L79 Cam Replacement

[Ron Fratoni]

Thanks to a lot of good feedback from an earlier post, I am going to replace the 270H cam in my recently rebuilt L79 motor with the stock 151 cam. The rebuilder put in Z28 valve springs. Do I need to replace these also? Does anyone know what the stock spring pressure was? 80#?

[Joe L.]

Re: L79 Cam Replacement

Ron-----

If the "Z28 valve springs" the rebuilder used were GM #3911068, these will work fine with the L-79 camshaft. These springs are 80# closed pressure. That's just about the same as the original springs.

[Duke W.]

Re: L79 Cam Replacement

There's no such thing as a "Z-28 spring". Z-28s used the same valve springs as two-barrel 283s. Both hydraulic lifter and mechanical lifter SHP engines used the same spring as every other small block. There was a change in spring part number circa late sixties, but the new 068 spring has essentially the same specs as the prior production spring and superceded the old part number for all prior small blocks.

Chevrolet did offer higher rate springs, and they were specifically designed for the "off-road" cams that were available over the counter.

Duke

[Joe C.]

Re: L79 Cam Replacement

Duke:

Federal Mogul lists the following for 327 engine valve springs:

VS677 : 67-69 : w/damper
VS739R : 67-69 : opt HD Z28
VS521 : 62-66 : w/damper

The 739R spring is also listed for various Z28 applications with 302, 305, and 350 engines.

The 521 spring is also listed for all 283 57-66.

Sounds like you are right about the springs; and I assume that you are saying that the GM counterpart to the 521 spring was replaced with a very similar 677 spring, for all applications. Are you saying that Chevy never actually installed 739R type springs? Then why were they made SPECIFICALLY for Z28 applications? Is there a significant difference in the rates of the 521/677 vs 739R? Assuming that the high rate springs will hasten valve seat recession, then how much of a redline boost could one expect with the 739R's?

[Clem Z.]

Re: L79 Cam Replacement

the #142 springs are referred to as Z-28 springs in the aftermarket because the were to be used with the #140 cam for the Z-28 racing. they are about 110# @ 1.700 and i would not use them with pressed in studs.they are fine for all chevy cams if you use screw in studs.

[Duke W.]

Re: L79 Cam Replacement

I don't have any data on those F-M springs, but if you look at a Chevrolet P&A catalog you will find that the 3911068 spring is listed as the replacement for all small blocks until they started installing valve rotators on the exhaust side in the seventies. An older book may still list the 3735381 for 55-66, but once stocks were exhausted the 068 serviced all applications.

The specs for these two springs are essentially the same.

Chevrolet SHP cams, both mechanical and hydraulic lifter are designed with very "soft action" so they can use the same spring as mild small blocks, but still have higher valvetrain limiting speed with OE valvetrain reliability and longevity. The aftermarket always offers options, and higher force springs are required for Chevrolet off road cams and many aftermarket cams.

Higher valve spring force with allow higher revs with OE cams, but you might pay a price in longevity and reliability, and not all engine configurations can benefit from higher revs depending on how the power falls off.

My advice is to stay with OE cams and valvesprings (or equivalent) unless you are building a racing engine.

Duke

[Clem Z.]

PS the #140 springs would be a rust/brown color *NM*

[Joe C.]

What Is The #140 Cam? *NM*

[Bryan L.]

Re: What Is The #140 Cam?

that was the over the counter cam hot cam for the 302. I think the lift was .512. It's often referred to as the #140 or off-road cam.

As a 16 y/o Mark Donohue wannabe, I bought a used #140 cam from a local drag racer ($25.00) and put it in a 67 Camaro with a basically stock 327 with only headers on it. It sounded fantastic but was so slow it wouldn't fall off a bridge. Live and learn.

BL

[Clem Z.]

Re: What Is The #140 Cam?

the full # is 3927140,it is what GM called "first design racing cam". it was a over the counter only cam.

[Joe L.]

Re: L79 Cam Replacement

Duke----

Not exactly. The GM #3911068 spring never "officially" replaced the GM #3735381. The GM #3735381 was originally used for all 1958-66 Corvette applications with small block. It replaced the GM #3837002 which was originally used for 55-57 Corvettes.

The GM #3735381 is a valve spring/flat damper assembly which has the following specs:

OD--------------1.228"
free length-----2.097"
wire diameter---0.169"
closed pressure as installed----76#
open pressure @ 1.26"-----------175#

The GM #3735381 valve spring assembly remains available from GM to this very day.

In 1967, Corvette and other Chevrolet small blocks changed to valve spring assembly GM #3896931. This spring was used throughout 1967 PRODUCTION but not thereafter. It was discontinued from SERVICE in March, 1968. I don't have specs for this valve spring, but I think that they are very similar, if not identical, to the GM #3911068.

Starting with model year 1968, the GM #3911068 valve spring was used for all PRODUCTION small block applications. The GM #3911068 also became the SERVICE valve spring assembly for 1967 small block applications which originally used the GM #3896931 valve spring assembly. However, it did not then become nor is it now the SERVICE valve spring for 1955 to 1966 Corvette small block applications. It can be used for 55-66 applications, though, and may even offer some slight advantage over the original springs for these applications. The specs on the GM #3911068 valve spring are as follows:

OD----------------1.239"
free length-------2.027"
wire diameter-----0.177"
closed pressure as installed-----80#
open pressure @1.25"------------200#

From 1968 through the 1991 model year, the GM #3911068 valve spring was used for ALL Corvette small block applications. It was also used for most other Chevrolet PRODUCTION Gen I small block applications right through the year 2000. This also included ALL 1968+ Z-28 PRODUCTION applications (1967 Z-28s originally used spring GM #3896931 which was replaced for SERVICE by the 3911068).

GM never used a valve spring on any PRODUCTION Gen I small block with a higher spring pressure than the GM #3911068. Other, higher-rated valve springs were used on SERVICE-only engine assemblies or otherwise available seperately in SERVICE. The GM #3927142 valve spring is one of these and is the one which is often referred to as the "Z-28 spring" since it was used with the "Z-28 off-road" camshaft GM #3927140. This spring has an OD of 1.273" and has a closed spring pressure of 110#. This spring can be used to advantage in a high reving small block, but at the expense of camshaft life.

The FM 739R spring has specs similar to the GM #3927142.

[Mark F.]

Re: What Is The #140 Cam?

Is the "140" cam still available? Is is better for road use than the "151" cam?

Mark

[Clem Z.]

Re: What Is The #140 Cam?

the #140 cam is a race only cam,you need open headers,high compression to make power with it.

[Joe L.]

Re: What Is The #140 Cam?

Mark-----

The GM #3927140 camshaft is discontinued. However, it's available in a Crane-manufactured reproduction and as part of camshaft kit GM #12364053. Would I use this cam for a street driven car? No.

[Joe C.]

Thanks Joe

These "142" springs are what the machine shop installed in my double hump heads for my '65 L76. The engine builder, who has an excellent rep, specializes in Chevrolet smallblocks. I had him install hardened exhaust seats, check for cracks, check the guides, etc etc. The springs he replaced were stock GM, and tired at 65 pounds, closed. The "Z28" springs are, as you said, 110 pounds (he showed me the old and the new on the spring setup tool). I was very concerned about pulling rocker studs, and asked him to pin them. He could have taken my money, but he said that it was not necessary. That convinced me of his sincerity and integrity. When I got the heads home, I pocket ported them, further unshrouded the exhaust valves, CC'd the chambers, polished the runners, and gasket matched. I also recontoured the valves, intake and exhaust, for better flow. Reassembled, and installed on the partially assembled engine. Obviously, these heads will flow much better than stock. I am not very concerned about reliability, because this car is only driven occasionally, for pleasure only. I am estimating that this engine will now rev to 7500 RPM before valves start to float (30-30 cam). I am not going to race this car, but it is nice to know that the power is on tap if I want it. Your thoughts, Joe.