When you guys degree in an aftermarket cam, are you installing them dot-to-dot with #1@ TDC or installing them on a lower LSA? The reason I ask is b/c I just talked to a buddy about why my car is sluggish off the line and he mentioned something about degreeing in the cam as if it had a 108* LSA when it's actually cut on a 112. I was just wondering about the validity of this. See sig for specs

 

Actually, that advice has some validity. Advancing the cam timing tends to improve the low RPM power at the cost of sacrificing upper RPM power.

 

You can't change the LSA of a cam by installing it different. The LSA is ground into the cam.

You can change the ICA. That's the point of when the cam is timed with the #1 piston at TDC.

107*-108* ICA is common for a cam with 112* LSA. It's normal for the cam company to grind 4* advance into the cam because most (not all) SBC's respond well with 4* advance.

You can easily change the ICA by advancing the cam, or retarding it. The normal amount is 4* either way. The rule of thumb is advance for low RPM grunt, retard for high RPM HP.

That's an exagerated example, since advancing the cam only 4* won't have a huge effect, but it gives you an idea.

 

I was told once they put 4 degrees advance in to compensate for inevitable chain stretch, which will retard the cam.

 

For the first few years of SBC production, the factory used a few certain cams, and a particular timing set. Then in about 1971, when they started really altering engines internally to make them meet the new emissions requirements, they continued to use the same cams (the 929 for example, first appeared in about 1965 283s, was used up into the 80s) but changed the timing set, to one that retarded the cam 4 degrees from where it had been before. Consequently, 4 degrees retarded became the new definition of "straight up". Originally the timing set put the intake lobe center line at 106 crank degrees after TDC; the OE 71-up set puts it at 110 crank degrees ATDC.

Replacement timing sets from anyone except GM continue to this day to be made on the old timing spec. So by definition, if you take an otherwise stock engine and put a replacement timing set in it, even if the chain was unstretched before, you just advanced the cam 4 degrees.

I would not recommend changing the cam timing during a rebuild, unless you are able and willing to put it together "straight up" with the timing set the cam was designed to be used with; measure the engine's torque curve on an engine dyno; take it apart and move the timing, put it back together, and re-measure it; and you know enough about what the car needs to be able to interpret whether changing it was an improvement or not, and are willing to take it back apart again and put it at the optimum location. I suspect, if you're asking the question as you asked it, that this is not something you have access to or knowledge of. So the best thing to do is leave it alone and put it together right because the cam mfr already did that for you.

LSA is Lobe Separation Angle. It is the degrees of rotation between the peak of the exhaust lobe and the peak of the intake lobe. It is ground into the cam and cannot be changed. You do not alter the LSA after the cam is already made.

ICL is Intake Center Line. Sometimes it has other names, but that one is most common. It is the number of degrees between some crank event (specifically the TDC occurrence other than the one at which the cylinder fires) and the intake lobe center line. That is the thing you can change by dinking with the timing set.

When the cam & crank sprockets are "dot-to-dot", the engine is at the instance of TDC during the engine cycle where #1's exhaust valve is closing and #1 intake valve is opening, not #1 firing. That point is actually #6 firing. Don't forget that as you assemble the rest of the motor (namely the distributor).

 

We should probably also make the distinction between "degreeing in" a cam and "installing the timing set". I've never built a really killer engine that was assembled to the nth in all details, and so have never degreed the cam. I've always installed the timing set straight up, with the exception of a '76 Buick 3.8 V6 that I retarded the stock cam an unknown but ~5 degrees to raise the power band a little (it helped).

You specifically said "SBC" with regard to the change in straight-up timing - same happen for BBC's? I've got ideas for this winter's "improvements" on the 396, and if I go through with them, will probably finally get the degreeing equipment and take that extra step. My current cam has never pulled to the RPMs advertised by the manufacturer. I've assumed that's at least partially because I'm running a small-bore/short-stroke/small-valve 396 when most BBC speed equipment is taylored for a large-bore/long-stroke/large-valve 454. But, this not-degreed-in may be a bigger factor - especially since smaller displacement tends to make a cam "more radical" than it would be in a larger displacement engine. The cam card gave all the info needed to degree it in properly, I just didn't do it.

 

Yes, I believe so. In fact IIRC it was universal throughout the GM product line, at least on V8s; I hve experienced it in Pontiac and Buick V8s as well as Chevys, at least.

Depending on what valve springs you have, those may be the culprit of less-than-spec high-RPM performance. BBCs are notorious for needing lots and lots of valve spring, with their heavy valves and high rocker ratio (lesser mechanical advantage of the spring on the lifter).

For the straightest possible scoop on big block cams, probably the best person to talk to would be Harold at Ultradyne. He's on the Chevelle BBS alot as UDHarold, at www.chevelles.com in the tech forums.

 

So should I have installed the cam dot to dot or not? Should I pull out the cam and degree it in advanced?

 

Yes.

No.

Just don't forget that dot-to-dot is #6 firing, not #1 firing.