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.