Mighty Whitey

dyno'd my car saturday evening after swapping to my 10-bolt with 4.10's for a comparison between a 9" and a 10-bolt.
when I went to start the car, and pull it on the dyno, it wouldn't start, just cranked; I knew then I was on borrowed time with an opti. let off, twisted the key again, and it started, I proceeded to pull it on the rollers.

upon doing so, I revealed that I have a stuttering problem right at 6k rpm and continues upward.. on top of that, my dyno numbers are 10hp and 20ft lbs LESS than before. obviously, something's up.

I contemplated valve float being the problem, but my power peaks at 5700rpm, holds within 3 or 4hp of its peak till 6000rpm, and then DROPPED, I mean, 75hp within 100rpm DROP.

I really dont see how it could pull so clean, straight to 6k and then drop like a lead balloon. if it slowly lost power, then dropped 50hp over a 500rpm range, I could see it, but go from 345hp to 270hp when only going from 5900rpm to 6000rpm?

I replaced the opti today, and it doesn't have this problem in first or second gear, but still does it in the high gears (3rd and 4th) the car has plenty of gas in it as well.

I haven't changed the plugs in about a year (only 3k miles worth of driving though) and they are the stock replacement NGK TR55 plugs gapped at .050"
I am also running an MSD coil on the car.

should I possibly try a colder, NGK TR6 plug? or possibly buy another set of TR55's and try gapping them at .045" or .040" instead?

BlackScreaminMachine

iTrader: (17)

Check fuel pressure under load.

Mighty Whitey

would that make it drop off that quick?

BlackScreaminMachine

iTrader: (17)

Sure would.

LT1Falcon

iTrader: (3)

how come you are not considering the opti. you even stated "I knew then I was on borrowed time with an opti." it does have some opti symptoms

racerns

I had a high rpm miss and found it was a bad coil but there was no real way to test for it except to swap in another coil. My miss was starting at about 5200 rpm though.

Mighty Whitey

I've already replaced it with another unit, to no avail, the problem still persists.

I'm gonna hook a fuel pressure guage to it and see what's up. as far as I know, the car still has a stock pump in it (not the OE pump, someone already cut an access hole). lol, we've joked saying I'm next in line to receive a walbro 255, seeing as I've went longer than anyone else on the stock replacement pump w/o problems

I'm going to ohm-out all the plug wires and see if there is any excessive resistance in any of them (they are msd's) as well as change the plugs (something I need to do anyway) if nothing else changes, I'll check out the coil to see if it's giving me fits.

nighthawk15

If the fuel pressure is good, I'm gonna put my money on a weak coil.

Mighty Whitey

I hope not... I just put that stupid MSD unit on there a few months ago!

if it is, a coil is MUCH cheaper than a pump.

nighthawk15

Well I just noticed the part where it only happens in the upper gears...very odd. Still could possibly be a coil though I suppose but I don't know why it wouldnt happen in every gear. Much stranger things have happened I suppose.

chris77

man this is the exact same problem my cas is having. have you figured it out yet. i also have the MSD coil. i have a thread (cap and rotor) just a few threads down. i dont think it's a fuel pressure issue i am running a walbro 255

racerns

Sometimes a high rpm miss can be hard to detect in 1st and 2nd since you blow through the miss so fast before you have to shift.

nighthawk15

You make a very good point.

transamws6_97

iTrader: (5)

Same exact thing happens to my car but only happens when it gets warm. Like not regular running temp while driving but if you sit for a minute or so the engine gets hotter thats when i miss. I think it might be icm

chris77

morning bump..i realy need to figure this out

ThoR294

iTrader: (6)

you have to realize, that ONE COIL is trying to fire EIGHT cylinders at 6000+ RPMS. a distributor type ignition was not really built for that. If I was you, I would look into that LS1 coil-over conversion for the LT1s.

Mighty Whitey

I agree with your opinion here, BUT:

I've ran this car for almost 2 years, twisting it beyond 6k and have yet to have a problem like this. not to mention, countless drag cars still use a conventional distributor and single coil setup. it may not be ideal, but I doubt this is totally to blame

I've thought about the LTCC, or the LS1 coil conversion as an option. but both still rely on the rather problematic opti-spark.

I'm about 2 steps away from drilling a hole in the back of the intake and putting a TBI truck distributor in the back of the intake, converting over to a TPI computer and harness, and driving the wheels off the car!
how many people do you hear have problems with their tuned-port or even TBI truck's with re-occuring ignition problems? losing SEFI and going to batch-fire is well worth not being stranded on the side of the road, 400 miles from home when I decide to drive the car to the beach.

nighthawk15

The stock opti can do more than 6k easy. Now up toward 7k, like 6500-6600 you might wanna consider something else. Even at that though there is an aftermarket opti that can do it. Can't think of the name at the moment though. That said the coil-over conversion is definitely not a bad idea. In fact I may be doing it when my opti goes. My point is his opti should have no problem taking what he's giving it.

WhyHelloOfficer

iTrader: (5)

Check the harness connection that goes down to the opti along the passenger side of the intake manifold. Sometimes it can be corroded and cracked, or just old and dirty and that is causing the high rpm miss.

It wouldn't hurt to pull the plugs and take a look at them. A new set of NGKs won't run you more than $20 anyways.

How is the condition of the plug wires? Are you positive they aren't burnt/cracked/worn or touching any heat sources (or arching)? Try putting a little bit of dielectric grease in each of the plug wire boots when you swap the plugs out. It helps to keep moisture out and improve conductivity.

speed_demon24

iTrader: (6)

Post up the dyno graph. What is the cam and valvetrain setup (cam, springs, installed height)? Valve float makes the power look like it drops off a cliff so don't rule that out yet.