ChrisMPG

iTrader: (1)

Hey everyone. I just got my 305 rebuild started up the other night. I swapped out the old cam with a L31 (B body LT1) cam during the rebuild. I went to set my timing and I'm just not sure what it should be at with the cam swap. Specs on the hood label call for 0 degrees BTDC. I read the LT1 cam swap digest post on here and read that about 8 degrees+ is recommended. Is this accurate? What is the optimal degree adjustment for the swap? I have had the chip tuned for the new cam and made all the necessary replacements to the internals of the engine. Your input and experience is greatly appreciated! Thanks!

sofakingdom

iTrader: (1)

Wherever YOUR engine runs the best, according to YOUR preferences, for YOUR car the way YOU drive it in the place YOU are with the fuel YOU can buy.

"If it RUNS good, it IS good".

Don't get hung up on a number. We call this process "tuning": it's kinda like, you ask THE ENGINE to tell you what IT wants. It goes like this:

Put down "light" and forget about "mark". They're not accurate anyway.
Adjust it one way or the other.
Better/worse?
Better: adjust it some more the same direction.
Worse: adjust it back to where it was, then a bit more.

Better/worse?
Better: adjust it some more the same direction.
Worse: adjust it back to where it was, then a bit more.

Better/worse?
Better: adjust it some more the same direction.
Worse: adjust it back to where it was, then a bit more.

Better/worse?
Better: adjust it some more the same direction.
Worse: adjust it back to where it was, then a bit more.

Better/worse?
Better: adjust it some more the same direction.
Worse: adjust it back to where it was, then a bit more.

Better/worse?
Better: adjust it some more the same direction.
Worse: adjust it back to where it was, then a bit more.

Better/worse?
Better: adjust it some more the same direction.
Worse: adjust it back to where it was, then a bit more.

Better/worse?
Better: adjust it some more the same direction.
Worse: adjust it back to where it was, then a bit more.

Better/worse?
Better: adjust it some more the same direction.
Worse: adjust it back to where it was, then a bit more.

Better/worse?
Better: adjust it some more the same direction.
Worse: adjust it back to where it was, then a bit more.

Better/worse?
Better: adjust it some more the same direction.
Worse: adjust it back to where it was, then a bit more.

Presently you will find that no matter what further adjustment you make, no additional improvement can be made. At that point, it's {duh} as good as it's going to get. You have OPTIMIZED that particular parameter by TUNING it.

NOW: pick up "light" and use it to look at "mark", and write down what you see, so you can put it back in case you ever disturb it.

86LG4Bird

iTrader: (1)

Just a comment:
Probably doesn't matter to you since the motor is all built and done, but the L31 (Vortec truck) cam is not exactly the B-body LT1 cam. Same lobes, same lobe spacing, but the "drop-in" ICL is 106 deg on the L31 cam (5 deg advance ground in) versus 111 deg ICL on the B-body LT1 cam (0 advance ground in). So, the only physical difference between the two is the position of the dowel pin at the nose of the cam.
L31 cam gives peak power at 4500-4600 rpm under the (restrictive) stock L31 MPFI manifold. LT1 B-cam gives a peak at 5100-5200 under the (high rpm oriented) LT1 manifold.

For ignition timing, I did the "sofa" method (I always do ) for the Vortec motor swap you see in my sig, and it works pretty darn well Yeah, +8 deg is a good starting point.

Darkshot

iTrader: (4)

If you ever have to smog it, you'll fail if the base timing isn't at 0*.

This won't matter if you're into PROM tuning however, because you can tweak the timing tables however you want and they have no way of checking this. The computer thinks base timing is at 0 so you should leave it there.




Just for the record, this isn't always true and can lead you astray. It's important to compare the absolute numbers you come up with with others and get their input if you're unfamiliar with what is "normal". You may have a mechanical problem and end up tuning around it.

ChrisMPG

iTrader: (1)

Thanks for all the advice guys. I really appreciate it. I set it to 0 degrees and found it ran pretty well. It idled at about 1100 rpm on a cold start and mellowed out to about 650 once it had warmed up. Sounded identical to how it sounded when I had the engine stock. I found at 3 degrees advanced that it ran a little more aggressive and more responsive but the warm up idle stayed at around 700. After I get it smogged I think I'll be keeping it in the 3-5 degrees adv. range and see how it does on the road. Thanks again for your help everyone!