How much Timing does a 4.3 like?

[tiresmokinV8]

Im working on my timing tables and slowly bumping them up across the board and scaning for knock. I bump down whatever cells see knock. Just wondering if there is a general area that these engines like at cruising and wot? Right now Im 6* over stock running 93. Ive only had to bump 2 cels down a degree but nothing at cruising 0-20% tps and WOT.
Thanks

[Russ K]

Using 93 octane in a low compression engine and adding a lot of spark timing won't gain any power. The 4.3's that I've tuned are at ~22-24* at WOT using 87 octane. And I zeroed the PE spark adder table.

You can compare the spark table to this tune that I started on today.

Russ Kemp

[tiresmokinV8]

Thanks. Ill check it out when I get to my laptop. Does adding timing do anything to afr? My afr seems to be leaning out even though its been 15* hotter this past week then it has been in a month.

[turbine guy]

tiresmokinv8: Good question. I've raised mine to 35* @ WOT, and no KR. (On 87 octane.)
Same deal as you, I've lowered a couple points 2* at off-idle and low rpm cruise, but at the higher rpm's I've added a bunch and no KR. I'm using the '92-'95 "W" camshaft in my '03 4.3l Sierra, but otherwise stock. (WOT shift points @ 5300.) The old school guys using Vortec heads and distributors are using 34*-36* total, so I though I'd try it on my '03. Seems to be working, but like Russ K. says, some engines seem to make more power with less timing.(?)

turbine guy.

edit; I've thought about this all afternoon, and it's nagging me. Russ K., I highly respect your opinions and obviously you have extensive experience in tuning all manner of GM VCM controlled engines, but the timing discrepancy thing is bugging me. The 4.3l engine is a traditional small block Chevy, with two cylinders missing. Virtually identical to the L31 5.7l engine of 1996 through 1998, as used in the full size trucks. I'm old school, and gained the majority of my experience with carburetors and stand-alone distributors. All dyno work done on small block Chevies with the pre-modern "Vortec" heads, showed that the engines "liked" 38* to 40* total ignition lead, to yield peak power. The more modern combustion chamber profiles, including the "Vortec" style heads, all demonstrate peak power with 34* to 36* total ignition lead. This is what I've tried to emulate with my VCM controlled '03 Sierra with 4.3l engine. I'm showing no KR under load in the upper RPM ranges, and my truck pulls clean to the 5300 rpm shift point. Now the LSx engines are a different story, but to me, the 4.3l should have this high total lead. What say you? (And others of course.)

turbine guy.

[tiresmokinV8]

well I set the 95 cell to 35* for wot and got 2* of knock at 4k rpms and slight (under 1*) throughout the rest. Didnt pick up anything in 1st ad 2nd gear.
Ill back it down a smidge. Thanks,

[tiresmokinV8]

So turbine guy. How do you like the 5300 shift point? I have mine set at 5000 for all gears since it seems to die off after that, but then again the last time I checked it was before I messed with the timing. I ran the scanner and logged the mph and rpm. With each gear you could see that once it went past 5k the mph curve tapered off signifcantly. This friday im going to the track for the frist time with the hptuners so hopefully I wont be disappointed. It sure feels stronger. Before my times were way unconsistant even with consistant 60ft's My et were from 16.1-16,5 and my trap speeds ranged from 79-85mph. Last year I was running 15.6 at 86 mph. Hoping this tune is better then thewait4me tune I had that ran crazy lean.

[turbine guy]

I bumped my shift points after installing the '92-'95 "W" cam, which has 208*/208* @.050" .435/.450" valve lift. (Stock cam is about 195*/202* @ .050" .425/.435" valve lift.) It seems to like 5300 Rpm, and falls off fast if you rev any higher. I have two trucks at work, one bone stock, and this one with the cam only. The 4.3l Sierra with stock cam, you're right on the money, it likes to shift at 5000 rpm on the 1-2, and 4900 rpm on the 2-3 shift.
The 2-3 shift is the killer, if you rev out past the horsepower peak by much more than a few rpm, the acceleration curve falls off pretty quick.
It all appears to be camshaft related, as the intake system on the stock 4.3l is pretty darn good, and if you look at your logged runs, at WOT there is practically 0 manifold vacuum, indicating little restriction at higher rpm. The exhaust needs work, but in the GMT800 chassis, it too is not a real restriction until substantial mods are done to the engine. You will run into maximum duty cycle on the injectors, if more than a few mods are done, as again you can see on your logs, but for stock, and mild builds the 19#/hr injectors get the job done. Even with this mild cam, I lost a little bit of bottom end torque, as seen on 60' times and I had to pull a wee bit out of my VE tables on the bottom. Overall, though, I'm running the timing up there, and no KR to speak of. I'm about to try another cam, the CC 270AHR, combined with Rhoads V-Max roller lifters. If I can keep false KR at bay from lifter tick, the results should be interesting. Hopefully not lose any bottom end, or actually gain some, and pull hard through 5700 Rpm.

turbine guy

edit; I run at 2200' altitude, and Northern Canada where air temps are usually cool. This permits higher timing lead as well. For your altitude and temps, obviously, you will have to tweak it down some to keep knock at bay.

[tiresmokinV8]

I bumped down my 2-3 shift from 5000 to 4900. See how it works. Thanks

[turbine guy]

Well, trap speed is a clear indicator of effective rear wheel horsepower. If your trap speed goes up slightly with a 4900 rpm 2-3 shift, then it is a change for the good. I would suggest back to back runs with the 2-3@ 5000 and @ 4900 to see if there is a difference. Same goes for timing. Try the 30*+ table, vs. mid 20*'s That's the beauty of HPT, it only takes half a minute to load a tune. I was commanding my shifts, just shy of the rev limiter, (5600) after the cam swap, and looked at the logged rpm curve. I then changed the shift points to occur right before the curve "falls off". For me, that happened to be 5300.

Silveradomykk, was certain that pulling some timing off idle helped his 60', but I've not experienced the same. My truck wth the "W" cam will only flash the torque converter to 1750 rpm. The stock cammed truck, otherwise identical, will flash to 1900 rpm. I'm hoping that "variable duration lifters" will give a flatter torque curve which the 4.3 really needs. (Unless you have a looser converter.)

turbine guy

edit; Have you checked your fuel pressure? Sounds like it may be falling off on the high end. A mechanical gauge may be helpfull here. Those little 19#/hr poppet injectors need all the pressure they can get. Might account for some of your inconsistancies too.

[tiresmokinV8]

Yea Ive been told that about the fuel pressure by a few friends. I have a buddy with a fuel pressure gauge that said he would check it the next time he's in town. Need to change my fuel filter too with 88,000 miles on the original. Just the threads and about 1" of the fuel line going into the filter look pretty rusted. But the rest of the line is clean.?