser passages are not blocked as Clem suggested. It's okay to wire the valve open, but don't block the passages.

Check the numbers stamped on the vacuum advance bracket. A correct Delco is stamped "236 16". If it is not this or "B28", buy a NAPA Echlin VC1810.

The ignition advance map intial timing, vacuum advance and centrifugal advacne should be OEM as specified in your '66 Chassis Service Manual, except for maybe a quicker centrifugal curve.

If it is a California model with AIR you'll need to make some changes.

Duke

[Patrick H.]

Re: L79 Cam Replacement

Yes.

[Joe L.]

Re: L79 Cam Replacement

Ron-----

I really can't see why the Comp Cams 270H should cause you any problems, at all. The 270 degree duration is the ADVERTISED duration. The duration @ .050" valve lift, which is the more important spec, is 224 degrees on both the intake and the exhaust. That compares with 221 degrees on both the intake and exhaust for the '151' stock cam. I can't see that making a lot of difference. The valve lift is .470" on both intake and exhaust. That compares with .447" on both intake and exhaust for the '151'. The difference here should help you make more power with little difference in idle quality or driveability.

The lobe seperation angle for the 270H is 110 degrees; the lobe seperation angle for the '151' is 114 degrees. That factor could make a significant difference in the characteristics of this camshaft over the stock '151' cam. However, I don't think that it should make enough difference to make the cam perform unsatisfactorily. Like the mechanic says, the 270H is used in a lot of small blocks with great results. It's very popular, as a matter of fact.

Another option for you would be the Comp Cams "N+L79H" grind. This cam is specially designed to emulate the sound and character of the '151' cam, while producing extra power. It's specs are as follows:

intake duration @.050 lift= 229 degrees

exhaust duration @.050" lift= 236 degrees

intake valve lift= 0.468"

exhaust valve lift= 0.462"

lobe seperation angle= 112 degrees.

I have no experience, whatsoever, with this camshaft, but I think it will do as advertised. Overall, it's a "hotter" grind than the one you have, though.

Or, you could just go to the stock '151' cam, as I mentioned before. But, as I say, I really think that your problem lies elsewhere besides the 270H cam. I just don't know what it is.

[Duke W.]

Re: L79 Cam Replacement

The problem with these aftermarket cams is they have too much overlap. They are designed to work with headers and open exhaust, but kill low end torque with the OE exhaust manifolds and mufflers

The story is in the lobe centerlines. The L-79 cam is 110 ATC for the inlet and 118 BTC for the exhaust. Lobe center angle is the arithmetic average of these two numbers or 114 degrees.

The Comp Cams catalog doesn't even provide complete info on the 270H or their other cams to adequately characterize and compare them to other cams. They just state a LCA of 110, so you have to guess the actual centerlines, and I think they are 110/110.

The fact that all the cams in this series are speced at a 110 LCA tells me that they grind several different finished cams of different duration off the same blank. Chevrolet developed their SHP cams with OE manifolds and each finished cam starts with a unique blank.

You can get an idea of what lobe phasing does on simulation programs. Based on my research I reindexed the cams on my Cosworth Vega by retarding the inlet 8 degrees and advancing the exhaust and equal amount. They were indexed with excess overlap to control NOx emissions. The old centerlines/LCA were 102/110/106 and are now 110/118/114. NOT coincidentally, these are the same as the L-79 cam. These centerlines work well on both engines by offering excellent torque bandwidth for a street high performance engine because they both have very nearly the same stroke.

With the reindexed cams, the engine's character has completely changed. Now it idles butter smooth at 900/18" instead of 1200/12". It has better throttle response, particularly at low revs, and SOTP pulls stronger from off-idle to 7000+. Next summer I'll have dyno numbers to compare with dyno results prior to the reindexing.

BTW, NOx emissions doubled with the new indexing, but since they started out at 1/4 the max allowable, then are now only at 1/2 the allowable.

Duke

[William V.]

Re: L79 Cam Replacement

Ron

You probibly should change the cam as suggested. However, I suggest before you do, you degree the ballancer timing mark to ensure the accurate timing. Balancers are often found to be several degrees off.

Good luck

[Craig S.]

Re: L79 Cam Replacement

Joe - is the 151 cam through GM still the original GM cam as in days of yore, or a blueprint grind from Crane or anohter aftermarket supplier? Just curious if I ordered one for my 67 L79 if I am just paying a higher price for a blueprint grind sourced through GM. I ran into this before when I ordered a small blow high volume oil pump (seems like it was the LT1 part) and I got a melling pump in a GM box...think it said M55 HV on the casting or some such thing. Not that they are bad pumps, just wondering if GM is playing the parts substitution game on some of these parts compared to say what you got 10 years ago when ordered...Craig

[Craig S.]

Re: L79 Cam Replacement

"small blow".....jeesh...if only I could type...."small block"

[Joe L.]

Re: L79 Cam Replacement

Craig------

I really don't know what cam that they're selling under the '151' part number these days. Certainly, it's not one manufactured at the old Flint V-8 engine plant as many of the original PRODUCTION and SERVICE cams were. However, it could still be an in-house part manufactured at the Toluca, Mexico engine plant or the Bay City, MI engine components plant. If they were going to sell reproductions under the original GM part number then I don't see why they would have needed to discontinue any of the big block and small block cams. Just about all are available in reproduction. But, they have discontinued most of these under the original part number.

I think there's only one way to find out. Sometimes, lessons are expensive. I have purchased a LOT of parts just to learn "what they were all about" and examine them for manufacturing and configuration details. Usually, obtaining the part is the best, and sometimes ONLY, means to do this. Of course, when the inspection and research is over, they enter my collection.