400ex plug #

[milburn]

hey whats the spark plug part number so i can just pick one up without having to go to the honda dealer?

[Wildbrain]

DPR8Z is the one that I am using

[Pipeless416]

i believe NGK DPR9Z's are stock, but im running an 8Z too because i think it keeps the head a bit cooler.

[GPracer2500]

NGK DPR8Z is stock.

[milburn]

THANKS

[Pipeless416]

Originally posted by GPracer2500
NGK DPR8Z is stock.

so its the opposite? i cant go out to the garage and look while i'm at school. :p

[GPracer2500]

The 8 is the stock plug. The 9 is a colder heat range plug that can be used for engines that need it. There is no performance benefit to running one heat range vs. another. Spark plug heat ranges are about matching the plug's heat transfer characteristics to the engine's heat output so the plug itself stays at the correct temperature. You don't want a plug to operate at too cold a temperature nor do you want it to get too hot.

[Pipeless416]

ah ic. so how would you determine which plug to use? i just read somewhere that mickey dunlap recommends the 9Z during summer uses.

[GPracer2500]

You can test a plug and examine it for indications of how hot it is getting. Doing that isn't exactly straight forward but it can be done so you know for sure what's what. The alternative is to go off of engine modifications and/or riding conditions. Stock compression engines should be happy with the standard plug (DPR8Z) in all conditions other than the most extreme. I'd say "extreme" would include things like flat track racing or continuous sand hill drag racing--things that put extreme heat loads on the engine for extended periods.

If you've got a high compression piston and/or advanced ignition timing then the colder plug (DPR9Z) is likely a more appropriate choice. If your engine is putting out a lot more power than it did stock (and therefore making more heat) then the colder plug probably makes sense.

The danger of using too hot a plug is that it can overheat and fail (not common). Or worse, it can overheat so much it becomes incandescent and causes a pre-ignition condition (even less common).

Running too cold of a plug will make it more prone to fouling and more likely to build up deposits that could hinder spark performance (even if the jetting is correct).

[Pipeless416]

ah ic. ive been running the 9z during the summer because it gets to be about 95 degrees, and then switch to the 8Z during the winter. i dont have any extreme mods and my compression is 10.8.1. thanks for all the help gpracer! its nice to have somebody who really knows what they're talking about.