Im just wondering what a decent timing graph looks like?? The stock one Im about to modify seems to have a lot of dips and valleys and I was wondering if smoothing it out will help any?? Or if I should just increase it all in small amounts?? Thanks for the help. Its a mild 350 with an auto tranny

 

I've also noticed that a lot of stock timing curves have some very rough valley's and dips in them. I usually just smooth them out. Keep in mind there are several timing adders for WOT that need to be taken into acount when you start playing with your timing.


I like to zero out any timing adders and do all my timing in the primary timing table. On a swirl port headed stock motor I aim for about 28 to 30 degrees totall timing at WOT, 20-25 at idle and low to mid 40's under cruising situations. The taper and smooth from there. I'll see if I can dig up a timing curve I did for a buddies LO3.

 

Hey thanks alot Bmonte That would help alot. I just need to see what a decent one looks like so that I can smooth mine out. Im sure that the stock curve will need some work!! Thanks again.

 

Heres one I did for my vortec. Basically, you test alot under part throttle and WOT to determine what the engine likes best, and then blend the rest of it together so its smooth. When setting the table up, I wouldnt sweat the details alot. Just try to get a good curve that the engine runs well on.

Also, with timing, its not about having a certain ammount or alot of timing, but having the right ammount. You want to have the mixture fully burned right after the piston passes top dead center, and finding this point takes some testing. If the engine lacks power and heats the exhaust up alot, you have too little timing. If it runs rough/detonates/overheats, then you have too much.

Id just smooth the stock curve out for now using BMonte's info as a reference and try to get the fueling shaped up before diving into the timing so your not chasing alot of different parameters at once. Also, as you change the timing, you will also have to adjust the VE tables as the timing changes the engines actual VE.

 

Exactly, but the magic number is 15* ATDC. That is where peak cylinder pressure should occur.

 

Hey thanks again guys that is alot smoother than anything I have right now. I am just starting to tweak the timing on this engine and i appreciate your help. I'll let you know how it goes. Greatly apprecaited!!!

 

I've heard 7 deg ATDC as well... Wonder what the correct number really is

 

So what's your method of finding the sweet spot? If it were possible to hold the RPM and MAP (without a dyno), it would be easy. Since most of us aren't working with a dyno, how do you suggest us new-b's go about this?

To me, the VE tables are a piece of cake because there is a clear objective of making the WB read whatever your looking for. The SA is black magic surrounded by mystery.

When I first started tuning, I thought about just taking it to the knock sensor then backing it off a few degrees. Then I read Grumpy's post saying the knock sensor is not a tuning tool and that made perfect sense to me.

Also, I remember Fast355 saying that the vortec heads didn't like more than 28* timing, but your chart shows you've taken it up to 38*? That's a huge difference, so what am I missing here? Another table or something?

 

I would like to add that the best point in which to reach peak cylinder pressure is entirely dependent on the combo of parts. This is where rod to stroke ratio comes in and and most importantly chamber design. A fast burn chamber like the vortec's will burn much faster therefore pcp (peak cylinder pressure) can happen later and still get the most from the burning air/fuel charge. If you think about it max power is produced with the crank at 90* to piston travel. The trick is to keep the average cylinder pressure as high as possible over the entire 180 degrees of crank rotation.

When tuning timing your just looking for max power, unfortunately not everyone has a dyno in their back yard. A couple of ways to tune around not having a dyno is to pay particular attention to MAP and TPS readings under cruise conditions. If you know to maintain speed up a particular hill it takes 34% throttle and you have a map reading of 65kpa, then advance the timing 2 degrees and that drops to 31% and 60kpa, then you've made a move in the right direction. The problem with this is that when your map moves then your timing changes. For tuning purposes I have completely flattened the timing table in 3 or 4 distinct table tops. That way there is no timing changes when the map moves around a bit. The car will run funny, but once you figure out what timing is best under light cruise, heavy cruise and idle then you can smooth out the table between these optimum timing numbers. WOT is a special case where only g-tech, track or dyno time will really give you conclusive data.

 

At WOT, typically the vortecs wont need much more then 28-30. At part throttle, though, the cylinder pressures are low, so the burn speed is less. Youll need more timing to compensate for this.

With part throttle, a good percentage of the SA tuning is perception/drivability in addition to testing. Basically if the car is sluggish, requires lots of throttle when driving it around, sounds like crap, and drinks gas relentlessly or overheats/detonates/runs rough, your timing is off. When the SA is good, the engine will be responsive, have a smooth exhaust note, get good fuel economy, not detonate, and stay cool in hot weather. It takes some practice to learn how to distinguish what timing is right at part throttle, but if you take small steps, youll eventually get a feel for it.

 

OK, thanks. I got a stock timing table from a vortec truck that I'm running now, and it looks like it's about 35* max, so I may not be far off. The WOT timing is only like 18*, so that will need some work.

Thanks for the help.

 

let me know when you get it right...been working on it for a while now...lol
vortec heads don't need much on the top end, that's what I've figured out