weird dyno session... stumped and gunshy

[DSteck]

The car:

2009 CTS-V
LSX block (427ci, built, 9.2 compression, dynoed 610/550 on an engine dyno NA)
Mast 305 heads
2.9L Whipple
Injector Dynamics ID850s

This car has been through the ringers and is hopefully on the road to recovery. I'll set the stage of what happened yesterday.

This particular car used to have a 434 with a TVS 2300 blower at 15psi and only made 630whp. Why the numbers were so low was always a mystery. That 434 let go when a very thing part of the block let coolant rush into the oil pan. It was replaced with the 427 that's in it now, and was switched over to the 2.9 Whipple.

We street tuned it in the morning and got that down, then hit the dyno. We began making pulls, and started off pretty conservative at 11.3 AFR and only 12* of timing for 13psi. I slowly started adding timing in and took it to 11.5 (this is on 93 octane). Everything was fine, although the whole thing was still making low power. It made a best of 670whp with only 14* in it. Anyway, I took it to 16-17* and got a weird reading on the scanner, but also felt something similar to a misfire. It happened at 3600rpm and again at 6100rpm. It also lost power on that pull. Gunshy given the history of BS this owner has gone through, we aborted the dyno session because I had no idea what happened.



My main question is... Can anybody shed light on what would cause the narrowband on bank 1 to report a quick lean spike? I felt it (similar to a misfire) but cannot come up with an explanation of what happened.

We left the dyno, and driving down the street, the blower belt decided to shred a rib, which got bound up in the tensioner. We pulled that belt off and kept driving, got a new belt, and threw it on the car. We went out to test on the street to try and recreate the issue, and it happened again at part throttle, low boost (not wide open throttle). We hit the highway and tried WOT again, and it was fine then... pulled like a freight train until the tires washed out. Now it was also making 16psi instead of ~13.5. It never did that misfire feeling again, even on repeated WOT pulls.

Go figure, the damn NW102 started sucking shut at WOT. Threw on an LS3 throttle body, same problem. We ran out of time at that point. The owner is going to try an LS2 throttle body next, and a C5 102 as well.



Anyway, my biggest concern is what was happening with the picture I posted. I would think a misfire wouldn't show a lean blip on the narrowband, and am hoping somebody may be able to shed some light on what's going on.

[carlrx7]

dave, i think the lean spike is the misfire. when im on my 2 step it cuts ignition and goes stupid lean (according to the o2 sensor) as well.

sounds like he has an ignition issue. if you get another change, scan individual cyl for misfire count.

[DSteck]

One thing I forgot to mention... After we fixed the belt, and before the throttle decided to start closing at WOT, the car threw a code for the cylinder 1 ignition coil. I swapped that coil with cylinder 2, hoping to see the code come back on cylinder 2 (and hopefully kill two birds with one stone and see those lean spikes on the narrowband switch to bank 2)... but a code never came back.

This car is in Chicago, and I don't know when I'll next be there. A couple weeks, at least.

[383_Stroker]

I agree it looks like a misfire. Maybe you had a loose coil wire and you fixed it when you swapped the coils?

[Mez]

Had a similar issue with a blown C5 with a 427. Eventually made 800+rwhp.

Mis-fire was traced to broken wire on a fuel injector. Difficult to detect as the wire insulation looked fine but when we wiggled the connector, we noticed a spark to ground. Lucky we found it. Actually, the wire was broken in two spots. Soldered it and shrink wrapped it and it was fine.

Since its not consistent, tough to diagnose of course. But that would lead me to believe its a wiring issue. Since it lean, I would suspect its an injector cutting out.

[DSteck]

That's what I thought... Injector possibly cutting out. However, the torque curve looked like garbage on that pull even after it recovered.

I guess I would just assume a misfire wouldn't show a lean spike.

[Chris@HPTuners]

valve float will also show a similar effect

[IDRIVEAG8GT]

I've seen that repeatedly on my truck, and it's all been from crappy spark plugs or a spark plug wire not being pressed all the way on.

[SargeZ06]

CAT coming apart on that bank? If it has them.

[edcmat-l1]

I guess I would just assume a misfire wouldn't show a lean spike.

Why would you assume that?

[DSteck]

Why would you assume that?

I guess since the fuel is still there.

Car has no cats, so it isn't that

The engine has platinum TR7s. I did not think to ask the owner what gap they are set to. I can find that out though. If it happens again, I suppose the easy thing to do is first reseat all ignition connections. After that, swap all coils bank to bank and see if it switches sides.



I am still trying to wrap my head around why this thing seems to be missing 100-150hp.

[edcmat-l1]

I guess since the fuel is still there.

Yeah, but so is a burst of raw O2. Just an FYI, there's no better indicator of a misfire than the narrowband O2s. You can disable your misfire tables, but you can't fool a properly functioning O2 sensor.

The engine has platinum TR7s. I did not think to ask the owner what gap they are set to. I can find that out though. If it happens again, I suppose the easy thing to do is first reseat all ignition connections. After that, swap all coils bank to bank and see if it switches sides.

First thing should always be to check the plug gap.

I am still trying to wrap my head around why this thing seems to be missing 100-150hp

Does the airflow model match? Fuel flow? You're surprising me here with all the basic stuff you seem to be missing. No offense, but this is typical of most "tuners" that have no diagnostic capability.

[383_Stroker]

Agreed, the O2 Sensor only senses O2. So, if you don't light it, in the case of a misfire, even though there's excess fuel now, there's also a ton of unburnt 02.

[DSteck]

I always try to leave O2s in for the fact that they show things a lot faster than a wideband.

Pulling plugs on this setup is a nightmare, plus the engine builder believes he is infallible. I ASSumed it'd be set appropriately.

Airflow and fuel consumption is on par with a 2.9 Whipple CTS-V with a forged 376 making 800+ whp (one of the guys who sells the CTS-V Whipple kit). You can see the dynamic airflow and cylinder air mass in the picture I posted. For reference, my car with IDs made 770whp at just over 90 lb/min of airflow. 1000s instead of 850s, but the injectors are flowed and tested on the same equipment. This "basic information" you're asking is pretty clear in the screen shot IMO. I've done lots of problem solving (did it for a living for Chrysler), but this was a new thing to me, and I didn't want to press my luck in this case. The worst part is the inability to recreate the problem.

[edcmat-l1]

If the engine is consuming enough air and fuel to make XXX amount of power, and it's not, it has to be parasitic loss. Has to be. Power doesn't evaporate.

If the numbers add up to the power it is making, then something else is most likely wrong. Limited airflow either in or out, some type of restriction.

Again if the numbers add up, but the power is not there, something is pulling it down. Good move to stop at that point. Numbers don't lie.

[DSteck]

If the engine is consuming enough air and fuel to make XXX amount of power, and it's not, it has to be parasitic loss. Has to be. Power doesn't evaporate.

If the numbers add up to the power it is making, then something else is most likely wrong. Limited airflow either in or out, some type of restriction.

Again if the numbers add up, but the power is not there, something is pulling it down. Good move to stop at that point. Numbers don't lie.

Had we kept going, that blower belt would have let go in a blaze of glory. It took a dump just a couple miles down the road at 2000rpm. Good thing we did stop. The pieces got bound up in the tensioner and it sucked pulling that crap out on the side of the road when it was 20* out.

I thought the same thing about parasitic loss. It has a 3.73 rear end from Bubb Rubb (anybody remember whistle tips?...). Err, Jesse Bubb. The whole car used to be a Jesse Bubb build, and everything he touched has broken so far (434 LSX, TVS2300, "built" 6L90...). Only the heat exchanger and differential are left with his name on it, and the diff whines. The car now has a built Circle D trans, but like I said, still the 3.73 rear from dipshit. This car made 730whp with bolt ons and heads/cam with the original LSA and 1900. I just want this customer to get the results he has been chasing.

[Alvin]

Has it been said where the plug gap is now currently? Have you done a plug check? I would think a 7 would be pretty hot for what you guys are trying to do.

[eficalibrator]

As mentioned, misfire will show up as lean on either type of O2 sensor. I also agree that it's time to pull a plug or two and look at them. Check the gap. High pressure supercharged engines usually end up liking surprisingly small plug gaps, even with an iridium tip.

When comparing raw airflow to measured power, don't forget to subtract both driveline loss and blower drive losses. Blower losses at a P-ratio of 2.0 can be BIG, especially at your airflow level. 100+hp loss (~10lb/min) is normal here. This is tax you've got to pay just to turn the screws before you have brake power available at the flywheel. ...now you know why I like turbos.

[edcmat-l1]

Has it been said where the plug gap is now currently? Have you done a plug check? I would think a 7 would be pretty hot for what you guys are trying to do.

I agree. However, if the power loss was due to the plug gap, you would think it would break up badly.

[DSteck]

It wasn't breaking up at all on the street. It pulls very linear and smooth until the throttle body gives up. My Z06 runs a TR6 at 0.028" on 16psi with just 93 in the tank and pulls clean to 7000rpm without incident. I agree a 6 is too hot for mine but laziness gets the best of me when it comes to my own car.

As mentioned, another V running the same airflow is making over 800 through an A6. I can appreciate drivetrain loss but this scenario seems extreme. The only other possible aspect is the fact that this was a Mustang dyno, and the dyno was flipping out the whole time (rpm pickup failure, aborting the pull early even in mph mode, and tire spin). I didn't have a lot of faith.

0.030" on the plug gap. I wanted to pull plugs but he said because of the Mast heads, it is a production to get the plugs out. I wasn't going to tell him to GTFO of my way. Lol.





Well, I'm glad to hear its a misfire (just for the sake of having an answer). Learn something new every day! Now I just want to make sure it doesn't come back.