I have some familiarity with CDI ignition systems, but not enough to get through these weeds. The Fiero ignition seems to have a circuit that controls ignition below a certain RPM. Then ECM takes over. The '87 '88 Iron Duke had distributorless ignition, so any system would have to control two coils. Must pluck the tachometer off of one of the coils.. but this might be the easier part.. Tach pickup: cam versus crank, two versus four pulse.. likely will matter too. ECM handles all advance, so should the CDI provide none?
0 value to add CDI on most stock engines. More so 2.5. Many buy the Myth that CDI products, like MSD products, are "better" and waste money for nothing.
Problem is HEI have many myths by people w/ no clue or far worse, push by Accel MSD and others to sell "improvements." Example: Many people here repeatedly just replace Ignition Control Module, complaining all ICM suck, and never even try to fix the part that's kill the ICM. Iffy plug wires, lose/rusty IM screws, iffy coil, bad tach filter, etc, can/will kill the best ICM very fast.
87 and up DIS engines w/ multiple Coils has about same problems. DIS Is Not a CDI system. Bad plugs and/or wires can kill DIS coil(s) and even the ICM.
OK, I buy the hype explanation. And how much spark energy could it take to provide 90HP? I will look harder at the stock system to see if I can uncover faults.
Originally posted by Chris Eddy: OK, I buy the hype explanation. And how much spark energy could it take to provide 90HP? I will look harder at the stock system to see if I can uncover faults.
I will explain some of this. The ICM controls cranking, once the engine is running the ECM provide a 5volt signal to the ICM and then takes over spark control. I used to hate DIS until I totally understood the spark system in the ECM. DIS is SO much better than HEI/CDI, it makes worlds of improvement over old systems. Like ECM vs carbs, carbs suck big time, they have no control over different environments. You want to stick to DIS, I REALLY mean it. Coils have more dwell time, more time to cool off. DIS does have a waste spark but it takes almost no energy to waste spark and takes MOST of the energy to spark under compression. To expand even more the secondary coil has both leads going to the spark plugs, so it has to fire both to work, but since compression take more power to spark more power goes to compression than it does the waste, there is no draw back. it fires harder than a single coil, the computer has more precise control over timing as it takes out all human error, IE timing the distributor, worn distributor, points, gap, etc. I swapped my 3.4 to DIS and it was a world of difference. It pulls to red line, where the stock coil get real weak after 4500RPMs. GM spent millions to develop/perfect this, it does work well, do NOT go backwards in tech. You will regret it. Now when I had DIS on my old 88 coupe the best upgrade was to go low ohm wires, I used Jacobs, but now that they got assimilated by Mr. Gasket Co. go with Taylor wires, the only down fall with the Taylors is you have to get a V8 set to get the 45deg angle for the boots, but then you have an extra set. do not use 90deg boots as they will degrade over time and look through the boot. get the good wires and all your DIS problem will go away.
[This message has been edited by sardonyx247 (edited 06-26-2016).]
Originally posted by sardonyx247: I will explain some of this. The ICM controls cranking, once the engine is running the ECM provide a 5volt signal to the ICM and then takes over spark control. I used to hate DIS until I totally understood the spark system in the ECM. DIS is SO much better than HEI/CDI, it makes worlds of improvement over old systems. Like ECM vs carbs, carbs suck big time, they have no control over different environments. You want to stick to DIS, I REALLY mean it. Coils have more dwell time, more time to cool off. DIS does have a waste spark but it takes almost no energy to waste spark and takes MOST of the energy to spark under compression. To expand even more the secondary coil has both leads going to the spark plugs, so it has to fire both to work, but since compression take more power to spark more power goes to compression than it does the waste, there is no draw back. it fires harder than a single coil, the computer has more precise control over timing as it takes out all human error, IE timing the distributor, worn distributor, points, gap, etc. I swapped my 3.4 to DIS and it was a world of difference. It pulls to red line, where the stock coil get real weak after 4500RPMs. GM spent millions to develop/perfect this, it does work well, do NOT go backwards in tech. You will regret it. Now when I had DIS on my old 88 coupe the best upgrade was to go low ohm wires, I used Jacobs, but now that they got assimilated by Mr. Gasket Co. go with Taylor wires, the only down fall with the Taylors is you have to get a V8 set to get the 45deg angle for the boots, but then you have an extra set. do not use 90deg boots as they will degrade over time and look through the boot. get the good wires and all your DIS problem will go away.
Trying to "upgrade" HEI to DIS is a lot of work for 2.5l duke w/ little or no benefit. Dukes "red line" at 4500-5000RPM but reaching that often can break many of them. Older versions you get the problems they often have. 84-86 are known to have weak rods, heads and head bolts for a start. First version of 84 fire recall even said look for cracks in the block starting in the lifter gallery area.
If you want DIS then better to use a 87 and up DIS engine w/ newer TBI and ECM too. Electrically is almost PnP for 85-86 w/ same trans but have to fix fuel lines, throttle cable, etc. Is about same effort to switch to a OE V6.
Most changes for each year are in my Cave, Duke Quick Ref
You can get bad wires or plugs on DIS regardless of who made them. (This version is called GM DIS2 in some documents. Was an older version called C³I and type 2 use same coils.) DIS coils output are ~2x spark energy vs HEI and will find any weak wire insulation faster vs. OE HEI. DIS doesn't like to outright hates some plugs even tho plug makers say plugs will run on DIS. (AC and Autolite Double Platinum works well.)
One of the biggest misconceptions about ignition systems is the idea that a hotter spark will make more power. That's not true, not unless the ignition system was having trouble igniting the fuel / air mixture. The ignition system needs enough power to reliably ignite the fuel / air mixture. After the mixture ignites, the spark is irrelevant. And it will burn at the same temperature regardless of how hot the spark was.
An engine with high compression and/or forced induction may need a more powerful ignition system to ignite the denser fuel / air mixture. But on a stock Duke, it's completely unnecessary.
Originally posted by Blacktree: One of the biggest misconceptions about ignition systems is the idea that a hotter spark will make more power. That's not true, not unless the ignition system was having trouble igniting the fuel / air mixture. The ignition system needs enough power to reliably ignite the fuel / air mixture. After the mixture ignites, the spark is irrelevant. And it will burn at the same temperature regardless of how hot the spark was.
An engine with high compression and/or forced induction may need a more powerful ignition system to ignite the denser fuel / air mixture. But on a stock Duke, it's completely unnecessary.
Yup. DIS haves more energy to make sure the Real spark is good all the time. Waste-spark eats some power but very little in a good system on a good engine. DIS having extra spark energy helps when engine is burning oil a bit, etc, and does basically nothing to overall engine output.
