Car went on the rollers last weekend. Put down some decent numbers and I'm very happy. But, what's with the dip in the torque curve between 3800~4800rpms? Just curious...

 

Do you have a log of the AFR?

It seems to have slightly effected the HP too, not much but you can see a dip in the curve.

If AFR is spot on, could thr wheels of slipped a little ont he rollers, not sure just guessing, but I tihnk the AFR may show something.

 

Looks like it did it twice, once at 3,800 and again at 4,800. What valve springs are you running? What rockers? Was it in 3rd gear locked?

 

Nevermind...I'm retarded. It's no where nearly as bas as it looks. The scale from one to the other is different. I replotted the same data in Excel and came up with the attached pic. I think the issue is with the timing. The original tuner took 2~3* out of the timing curve for the affected area. I'm going to bring it up a couple degrees and make sure AFR is still under 13.0:1. That should get rid of the imperfection in the curve.


(Stock rockers and Patriot Gold Dual Springs BTW)

 

that still doesn't look right, something is going on. check your spark and fuel...

 

Do you know why he took away timing? was there a problem? Knock?

 

Timing got pulled for sure.

 

It was probably because he started with a stock table from a pre-01 car and never upped it enough. I shouldn't get any KR with 27~28* there as long as I keep the AFR around 12.5:1. The original tuner had timing at 25* there and the car was running a lean 13.5:1 when I got it. It's a work in progress. But, the dyno looks to be a helpful tool...

 

Timing should be getting ADVANCED with RPM- when you start PULLING timing, something is wrong. Get to the bottom of that!

 

I talked with a couple of other tuners to compare notes. After speaking with them and seeing the car on the rollers, I now know where I need to set my timing. The original tuner did tune it to run fairly well, but not to the level where I think it should/can be at. I'm just looking to iron out the remaining bugs so the car performs at a consistent level. Yes, the timing should advance with RPM until MBT is reached. However, because of rising and falling cylinder pressures, temperatures, and a handful of other variables, what you typically see on H/C LS1's is the timing curve tends to flatten out around 3,000rpm. Mine had a dip most likely due to the fact that the AFR was a little on the lean side and couldn't run the optimal timing. So, the tuner took out a couple degrees. Thanks for everyone's input. I'm on the right track to getting this puppy dialed in.

 

BMT is not a constant spark time. Combustion is fairly close to a constant time process in a given engine, especailly relative piston velocity. If it takes X seconds to burn, you need to give it more "distance" or "degrees" at different rpm.

Generally, you want peak presures at ~15* ATDC, give or take a bit for different engine configs. The piston will get to 15*ATDC in less time at 6k rpm than 2k rpm, but it takes the same amount of time to burn. Hence, the need to advance timing as rpm increases and why BMT isn't a constant spark timing. Ideally, you're always at BMT- part throttle or WOT, 1500rpm or 7500rpm. emissions and whatever play a role though

 

I understand the theory. But, I'm willing to lean more towards proven practice and proven practice by multiple tuners says otherwise. Besides, burn rates at different rpms are difficult to compare since cylinder pressures are different. The burn rate at peak cylinder pressure (aka peak torque) is typically faster than just before or after that point in the rpms...hence the reason why the stock timing curve had the dip down to 25 degrees around 4000~4800rpm. Looking at the way my timing was set on the dyno and where the dips were in the curves, a couple of degrees in the affected area should solve the problem (assuming no KR pops up on the scanner). The rest of the curve looks relatively good, so why mess with a good thing???

 

While pressure does effect burn rate, the chance is pretty minute when compared to the chance in "degrees time" from RPM. No way your timing should get retarded unless its getting sprayed or you have some serious boost lag or something of that nature.