Magic Fingers

Hey guys, I just replaced a 1993 LT1 motor that had spun a rod bearing and have a tech question. The replacement engine is bone stock with 62000 miles, I installed the old motors GM opti-spark-Cap-Rotor and water pump (which have only 500 miles), onto the replacement motor. The only add on is headers, which where on the car with the old motor. The car is at the tuner-dyno shop. They called and said the car is tuned, but won't rev beyond 5500 RPM without dropping off. They told me to replace the opti-spark. I saw the dyno sheet and the baseline pull went beyond 5500, and the tuned pull dropped off at about 5500, with an add-on flashed program. They claim it was because the base line pull was not under load, and the tuned pull was. I find it hard to grasp that the opti-spark distributor fails under load rpm, VS free reving RPM. It sounds to me as though the coil, wires, rev-limiter, (which was said to be raised at time of tune), or possible valve float may be the root cause. Air fuel ratios were good all the way up. They have not provided any definative reason to replace a 500 mile opti-spark, other than, bad opti-spark historys. Does the opti-sparks ability to deliver a signal to the coil truly degrade at higher rpm only under load, if infact it is defective. Thanks, Larry

PLRX

The LT1 has a factory 5,600 RPM limiter. It can be removed with a tune.

When I took Betty Boop to the Mustang Dyno, the tuner told me the engine pulled great to up to 5,300 RPM. The AFR was dead on. Yes, that Vette would rev to 6,000 RPM but, its power band peak is 5,300 RPM

95 LT1: 280 RWHP

bjankuski

Yes it does require more spark energy to fire the plugs at WOT then free reving the engine so an ignition problem can show up at high RPM high load that is not seen at low load high RPM. That being said I do not know if the opti is your problem. I would need to see your dyno curve, I stock LT1 is all done by 5500 RPM and has aready reached its peak power output so the curve will be declining. If the AFR is correct and the curve is smooth and declining in an smooth bell shaped curve, then I would suppect everything is OK and what you are seeing is a normal power decline once you crest the peak of the power curve. If the decline is very severe and not smooth I would suppect some mechanical or electrical problem.

Magic Fingers

The decline is severe. I just don't buy into the opti-spark failing under load. Its delivering a signal to process a spark, not create a spark. So I too believe the issue is down stream of the distributor, being mechanical, coil, or wire related. Thanks, Larry

Nathan Plemons

The factory rev limiter is actually closer to 5800, and it should rev there no problem. My experience is that a dynojet will actually show about 1000 RPMs less than you're actually revving.

However, LT1 valve springs are really notoriously weak. If it has been bounced off the rev limiter a few too many times (say by some shop that was trying to tune a stone stock engine that needed no tuning whatsoever) they could have weakened the springs to the point that you are now getting valve float above 5500 RPM, this would show up as a drop in power.

Let me post a few pics here to illustrate:

Stone stock LT1 dyno. You can't really see it because of the txt but it pulls to an indicated 5600-5700 RPM with no dramatic drop in power.


Next is a dyno graph of the same car with heads / cam. I know for a fact the rev limiter is set to 6500 at this point, but the dyno graph stops at 6400. Notice, however, that after 6000 the graph drops a little? That's valve float due to crappy springs.


Here we go with the same car again. A few more mods but the same valve springs. Notice it again stops at 6400 and has the drop off after 6000.

Here is the same car again, different dyno, different day. Notice it still stops at 6400 and still falls off at 6000.

Here we have the same car with upgraded cylinder heads and much improve valve springs. Rev limiter still set to 6500 although the dyno doesn't show it getting that far, notice the valve float is gone.


And finally here is the same car with 1.7 rockers. This puts more stress on the valve springs, but the springs are good enough to take it. Same story, no valve float but the dyno doesn't show 6500 either.



So what does it all mean? Very likely the shop doing the tuning doesn't know what they are doing and / or they've stressed your springs so much that you're now losing power.

Magic Fingers

Awesome explanation, thanks for the excellent details. Larry

Nathan Plemons

FWIW, load vs no load is probably meaningless. If they dyno'd it "before" it would have had to have been under SOME load otherwise the dyno couldn't take a reading. It would be exceedingly stupid to change the configuration of dyno in the middle of back to back testing.

Certain dynos have the ability to add additional load to simulate real driving conditions. If this is the case, however, they should be able to turn off that feature and dyno it like they did on the first go round. If they are correct, the problem will go away when they go back to "no load."

It sounds like they either have no idea or are attempting to feed you a load of crap so they can charge you a ton of money to do the repairs.

93Rubie

Dumb question, what are they basing this RPM on the Dyno Tach or the actual Tach?

The cluster tachs are somewhat inaccurate or at least can be.

Fuel Cut off on stock LT1 is 5850. Peak HP occurs around 5K. No need for 6K on a stock LT1, your out of your power curve.

Great dyno curves. I LOVE the fact the LT1's torque curve is flat as a table top from around 2k to 5k.

mixalive

Great info