mrmagrath

I'm doing the bottom end on my 74 L48. My timing gears and chain were knackered, so I bought a Cloyes multiple keyway set. Is there going to be any advantage to advancing or retarding the cam timing on this engine, or am I better to time it straight up, as per original. The engine has no other mods, just stock. There have been discussions in the past regarding the factory retarding the cam timing 4 degrees for emissions purposes. What are your thoughts please gentlemen?

7t9l82

in your case there would be less benefit because your cam is weaker than the L-82 cam. if your that far in the motor I'd get. an aftermarket cam. you would pick up some power but you can definitely do much better.

billla

A simple search would have provided more information than you could imagine.

The simple answer is that the keyways usually represent a 4 degree change in cam timing.

Advancing the cam 4 degrees typically moves the power peaks down about 500 RPM.

Retarding the cam 4 degrees typically moves the power peaks up about 500 RPM.

There are additional side effects to both of these changes. If you want different power peaks, buy a different cam.

I have yet to see a reference regarding factory installation of cams other than straight up despite a number of requests. If someone has it, post it.

mrmagrath

Yep!
I did search first but with no answers specific to my particular issue, L48 cam timing, but thanks for the replies!
At the end of the day, I'm stuck with the cam I have. L82 cams and timing have come up in the past, but I was wondering if anyone could give me a definitive answer on the L48 cam and advancing or retarding. I should have said that I realise what advancing and retarding the cam timing does, I was keen to find out if anyone had an opinion on what would be best for my situation. I'm at the stage where I'm about to put the timing gears on, and wondering what your thoughts are on the position of the gears. I hope I don't come across as ungrateful!!!! Not my intention!
Cheers
Grant

billla

I'm unsure what you're asking - you understand what advancing and retarding the cam does, but you're unsure how it applies to the L-48 cam?

Maybe I'm missing something...but look at the specs for a stock L-48 and if the peaks aren't where you want them, move them...or as noted, better yet buy the cam that delivers the power peaks you want. I think the HP peak is around 4000, and TQ peak is around 2500...?

DRIVESHAFT

The cam that came in the L48 cars was used before emissions was too big of a deal.
I would bet that if the factory decided to retard them for emissions purposes, they did it with the timing chain gears.

But if you want to know for sure where the cam is installed, degree it.
The intake centerline for that cam should be at 108° ATDC.

7t9l82

if you can't change the cam definitely advance it. it will benifit you in several ways. i worked on making these cars run better back when they were new and this is one way we would do it. retarding the cam and ignition was the way they got emissions down. as a result they cut the compression to keep these cars from detonating on regular fuel. anyone who drove cars of this era remember turning the ignition off and the car would diesel on.boy did they run bad.

billla

What ways, specifically?

You've referenced a number of times that the cam was installed at the factory retarded - do you have any documentation on this?

aussiejohn

Grant,

I'm surprised that you first did not contact Competition Cams, or Crow Cams in Australia. Their technical staff will be able to answer any queries you have. How about you do that and post their replies back here?

Regards from Down Under.

aussiejohn

mrmagrath

When searching before I posted, I came across a thread where a guy was working on his 77, and someone said the factory retarded the cam timing 4 degrees to help with emmissions. My research indicates that if I advance 4 degrees, I'm going to have better torque, which suits me fine, since I'm not going to be going much faster than 65mph, but enjoy taking off fast! Maybe on the next Vette, I'll build a performance engine, but for the time being, I'm short on cash (bloody kids with expensive tastes!), and keen to get some miles under my belt before the end of our summer. So, I'll install the gears 4 degrees advanced, and let you know the result. I'll leave the timing gear cover off for a day or so incase someone has a great reason to do otherwise!
Cheers
Grant

aussiejohn

[QUOTE=mrmagrath; I'll leave the timing gear cover off for a day or so incase someone has a great reason to do otherwise!
Cheers
Grant[/QUOTE]

Don't do that, one of those elusive Moas might jump in there.

Regards from Down Under.

aussiejohn

scottyp99

For those of you who may be confused by some of the things mentioned in this thread, I just got a call from Captain Know-it-all, and he told me that down in the southern hemisphere, (bear in mind we are dealing with Australians and New Zealanders) the seasons are reversed (they have summer during our winter) and the Moa is a flightless bird that lives in New Zealand.

http://en.wikipedia.org/wiki/Moa

In the interest of staying on topic, I would say that the only way to really know for sure, as far as the whole factory retarded cam thing goes, would be to degree the cam and actually see what the numbers tell you.

An interesting experiment, for someone who has several old SBCs laying around, might be to take a cam gear from an older, pre-emmissions era engine, and one from a later era, mid or late '70s, say, and compare them. Let us know if there is any retard built into the newer cam gear.

BTW, when degreeing your cam, don't forget that, on SBCs south of the equator, the camshaft spins backwards because of the Coriolis Effect!!


Scott

jb78L-82

I am with your thoughts on this issue! I am no cam expert but people I know who are experts-build engines and mod cars from the 70's/80's-all say that the cams were/are retarded from the factory-specifically GM and Ford. They also say that advancing one of those cams DOES NOT just move the RPM band up or down 500 RPM. Advancing the cam increases compression and thus power which is the real reason the motors produce more power and the cars are quite a bit quicker. According to them moving the torque/hp band 500 rpm would hardly be noticeable on a factory street car. The V8 Fords from that era according to my engine builders typically produce 30 more HP with a factory cam 4 degree advance. I am intending to advance my factory l-82 cam soon 4 degrees and will be going back to the dyno to see if there is any HP change from the advance only. My dyno man says probably about 25-30 hp-we will see and then maybe we can put this to rest.

billla

Let's back up for a moment

There's 3 ways that come to mind that would document the cam installed retarded; either the cam/timing gears were different, or the cam was installed with different-than standard positioning of the cam/crank sprockets. The first two would generate alternate part numbers, the latter would be clearly documented in the factory service manual - fair? I would think anyone making this claim could do a little research and easily and quickly find out how (and if) this was done using standard and readily-available references

Second, the impact of changing cam timing is very well-documented and well-understood; it's not a mystery and not subject to interpretation. A little searching on any of the cam sites will provide everything needed. Aside from the powerband change, advancing the cam will provide very slightly more torque, at the expense of some horsepower. Note that we're only changing the advance, i.e. the relationship between all the cam timing events remains the same - they just all happen earlier or later.

A couple of small references to start folks that would prefer to research are below; there are many others

http://www.compcams.com/Pages/416/va...-tutorial.aspx

http://www.cpgnation.com/forum/effec...tion-2630.html

69 Chevy

I was shocked when reading 'effects of altering camshaft timing' at this link http://www.cpgnation.com/forum/effec...tion-2630.html NOT to find an increase or decrease in compression ratio.

DRIVESHAFT

Why? Moving the cam position doesn't change the compression ratio.

jim2527

No, but if the valves are staying shut longer during the compression stroke.....

DRIVESHAFT

Of course that can change, but 69 said "an increase or decrease in compression ratio."
As I mentioned, the compression ratio doesn't change.

bashcraft

The static compression ratio doesn't change, but the dynamic compression ratio does.

scottyp99

Here is a link to a page that explains dynamic compression ratio. At the bottom of the page is a link to a compression ratio calculator that will let you compute dynamic compression ratio.

Say you have a cam like the old 929 cam (L48) the specs commonly given are 112 lobe separation angle and 108 intake lobe centerline. I think this means the cam has 4 degrees of advance "ground in". I can never remember which way it goes, off the top of my head. Anyway, using the compression ratio calculator, it would be easy to see what advancing and retarding the cam would do to the dynamic compression ratio by playing with the intake lobe centerline number.

Edit: Oops! Forgot to post the link! Derrrrp!!!!!

http://cochise.uia.net/pkelley2/DynamicCR.html


Scott