So I've been reading about the pulley. What is the science behind this???
"By Design" - the 104 & 107 cams are designed for 104 MOP timing. That's were they run the best.
All Non-Federal / Non-Emissions Carb Turbo 910s
Intake 104 (green) / Exhaust 104 (green)
nominally correct / nominally correct
The cams were re-timed for emissions purposes, first in the USA, later everywhere. The 910, in good condition, will pass the emissions test in most areas in the original, design-correct 104 (green dot) MOP condition. However, Lotus was on the hook for warranty costs (including emissions) for the long term, and wanted a wider margin of safety. Therefore, the engine was designed cleaner that required so that age, wear and lack of maintenance wouldn't push it into a failure condition during the warranty period. The changes included retarding first just the intake cam, and later both cams.
1983-85 USA Emissions Carb 910, and...
All 920 2.0 Turbo
Intake 110 (red) / Exhaust 104 (green)
......... 6° retard / nominally correct
1986-88 USA Emissions Bosch 910, and all...
1989-96 SE, S4, S4s GMP4 910.
Intake 110 (red) / Exhaust 100 (blue)(really 97)
......... 6° retard / ............. 7° retard
The 100 MOP blue dot pulley is really 97 MOP, they just rounded it up when naming it. All other pulley MOP's are accurate as marked.
The following doesn't apply to the Turbo engines, but while I'm at it...
Federal 907 Emissions (C-Cam wants 110 MOP)
Intake 100 (blue, 97) / Exhaust 110 (red)
.......... 13° advance / nominally correct
....... (110 - 97 = 13)
On the 907, advancing the intake increased the overlap by 13°, increasing the amount of co-mingling between the intake and exhaust gases. The net effect is a lot like Exhaust Gas Recirculation, and reduced the oxides of nitrogen.
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The effect of an MOP change is opposite on the two cams.
Intake cam: LARGER MOP retards, and SMALLER MOP advances.
Exhaust cam: SMALLER MOP retards, and LARGER MOP retards.
Advancing the intake and/or retarding the exhaust increases the overlap.
Retarding the intake and/or advancing the exhaust reduces the overlap.
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Converting back to 104 MOP (green dot) pulleys simply puts the cam timing back to were it was originally designed to be. The result doesn't unleash a horsepower monster, but there is some power improvement, and a noticeably increased sense of urgency about the engine. Better throttle response.
Boost covers a multiple of sins, and more boost makes more power ("Boost... it does an engine good"). So once the boost comes in, the difference between green dot and emissions timing becomes less noticeable, as the difference is lost in the greater boost power surge. But the engine will feel more responsive and powerful off-idle and during low boost conditions, like driving in traffic.
You'll also detect a little more cammy idle, and a gruffer voice off-boost... not bad things when you're idling through the drive-in, trying to impress the push-rod V8 boys.
If you want more power for your buck, up the boost (and chip the GMP4 cars). If you just want the engine to run better, and feel more responsive, especially in traffic, the pulley swap is easy, and doesn't put significantly more stress on the engine & drivetrain (like more boost would).
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All Turbo 910/ 920 engines came from the factory with a 104 MOP (green dot) pulley on the Auxiliary shaft (distributor/ oil pump). So, as a freebie (over-looking a little extra labor while you're replacing the timing belt), you can swap the Aux and Intake pulleys, putting the green dot on the Intake.
The intake cam has more impact on the engine's personality and power output than the exhaust cam has. Off boost, atmospheric pressure is all there is to push air/fuel into the engine, and any intake improvement will be felt.
After combustion, pressure in the cylinder is so high that the gas "WILL" flow out when the exhaust valve opens (it's called "Critical Flow", and no minor obstruction is stopping it !).
Yes, changing the exhaust cam timing also makes a difference on the dyno, and if you want the maximum power/ best running engine, then buy another green dot pulley and re-time the exhaust cam as well. But the improvement due to the exhaust cam timing change will be smaller, less noticeable than produced by the intake cam timing change.
If you're on a budget and just want to capitalize on a relative freebie, then simply put the green dot auxiliary pulley on the intake cam. If you're just going to change one, then the intake is the one. Not as good as both, but the price is right.
There is a front and a back to the pulleys, and you have to make certain you install them the correct way around. Don't screw that up!
Regards,
Tim Engel