Coils

2.8 ohms would be fine unless you are running a four cylinder at 25,000 rpm.

3.5 ohms is fine for our twins.

Lower resistance for the most part only means faster charge time on the coil. Output voltage is not a function of resistance, it's a function of input voltage and the ratio of windings between the primary and secondary windings.

Lower resistance provides quicker charging but it draws the battery down to a lower voltage, the lower voltage to the primary winding causes lower voltage output to the secondary windings.

A 5 ohm coil consumes about 28 watts, a 3.5 ohm about 40 watts. In general I always opt for the highest resistance with that will charge in the appropriate amount of time. Other wise you are just using up more battery power that get's wasted heating up the coils and sending more current to the plugs, which you don't want, you want voltage.

Lower resistance only provides more voltage in a situation where a higher resistance coil doesn't have enough time to fully charge. 2.8 ohm would work, just might use a little more juice. 5 ohm is too high for our twins. 3.5 is just about right, kind like the Three Bears story.
JohnnyB

What lower resistance does is allow the coil to more fully charge than would a higher resistance coil. The net effect CAN be more voltage. But if you took a 5ohm coil and a 3 ohm coil, both with the same winding ratios, and fully charged them the lower resistance coil would send no higher voltage to the plug.
But seeing as very few OEM coils are choosen to be fully charged between each discharge you can cheat by using a lower resistance coil, faster charge time, stronger magnetic field to collapse, more voltage. But there is a limit to that.

Performance coils use a lower resistance to drive a higher winding ratio. To simplify it, if your primary coil has 100 turns and your secondary has 10 turns and you pump 12 volts into the primary you get 120 out of the secondary (or maybe it's the reverse). To boost output voltage you have to increase the number of primary turns, more primary turns takes longer to charge up, so they use a larger wire with less resistance which allows more current to flow into the primary faster.

The higher voltage output of a low resistance coil is not a function of the low resistance, it's a function of the winding ratio. Which REQUIRES a low resistance to provide an adequately low charging time.

However...the reverse is NOT true, a lower resistance coil does not by promise a higher voltage to the plug, unless the coil is designed to produce a higher voltage. That's why coils are usually rated by both primary resistance AND output voltage. You can have a 3 ohm coil that makes 30,000 volts, or a 3 ohm coil that make 50,000 volts. Depends on the winding ratio, wire gauge, core material etc.

But for the most part, you can be pretty sure, like JC suggests, that an aftermarket performance coil with low resistance is going to send more voltage to the plug.

But remember...an OEM coil with no markings that reads low primary resistance has probably just got some shorted windings in it. Which of course show a low resistance but don't do shit for output voltage.

I learned all this by watching Spiderman 2 tonight. It was that good.
JohnnyB