what does advancing / retarding the cam do ? if u do eather one of these do u need to run more base timing ? how does it affect the eng ? what does it do to the idle ?
if you had a eng an u didnt know if the cam was advance,retarded, or just stock . what would be a good way to guess if it was one of these ways ? or is there no way , but to tear the eng open ?
if u can answer any of these questions please do .
thanks chad

 

OK...

Lets say the LSA is 112*

The Intake Center Angle (ICA) is how soon the intake valve on the #1 cylinder reaches peak lift after the #1 piston is at Top Dead Center.

If the LSA and the ICA are the same (both 112*) then the cam is said to be installed "straight up".

Most (I say most losely) SBC work best with the cam advanced 4*. So on our cam with 112* LSA , 4* advanced would put the ICA at 108*. A lot of manufactures grind the cam with 4* advance, so all you have to do is install them.

So, for example, a Comp Cams XE268 is ground with 110* LSA, and an ICA of 106*, therefore advancing it 4* out of the box.

You can advance or retard it yourself, but you need a timing set that will allow you to. Honestly, it's best left to pro engine builders.

General rule of thumb is advancing the ICA will move the power band down in the RPM range.

Retarding the cam will move the power band up in the RPM range.

 

if u retard your cam do u need to run more base timing since the valves are not opening up as if the cam was set stright up ?

 

You set the timing the same you would with any other engine.

Why are you asking this?

Elaborate more. As I said, it's best left to a pro. Don't mess with it thinking you're gonna notice a big difference.

 

What would happen if you advanced a cam 3* that was already ground 4* advanced?

 

The only way to know whether a cam is installed "advanced", retarded", or "straight up" would be to (a) have the specs for the cam in question, and (b) measure it after installation to see where it is.

There's no universal "right" cam timing. Even factory cams have changed over the years; for instance, the 929 (pppuuuuukkkke) has been in use since the mid 60s, but in about 71, they changed the timing set to retard it 4° from where it had previously been installed. This was an emissions thing; it created some "passive EGR". So what's the right timing for a 929 cam? There's 2 answers, just from the factory alone.

Most aftermarket timing sets, even those sold as "stock replacement", are made to the pre-71 spec. So, if you get a cam that's ground in the normal way, i.e. with the intake lobe center at the usual relationship to the dowel pin in the cam nose, and put it in with a 71-up stock timing set, it will be considered to be at 0°; but if you use almost any other timing set, it will be at 4° advanced. That is, the intake lobe centerline will be moved 4° earlier in the engine cycle. That's what is usually meant when a cam is said to be "ground 4° advanced".

The distributor follows the cam, since it's driven off of a gear on the cam; but you set the ignition timing to the crank. So if you were to take an assembled motor, remove the timing set and change it out to move the cam timing, the distributor will move the same number of degrees; but if you then set the spark timing using a timing light to where it was before, it will be .... where it was before.

Most often, a cam is desgined by its mfr with a particular RPM range and torque curve in mind. About 99.999% of the time it's best to install it "straight up", that is, where it was designed to go. The only way to know whether advancing it or retarding it would be to install it at 0° and do a few dyno pulls or track runs; then advance it, and do a few more dyno pulls or track runs; then retard it and do a few more; then put it where it gives you the power vs. RPM curve you want. Obviously other changes such as fuel and spark curves may need to be made to re-optimize them each time after the engine's mechanical characterisitcs are changed. If you can't, or aren't willing to, go to that much trouble in fine-tuning a motor, then it's best to just put it where it belongs and leave it alone. Most of the time you'll make the engine produce less usable power by changing the cam timing from its design point, which of course you'd never know except by trying various settings and getting a consistent measurement.

In short, put it where it belongs and don't try to outsmart yourself or the cam designer, unless you've already got the car so finely tuned that you can actually measure the effect of that variable without it being swamped by other things that affect the car's performance more than the relatively tiny effect that the cam timing will produce.

 

the reason i have started this topic is that my engine seams like it wants more timing an its a already at 12 degrees base.
i bought the car with this eng in it. i am trying to figure out why this thing wants so much timing . its big block with a big cam.
the guy told me the cam was a isky custom grind 340 duration with 580 lift hydrolic. compression is some were around 11:5-12:1 . my engine doesnt want to stay at a idle . i had the timing at 10 then moved it to 12 an it stared runing better but still doesnt want to hold a idle. will idle for a min then wants to die . i have tryed every thing i know . now i am shooting in the dark by asking this topic . i am open to any ideas .
thanks

 

well it could be alot of things. the carb could be loading up your motor. just from the breif description you.ve given thats what i would check first. try a new power valve if your running a holley, around a 5.5 Hg. next if the harmonic balancer is a factory unit( rubber sandwiched between two metal rings) then the outer ring could be slipping. plus what are you running for an ignition? is the distributer a stocker or are all the parts inside new? most HP engines like more initial timming and less over all timing(big blocks) most will like 12-16* initial with 34-38 total timing.

 

ok, to clarify:
If i have a 112LSA w/a 108 centerline, then it already has 4degrees advance in it....SO, if the centerline was actually at 107 when i degreed it, then I would be a total of 5 degrees advanced. This in turn would make more bottom end, then if it was at 108 or 4 degrees advanced, right?

 

Yes

 

Ditto to,, and to elaborate on what 408TA said:

I tuned a ton of other people's engines way back when,, and your scenario was VERY common. The main "problem" with "setting the timing" is there is no real way to know for sure if the guy that put the engine together set the timing tab on the engine to zero while the engine was at TDC (or if the tab was bumped later,,, etc. Plus,,, that "problem" can be compounded by aftermarket timing tabs and balancers (even if they're in good shape),, that may or may not been present at the orginal build. I set cars based on the tab at 22 degrees initial and 44 degrees total that I KNOW would have been hammering the crud out of the engine IF it had REALLY been set that high. (on a semi-related note,,, that's why you CAN NOT go by what your buddy on the internet tells you he has his timing at,,, no telling where it really is!!!)

I agree iwth 408TA timing range on a typical BBC with an accurate timing tab. Typically the larger cams like / need more intitial timing. Still,, it's not uncommon with the tab set right to have 14 - 18 degrees intitial timing on a 11:1 engine (with as high as 34 - 38 total),,, depending on the fuel and how large the cam is.

Basically,, there are two things you can do:
1 - give the engine what it wants (by dailing it in at the track) and don't worry about what the timing tab said it has. FYI--- Unless I build the engine and know the tab is set on zero,,, then what the tab says is the timing,,,, is nothing more than a reference point as far as I'm concerned. If it says you have 22 intitial and 44 degree total,, and the car is not pinging and you ran faster at 22 than 20,,, so what? Do you HAVE to know what the timing really is? If so,, see #2.
2 - get a piston stop and degree wheel and make sure the timing tab is really at Zero. You're still going to have to dial it in at the track,, but at least you could share CORRECT timing numbers.

Also as stated your idling / running issue might be fuel related and have nothing to do with timing. I saw plenty of examples of high initial timing settings to compensate for a carb issue.

Hope that helps.