In as far as glow plugs go, I've been using the A5 OS in my .46's except in my evo .46 which I have a horizon hobby plug ( guys at the LHS says that the evo runs best on the horizon plug.
Now, some of the plugs are rated as "hot" and some are "cold". When should you use each type? I also have a fox plug in my K&B .65 because the LHS was out of OS #8's.

Dave...

If your engine is running o.k. with your present setup, don't change a thing. To my knowledge, there's not a chart or list that gives comparative heat ranges of different mfg's glow plugs, although it would be nice. Different engines have different performance,too and a plug that works well in one under a given circumstance may be completely different in another. It's a lot of trial and error, seeing what works best.

A 'hot" plug or "cold" plug is relative; one plug amy be hotter or colder than another. What determines this is how well a glow plug retains heat from one ignition cycle to another. A "hot plug will stay hotter than a "colder" plug, allowing for more reliable ignition (as in a 4-stroke, since the igniton occurs half as often at a given rpm), or earlier ignition, as part of the process is due to catalytic action of methanol and platinum, part due to plug heat, which actually "advances" ignition timing. Higher nitro content will also have this effect, because it lowers the flash point of the fuel, allowing it to ignite at a lower temperature.

You can see how changing things around can increase power, or can cause preignition or detonation, sometimes by doing the same thing in different circumstances. So, unless you are trying to hot rod, or fix a power delivery or an idle problem, if what you have now works, don't fix it.

I believe that you would run a colder plug In a high nitro engine. I also believe they run a colder plug in high RPM set ups. You need to go hotter as the nitro level is dropped or you are running a lower RPM engine. 4 stroke plugs generally are the hottest. If the plugs you are using work fine with your engine/fuel combination then by all means continue to do so. I personally have had good luck with Fox plugs. I like them best so far. OS plugs also have a sterling reputation. I have a few to try some day.

Mark Shuman

If you cant get a hangar 9 plug for your evo try a OS F plug ,which is actually a four stroke plug.It is the same as a hangar 9. We can't get Hangar 9 plugs in Australia so I use the OS F plugs and they work great in my Evo's. I also use these plugs in other engines that require a long reach type plug and have found that transitioning from idle to full revs has improved dramatically. Also as stated earlier by others the nitro content of your fuel is the governing factor to which heat range plug you use. If still in doubt just tell the guys at your lhs what fuel you are using etc and I"m sure they will help you select the right one.Good Luck.

I agree, Phread, a colder plug in a high rpm engine will help prevent pre-ignition because it is able to shed enough heat between combustion cycles; otherwise it would still be glowing hot enough that the fuel would flash spontaneously as soon as the fuel charge entered the cylinder. Likewise going to a colder plug with a higher nitro content would serve to counter the "timing advance" effect of either higher nitro or hotter plug. There was a great article in RCM, or maybe MAN, a couple months ago that talked at length about glow plugs and heat ranges.

A short article on glow plugs and a large chart with the plugs arranged by heat range are in the July 2002 RC Report starting on page 19 with the chart on page 22. There was another good article in one of the other mags this year, but I don't have time to look it up tonight. E-mail me and I may be able to send a copy of the chart to you.
Fred B

I believe MAN (Model Airplane News) ran a two-part article in August and Sept. of last year. I found the information to be of great help. The only thing I would add is that an increase in lubrication would also facilitate using a hotter plug. Have a good one.
XS