GTA1990

iTrader: (1)

101 question as I try to improve how my L31 Hotcam'd firebird is running. I've set the base timing to 10 deg BTDC with a timing light and the EST connector disconnected and updated the relevant scalar/ table in my bin ('SA - Initial SA' via tuner-pro) to reflect this.

Car starts up easily and seems to idle okay but has a slight hesitation when taking off (T5 manual car) that I'm thinking increasing timing might help with; I'm thinking of aligning with adage 'give her as much as she can take' whilst keeping an eye on knock counts. Question I have, is there any difference with increasing timing on doing this via the distributor or the SA tables, assuming I'm updating the 'SA - Initial SA' number if doing it mechanically.

thanks

sofakingdom

iTrader: (1)

Not really any difference. All the engine will see, is WHEN the spark occurs; it has no way to know HOW it got there.

Obviously if you do it with the dist, timing at EVERY RPM will be affected equally, for better OR worse; whereas if you do it in the tune you can be more detailed about it.

You can try bumping the dist and see what happens to the specific symptom you're working on. Might not actually do any good. Or at best, just partly cover up some other problem.

"Hesitation when taking off" is almost always fuel related. Specifically, too lean at low RPMs and low vacuum.

ULTM8Z

On a speed-density car, with a LT4 Hotcam, you're going to need some tuning anyway. If you're into the calibration on Tunerpro, probably start looking at...

AE vs delta TPS and start increasing the lower range of delta-TPS values. On the 4th gen LT1, GM had those initial values as high as 1.50 up from the TPI's being 1.00.

GTA1990

iTrader: (1)

thanks both. Yes I have the EBL P4 on VE learn (narrowband) and it has pulled quite a bit of fuel out which is of course unsurprising given the cam and I also have 30lb injectors (i've set my bin to reflect the injector size). On hot idle my wideband does hover in the 14-15 range so I suspect by flipping to WB for learns it may add some fuel to your point sofa

Thanks ULTM8Z. I have a bit of reading to do on tuning this thing.

For timing, I'll go ahead and advance it via the distributor to point of knocks and back it down a few degrees and reflect that in the initial SA value so as not to increase total timing (as I understand). I'll report back here for future reference.

ULTM8Z

Yeah, on my Miniram, it likes to idle around 13.5 to 14:1. That plus the 4th gen LT1 accelerator enrichment tables provide silky smooth take offs from idle.

If you're still running TPI, you may not need to go that far... possibly just a tad more on the AE delta TPS to cover the hole.

BTW, keep an eye on your wide band during throttle tip in to see what the AFR is doing. You can also have too much fuel on tip in as well as too little, which can cause bogging and hesitation as well.

sofakingdom

iTrader: (1)

Too much fuel will show up as the engine continuing to "run", but making no power; and a puff of black smoke out the tailpipe.

Too little fuel will give you that "fall on its face" "die" "stop running" sensation, maybe an occasional pop or spit-back into the intake, then a second or so of rough uncertain running, then running better.

From your description it sounds like too little fuel.

What happened when you twiddled the dist acoupla degrees? Did it get better? Worse? No change?

Better = needs more timing in the tune

Worse = (you do the math)

No change = that's not The Cause, put it back and find the REAL problem, which is likely to be fuel

This is how we used to tune a carb in the Old Days. Doesn't too much matter what the fuel/spark delivery method is though, the effects of failure on how the engine behaves from either root cause, are exactly the same.

AlkyIROC

Base timing is adjusted to best suit the camshaft that is installed. Total timing needs to be programmed electrically or mechanically depending on the distributor since increasing the base timing will also increase the total timing which the engine may not want.

10- to 12-degrees of initial timing when the duration of the camshaft is less than 220-degrees @ 0.050” of valve lift
14- to 16-degrees of initial timing with a camshaft duration of less than 240-degrees @ 0.050”
18- to 20-degrees of initial timing when the camshaft duration is less than 260-degrees @ 0.050” of valve lift.
More than 260 degrees @ 0.050" is normally a full race cam and a locked out ignition set to full advance with a start retard is generally used but you can still use a timing curve if you want.

Where your engine wants the total timing is best figured out on a dyno or dragstrip. 32-34* total timing is most common but higher or lower numbers can be used depending on how the engine is built and used. Power adder engines usually want less total timing while NA engines usually want a little more but there are a lot of other factors in there also.

Once your base timing is set based on your camshaft, it never needs to be changed. You adjust the advance to increase performance depending on when components used in the engine. Since not all engines are the same, nobody can tell you how much total timing your engine needs.

As for your hesitation, what type of distributor and how old is it. They do wear out over time and simply replacing the distributor can make a big change in performance depending on the condition of the old one.

GTA1990

iTrader: (1)

Running a pro flo xt so yep more air than a stock TPI. It’s a sight stumble but don’t believe there’s a lean pop (was having these with bad IAC wiring (stuck open) so somewhat familiar with the sound)

will switch to wideband AE learns and have a look at those TPS tables; thanks. If AFRs still straddle into the 15s I’ll have a look at enriching the commanded AFR

LT4 hot cam is 218/228 duration I understand so seems like my base timing at 10 may not be far off. Distributor is original to the car from what I know so at around 120k miles and over 30 years old

i next get to the car this weekend so will work through these suggestions and report back