Hi,

As per my sig I own a Firestorm and Hellfire. The stock engines come with HPI R4/R5 glow plugs which are short. But I know loads of people are running long plugs, such as the Mccoy MC9 cold plug. I'm looking for a cold plug to run in mine, should I just get the HPI ones?

Is there a problem with running long plugs in place of short ones? I would expect it would increase compression in the engine? I'm looking for a good cold plug to run in both my engines, I'm running 25% Tornado fuel, so any recommendations would be great.

I've used shorty (OS) plugs to help reduce the compression ratio in engines that use long plugs. Havn't done it the other way around. I would check to make sure the long plug doesn't hit the piston on an engine designed for a short plug. Even then, you might have hot spots on the protruding threads of the glowplug and that could cause running/tuning issues.

By the way, a lot of people recommend the MC8 for 20% nitro and the MC9 for 30% nitro (in big block engines). I can't say if this is correct or not, but you might want to do a SEARCH and see what others say.

I think my MC9 fits my HPI 4.6Ho without sticking out into the engine. But I don't know what plug is stock, I don't think HPI do long plugs so I'm looking for a good cold short plug. If a long plug is okay I'll use the MC9 in both.

Are long plugs the most common?

Got my new GO .25 6 Port today. Here I tried out 2 lengths of glow plug.

And a HPI R4 medium cold plug:


A long RB Concepts 6 plug:


You can see the longer RB plug sticks out a lot from the base of the head. Surely this would be a problem? I think I will run with the HPI short plugs with this engine

An easy way of using long plugs in an engine that is not designed for them is just to simply put another sealing washer under the head of the glow plug. This is also a handy idea to reduce compression when first starting a new engine for breakin.

Cheers for the reply. I guess I will just have to do that from now on, double up on washers, and triple up for my break in!

It's best to run the right size plug as the combustion chamber of your engine is a very important piece to the puzzle and you want it to work right as it was designed to. With the right size plug the base of the plug should be even with the head button/combustion chamber. I as well have used the 2 washer trick with long plugs on a short plug application. Works well when in a pinch

run the os range of plugs in the go25,i run os a5 in mine