I would like to start a debate on cam timing vs. spark timing.

Because of the issues i'm having with my LT4 with hotcam, I have come to the conclusion that people saying i put the cam in a tooth off because I'm having to add lots of timing in the programming are nuts. Cam timing is different... it is the physical timing of the vales opening/closing. If this was off by one tooth (approx 5-7 degrees maybe), the engine would NOT run right at all and could possibly have valves smashing into your pistons. Timing of the spark as controlled by the computer is a totally different. This determines when the A/F is ignited with the valves closing/opening at the exact same time everytime.

You can't change the phsysical timing of the valves with the computer (at least in a non-VTEC engine). SOOOO..... once again, I'm positive I put my engine back together correctly and that is not an issue at all. I wouldn't miss something like that, especially after all the work I've done to this car and the '94.

The engine runs, and it runs well.... just not extremely well like i was hoping, but then again it doesn't act like a finely tuned machine because it isn't any more. This completely boils down to tuning.

:rant:


[Modified by GlockLT4, 3:39 PM 4/7/2004]

 

You are quite right for a SBC with dist but I'm not sure how the opti is tied in. Did you install your cam "straight up"?

As I'm sure you know, cam timing can make a difference in performance; you can move the torque peak up/down by offsetting the valve events timing. There is a table on my site taken from an old mag. article on an L98 buildup which shows the effects of cam offset on 1/4mile performance.

Maybe a few degrees of offset would "wake up" your new cam??

 

yes, i installed the cam by aligning the timing marks. I'm positive they were lined up properly. I did not "degree" the cam. No room really to even get a degree wheel in there with the engine in the car. I've talked with many people who have done this. How would you adjust it anyway?? Can't adjust the teeth on the cam or crank ... or the chain. I see absolutely no way of ajdusting anything besides a different grind cam (which is the way it should be).


[Modified by GlockLT4, 3:55 PM 4/7/2004]

 

i disagree on a couple of counts.

first, an engine will actually run decently with it being a tooth off on the cam timing. i know because i've seen it happen. i've fixed this on cars where people thought it was weak, but didn't really know why. it would still idle and drive fine. especially if it's retarded a tooth, it can be difficult to tell. advanced a tooth and it will detonate easily and be further down on power, so it's more obvious.

second, the LTx is in a completely different ball game, since the ignition timing is indexed off the cam. so if the cam is one tooth off, say retarded the 10 degrees or whatever it is, suddenly now you have the double whammy of both the cam timing and the ignition timing being off. that's just a function of how the optispark is being driven, and the fact that the ecm uses the opti trigger for ignition timing.

10 degrees isn't really enough to cause serious, obvious problems, but it's enough to cause power loss and other quirks. i think a tooth off might be more than that, though. let me look for some pics of the LT4 timing chain set...

ok, ya, one tooth is 20 degrees. a cam retarded 20 degrees isn't actually that bad, you can run it on desktop dyno to get an idea for that. but if the timing were retarded 20 degrees, you'd lose a lot of power. probably more than what you're seeing, so maybe it's not a cam install problem, but it's free to check by just putting a dial gauge on the pushrod.

but if you're putting in something like 45-50 degrees of advance at WOT, that's a pretty good indicator that the ignition timing is much further retarded than you think. it's possible there's another cause, but the timing/cam issue seems the most likely/probable one.

i can't remember how much timing you were putting in, was it that much? no, it was less, right?

-michael

 

i just realized i checked the number of teeth on the extreme duty chain. the stock LT4 chain set might have more teeth, thereby making it less than 20 degrees per tooth off.

anyone have a pic of one to verify?

-michael

 

Yes, I had up to 50 * at WOT around 6000rpm. It is at 37 max right now (i think without looking at that first table).

And yes, I agree that both cam timing off and spark timing off would pile up against itself.


[Modified by GlockLT4, 4:03 PM 4/7/2004]

 

I have a .bin file from a site which claims to be done by Modern Musclecar which does have up to 50 degrees of advance.

http://www.bfranker.badz28.com/LT1Edit/LT1files.htm

I think my advance tables from PCMFORLESS were as high as 44-45? I'd have to look.

 

efi stuff can be really tricky. you often can't trust that it's doing what it says or should. but the symptoms would make me check the cam timing. it's not too difficult, you can just pop off the passenger valve cover, take the rocker off the #2 intake valve, and put a dial gauge on the pushrod.

of course, in order to know the crank angle you'll need to take off the bellhousing front cover so you can see the teeth on the flywheel, and find tdc, then figure on 2.35 degrees per tooth of movement.

basically, as you turn the engine by hand, the dial gauge should show .050" of lift as you get right to TDC, with the HOT cam. if it's more than about 5 degrees (2 teeth on the flywheel) off, there's a good chance the cam is in wrong.

it's simpler than i made it sound, the hard part is just marking TDC correctly on the flywheel relative to the bellhousing. if you buy a piston stop then it's really easy.

-michael

 

How are you determining that you are getting 50 degrees of ignition advance at 6000?

Jake

 

That's what my extended spark tables are set to and that's what datamaster reads.

See the tables above for an example. The first one i posted is what i just loaded in the computer and I'm going to try tonight if i ever get home from work and if it stops raining.


[Modified by GlockLT4, 5:28 PM 4/7/2004]

 

I asked because I got reading of 73+ degrees in reference to TDC running Diacom Plus at 6375 RPMs.

I KNOW that's can't be right. I posted about it but never received an explanation other than Diacom Plus was having a mental breakdown.

I can only suspect that I made the WOT blast before the engine went into closed loop and that somehow effected the ignition timing Diacom was reporting. I was only getting .9 (point nine) degrees of knock retard at that point so I know the engine wasn't detonating.

Jake

 

yeah, with 50* i was getting like .1 degrees of knock...

 

The Hot Cam should run pretty well on your stock program. The only way you'll know for sure if something is up is to:

A. Take it to a dyno
B. Take it to the track
C. Rip the timing cover back off and double check your timing marks.

Do you remember when I was having some strange problems over a year ago? It turns out that I had somehow managed to put the cam in out of time. Since opti only goes on ONE way it was off by exactly as much as the cam was. The car started and ran fairly well. I did have some bizarre issues with starting it when hot, etc. Ultimately the car still felt pretty quick to me though.

I didn't know for sure something was wrong until I took it out to the track. My car which had run 12's was stuck in the 14's. Now I suck as a driver but even I can't make a 14 second car out of a 12 second car without trying.

 

I had 296hp stock, when i dynoed it with stock programming after the cam swap i was down to 289hp. So that's not enough of a loss to cause a 2second gain on the quarter mile.

 

My laptop died shortly after I made that run so I haven't been able to try again.

I just went back and checked a couple of other traces I'd saved and they show the same thing, and ONE of them was done in closed loop mode; so that kills that theory.

The extremely high timing readings are in "SPARK ADVANCE RELATIVE TO TDC" column, but SPARK ADVANCE RELATIVE TO REF" stays in the low to mid 30s and only goes as high as 40 degrees at about 5000 RPMs and above.

Crazy!

Jake


[Modified by JAKE, 9:05 PM 4/8/2004]

 

did it only happen that one time, or is it consistent?

-michael