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Modifying the Ignition Timing on a Jeep Wrangler 2.5L TBI Engine

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I understand that many people get "noticeable" power gain by tuning the ignition timing on the Jeep 2.5L TBI four-cylinder engine. Some relocate the IAT in a cold place (cheap but small power gain) and others drilled a new position for the CPS to get 3-4 degrees variation. 


CPS relocation mod made me realize that timing can be tweaked by editing the signal going from cps to ignition module. I'm not into drilling the engine, so I called the italian AEB to get a controller (to be installed on the CPS line going to the ignition module) to adjust timing and they answered that is definitely doable, but they need a oscilloscope report of the CPS signal to give me a module that fits. I'll let you know if I get any further in this direction.

In the meantime, do you know of any kit/ecu/module/processor designed for this purpose? Googling "adjust timing on yj" mostly returns a useless "you can't adjust timing by turning the distributor".


Happy New Year!



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Hi, Claudio...This is very interesting and not a complicated approach.  The Crankshaft Position Sensor indexes with TDC for #1 piston.  (Actually, this is a Hall-effect type reference; the open notches on the flywheel generate a TDC reference signal: http://en.wikipedia.org/wiki/Hall_effect_sensor.)  Your inline module is a simpler alternative to physically moving the CPS at the back of the engine. 


On Mopar EFI conversions for the 4.2L inline six, the kit includes a crankshaft reference pickup that mounts next to the crankshaft damper (front of crankshaft).  The CPS pickup can be moved, providing room for altering the ignition base timing.  The distributor housing has a fixed position at the engine block, similar to your 2.5L TBI four.  Since these engines each use a distributor for spark delivery, there is obviously a limit to the timing adjustment.  Technically, the distributor housing could be re-referenced or indexed to match the new timing position if necessary. 


Note: You could even create a reference pickup driver for the front damper and make a fully adjustable and independent ignition timing trigger point.  For a 2.5L Jeep 2.5L four-cylinder engine, however, the amount of added timing advance that we're considering would be too insignificant to justify installing an independent trigger system. 


As for the inline CPS signal module you describe, check out the products from MSD Ignitions.  They offer a full line of electronic interface performance products that include triggers and also adjustable timing controls


You may even be able to modify the spark timing advance curve with a reprogrammable aftermarket ECU/ECM, although I advise against doing so.  The factory ECU has software with infinite timing possibilities already programmed by engineers at a test lab.  Making your own ignition "map" is very involved and should be limited to racing engines with distinct loads and calculated throttle positioning.  Optimally, it takes an engine dynamometer and electronic test equipment, including an oscilloscope and four-gas exhaust analyzer, to create a fuel-spark map.  This leaves Jeep 2.5L TBI engine tuners with the stock OEM ECU programming and fiddling with the ignition base timing point and TDC reference point.


With regard to how much performance gain you can create here, I would expect only moderate results.  The reason is simple: Fuel octane dictates spark timing advance.  Timing equals horsepower: the more spark timing advance, the higher horsepower output.  The limiting factor is detonation or spark knock


In an octane lab where gasoline is developed, fuel octane and a fixed spark timing get pitted against the compression ratio.  You're working with two fixed variables on the Jeep 2.5L TBI engine: compression ratio and available fuel.  You can advance the ignition timing only to the extent that the engine will tolerate the additional spark advance. 


If we're assuming that the factory spark timing "curves" are engineered for both emission requirements and fuel limitations, then there's an obvious limit to what kind of base spark timing the engine will tolerate.  Moving the entire spark advance curve upward, which is precisely what repositioning the TDC reference will do, must be consistent with available octane fuel and the engine's compression ratio.  Harmful detonation ("ping" or spark knock) will otherwise be the outcome.


I do have a solution here.  Years ago, in the early 1990s, I played with the J&S Electronics SafeGuard device.  (See that company's website for details:  http://www.jandssafeguard.com/.)  I was among the first automotive journalists to "discover" the merits of this aftermarket solution, and I publicized J&S Electronics in magazine articles, my Q&A columns and even my Bentley Publishers books. 


This ingenious approach relies upon a knock sensor on the engine that listens for spark knock or detonation.  In milliseconds, the ignition timing retards per cylinder to stop the knock. Ultimately, this does not retard all cylinders the same amount.  The aim is to stop the knocking at the culprit cylinder(s).  Each of the other cylinders is allowed to maintain its highest possible spark timing advance—just below the point of detonation or spark knock.  Visualize each cylinder running at its own maximum timing advance without detonation!  This is all governed by the knock sensor signal.


All of this takes place seamlessly.  On older conventional distributor applications (centrifugal and vacuum advance units), when I installed and tested the J&S SafeGuard, the strategy was to manually set the ignition base timing well ahead of the stock position.  The timing could be advanced considerably, as the SafeGuard device retards timing in accordance with detonation per cylinder.  When the knock disappears at a given cylinder, the timing stays at that point.  If you were to look at this on an oscilloscope, each cylinder would have its own timing position.  Whether a conventional distributor or electronic timing maps, the spark timing curve follows the factory programming.


Today, J&S Electronics has applications for OEM fuel-and-spark electronic engine management.  In the case of a 2.5L Jeep TBI ignition with distributor spark, I would set base timing by repositioning the TDC signal as you describe.  You could advance it, say, 12-degrees beyond stock for a false reference signal of 12-degrees BTDC instead of zero degrees or TDC.  The engine management computer would run the engine well above the timing advance norms, and the J&S Electronics SafeGuard would pull back the timing for each cylinder to just below the detonation threshold.  This is the peak horsepower point for any engine.  You could use any practical fuel octane, as the SafeGuard would simply retard timing to compensate for lower octane fuel...and advance timing as the octane increases.


Eventually, the OEM systems on Porsche and Corvette developed strategies similar to the J&S SafeGuard device.  The key to maximum performance here is not simply advancing the spark timing:  The engine has individual spark timing advance per cylinder, and each cylinder operates at peak performance.  Not just advancing the overall spark timing curve, J&S SafeGuard takes engine performance to its peak for each cylinder.


Happy New Year!



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