timing questions

[steveo]

the last thing i have yet to perfect in my tune is my timing table, mostly in cruising range.. and im hoping to get some advice

im running a timing table that was made by a tune-by-mail guy for someone with the same cam on a similar engine

the table seems to mimic a distributor with no vac advance; it runs 28 degrees flat up to 2000, where it advances straight up to 35 degrees at 2800rpm. there's no variation in map vs advance in the table at all.

this map is exceptional for hard driving. the thing flies, snaps you back in your seat, and it'll smoke the tires no problem in any gear.

my afr is definitely alright, i've tuned with plug chops and narrowband in driving range.

i've noticed at part throttle high vacuum at lower rpms has a pretty decent surge, and there's a hell of a rough spot around 2200 rpm no matter what the MAP (that might just be rotating imbalance, though) worst of all, it seems like my fuel economy sucks bad for cruising. im almost certain it's a timing thing.

to me, it seems all wrong. the lt1 should be able to handle quite a bit more timing advance, especially at higher vacuum... weird thing is, it seems like any attempt i make to 'improve' the timing table logically in non hard driving ranges (say.. increase advance around 1500-2200 but high vacuum) is met with harsh cam surge, or really laggy throttle, the car 'hates' it.

also idle timing is very tough on this thing, it seems that 20-22 degrees is freaky rough but clean burning....23-25 degrees smooths it out, but almost makes your eyes burn, making my exhaust all misty from the reaction. there's no in between..

so.. any tips?

here's some cam specs:

Advertised Duration (Int/Exh): 262/270
Duration @ .050 (Int/Exh): 211/219
Gross Valve Lift (Int/Exh): .507/.515
LSA/ICL: 112/106

stock compression and all that stuff..

[dave w]

I would use a timing table like the one pictured below for the cam specs you posted. See attached Excel Spreadsheet for the same timing table to copy / paste into TunerPro.

dave w

[EagleMark]

What PCM? Mask ID? MAF? S/D?

94-95 $EE have you tried a stock aluminum head timing table?

For idle have you tried the individual fuel cylinder trims and use a heat gun to read header temps?

[steveo]

the stock timing table is pretty limp when used with this cam.

EE MAF

i'll give that one a run and see what happens

[bobdec]

Stevo. With my 214/228 112 (Hotcam) and 1.6 RR's .525/.525 lift I'm running low 30's at 750 RPM 49 Kpa at idle , in the over 2K RPM cells I'm up in the low 40's at mid range 50-70 Kpa and slowly drops to the mid 30's at 100 Kpa. At 2800 65 Kpa I'm at 40. As far as low end my tuner cranked up all the low (below 50 Kpa) cells way up into high 40's to 50. Never figured that out, but since idle is at 49 Kpa anything below 50 Kpa is deceleration. What I did was print out a copy of my timing tables, then logged a WOT run and using the log RPM/MAP readings I circled the timing cells on the printout as I went through 1-2-3 gears, then connected the circles with lines so I could follow how the timing flow from a stop up to top end. Was looking specifically at any jumps between closed TPS and entering main tables, then any big jumps or RPM abnormalities through the run also keeping an eye on knock or retard. Then did the same for a normal stop sign to 55 MPH street drive. All of this is seat of pants, I think I could drop timing a but w/o any power loss problems, by since I don't have any knock I have never addressed it. With reverse flow cooling and my running temp at 185-195 these LT1's can take advance. By the way there is a max advance setting in the TC constants mine is set at 50.

[steveo]

oh hey bob, didnt see you were over here too

i pull 35kpa of vacuum at idle and 27deg seems to be fairly stable @ 700rpm, minimal lope even with an aluminum flywheel. my only bitch about 27 degrees of timing is the exhaust reaction is intense. i swear i've thought i had a blown headgasket just from the white smoke and water pouring out of my pipes. if i run sub 23-24 degrees, it goes away, but it lopes like a bastard. there is no happy medium.

seriously though, you're 214/228 112lsa and im 211/219 112lsa and a bit more lift.. your cam is bigger for sure, but i'd never guess i'd pull 15kpa more vacuum than you at idle. i'd never have figured.

is there some kind cam calculator i dont know about, or do these guys just construct engines in their head? man, im in awe of tune-by-mail guys.

now, i guess part of that might be that you have inherent load from your torque converter, whereas i have true neutral and a very light flywheel. but my cruising is 35-40 most of the time too.

i got off topic.

i tried that map you posted. cruise was a lot better, but high end had some limp spots.

so i took my old map, added a whole 5 degrees from 30-40kpa, 3 degrees at 45, and 2 degrees at 50. i did a bunch of slow lugging low rpm driving and couldnt detect any hesitation (which it had a bunch of before). i think im getting there.

i agree lt1s are timing hungry, but you do hit cam surge pretty quick if you go too far. the stock map has tons of high vacuum advance, and it runs like a turd on my car, very rough while cruising.

anyway log and timing map attached if anyone is interested, i guess i'll see how far i can push it