when degreeing a cam in an LT1 and using an offset cam bushing, what 'really" happens to ignition timing, opti, injector firing, etc?

Is this easily fixed during tuning or what is the deal?

Anyone know?

Lloyd

 

From Bryan @ PCMforless, I know this is with a crank key instead of bushing, but really the same concept right?

 

I just installed one in my LT1. I'm going to run it this afternoon. All I plan to do is pull 3 deg timing out of the advance table.

Mike

 

It changes your injector timing and spark timing by 4* if ya move the cam 2*.
Weather that will screw up the running????????.
It can be fixed in the computer.

 

Can you explain more LR? I would think this would only be true on the '96+ motors with a crank sensor, and only if you adjusted timing with a crank gear not with the cam offset bushing...I would think that changing cam timing with the bushing in the cam gear would have no effect on tuning because there is no cam sensor to know that something is amiss....what am I missing?


Ohh - The opti - right? It is the cam sensor.....

 

The crank sensor is "dumb".. it has no other function aside from detecting misfires. No effects on timing, etc.

 

Yeah the crank sensor is basically worthless.

I'd like to see what people get for the optimum timing on motors with and without a bushing in there, the timing is a product of the chambers so it's basically set in stone, you just have to get the computer to put the right amount so in the end the motor gets what it needs. I tend to agree with Bryan on this, I don't get where it screws things up.

Bret

 

If ya have a cam with a 110ICL and the opti is at xxx timng back the cam up or advance it to a 106/108 it moves the opti and changes spark and injector timing.
Doesn't matter if ya move it with bushings or crank sprocket it is still the same,ya moved the cam, which drives the opti thus ya move the opti the corresponding amount.
If the computer is able to correct internally for the amount the opti is moved(this I don't know) then the signal on the other end(injectors and spark) will be the same but if not the opti is off with the crank whatever amount ya moved the cam.A dyno tune would fix it as most of the time the timing is recurved anyway. Now how far the injectors can be off and still function correctly,I don't know,but a set of Hilborns just lay the fuel on the intake valve and whenever the valve opens ya get fuel.

Do ya have to reset a distributor after advancing a cam to make the spark timing the same?
Yep.

 

Everybody KNOWS that.

Loyd implied a performance cam and I have NOT seen one ground to factory spec's yet.....

Explain to us what happens with the computer when ya put a bushing in a cam with a 108 ICL and advance it to a 104 ICL The pin for the opti was correct but ya want to try more advance. Or the cam isn't on the correct ICL and ya want to put it where ya ordered it and that moves the opti. What happens then?
Explain if the computer self corrects fuel and spark for the movement. Do ya need another tune? Be OK? What?

 

Ya ain't posted ANYTHING 'bout any computer tuning....
Some DO KNOW how to degree a cam. Even using the OPENING and CLOSING numbers ya are so fond of. That part is NOT exclusive to you, ya know.
NO BODY said anything 'bout duration.

 

Here's how I got it figured:

You set the cam in relation to the crank to get the valve timing using the key or bushing, if that changes ignition & injector timing because of the dowel position, you adjust the ignition timing in the computer, and live with the injector timing being off my some miniscule, insignificant amount. As was alludes to, mechanical injection, TBI, & batch fire injection just splooges the fuel in there and SFI might have helped emissions or low speed driveability but didn't really make any more power, did it?

 

 

There are advantages to SFI, and there is HP to be made by optimizing the injector timing. With a stock PCM, that's not possible.

Just to see who's paying attention.... is the answer to the original question the same for both a 94 with the Opti driven off the cam sprocket and a 95-97 with the Opti driven off the cam dowel?

 

A '93-'94 being driven from the center will not move the opti as much.
They are also batch fire which wouldn't effect the injectors as much.

 

No, because the pin isn’t moving but the sprocket is.

As far as the rest of this discussion… does it matter? If you made this change on a carbureted car you would tune it for best power which would probably result in changing the timing any way. Same with an LT1… just tune it for where it works best and don’t worry about it