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You are right it’s definitely a good idea. I will go over everything I can think of again.
Car seems to run really well though and isn’t setting any trouble codes.
Anything in particular you think it could be. Other than something like a plug wire or poor connection somewhere.
Running out of ideas on this thing.
Definetly though compared to the curve in the graph you posted something seems wrong.
It definitely seems to flat line.
Those bumps in the torque curve up top are odd - like it is an intermittent issue. Valvetrain instability would not have bumps up in power at the top like that. Can you ask the shop for a STD graph smoothing at 0? The first bump I'm estimating to be around 6000 rpm, and looks to be 15 ft lb of torque. At 6000 rpm a change of 15 ft lb should take it from 394 hp to 411hp. The power graph is being completely smoothed over you can't really tell what is going on. The HP curve doesn't increase at all where the torque bumps are. The engine did not really flat line...it nosed over and tried to recover a couple times.
The way the torque starts dropping rapidly, and then has those bumps leads me to think spark more than anything else. Intake restriction or valvetrain instability would not pop back up like that.
High resistance in the spark plug wires like a broken or loose wire, or even a broken spark plug porcelain can run fine at lower rpm, and only show issues up top. Sometimes you can also get plug wires arcing to the header tubes causing misfires at high rpm.
Also note - the TSP dyno graph is on an engine dyno. Engine dyno graphs simply interpolate between points that is why the curves always look so smooth. The dyno records data on the dots and draws lines between them.
Those bumps in the torque curve up top are odd - like it is an intermittent issue. Valvetrain instability would not have bumps up in power at the top like that. Can you ask the shop for a STD graph smoothing at 0? The first bump I'm estimating to be around 6000 rpm, and looks to be 15 ft lb of torque. At 6000 rpm a change of 15 ft lb should take it from 394 hp to 411hp. The power graph is being completely smoothed over you can't really tell what is going on. The HP curve doesn't increase at all where the torque bumps are. The engine did not really flat line...it nosed over and tried to recover a couple times.
The way the torque starts dropping rapidly, and then has those bumps leads me to think spark more than anything else. Intake restriction or valvetrain instability would not pop back up like that.
High resistance in the spark plug wires like a broken or loose wire, or even a broken spark plug porcelain can run fine at lower rpm, and only show issues up top. Sometimes you can also get plug wires arcing to the header tubes causing misfires at high rpm.
Also note - the TSP dyno graph is on an engine dyno. Engine dyno graphs simply interpolate between points that is why the curves always look so smooth. The dyno records data on the dots and draws lines between them.
I will hit the shop up tomorrow and see what they say. Hopefully they have that saved.
It does have new wires and plugs in it, probably less than 1000 miles on, and I pulled all the plugs today to check compression. They all looked good though I admit I didn’t check that close for cracks.
That’s interesting about engine dynos. Didn’t know that.
I guess that is one good thing about having problems. I always lean a lot more then if everything goes smoothly.
Those bumps in the torque curve up top are odd - like it is an intermittent issue. Valvetrain instability would not have bumps up in power at the top like that. Can you ask the shop for a STD graph smoothing at 0? The first bump I'm estimating to be around 6000 rpm, and looks to be 15 ft lb of torque. At 6000 rpm a change of 15 ft lb should take it from 394 hp to 411hp. The power graph is being completely smoothed over you can't really tell what is going on. The HP curve doesn't increase at all where the torque bumps are. The engine did not really flat line...it nosed over and tried to recover a couple times.
The way the torque starts dropping rapidly, and then has those bumps leads me to think spark more than anything else. Intake restriction or valvetrain instability would not pop back up like that.
High resistance in the spark plug wires like a broken or loose wire, or even a broken spark plug porcelain can run fine at lower rpm, and only show issues up top. Sometimes you can also get plug wires arcing to the header tubes causing misfires at high rpm.
But if it was misfires, wouldn't you'd see a series of spikes up and down? And it starts to go down within a .8s period, fluctuates up in .1s, levels for .1s, then drops back to where it within .2s. Really strange. Unless the software is smoothing it or something.
Those bumps in the torque curve up top are odd - like it is an intermittent issue. Valvetrain instability would not have bumps up in power at the top like that. Can you ask the shop for a STD graph smoothing at 0? The first bump I'm estimating to be around 6000 rpm, and looks to be 15 ft lb of torque. At 6000 rpm a change of 15 ft lb should take it from 394 hp to 411hp. The power graph is being completely smoothed over you can't really tell what is going on. The HP curve doesn't increase at all where the torque bumps are. The engine did not really flat line...it nosed over and tried to recover a couple times.
The way the torque starts dropping rapidly, and then has those bumps leads me to think spark more than anything else. Intake restriction or valvetrain instability would not pop back up like that.
High resistance in the spark plug wires like a broken or loose wire, or even a broken spark plug porcelain can run fine at lower rpm, and only show issues up top. Sometimes you can also get plug wires arcing to the header tubes causing misfires at high rpm.
Also note - the TSP dyno graph is on an engine dyno. Engine dyno graphs simply interpolate between points that is why the curves always look so smooth. The dyno records data on the dots and draws lines between them.
Not saying the smoking gun is valvetrain, however valvetrain instability absolutely can do that. I had an engine that lost 40 hp from 6,150 to 6,350 then magically at 6,500 it had gained 25 hp back. On the dyno you could barely hear it, but on the street you could somewhat feel it and you could certainly see it in the MAF readings. It had numerous smaller dips and spikes starting 1,000 rpm below that. Changed the springs out and it was smoother across the whole rpm range and even made more torque earlier at peak torque. Spring harmonics and natural resonant frequency can work in weird ways.
So I have some questions that hopefully someone can answer.
The car is an 01Fbody M6 with stock rear end.
Engine is a LS6 block with built bottom end.
Heads are patriot performance ported LS6 heads with dual 660 springs, ported Fast92 intake and pacesetter LT headers.
Until recently it had a 228 cam but recently changed to a TSP torquer V2 112 LSA cam.
Previously the car was tuned but never dynoed.
Just got back from getting the new setup tuned and dynoed. Car put down 394hp at 6100 and 382TQ at 3600
Was rather disappointed with the numbers and it honestly feels like it had more power before.
To me, how low rpm the TQ peaks at as well as it running out of power at 6100 and of course the low numbers all indicate something is wrong but no real idea what.
Any help anyone can provide would be greatly appreciated.
Thanks
Since it was on a DynoJet ask if then can send you the runfile. Likely its plotted vs time instead of rpm as the tach signal was a little noisy and that could also explain the mysterious bumps in the tq curve at higher rpm. Can't quite see the left hand side of the graph but it looks like both power and tq are on the same scale so where they cross is 5252. Pretty easy to get a decent idea of rpm by some simple math and plotting a few points manually on the hp and tq curves. Hp = (tq*rpm)/5252 so rpm = (Hp*5252)/tq. Easiest thing to do would be to have them send you the runfile and download the DynoJet software yourself and then you can change the bottom axis, correction factor, as well as the smoothing and see how the curve changes.
07-25-2023, 04:09 PM
