BRAKE

iTrader: (14)

I’m really dumbfounded with this and I’m going crazy trying to figure out what’s going on here. Last year this set up made 501/485 on a dyno jet, it peaked at 501 at 5500ish rpm and died off. The tuner said it was because the prc .650 springs more than likely weren’t enough spring for the ramp rate of this cam, he’s seen it before and told me he’s used the Livernois .690 springs several times with this cam and achieved the results he was looking for. So we left it alone for a while, I changed the springs to the .690’s and went to get re tuned. The car baselined this time at 443rwhp and finished at 460 rwhp (on a mustang dyno) I immediately put it back on the original dyno it made 501 on and it made 475/467. As you can see from the dyno graph, its not pulling like it should. This cam should pull past 6600 rpm no problem. This set up should have no issue hitting 500-520 rwhp, there are many people running this that make that power. The choppy dyno graph looks like spark blow out, but I changed the plugs on the dyno before the 475 pull (TR6 gapped at .035”). I just don’t get it, any ideas on what could be holding it back and cause the power to fall off so early?? Literally the only thing I changed was the springs and I don’t see why a slightly stiffer spring would cause this type of loss. I logged it last night and everything looked fine, no kr, plenty of fuel, timing curve looks amazing. The motor is in great shape too, the 501 dyno was at 800 ish miles and now I have little over 4,000 miles on the motor, good compression, nothing srong mechanically.

Does anyone have any idea what's going on here? Anyone have a similar issue?

Mods:
408 11.0-1 comp.
Callies crank/rods
Diamond Pistons
G6X3 cam
1 7/8’s tsp long tubes/ory
GMMG cat back
L92 heads milled .020”
L76 intake
92mm t body
255 in tank/LS3 injectors (@86% duty)

Sic_C6Z06

iTrader: (1)

If needed I can post a log of Wide open throttle recorded last night with 93 pump in the tank. 0 KR. Geometry of the Valve train appears to be good. Only modification since dyno time has been 160 thermostat. Fans have now been adjusted to run more frequently. Peak timing is 26 right now. You can not hear the car break up. AFR right around 12.6 at WOT

Points of interest possibly left would be

failing lifters
(LS7 stock replacements with only a couple thousand miles on them)

Exhaust flow restriction
( doesnt explain drop in power, 501 was made with same exhaust set up)

Wheels and tires were changed from A mickey thompson ET street II with an 18 inch c5 Z06 replica wheel to a 15inch weld RTS with Mickey Thompson ET street. Shouldnt effect the reading 30-40 rwhp.

Dyno graph isnt really showing any signs of the clutch failing/slipping or the trans failing.

BRAKE

iTrader: (14)

Thanks Todd.

redtan

iTrader: (5)

If I'm reading this correctly, the only other change you made was going from the PRC .650 springs to the other .690 springs.

Could it be too much spring for that cam? What type of lobes does it have? Dunno why a PRC .650 spring would not be good enough for a .60x lift cam, so wondering why your tuner had you go to an even beefier spring only to lose lots of power.

Sic_C6Z06

iTrader: (1)

Original tuner was seeing valve float in the dyno graph i believe. Or may have been picking it up in logs while on the rollers.

LG has stated the cam was designed around the COMP 921 spring IIRC (cant remember if thats the exact one off the top of my head) but the Livernois .690 spring is very comparable. slight seat pressure differences and coil bind differences are all minimal.

I feel the tune is strong honestly.. I am going to take a second look at the commanded timing tonight and compare.

I feel if pushrod length was incorrect we would be seeing other issues.

BRAKE

iTrader: (14)

The reason he suggested the bigger spring is because of valve float with the .650s. He said he's seen it happen before and by switching to the .690's the cars he has tuned were able to pull all the way through and make good power. Problem is, my original tuner is MIA and I went to another very reputable shop to have it tuned. Which they did a great job tuning, but the cam should pull to 6600-6900 no problem.

1lejohn

iTrader: (9)

Just some thoughts. My car made 508 with about 1k miles and 18" street radials. We dynoed again with a 16" Hossier slick and made 487. Tires do make a difference

The car peaks at 58-6000 with a L76 intake. It will pull all the way to 7k but power drops off. way off. My build is very intake limited. I think yours is also.

The heads also seem to stall eariler then cath. port builds. Hence GM developed the 7 head.

I also found 2 tenths and 2-3 MPH in giving the car more timing. I was running 24 bumped it up to 32-34 and it liked it. I street drive it at 30 and use 24 for my N2O base.

My knock sensors are deleted , running PAC dual 1521's ? , and a FTI hyd.roller. I'm still running a MAF and 58 Fast injectors.

Mamo ported 102 on the way. I'll do a before and after dyno test same dyno probally not the same day.

jmilz28

iTrader: (13)

Check the basics. Do you have cats? If so, check for clog or simply replace, could be clogged but not totally. Replace the fuel filter. Did you check fuel pressure during the run? I've seen them work fine but then stumble at higher RPMs. Maybe even a bad ground could have caused this. Collapsing intake tube? O2 sensors? IAT sensor? Dirty injectors? Heck, you may have even gotten a bad tank of gas. This feels like something simple.

Sic_C6Z06

iTrader: (1)

so the tires could make that big of a difference. interesting. I figured it would effect it but not that much. learned something today. thanks for your input there. I do notice that many of the plastic intake set ups are limited to where they make peak power so that may definitely be an area to address.

I have also wondered if the L92 cylinder head has a valve that is just too big for a 4 inch bore 408. basically stalling itself out and cant force anymore air past the valve BUT i saw it pull to around 6600 on a mustang dyno a few days before struggling to make power past 5500 on a dynojet.

Im interested in the wheel and tire difference making that much of a change.. ima do some more research on that. the rotating mass makes sense though. Ive also seen cars make damn near 10 whp more one pull to another on how u strap the car down so I havent been overly overly worried about the actual numbers, i was more so worried on why its falling off so hard. Numbers are great but i know for a fact it makes "less" recorded power but runs harder.. its a damn good race for my Z.

1LEJohn it looks juding by ur sig ur a 416 whats ur bore size? and do u run 93 pump or mix race gas or just run 100 plus octane all the time? ive been planning on bumping up the timing with some good gas in the car to see what it does for him

Sic_C6Z06

iTrader: (1)

No cats, like dogs better anyways.

Fuel filter and rails can be checked still

Fuel pressure was recorded during run and its solid throughout the rpm band.

intake tube is solid.

o2 and IAT's look to be good but can always double check.. again recorded those also via HPT.

bad tank of gas is possible. put good gas in it last night to be sure, havent made any dyno pulls on it yet though to see for sure

Gabbiani

iTrader: (20)

Most the board will argue with me but I have seen similar jaggies many times with tr6 plugs. I won't run them anymore.

BRAKE

iTrader: (14)

Thanks for all the in put. The reason I'm scratching my head is because all of the little sensor issues have been checked, I assume the fuel filter is good, checked the fuel pressure on a dyno pull, gauge didn't drop at all. Injectors have 4,000 miles on them, I could check them but the afr is rock solid. Air filter is clean. Speed density car with no collapsed tube.

The only thing I havent checked is cam and crank sensor, but I figured if those were malfunctioning the car would be showing major areas of concern. Is that not the case?

BRAKE

iTrader: (14)

What plugs do you use now?

I literally ran the car on the dyno twice, changed my old tr6's (gapped at .055" for some reason) put in a fresh set of tr6's gapped at .035" and NO change at all.

ArklaTexan

iTrader: (24)

subscribing

Sic_C6Z06

iTrader: (1)

i think the sensors are fine.

1. no codes
2. its commanding 26 degrees of peak timing, and its hitting its mark

Sic_C6Z06

iTrader: (1)

Could there be a benefit to running a TR7? step colder.. car seems to run strong though with the TR6, unless its losing spark up top but i havent been seeing anything really related to that on the logs.

Sic_C6Z06

iTrader: (1)

and no issues with cranking or missing either for crank or cam sensor to be bad

1lejohn

iTrader: (9)

Bore is a stock 4.065 L92 block. 93 on motor , mix when spraying

Gabbiani

iTrader: (20)

AC Delco or whatever stock replacements they have.

jmilz28

iTrader: (13)

Hmm, I'd sling a fuel filter on it just to be safe. It's a long shot but a cheap thing to rule it out. You COULD have a restriction and still get acceptable pressure but not enough volume. They're not the same. Double check the fuel pump harness/circuit including ground just to rule it out. IAT could still be an issue. Do you have a known good one handy you can swap it for testing? Cam or crank sensor would almost certainly throw codes and/or have misfires. Man, did this thing have fresh plugs? Could they have fouled quickly if they are new? Worth pulling one or two to rule it out.