What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Which PMAG automotive spark plug for IO-390?

Gash

Well Known Member
I've been reading conflicting recommendations on which long reach automotive spark plug to get for the IO-390 with PMAG ignition. Is it better to use Denso IKH24 or IKH27? Does it matter? Please share real world experiences and data if available. Thanks.
 
I use the Denso IKH 27. I don't know the difference between the 24 and 27 but as you mentioned, on the IO-390, you must use long reach plugs. 95 hours so far, no issues.
 
Last edited:
I have only used the IKH27 plugs which is what came in the engine from Lycoming and have had no problems. The IKH24 is slightly hotter but have not felt any need so far to experiment and try something other than the 27.
 
Spark plug thread drift

Denso IKH 27 long reach, the correct one
Denso IK 27 is a short reach plug
Sourced the former at sparkplug.com
 
Yep, thanks. Trying to figure out if it's alright to go with the slightly hotter 24 instead of 27 since I've seen both recommended.
 
-27 works for me

I use Denso IKH01-27 in my IO-390 with great performance. I tried another brand a few years ago--NOT doing that again.
 
Here's a website with a chart comparing heat ranges for different automotive spark plug brands.

It's worth noting here that the Denso 24 heat rating is the same as the NGK 8 heat rating (which is what most people are using in their short reach applications, i.e. the NGK BR8ES).

Many here are using the Denso 27 heat rating plugs, which is the same as the NGK 9 heat rating. I'm concerned that these spark plugs may be too cold for normal applications like an RV-14 with an IO-390. By "normal" I mean not racing and not being pushed hard with high CHTs. It seems to me that the Denso IKH24 (same heat rating as the NGK BR8ES) would be a better choice to have less lead fouling. I'm interested in learning others' thoughts here. Thank you.
 
Last edited:
Karl, the selection goal is a plug whose center electrode runs hot enough to burn away carbon, without being hot enough to cause pre-ignition, i.e. ignition of the fuel air mix by glow, rather than spark. The rule is "hot enough, and no more".

After monitoring IKH27s for a while, I would not consider 24s.
 
Last edited:
Thanks Dan, what are you monitoring on your plugs? I saw one of your posts on another thread showing a photo of one of your IKH27 plugs. It looked clean, so that's good. Is there any other data? Have you done a comparison between the IKH27 and any other heat ranges and/or brands?

I keep going back to the fact that the Denso 27 heat range is the same as the NGK 9 heat range. This is colder than the NGK BR8ES plugs (8 heat range) I have run successfully for over 1,500 hours on my last two RVs with IO-375 and IO-360 engines with 8.5-1 compression jugs.

There's also the fact that SDS sells the IKH24 plugs with their electronic ignition for engines with long reach plugs. Therefore I still suspect that the Denso IKH27 plugs actually aren't hot enough for an IO-390 that's not being pushed hard (like racing, etc.) I also suspect that everyone is installing the Denso IKH27 plugs simply because that's what is listed on p. 9 of the PMAG installation manual. But even PMAG writes in their manual that it's just a plug that OTHERS have reported that works. There does not appear to be any test data to support their recommendation. If possible I'd like to get more data to make an informed decision.
 
Last edited:
Thanks Dan, what are you monitoring on your plugs? I saw one of your posts on another thread showing a photo of one of your IKH27 plugs. It looked clean, so that's good. Is there any other data? Have you done a comparison between the IKH27 and any other heat ranges and/or brands?

I keep going back to the fact that the Denso 27 heat range is the same as the NGK 9 heat range. This is colder than the NGK BR8ES plugs (8 heat range) I have run successfully for over 1,500 hours on my last two RVs with IO-375 and IO-360 engines with 8.5-1 compression jugs.

There's also the fact that SDS sells the IKH24 plugs with their electronic ignition for engines with long reach plugs. Therefore I still suspect that the Denso IKH27 plugs actually aren't hot enough for an IO-390 that's not being pushed hard (like racing, etc.) I also suspect that everyone is installing the Denso IKH27 plugs simply because that's what is listed on p. 9 of the PMAG installation manual. But even PMAG writes in their manual that it's just a plug that OTHERS have reported that works. There does not appear to be any test data to support their recommendation. If possible I'd like to get more data to make an informed decision.

First, heat range comparisons between manufacturers are not very accurate. Second, as Dan mentioned, the plug only needs to be hot enough to burn off deposits at all/most operating ranges of any specific engine. Being any hotter provides no benefit and reduces the margin before the onset of pre-ignition. Third, each cylinder head design and CR will have different requirements, relative to plug heat range. Finally, the right plug can only be found by trial and error, usually done by the engine manufacturer. Good engine tuners do this themselves for non-OEM builds.

Dan is a pretty reliable engine guy and I would trust his expeience input. You will only find anecdotal data, as the OEMs don't do testing for this type of plug.

Larry
 
Last edited:
2,100 trouble free hours with -27 for me

2,100 trouble free tach hours with -27 as a go to plug for me. I have run all kinds of conditions with extensive efforts to gain performance with plugs and timing (plugs never made any measurable difference). During that time I have tested several other plugs and all of them did okay too.
 
I use Denso IKH01-27 in my IO-390 with great performance. I tried another brand a few years ago--NOT doing that again.

The Denso IKH01-27 are about three times the cost of the Denso IKH-27, does the additional cost offer that much of a performance difference?
 
Cool plastic cases

I tested 8 of these (Denso IKH01-27) extensively for racing and found no benefit... but they do come in neat individual plastic cases.
 
NGK BR9EIX. Run them since install 10 years ago. Swap them on the annual, probably don?t need to but O?Reillys price is sweet enough !
 
Thanks for the responses guys...especially Christo: your experience and methodology is a confidence builder for sure. I will go with the Denso 27 heat range.
 
Has anybody found the best (cheapest) source to buy Denso IHK27?s? BTW, these are the spark plugs that Thunderbolt provides if you buy a PMAG through them.
 
Impossible to buy

Trying to buy 4 Denso IKH27 plugs to match the orig's that came with the T-bolt 390 now with dual Pmags.
Unbelievable how hard these are to find online.
Anybody have a source that's not Amazon?
 
I bought 8 from sparkplugs.com in April for $9.50 each. I don’t know if they have the absolute best prices but their website/delivery was quick and easy.
 
Back
Top