i may be repeating the question but i want to make sure my thinking is correct. if i have my timing at 30 deg. advance then change my cam by advancing it 4 degrees w/adjust timing set.
would my ignition timing through the opti now be 26 deg.?
30 deg.?
or 34 deg?
if it is off can i just change the timing through my FAST computer.
thanks

 

I would like to know this as well. My personal feeling is that it would NOT change the ignition timing, but there has been much debate on this issue. Then again, the opti is controlled by the cam on the end, not on a gear. So, ?????

 

now thats a pretty interesting question. at current i'm thinking that by advancing the cam 4 degrees you would effectivly retard the timing 2 degrees. its 2 degrees because the cam only spins at 1/2 the rate of the crank 4/2 = 2. so your timing would be 28* the reason i think you'd be retarding your timing is that when you advance the cam timing it closes the intake valve "sooner" in relation to the pistons position (really both valves but its simpler to use the intake only here). by making the cam close the valves sooner you effectively "roll it back" which would make the rotor "late" at reaching the pin compared to where it would if the cam was installed straight or retarded 4*.

thats how i see it if anybody has a model or another idea i'd like to hear it. never built an lt1 myself always with a carb and old school dist for me. i hope somebody who has had experience tuning an lt1 speaks up. it would seem to me having heard that lt1's need more timing with a cam could be a falsity based on this, if you take into consideration that the shelf cams out there usually have 4 degrees advance ground in and much narrower than stock lsa's it seems no wonder cammed lt1's love their timing, because maybe the computer isnt reporting it correctly given that the rotor is in the "wrong" position?


EDIT: on second thought maybe comp and other manufacturers adjust for this by moving the dowel position in the end of the cam. anybody?

 

Since the rotational rate of the crank is 2X the cam, the crank would rotate 8deg when the cam rotates 4deg. And, move the Opti sensor "forward" in reference to the crank, and the spark will occur sooner with respect to TDC... more advance.

At least that's the way it looks at this time of the morning..... might look different when I wake up.

 

kind of confusing isn't it? that's why i have to be sure. i think the hardest thing is keeping the advance of the cam seprate from ignition timing in your brain yet since the opti is dowel to the cam they must be connected. i guess the only true way would be for me to put a timing light on it.
edit: the adjustment is made by way of an offset pill around the dowel

 

IF the dowel is moved in the process of changing cam timing, the ignition timing will be altered. Advance is advancing, retard is retarding. Advance or retard is in relation to crank timing. It applies to ignition as well as valve timing.

edit: the adjustment is made by way of an offset pill around the dowel

In this case, if an offset bushing is used in the cam gear, the cam gear remains stationary, (in relation to the crank) however, the cam dowel WILL be moved. Soooo, the camshaft will move. This dowel not only controls camshaft timing, it controls ignition timing.

 

so what you're saying is that the timing would now be 34 deg. and i would have to set it at 26 to get 30.

 

Hold on..... are you advancing the cam the way people usually refer to it- ADVANCED 4* RELATIVE TO CRANKSHAFT POSTION?

An example might clear this up- a cam spec'ed for a 112* Intake Centerline on the cam card is said to be installed "straight up" if actually installed at 112*. If it is instead installed on a 108* Intake Centerline the cam is said to be "advanced 4 degrees." That does NOT mean that the cam is moved by 4*. It's only moved 2*- which TRANSLATES to 4* at the crank. That's what most people are talking about when they refer to advancing or retarding the cam.

If you've advancing the cam "4*" as described in the above example (and you almost certainly are) then your ignition timing will also advance by 4* at the crank, where ignition timing is measured.

All this assemes that the distributor is locked to the cam- no offset dowels or other methods being used to move one independently of the other.

 

okay, so i'm buying into the logical theory of what you and arnie are saying but now comes the weird part. the motor likes timing advance.
the timing now is at 32 (which would be 36 according to the theory) and is making power till 6200 rpm. if i drop 2 degrees i loose about 15 hp, two more deg. and another 10hp plus it drops off around 5800rpm.

side note: i picked up 50 rwhp by advancing the cam the 4 deg. i did not install the cam and did not degree it. i assumed it was done by the builder. but i was experiencing symptoms similar to having a cam 1 tooth off. i check timing with timing light-dead on. cam was dot to dot. so then i experimented with the advancing w/adj. timing gear.

 

Advancing cam timing advances ignition timing (should be 2x as much as the cam timing). Both should result in higher cylinder pressures at low rpm’s, so both should add up to more power as long as you don’t take it too far or run into detonation.

 

If instruction/info was effectively sent, and received as intended, I believe you are now aware the answer is 34*. As mentioned, this is figuring the cam advance is/was measured in crankshaft *. BTW, you didn't mention the specific cam or cam timing you altered to obtain the 50 rwhp gain. Also, 36* advance is not out of the ordinary or excessive to obtain max hp.

 

So..... if I understand this correctly...... you advanced the cam that was already in it 4* from whereverthehell it was set at before (and therefore also advanced the ignition timing by 4*) and the combination of these two factors added 50 RWHP? Do I have that right?

That sounds quite..... amazing, actually. That's quite a bit to pick up from only 4*. Not saying it isn't possible- some combos are quite touchy. I've just never seen that kind of change from only 4* (then again, I work on older engines that you can rotate the distributor independently from the cam so you don't worry too much about altering ignition timing from a cam timing change- you just turn the distributor to set the timing back where you want it).

 

the question of where the cam was is my biggest flaw. i didn't and still don't and probably wont till i take the motor out and degree it. i'm just trying to figure out how far i can push it. i would go a little further if the timing was retarded. in other words at 26 when the computer says 30. i felt like 32 degrees advance (which is where i stand now) was creeping up there in the danger zone. maybe we'll lpush it a little further.

 

OK, I understand. Doing things "trial and error" is a perfectly valid way of going about things. If 4* advance added 50 RWHP then try another 2* and see how she does. If you've got a scan tool keep an eye on the knock retard. If it starts coming up you probably went too far. At least on the ignition timing. The cam might still want more. Don't know. It's possible the cam might have been ground wrong, the timing chain is off, etc, etc. Without degreeing the cam in, trial and error is all you're left with.

 

i hear ya, i was think of going a little more but wanted to know what i was doing to the ign. timing in the process.
thanks for all the help guys
will post results in the future