Squeezing a few extra ponies out of a stockish LS6

[Supercharged111]

My vette only has a CAI, so pretty well stock. That said, I'm still going go over the VE and MAF to make sure they're as good as they can be. Lowering fan temps and killing that stupid skip shift alone was worth the $125. So far I don't even have a wideband installed because I'm planning to get some longtubes in the next couple months, so I figured I'd waste my last bung on that. As of now, the only thing that I've done to the tune in an attempt to increase performance was to lean out PE a bit, hoping the stock engine with stock VE tables is accurate enough to make a positive difference. The real reason I'm posting this is because I've seen a lot of dynos where simply bolting on the headers only nets ~10whp, but a tune pulls out another ~15whp. The fuel I can handle, I know what I'm shooting for there, but the timing isn't as intuitive. From my research, it seems in stock form they're quite fond of their 22* WOT and increasing it further wouldn't really do anything for power (sea level thinking here, I get more at 6200'). I have a difficult time believing that correcting the fueling for the headers alone is worth 15whp, so is anyone willing to divulge how much the engines desires change for spark with headers?

[Supercharged111]

I've scoured the 4 corners of the internet and still nothing. Maybe I suck at searching but google, this forum, and the Corvette forum return absolutely nothing when I try to find out the ignition timing desires of an engine before and after long tubes are fitted. Since the stock cam has negative overlap (204/218 @ .050" w/117.5* LSA, don't know where GM installs it as that seems to be top secret as well), I find it difficult to believe any scavenging is occuring. I can only assume the chambers are evacuated more efficiently thus lowering EGTs, but after that I'm drawing a blank.

[joecar]

GM installs 5308 at -2.5° [ Correction: 5308 has -2.5° builtin ]

With negative overlap, headers still do produce some scavenging, but it is gated by the the combustion chamber being momentarily isolated

[ low pressure pulse travels back up header primary, past EX valve and into CC, the EX valve effectively closes and traps the low pressure pulse in the CC, in a very short time (not enough to significantly leak the low pressure, especially at higher rpm) the IN valve effectively opens and exposes the low pressure in the CC to the intake port ]

If you want to play with AFR (leaning a little when transiting from peak TQ to peak HP), you really should use a wideband, otherwise it's like being blind, and you may risk knocking and elevating CC temperature.

Without positive overlap, keeping stock timing is a good idea... some people advance a few degrees, but you have to pay close attention to avoid knock, and it may or may not really increase peak torque (use a dyno to see which is the case), and may put you on the risky side of MBT (no safety padding for the boundary conditions).

[Supercharged111]

I only leaned it to command 12.5 at peak torque and tapering to 12.7 after that, so even if it goes 12.7 to 13.0 I'm still safe. Much better than the low 11s it commanded stock, seems to stink less when I jump on the skinny pedal too but that could just be me. Once the headers are in, I'll stick the Tech Edge in as I have a leftover cable for that one to log serially.

As for the timing, it seems you can advance it but a lot of people advocate leaving it alone. They say further advancing it, even without detonation, yields no more power (on a bone stock engine). Weird because when I compare the 2001 LS6 tune to the 2004 LS6 tune, the 2001 commands a bit more timing. The only engine differences that I'm aware of is more lift on the cam and possibly different cats (thought all 2001+ got the new cats though). The thing I'm trying to wrap my mind around is why oh why pre-tune do the headers only produce ~10whp but once they're tuned, a total of ~25whp is gained. If the car had the stock pig rich tune and you fitted the headers, it seems to me it'd run a little leaner and closer to where it needs to be and be that much better. If the timing requirements remain the same, then where is all that extra power? Or are the headers producing false knock and simply desensitizing the knock sensors gives that extra ~15whp that you should have had all along? It's obvious that I can't bolt them on and be done, seems that's only getting me halfway there

One more thing, is -2.5 retarded or advanced 2.5*? Advanced seems more logical to me, but you never know.

[joecar]

GM installs 5308 at -2.5°Correction: 5308 has -2.5° builtin.


One more thing, is -2.5 retarded or advanced 2.5*? Advanced seems more logical to me, but you never know.

It is retarded 2.5°...

in the attached image, 5308 is the lower diagram.

[Supercharged111]

Wow, never seen a camshaft installed at 120 before. I was thinking of getting an aftermarket 22X/22X with ~.580" and 112 + 4 or 114 + 4. I guess that's how an aftermarket can match the low end of a stock cam, they're advanced quite a bit more. Armed with that info, I can now compare the IVC of the stock cam to aftermarket cams. I inadvertently found some better results by searching detonation instead of timing, but still nothing definitive. I spoke with a local tuner today who indicated my suspicions on the stock curve, it shoudln't have more timing at peak torque than at peak hp (which mine does). I have 30 degrees at peak torque and it ramps down to 26 after that. Up here it seems most like about 30 degrees. My plan then is to install the headers, sort out the fuel on the street, and rent his dyno for an hour to nail down the timing. He's not too terribly secretive, so I think he'll probably provide some pointers along the way.

[Highlander]

I only leaned it to command 12.5 at peak torque and tapering to 12.7 after that, so even if it goes 12.7 to 13.0 I'm still safe. Much better than the low 11s it commanded stock, seems to stink less when I jump on the skinny pedal too but that could just be me. Once the headers are in, I'll stick the Tech Edge in as I have a leftover cable for that one to log serially.

As for the timing, it seems you can advance it but a lot of people advocate leaving it alone. They say further advancing it, even without detonation, yields no more power (on a bone stock engine). Weird because when I compare the 2001 LS6 tune to the 2004 LS6 tune, the 2001 commands a bit more timing. The only engine differences that I'm aware of is more lift on the cam and possibly different cats (thought all 2001+ got the new cats though). The thing I'm trying to wrap my mind around is why oh why pre-tune do the headers only produce ~10whp but once they're tuned, a total of ~25whp is gained. If the car had the stock pig rich tune and you fitted the headers, it seems to me it'd run a little leaner and closer to where it needs to be and be that much better. If the timing requirements remain the same, then where is all that extra power? Or are the headers producing false knock and simply desensitizing the knock sensors gives that extra ~15whp that you should have had all along? It's obvious that I can't bolt them on and be done, seems that's only getting me halfway there

One more thing, is -2.5 retarded or advanced 2.5*? Advanced seems more logical to me, but you never know.

Why would you do this????????

[Supercharged111]

Why wouldn't I? The commanded PE ratio was excessive and the car reeked something fierce every time I stepped on it. I know it's PE rape, but that's why I didn't command 13.0:1. Since the motor is stock, it should be close if not dead on unless I'm missing something here.

After a bit more poking around, it seems that correcting the tune rather than altering it is what unlocks the power with the longtubes, i.e. desensitizing the knock sensors to filter out false knock and of course correcting the fuel that will be inevitably out of whack. Is this a fair assessment?

[Highlander]

You command a richer mixture at peak tq and leaner at wot?

[Supercharged111]

At WOT, I command a richer mixture at peak torque than I do before or after it. The factory did it, and it's advocated quite a bit by more people than GM. Do you know something different? From what I know, richer = more torque. Richer also cools the cylinders more, which is most important at peak torque/peak cylinder pressure.

[Highlander]

Peak Heat is achieved at Peak HP. Cylinder pressure should be controlled with timing and not fuel. You should use more fuel if you run into a consistent knock which retarding timing will simply put you out of your power "target" and you can aid in detonation with a cooler combustion chamber but with adequate timing.

And when I refer to target power, this comes with experience, where you actually know how much power should that combo achieve and your issue might be the fuel quality from the different vendors.

Too much fuel and you loose power... too little and you over heat the pistons...

What you are doing by increasing fuel is actually slowing down the burn, which is the same as retarding the timing. I'd suggest you rework your timing table and pour in fuel if you get issues with knock and can't achieve a MBT.

But your fueling should be the other way around. Heat is a function of power and not torque.

Now.. eficalibrator can correct me if I am wrong.

[Supercharged111]

But I didn't add fuel, I leaned it from a richer stock setting mimicking its curve and have logged no KR yet. Based on all the dyno whores over on the Corvette forum, this combo should be getting me ~36Xwhp and headers should have me at ~38Xwhp. I've given up and plan to hit the dyno as soon as I get the headers installed and fueling corrected via wideband. My main question pertains to headers though, and whether or not they'll want a different timing value. I've seen a number of cases now where they induce false knock and require desensitization of the knock sensors, but nobody's really confessed if they want more, less, or the same amount of timing. I'm really not after a specific value as 2 of the same car might want different amounts of timing, just a trend of more, less, or same. I have seen a few people tuning to a target of 13.0:1 WOT, so I ASSume if I get no KR and land in the 38X whp range that I'll be on the right track with that.

The cylinder pressure thing has me a little perplexed here. I understand that having the timing in the right spot creates the most energy, but the engine itself is able to ingest the most air per every other rev at peak torque, which I thought inherently created the highest cylinder pressure. At peak hp, it's burning more fuel per second, so I'm guessing that's why you say it also creates the most heat. It sounds like there are 2 schools of thought at work here, because I have heard a number of people advocating higher advance after peak torque and also leaning the mixture after peak torque. Your explanation of shedding heat is logical, so don't get the idea I'm attacking you here. I'm sure you'll forget more than I'll know about tuning as I'm not running a business here.

[Boost]

The tiny bit that I know tells me that heat = pressure or at least proportional and most factory tunes have the least timing at peak torque RPMs which leads me to guess that's when dangerous pressures are occuring. I could see peak heat at high RPM with less time to cool maybe.

Really appreciating the insights from the wise experienced crowd here!

Supercharged1111 don't make the classic blunder of tuning timing before fuel with a wideband, and also listening to (the rest of!) the internet.

My 2 cents

[joecar]

Peak torque is where the engine sees the highest pressure (MBEP) and is the most sensitive to knock and where it sees higher combustion chamber temperatures.

[Boost]

Thanks Joe!

[Supercharged111]

I've kind of come to the conclusion that the power gains realized from simply bolting on headers comes from correcting the fueling and finding/fixing any real and/or false knock. I'm betting if I can bump timing that it's less than 10whp. Therefore my first step in tuning is to correct the fuel and chase down knock. Then it's off to the dyno with a couple guesstimated canned tunes to see what it likes. I hope to learn a lot and increase the efficiency with which I tune.

[joecar]

Let us know how you go.