1st of All I'm the Tuner on this car and I've run out of ideas, every day I learn something new, so someone have an idea?

Why the Dip in Power??



The car is a 2002 Z28 M6

The Mods are :

Patriot Heads w/2.055 intake valves & 1.6 Exhaust Valves 10.5 to 1 Compression
XE-R lobe 232/234 .598 lift 112LSA
Comp 1.75 Roller Rockers, Total lift is about .615 with these Rockers
Stock LS6 intake
Lid w/K&N Filter
Long Tube Headers w/Cats
10 Bolt Rear with stock 3.42's

Car runs very well, very strong through out the rpms. But you can feel a slight hesitation or dip in power right at 6000 rpms. Only did a few pulls on the dyno, as we thought the dyno was not working correctly, so pulled the car off early. Owner of the dyno swears it's not the dyno, that it's something else..

1st off, I've done this (tuning) for a few years now and I have never encountered this before. A/F with a Wide Band is right at 12.9 to 13.1 during the pull. Total of 30 Deg of Timing, No KR at all. No changes in the timing cells to make a drastic change. VE tables are nice and smooth, PE Tables are right on the money, no drastic changes, MAF tables nice steady curve upward, no changes.

Tuning on the street with a Wide Band, changing the timing down or up, fuel tuning more or less changes the A/F ratio but the hesitation is still there. Seems to get worse as the motor gets hotter..

Rocker Geometry looks good, using stock push rods.

Any ideas?

Anyware in the tuning that I might be missing?? Someone else have this problem before?

Let me know..

Clint

 

Maybe the valve springs not good for that much lift?

 

I'm not sure, but the Valve springs are a dual spring rated at .650 lift.

Plus the drop off in power is a nice smoth curve, like the motor all of a sudden was under adtional load, like the AC compressor came on, something like that.

Also, don;t think it's valve float as the dyno line would be a little more choppy, like the rocky Mtns.

Clint

 

It sounds (based on what you're tellin' us) like it's mechanical. My friend wants to run a very similar setup (1.75's plus a torquer v.2 cam & patriot stg 2 heads). We were worried about PTV when we put it together. But, Trevor@TSP thinks it'll be ok. Are you using stock pushrods or the stock pushrod length? Stock lifters? I'd first wonder if the lifters are not holding up with the XE-R lobe and the 1.75 ratio combo??? Either way, it sounds mechanical...maybe clutch slip?

 

Good Ideas..

Adtional Info.. There's another Car a 2001 SS w/an Automatic that I have tuned that has the same exact problem.. This was the reason I pulled the 2002 Z off the dyno early because I thought the Dyno was suspect, but you can feel it slightly on the street when doing a pull.

The 2 cars do have things in common.. 1st Off ME.. I did the tuning on both. The Same Installer/Shop of the Heads, Cam, Rockers, but he's a friend of mine and I know he does quality work, and knows what he's doing. The Same Rockers, Comp 1.75 and Same Head Setup. Cams are slightly different, but both with the XE-R Lope..

The 2001 SS was in search for 400RWHP with an Auto. I tuned the car with the stock lifters to 393RWHP, No Dip in Power then at all 1 month ago, just didn't hit the 400RWHP mark he wanted. So the owner bought the 1.75 rockers, to get more power. Re-Dynoed, The car then showed a dip in power, same Dyno, did not hit the 400 RWHP mark but did show a slight improvement durring the pull of a few HP, but the Dip in power is all of a sudden and new... I even loaded the old tune to see if it's me, the old tune where he got 393 RWHP with no dip in power 1 month ago, but now shows the dip. After the 2nd car showed it as well, Thought the Dyno was a suspect so I did not dyno any more that day. NEW INFO:The owner of the 2001 SS thought is was the 1.75 rockers so he put the stock ratio 1.7, but they are the Harlan Roller Tip 1.7 not the Stock ones.. Went back to the Same Dyno Shop, the dip was still there..

So with 2 cars having the same problem, changing the rockers on one did no effect. Same Dyno but he swears it's okay, also you can feel it slightly on the street. Tuning was done by me and all fingers pointing at me, I don;t mind, but I have never had this problem before.. but I did have the old tune with no power dip re-Flashed HP Tuners, still shows the dip, was not there that dyno day, owner just went in to see if dip was there. He reported it still was.

Any more ideas?

Clint

 

Were the heads milled? If so, how much? Did Patriot advise on using stock push rods?

 

Anyone have ideas on actual tuning. Am I missing something somewhere in the tuning? Timing and Fuel exactly the same. To make a drastic change like this, a 20HP drop in power then right back up 800 rpms later is of concern. You can actualy hear it on the dyno as there's a slight hesatation durring the pull..

Thank you..

Clint

 

Perhaps it's the rockers/springs? Check this thread out: https://ls1tech.com/forums/generation-iii-internal-engine/445464-600-220-duration-too-much-lift.html

Just a thought...

Mark

 

I've seen these kind of dips sooooo often when running those aftermarket rockers. Re-install the stock rockers and redyno. You will most likely find your missing power.

 

As far as you can tell, what are the physics or events that contribute to this with these rockers?

 

I meant stock length pushrods which looks like what he has...

 

More weight over the valve with most aftermarket rockers. This puts way too much stress on the valve springs. Here's what I'm talking about.

Stock Rocker - 140.1g overall, 8.2g at the valve
H/S 1.7 Non adj - 132.8g overall, 22.5g at the valve
Crane LS1 1.8 - 149.1g overall, 22.3g at the valve

So even though the other rockers weigh the same or sometimes less, look how much heavier they are over the valve.

 

Wow, I'd never seen that before... thanks Patrick.

 

So seems like Rockers or the springs can't keep up with the .615 lift and the aggressive ramp rate of the cam.. Again I never encountered this problem before on the tuning, but I'm completely open to the fact it was me..

Anyone out there that can say yes it is the tuning.. I'd be willing to send the bin file if necessary. Trying to eliminate a tuning issue here as no matter what I flash into the VCM it still has the dip..

Clint

 

If you had the AFR graph to go with the dyno chart, then yes. Lack of fuel, timing being pulled, etc. would all cause a dip in power. Your tune might be fine, but what about fuel pressure, clogged injectors, weak plugs/wires, etc.

 

Never checked Fuel Pressure. Don't know how old the fuel filter is. Clogged injectors probably not possible with 20k on the motor.

But these issues you mentioned, timing/KR issues would show up on the HPTuners scanner during the pull. That was clear. The A/F would change if an injector is failing, running out of fuel pressure, or spark plug problem, the A/F was stright and right on the money at 13.0.

+ it only happens at 6000 rpms every single time. Spark Plug and other ignition issues would be seen anywhere in the power band not just at 6K rpms.

So, back to Tuning or Valve Train Problems. I personally believe it's Valve Train, but trying to eliminate me as the cause here for the owner of the car...

Clint

 

I meant if you had those problems (lack of fuel from tuning, KR), you would have seen it in the AFR graph... "An AFR graph that matched the dyno." Sorry, I had stated that awkwardly.

You could still have "other" problems, but I agree that you're most likely experiencing mechanical issues. Fuel pressure from clogged filter, or weak wires *could* be the issue, but instinctively I thought valve springs or rockers just like everyone else.

 

Do you still have the logs for the dyno runs where this happened? Did you see what the delivered spark was to the #1 cylinder? Granted, you might be commanding 30* or what not at WOT. But, is that what the motor is really seeing? Abuse MGT/Engine Protection disabled? COT disabled? If it doesn't do it at cooler temps, check your spark modifiers. If you're AFR is truly spot on and spark is at full force, then it has to be mechanical.

 

Thank you for the nice post.. To answer your questions..

Q: Do you still have the logs for the dyno runs where this happened?
A: No but I remember them, the pulls looked perfectly fine from a scan stand point, but the dip could still not be corrected.

Q: Did you see what the delivered spark was to the #1 cylinder?
A: Yes #1. The car is seeing 30deg of timing at WOT durring the pulls. No KR..

Q: Abuse MGT/Engine Protection disabled?
A: A Little Unclear here. Are you asking about Torque Management Abuse Mode RPM,TPS,Speed? All these are correct. As I didn't find a MGT/Engine Protection perameter.

Q: COT disabled?
A: Yep it's Off

Spark and Fuel are in Full force.. Thank you..

Regards,

Clint

 

valvetrin harmonics, but geez thats a lot of lost power. try playing with preload. from what ive seen the aftermarket stuff is really tempramental when it comes to that stuff.