L79flight327

camshaft advice for a 327/350 L79 with flat tops.

I have a 1966 Coupe with a 327/350 L79 that was rebuilt by a prior owner. I don’t like the way the whole car shakes at idle and lugs below 2500 rpm. I want good street performance for some spirited driving. Yea, I occasionally drive it like I stole it, but it lives at 3000 not a 6000.

The engine set up is:
Carb: Holley 600 with vacuum secondary
Intake: OEM Winters
Heads: OEM 461 2.02/1.60 heads stamped long slot rockers
Pistons: Forged flat tops with four valve reliefs, 9.25:1 CR
Cylinder pressure: 155 to 160 psi with current cam
Exhaust: 2.5 inch manifolds and side pipes
Timing map: 16 base at idle and 32 all in by 3000 rpm.
Trans: M21 CR with 3.70 gears.

I did not get a cam card with the build sheet so I bolted a degree wheel to the balancer and gave the crank a spin. The basics cam specs are:

ISKY Cam: 205/226 .416/.465 LSA=108 Intake CL = 110

Intake Open 15 BTC, Open @.50 4 ATC / Max lift: .277 (.416) @ 110 ATC
Intake Close @.50 29 ABC, Intake Close 48 ABC
Intake Duration 243 degrees/ 205 degrees @.50
Exhaust: Open 72 BBC, Open @.50 42 BBC / Max lift: .310 (.465) @ 106 BTC
Exhaust: Close 50 ATC, Close @.50 4 ATC
Exhaust Duration: 302 degrees/ 226 degrees@.50
LSA = 108 degrees

I interpret these events to indicate:
1) ISKY Cam is retarded 4 degrees since LSA – ICL = -4
2) Overlap seat to seat = 65 degrees (50+15)


I called Comp Cams for their recommendation (XE268H). I think the 268 may be too aggressive for my set up. I am considering the L79 OEM (151), the L79 Nostalgia Plus, and the XE262. There events are as follows:

151 L79 OEM Cam: 222/222 .447/.447 LSA=114 Intake CL = 114
Intake Open 61 BTC / Intake Close 101 ABC
Intake Duration 342 degrees / 222 @.50
Exhaust Open 109 BBC / Exhaust Close 53 ATC
Exhaust Duration: 342 degrees / 222 @.50
LSA = 114 degrees

I interpret these events to indicate:
1) Cam has no advance or retard since LSA = Intake CL
2) Overlap seat to seat is 114 degrees (53+61)


L79 Nostalgia Plus Cam: 229/236 .468/.462 LSA=112 Intake CL = 108
Intake Open 30 BTC / Intake Close 66 ABC
Intake Duration = 276 degrees / 229 @.50
Exhaust Open 78 BBC / Exhaust Close 26 ATC
Exhaust Duration: 283 degrees / 236 @.50
LSA = 112 degrees

I interpret these events to indicate:
1) L79 Plus Cam is Advanced 4 degrees since LSA-ICL = 4
2) Overlap seat to seat = 56 degrees (30+26)


XE262H Cam: 218/224 .462/.468 LSA=110 Intake CL = 106
Intake Open 25 BTC / Intake Close 57 ABC
Intake Duration = 262 degrees / 218 @.50
Exhaust Open 21 ATC / Exhaust Close 69 BBC
Exhaust Duration: 270 degrees / 224 @.50
LSA = 110 degrees

I interpret these events to indicate:
1) XE262 Cam is Advanced 4 degrees since LSA-ICL = 4
2) Overlap seat to seat = 46 degrees (25+21)

At first glance the three cams have similar lift and duration @ .50. However their valve events, LSA, and seat to seat overlap are very different.

ISKY = 65 degrees of overlap, LSA=108
151 = 114 degrees of overlap, LSA=114
L79 Plus = 56 degrees of overlap, LSA=112
262 = 46 degrees of overlap. LSA-110

I have always thought that lopey engines had more over lap, smaller LSA, and result in less low end torque and more top end power. I also assumed my low cylinder pressure was a result of too much overlap. Until I read the LSA only test results by Hot Rod. The dyno test of a good flowing 350 using identical lift and duration changing only LSA (101, 106, 114) indicate the smaller LSA provides more torque at low RPM. The down side to smaller LSA is a rough idle and low vacuum. I don’t have power brakes or anything else that needs vacuum, so the low vacuum is not a concern, the rough idle and the car shaking is bothersome. I am also of the understating that cylinder pressure of around 180 would be ideal for a SBC.

Given my mostly OEM set up, use rec 90 fuel, I was thinking the best cam choice would be the 151 with its much longer overlap to help with filling the cylinder though my OEM 461 heads. I have read recommendations for installing the 151/ 4 degrees advanced (ICL= 110). How does advancing the cam effect the engines performance? Will doing do increase my cylinder pressure?

The engine rebuild has only 15k mile on it, so I am thinking all it needs is a cam and lifters. What cam would you suggest? And why? THANKS in advance for taking the time!

Bluestripe67

Welcome to the CF. Look for a response from SWCDuke....this is calling for his engineering expertise. Dennis

Sky65

Have you tried putting a little more initial timing in it? 32 total seems a little low and you may be able to handle 18 at idle. Do you use a vac advance? Is it on manifold vac?

I like the 262H for your build with no other changes.


Tom

SWCDuke

This was hashed out a month or two ago. Use the L-46/82 cam. It's very close to the L-79, but with better lobe dynamics and the IPOML is indexed four degrees later, so it should be installed with the OE replacement truck roller chain set (made by Cloyes and available at NAPA for about 25 bucks) that has +/- 4 degrees adjustment and you want to advance it 4 degrees on a 327 to bring the IPOML back to 110 (same as the L-79).

This cam is used today in the GM 350/290 HP crate engine with 8.5:1 CR. Typical idle behavior is 14-15" @ 750. It has an excellent blend or low end torque and top end power, especially if the spark advnance map is optimized, and it can be very aggressive.

There is a nearby active thread on this subject, too.

Duke

Kevova

Interesting split duration cam, honestly never saw a sbc cam with those specs, the difference in lift has me puzzled. Most sbc cam are single profile L82 would be a good choice for a replacement

SWCDuke

The L46/82 cam has the same duration on both sides (224 deg. @.050 lifter rise vs. 222 for the L-79 cam, and both have a 114 deg. LSA), but slightly different lift. The L-79 uses the same lobe on both sides. The L-46/82 cam inlet and exhaust lobe dynamics were optimized for the specific masses of the inlet and exhaust valves.

That's the primary reason I like this cam better than the L-79 cam. Chevrolet learned a lot about valve train dynamics beginning in the early sixties with the Optron machine and this knowledge is evident in the L-46/82 lobe dynamics. This cam was actually designed in the 1966 time frame, but did not go into production until late '68 for the '69 MY, and it is still in production today for the 350/290 HP crate engine. Fifty years of production means GM is very satisfied with the design and so are the vast majority of owners.

Duke

L79flight327

Tom: Yes I played with the ignition timing, springs, weights etc when I first got the car. Yes, I run vacuum adv off the manifold. Yes, I have run it with as much as 18 even 20 base timing, but was afraid I would never here it detonate over the pipes so I backed it off.

L79flight327

Please forgive my ignorance... what is the L-46 Cam/82 Cam? Part Number? Where would I find one?

SWCDuke

Federal Mogul CS-1095R. You can buy it at NAPA or any retail source that sells F-M parts.

It was used in the '69 to '70 RPO L-46 350/350 HP engine and the L-82 engine (power rating depends on the year) from '73 to '80, and as I previously stated, in the currently available 350/290 HP GM crate engine.

Fifty years of proven performance and durability! While you're at it ordering the cam, order a set of F-M VS-677 valve springs that were used in nearly all '67-up small blocks including the L-46/82 cam and LT-1 mechanical lifter cam. Chances are your valve springs are higher rate, which is not necessary and may adversely affect valve train durability.

With proper OE spring setup the L-46/82 cam will rev to at least 6500, but unless your 327 has massaged heads, the useable power bandwidth will only extend to about 6000.

L79flight327

Duke: Thanks for the response. I started looking on line and was able to find retailers selling the L46/82 cam...224/224 .450/.460 LSA 114 looks like its just a tad more lift and duration than the L79. Please confirm you recommend installing this 4 degrees advanced?

SWCDuke

Yes, those are the specs and on 327s I recommend installing it four deg. advanced with the adjustable truck roller chain timing set. This brings the IPOML back to 110 deg. ATDC, same as the L-79.

Take a look at the following thread.

https://www.corvetteforum.com/forums...-advanced.html

Duke

Kevova

Super Chevy did an article comparing L79, L46,& L82 cams. It's worth a read.

Kevova

http://www.superchevy.com/how-to/eng...-dyno-testing/

GCD1962

I used this cam out of my 350/290 crate motor in my '62 with an LT-1 short block after the original cam broke in two pieces. This cam works great in my application

L79flight327

THANKS! Kevova : That was a very interesting article.. More real world data supporting the amazing performance of the L79! (in stock trim). If it weren't for the crappy fuels we have today or the costly alternatives to achieve 100 octane I am sure we would all put back those 11:1 one pistons!

This speaks to the purposes of my post... Since I don't have 11:1 CR anymore I guess the L79 cam is NOT the ideal cam for the same set up with 9.25:1.

In your prior post you noticed the odd lift and duration of the intake valve on the ISKY... I too thought it was odd. I even thought it might have been wiped..The ISKY cam lobe catalog has a .277 lift lobe (.416/ 1.5) in their circle track section . so I am guessing its not wiped..just not appropriate for the set up...

GCD1962

The GM part # for the cam is 3896962. That's what's installed in the 250/290hp crate engine

SWCDuke

There is no such thing as an "ideal cam" for ANY engine. Valve timing has a big influence on the shape of the torque curve. It's the primary variable that determines the tradeoff between low end torque/power and top end power.

Once valve timing is determined the maximum CR can usually be accurately estimated based on both experience and some approximate analytical techniques. The two big influences are the inlet valve closing point and available fuel octane, but other variables such a altitude, and maximum ambient and underhood temperatures also have influence.

Of the Flint-built L-79s that are still around, most of whose actual CRs are closer to 10.5 than 11, all that I've heard of run detonation-free on modern unleaded 93 PON fuel, especially with the lazy OE centrifugal spark advance curve, and this can usually be made more aggressive without detonation.

I suggest you search for threads started by me and download and read the tuning seminar to get a better understanding of valve timing, compression ratio, spark advance maps, and the different octane rating systems. Also read the compression ratio article.

Duke

IGO200

I used the Nostalgia plus with 10:1's. Nice lope, good bottom end, runs out of torque a bit above 5500, but for me it's a real good cam. I used the Z28 valve springs (everybody knows that p/n but I can't remember it right now ).

L79flight327

Duke: Thanks will look for those threads. Agree intake valve closing dictates compression and hence my focus on overlap in evaluating the three cams. I have not been able to locate a cam card for the L46 cam to look at its actual valve timming...can you please post one or tell me whete i can find one.

SWCDuke

Overlap has nothing to do with DCR. DCR is solely a function of SCR and inlet closing point, but there are different ways to compute it depending on what is considered inlet valve closing. I use DCR as a guideline, not a hard go/no go criterion.

Overlap is the primary determining factor in idle behavior

napaonline.com has specs for the CS-1045R, which is manufactured to the ...962 blueprint, 224/224/114/114/114/.300/306 Installed with four degrees advance will be 224/224/110/118/114...

L-79 cam is 222/222/110/118/114/.298/.298.

Duke