fbody_brian

Info about the car.
stock heads, pacesetter long tubes, ory, loudmouth, no cats. Ws6 hood and airbox (suncoast creations). 219/227@50 .534/.544 camshaft w 1.7 roller rockers. Hardened pushrods with guide plates. Crane dual valve springs set up at 136# on the seat, 340# over the nose. Walbro 255lph fuel pump with 41psi through the rpm band wot all the way to redline. Plugs are fairly new tr55 gapped at .040, wires are duralast about 3 years old. Optispark is non-vented chinese replacement case rebuilt using the original mitsubishi sensor and wheel from the original opti. Rotor and cap are about 2 years old. Rotor screws were attached with locktite.
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@5500-5700 rpm the car stops gaining power. The maf starts bouncing @ around 270-280 afgs and doesn't get any higher than that. The map doesn't show anything unusual, it drops a little bit in the upper rpm, but nothing out of the ordinary. AFR looks good, 12.8 to around 13.0, with a rich spike right at the shift point of 6150. The sound of the engine changes when this happens, but it is not super prominent and it's hard to describe, it's just different. The rate of acceleration just slows down at that point, but the engine keeps gaining rpm, and doesn't stutter any. I am not getting any knock retard when it happens.

I'm at a bit of a loss and don't want to just throw money at it blindly. I'm thinking of replacing the plug wires anyway, but could the opti be the problem? The coil? Does it sound like the ignition at all?
I could really use some ideas for what to check next.

The car feels strong. It ran a 12.81@108 with a DA of 1100, but I think I'm leaving something on the table here. It's done it with 2 different ICMs. It's also still did with a set of 1.6 rockers.

Help!!

BALLSS

Does it feel like a ignition miss or does the car just "nose over" aka stop making power?

even though the Chinese Opti has the Mitsubishi optical sensor in it...that opti "could" be the problem.

The Auto zone wires...replace with anything of better quality. Magnacore, Taylor or MSD

but I would first confirm the Crane springs you have are good for.600 lift assuming your cam lift specs of .544 are WITH 1:5 RR. With your cam AND 1:6 RR your lift is .580 and you really want .030 margin on spring lift above that. with the 1.7 RR your lift would have been .616

If you have exceeded lift of the spring you run...don't drive the motor, replace the springs

fbody_brian

It's .544 with the 1.7s
it does it with the 1.6 rockers at .512 lift also
it doesn't really nose over, it just kinda goes flat.

crane spring say: Maximum Net Lift w/.060" Clearance 0.710, so I would think that I am good there.

BALLSS

Good on the lift than with the Crane springs

If the motor has previously run right with this set up but now the high rpm issue, and given what you have tested, Opti would be the leading candidate to swap

My MSD opti did this...and being a new opti I assumed it was something else....several part changes later....it was the MSD opti. Started and ran fine but lost power in upper rpm's.

I have never had a failed or failing opti throw a code which was frustrating in finding cause of problem

lt1-xjs

The peak power on that cam may only be 5700 rpm. Lunati 54743

fbody_brian

I suppose it's possible that the cam is done by then, but I expected it to keep gaining power above 5500-5700rpm
cam card says:

• RPM Range: 1800-6000

I'm going to go through the ignition system, replace plug wires, cap and rotor, possibly the opti, and see if there is any improvement.

SS RRR

iTrader: (2)

Do you shift at 5700, or does it recover after 5700?

fbody_brian

shift is complete at around 6150.
doesn't really recover after 5700. It's hard to describe. I go WOT, car pulls good, datalog shows maf reading steadily climbing with rpm, between 5500-5700 rpm the maf stops climbing with rpm. It stalls out around 270-280 afgs and just bounces around that area, the rate of acceleration slows ever so slightly, though it never noses over or stutters. My first thought was valve float, possibly bouncing the intake valves off the seats and bleeding off compression. I threw on lower ratio rockers and the problem remained, with no change in the RPM that it happens.
I would think that if i were getting a misfire that it would be reflected in the AFR.
Just a little stumped.
I really think that I am going to swap out the ignition stuff and let her rip. If it doesn't fix it, I'll just have to live with it.
i also have had this strange issue with the logged rpm during WOT runs, where the RPM is really jagged, like the car is struggling to accelerate, but it's not, so I am thinking that thereis something wrong with the optical sensor or the shuttle wheel.
Probably going to try out a Pantera EFI optispark.
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SS RRR

iTrader: (2)

Just changing from one rocker ratio to another doesn't necessarily mean the springs are fine.
I know you said your AFR is good, but FP should be in the 43 to 45psi range.
I had an Opti issue where it would stumble in the 6500 range when I was shifting at 7200 with my last motor, so...

fbody_brian

I understand that, but i would expect that if i went from .480 lift to .512 lift to .544 lift by running different rocker ratios that, if it were valve float, the rpm it happens at would get lower with more lift, and also that it would progressively get worse the more i floated them, but it is always around the same rpm. That said, I have in no way been able to rule out valve float.
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SS RRR

iTrader: (2)

I've read it somewhere. Maybe shoebox's page or he's posted that before. Can't remember. Regardless, you are on the low end of normal FP for a stock engine.
You will need the specs of the springs, get a spring pressure tester and see where they're at.
https://www.summitracing.com/parts/mor-62390/overview/
Because of the cowl you will not be able to test all of them, but the ones you can, you may be able to get an idea if they're okay.
As far as opti, I just bought another and hoped for the best. However that was a few years ago when you could still get ahold of a quality opti for a reasonable price.

fbody_brian

Looks like that's really the only way to rule out valve float. The springs are Crane 99893-16 (10308-1 kit).
this is what LE had to say about them:
https://ls1tech.com/forums/lt1-lt4-m...l#post10499303

Appreciate the suggestions, hopefully I can get it all sorted out. This car has been one issue after the next. Glad it's not the DD anymore!

SS RRR

iTrader: (2)

You can also put it on a chassis dyno and see what happens in the upper RPM. If your power starts to significantly nose over after 5700rpm (usually the graph remains smooth) that could be a good indication of valve float.

BALLSS

if the springs have more than 20k mi on them ....they are "wear" items and need replacing with cams other than stock. Stock cams springs last longer but still lose their open & seat pressure and need to be replaced typically after the 75k mi mark if you want to spin the motor higher than stock

pull a few springs and measure their pressure...and confirm their "installed height" is on spec to provide the open & seat pressure. So many people just "throw" on springs without measuring for Installed Height....

not saying this is OP's issue but if springs have miles on them with a cam...check them

fbody_brian

Probably around 3000 miles on them.

NewOrleansLT1

I had a hair line crack in a spark plug and a loose opti rotor from the plastic rivers failing that caused similar symptoms in my car a few years ago.. Those two problems eventually took my coil out also. I guess the coil got over heated from the resistance maybe idk..

BALLSS

3k mi they should be fine

I and SSR had same issue with high RPM fade.....both cases was Opti.

SS RRR

iTrader: (2)

Oh, and to further explain the opti issue on mine, I changed the cap and rotor and had the same problem. It must've been the sensor or wheel that had issues with higher RPM.
If you chose to put the car on the dyno ignition anomolies may show up where the power line may continue to rise, but the line is really rough. Of course, do so on a wide band to ensure fuel is good.

Puck

iTrader: (17)

I ran those springs with a larger cam and never ran into issues. Pulled clean to 6300, and had over 20k on them when I put the car away with no signs of valve float or fatigue. I would be surprised if you had issues with those springs and that cam with only 3k miles.

Honestly sounds like opti...does it happen both when hot and cold? If only when hot it could be the ICM overheating and spacing it from the head with washers may help. I see its stock heads but has the HG been changed? If its higher than stock compression dropping the plug gap to .035 may help - my tuner recommended it with a thinner HG even though I had an MSD coil since we run pretty high compression on these cars.

Trying to find a way to not blame the opti like everyone always does but in this case... .

NewOrleansLT1

Op, how I found the bad cylinder was I let the car get up to operating temp then took a water bottle and poked a pin hole in the cap.. I then skeeted water on each header primary until I found the one that didn’t sizzle.. My car even ran a 12.23 @110mph when it was in that condition on 7 cylinders lol.. Once I fixed the issues it went a sec faster and gained almost 10mph..