Hmmm, I wonder if the pulley on the left is out of balance or a blem of some sort.

Glad you figured it out.

 

It doesn't seem like it is even intended to be the same part. The weight is significantly different as is the thickness of the "elastomer" in between the two metal parts. Both of those things would dictate the frequency range for which the pulley would be an effective damper. Either the one of the left is a very poorly executed cheapo knock-off, or I got the wrong part and this one is designed for low-rpm applications like trucks...

Cheers,

Dean

 

Awesome! That's 3 LS1's fixed because of a pulley. Glad we could help!

 

Another idea is it needed a crank relearn

http://www.performancetrucks.net/for...3&postcount=14

Basically, if anything along the crank axis is disturbed, a relearn should be performed. An aftermarket balancer of any quality will have a different vibration pattern than stock so when you're hammering on it the PCM sees this and compensates to the detriment of your performance. I had the same issue with a misfire under load from using a Fbody balancer on a truck engine and pulled my hair out for months because everyone slammed the notion of crank issues without a code P1336

https://ls1tech.com/forums/pcm-diagn...-problems.html

Also, FWIW a cheap balancer wont make any difference unless its of shoddy construction, then it can be dangerous as it flies apart. LSx engines use a neutral balance balancer so any can be adapted regardless of original application. If your knock-off balancer was out of balance then yes, its garbage. Being slightly heavier wont make any difference

 

I guess I'll try the crank relearn because I was WRONG about the problem being gone.

I had the car on the dyno on Saturday to do a couple quick pulls to get certified HP numbers to classify the car for a HP/wt class. Well, the power dip is still there, unchanged from before. I guess I let my optimism get the best of me and wanted to believe it was gone when I drove the car on the track

Back to the drawing board.

Cheers,

Dean

 

What? I can't believe it. Damn that sucks. Mine is still doing just fine with the stock pulley back on.

 

find a fix?

 

Sadly, no. But, honestly, I haven't been looking lately. Other projects have taken priority.

My next plan of attack is to make a list of all the things that haven't changed between the two motors and then start eliminating them one by one from easy to hard/$$$.

The list of things that have moved from the original used motor to the new crate motor (both have exhibited EXACTLY the same behavior) and have not been swapped out and tested:

- entire Painless wiring harness
- headers and exhaust
- coil packs
- throttle body
- intake manifold (everything in front of the TB has been changed, altered, etc. with no effect)
- flywheel
- knock sensors
- IAT sensor
- O2 sensors
- fuel pump
- fuel filter
- water temp sensor
- engine mounts
- water pump

That's it, I think. Every other part/sensor has been changed at some point with absolutely no effect on the problem.

I've just installed a set of used D585 coil packs. The car runs fine on them, but I haven't had it on the track or dyno yet. Will fix that on Aug 12th.

Fresh thoughts and ideas are welcome and appreciated!!

Cheers,

Dean

 

You have quite a few aftermarket gauges in the car. Have you tried unplugging them (make sure not to ground anything accidentally) to check that they aren't causing any electrical feedback?

How is the tach wire and alternator wired up? Tach may have a pullup resistor to 12V and the alternator wire goes through some resistor (and maybe a diode?).

I was swapping out my cluster yesterday and trying to figure out why my tach was working fine on the stock gauge. Looks like someone added a pullup resistor or some other circuit to increase the tach signal V. I believe the ls1 signal is ~4.8V and mine was at ~12.5V. I read somewhere that the pullup resistor can cause weird speedometer blips in the miata which is a problem I used to have. That got me thinking about your problem.

 

I don't have a LS1 car anymore (got a G8 GT now) but I have the same problem on a totally different motor in my old school 71 Torino.

The good part for me is fewer parts to check. I've been down the same road as you, checked electrical, timing, fuel pressure, voltages, timing, blah blah blah. I have an MSD box and a magnetic distributor that is solid. carb is perfect, fuel pressure is great, wires are known good making 650 hp on another car with a similar motor.

This is the dyno from my mild 545 powered 71 Torino. It has a 232/242 @0.050 Lunati cam with matching springs(they claim) and Procomp(!) roller rockers.

The only thing I have not done yet is replace the valvesprings/rockers. Now that I found someone else with the same problem I can only draw the conclusion that we are experiencing a harmonic issue in our drive trains. I'm yanking my springs and cheapo roller rockers soon to confirm but I am willing to bet you that this is the problem.

I'll check in here after I done the swap but it will be in a month or two,
Pete

 

Pete,

Thanks for posting. Although I'm sorry you're having problems, misery does love company! ;-)

Theoretically, I agree 100% with you that the evidence really points to a harmonic. The most important piece of evidence for me that leads to this is that a partial throttle dyno pull exhibited the behavior on my car. The dyno I used will allow me to hold the car at a particular RPM under load - and this would definitely give us more evidence if the power dip is constant, not transient, in that RPM range and if the power comes back above/below that RPM range..

I just *cannot* wrap my head around the fact that this is happening on a BONE STOCK CRATE LS1... which is the only reason I'm still skeptical.

That being said, buying and installing different springs and rockers is about the same investment of time and money as scheduling time on the dyno, getting the car there, etc. so I might as well just do that! Better springs and rockers certainly aren't going to hurt the motor in the long run, anyway...

Please keep me posted in the event you get to this before I do. And I'll do likewise.

Cheers,

Dean

 

Not to spread FUD, but I'd be wary of installing any aftermarket springs or rockers on your stock cam race motor. Every once in a while some spring and rocker company has a serious metallurgy issue and kills a bunch of motors. I had 3 broken aftermarket springs after 2000 miles of street driving on my gto. Maybe consider the GM yellow/ls6 springs?

 

I may be wrong, but I will share my knowledge and experience in the matter, maybe it can help in some way or another.

Around 4-5Krpm on most engines is where maximum engine load is made (where the torque and power curves cross). This is also the point where the highest demand is made on the ignition system as a whole. If you have a weak link (ie, wire that is arching through or poor ceramics on the spark plugs) your spark energy will suffer and power output will drop until engine loads lessen 5.5K rpm+ and the band starts to clean back up. For example, we have seen in testing, especially with aftermarket headers that get close to the wire, certain manufacturer wires can arch right through to the header.

Below is a dyno where we were assisting a customer with a YSI Vortec blown engine with the exact same problem. They could never get it to run without the dip. We swapped in some new wires and coils and voila, problem 'almost' gone. I am sure if we changed the plugs and lowered the gap a touch from where he had it the problem would be cleaned right up. I also noticed you were going lean, possibly a symptom of the few poor, or missing, ignition events in the cylinder?

You guys did mention an interesting thing with the dampner. We have often seen the UD stuff can potentially lower available system voltage causing a similar situation. In that case the dwell time of the coils, during lower voltage situations, may need to be altered as they will generally not provide as much ignition energy as when the charging system is at a higher voltage. It may be where the link between the 2 situations is?

Hope you get your problem worked out.

 

Thanks for the ideas and info. This one is definitely a head scratcher.

My problem definitely occurs very close to the torque peak. However, I've duplicated the problem over the exact same RPM range with a half throttle dyno pull, meaning that the demands on all the components weren't even remotely close to maximum.

Also, I've tried different brand plugs, different heat range, a wide spectrum of gaps from very small to very large, different plug wires and known-good different coil packs. None of these had ANY effect. That is, not only didn't they solve the problem, but you could hold the dyno plots up to a light and they're exactly the same on every single pull.

That's what leads me back to agreeing with Pete - a mechanical rpm related problem is one of the few that will be incredibly repeatable across temperatures and loads. I'd expect an electrical issue to be somewhat intermittent. I've swapped alternators and batteries, and if the problem was electrical load related, I'd expect it to behave even slightly differently across these swapped components or even different with alternator good and hot (end of a 30 minute race) versus stone cold (first dyno pull). However, the symptoms NEVER change with conditions. They are always exactly the same.

*sigh*

Going to put a vacuum gauge on the engine, hopefully this weekend, which should give us some evidence of valve float if that is indeed the issue. We'd still need to find the cause for any harmonics, but that could be the flywheel or clutch or something like that.

Cheers,

Dean

 

I just went back and started to look at this thread from the beginning, and I realized I have lost sight of one thing: This problem used to be intermittent, but now it is 100% repeatable. Probably the last 50 pulls on the dyno and every time revving through this range on the track for the last 6 months - it's happened every single time. Hmmmm...

Just remembered what might be important information. I originally had a used motor of unknown mileage (craigslist seller said low, low miles, of course). This motor exhibited the problem all the time, I think. I spun a bearing on that motor and bought an LS1 crate motor from GM. That motor is the one that the dyno plots are from. It had the problem sporadically. I overheated that motor and warped the heads, so I put the heads from the old motor (along with the old springs, rockers, pushrods) on the new block. This combination exhibits the behavior EVERY SINGLE PULL.

Wonder if the more tired valve springs in the older heads are the difference between occasionally exhibiting the behavior versus reliably exhibiting the behavior?? Springs of different effective rates, old/new, would have slightly different harmonic frequencies.

Thoughts?

 

I have been helping Dtfastbear try to solve this problem.

One of the symptoms is after running through the trouble area (5,200-6,000)and lifting the throttle, idle is choppy. The theory is the valves float and the lifters get pumped up so they don't seat right after dropping to idle.



http://www.youtube.com/watch?v=p-2LdoXIZWI
Motor run to approx 6,000 RPM's then back to idle. Watch the vacuum drop from 22 to 20 as it encounters the trouble area in the 5,200 to 6,000 range. Then notice the vacuum at idle bounce from what I suspect to be pumped up lifters from valve float.




http://www.youtube.com/watch?v=Jpz61dbj-i4
Motor run to approx 4,500 RPM's then back to idle. Watch the vacuum stay steady at 22. Then notice the vacuum at idle does not bounce.

 

I usually stick to electronics, and I might be wrong, but you can hear either lifter tick after your off the throttle or a bearing that isn't doing to well.

I had a similar sound, ended up being a slightly tapered journal on the crank which eventually spun a bearing.

 

The problem is exactly the same with 2 different motors. The first motor was a "junk yard lump" that failed for another reason altogether. The current motor is a new crate motor with the externals swapped over. I am still thinking harmonic induced valve float.

 

Stupid idea.. what kind of oil are you using? Not sure if there is an issue with GM but I know that the newer Ford V8 doesn't like heavy weight oil for the lifters at high rpm or lifter pump can occur.

 

NO idea is a stupid idea at this point. I'm convinced the solution is going to end up being dead simple when we eventually find it...

Running 15-40 synthetic. Have run lighter non-synthetic, too, prior to this. No difference in the power dip behavior across oil weights and brands.

Cheers,

Dean