Spark Plug for twin-plug w/ high ECR+boost

[993Speedster]

Looking for recommendations for spark plugs. Did plenty of searching on the forums and web but no one has my exact setup and goals so I figure the high boost 930 area is the best place to ask.


I'd like to see what you all have to input, but here are the stock replacements as per recommendation from the NGK website based on engine code:

N/A 964 - BCPR7ET
930 - BR8EIX - optional BR8ES
M30 (turbo 964 3.3) - B8ES - optional BR8ES

If anyone is a master at deciphering the letters/meaning and which are of use to us 911 owners and our head designs, that would be appreciated. I understand R = resistor plug, IX = iridium (which I don't want) P = projected tip? (how does that affect anything if at all?) etc...

My setup is a twin-plug 3.6L 964 engine at stock compression (10.8-11ish) that is going to be giving as much boost as I can with the given octane (of which there will be plenty/infinite with both E85 and methanol injection).

If it helps; here are the current power goals for this stage/set of plugs. Power delivery will be limiting boost at spool-peak tq. to 0.5 bar and a rising boost curve as the cam/heads run out of steam; so peak cylinder pressure shouldn't be insane. That being said, I'm also not upset about fouling out a cold plug to start with. Better safe than sorry right? Rule of thumb is 1 step colder for every extra 100hp over stock so that would put me at a 9 or is that just and ice cold plug for what I'm doing?

My brain is saying a combination of the letters and numbers between the above shown plugs + 1 step colder (ie. 1 number higher on NGK and 1 number lower on Bosch)

So..

BPR9ES?

[993Speedster]

Here's an excellent description for all the letters and numbers. At least for NGK that is... as all are different. This is the most inclusive chart/description I could find (but can be a little confusing with how many options there are) courtesy of the NGK website.







Also spent awhile going through Worldpac parts catalogue at work seeing what is listed as potential cross-over's etc and noticed some weird trends; almost anything twin-plug and 3.6L has some sort of multi-ground strap construction. Triple ground strap + projected tip for stock 964 motor and double ground strap fine-wire platinum for 3.6 turbo.

I'm thinking I don't wan't any exotic metals (ie. iridium, platinum, nickel-alloy, etc); regular copper plugs for their heat absorbing ability. So last letter would be an S. The next option is projected tip or not? Old 911's seem to not have it, while 964 and up seem to? I've heard projected tip can be bad on engines that are raced though so IDK here... I know I need resistor plugs though, as pretty much everything with my style of ignition denotes that. And reach (the letter E in this case) is pretty much set in stone based on head/spark plug bore depth.

Based on all my babbling, seems like BPR9ES right?

[Neil Harvey]

The choice of plug is determined by many factors.

If you have a stock engine with normal street use, go with the recommended plugs.

If you have a performance engine, you need to decide upon a plug based on the engines use, level of performance and the type of ignition used.

Let’s start with ignition type. If you use an Inductive ignition, Igniter and Coil, a fine wire plug is the best as it requires less energy to be transmitted through the firing electrode. A thicker firing electrode requires more energy. Precious metal like Iridium are used as they withstand the shock and erosion better. Don’t rule out these plugs. Many have opinions against these plugs, but most are based on false information and opinion.

Projection is as it states. The firing end is projected into the chamber further, helping combustion. Pro’s for this are better combustion, running a slighter hotter plug temperature if required without changing heat range, better ignitability and throttle response. Cons are, issues with boost pressure causing misfires, and piston clearance. In most cases the electrode is un covered more which allows the plug to run at higher temperatures. This can cause mis fires as the plug can over heat. Air cooled engines are a whole different animal as the heads are cooled by air which can change by road speed ambient temps etc., where water cooled heads typical stay at a consistent temp. “Pulling” the firing end back into the plug’s shell will run the plug cooler regardless of its temperature range number.

Don’t confuse projection with the plugs reach. In the earlier Porsche engines, ¾” is typical for reach.

Most plugs today have resisters fitted. Finding non-resistor plugs is getting harder. Especially in 14.00mm sizes. Resistors can be removed too. Resistors do several things, one of them unfortunately is restricting the energy released at the firing end. In street engines this is not a problem, but in a performance engine it can be.

The best way to find the plug you need to start with its size and reach. 14.00mm x ¾” for example. Then decide if you want a resistor or non-resistor plug. Then the engines performance level, boosted and how high a boost level will be used, or normally aspirated, plus known piston clearance. Having decided on a plug, start with a cold plug and work up from there. You need to know how to read a plug otherwise all of this is useless. I have a write up on my web site about reading plugs.

NGK and possibly other can confuse some with their terminology. NGK show both the heat range and the gap as the same number. For example, BKR7EIX-11. The heat range is 7 and the gap is 1.100 mm.

Unfortunately, reading spark plugs is becoming a lost discipline. In my earlier days it was the only way to understand the engines running condition. Today we have knock sensors and Lambda sensors to tell us the running conditions. But this data can often be skewed, the plug is still the best way to know what is going on inside the chamber.

The plugs heat range adds zero to the engine’s performance. But it can hurt it.

[pkabush]

Nice write up. Thanks for taking the time.

[993Speedster]

So glad I took the time to ponder it over and go through the spark plug shelf at work and compare/contrast for several hours yesterday.

My choice of BPR9ES does not even fit in the head. The "B" by itself has correct REACH and correct thread pitch+diameter (ie. 14mmx1.25) but it has the big size nut/body >>> 13/16" socket does NOT fit in 964/993 twin plug valve covers.

You MUST have the 5/8" size hex nut but also 14mmx1.25 so that means either "BC" or "BK" starting letters - so here lies the issue and a bunch of confusion.

- It seem's impossible to find anything 8 or colder in the correct reach, pitch, and hex-nut size that STILL HAS 5k OHM. Everything that cold is a NON-resistor style plug and I'm about to switch to VEMS engine management. I don't want to cause any extra issues from EMF

- BCR9ES doesn't exist.

- Anything with BCR starting letters doesn't exist for that matter.

- So projected tip then... OK; BCPR >>> anything BCPR stops at 7; BCPR8ES has been discontinued and I'm sad (so are the Skyline owners apparently...)

- So that leads me to BK starting letters... you WON'T find a P for projected tip because all BK are "iso" height which is almost exactly the same as projected tip for what it's worth. Consider BCPR and BKR basically the same... projected tip, 5kOHM resistance, 14mmx1.25 with the 5/8" nut

- BKR6ES-11 I had laying around matched my BCPR7ET (stock OEM plug) in size exactly minus the triple ground strap which I'll be ditching anyhow.

- So on with the search into the "BKR" denominations...



What a goose chase... anyone have a good list of what plugs are ACTUALLY available laid out in a sensible matter? Searching for part numbers based on what I think I want, only to find out that combination doesn't even exist is getting tiring.

[Neil Harvey]

You are asking for an old style plug. Most are now obsolete. Thick electrodes are a thing of the past. Look at a fine wire plug. If you are moving over to an EFI system, the fine wire will be OK. It may even help if you are going to Inductive ignition. As for non resister, I do not think you will have an issue. Your alternator puts out way more RF than the plugs will.

If you want to stay with NGK, you have some options. Mill the 13/16" hex down or machine your plug socket down. The socket would be the easiest. Typically the larger hex plugs will fit with a turned down socket.

Plugs offered today are mostly for street cars and are built for street cars. If you want "colder" old style plugs you generally have to look at race plugs. But expect to pay a lot more.

Try a R5671A-9 but this is a non resister. or try R 7433-9, which has a resister, but be careful as this may run hot in a turbo engine. Without knowing your engines' performance one cannot recommend a plug.

Race plugs are discontinued too so check availability. I cannot think of a recent race engine that still uses a 14.0mm plug. Here is your biggest problem. Your engine is a dinosaur in respect to race engines. Street engines are mostly 10.00mm or 12.0mm fine wire and are warmer than what you are asking for.

Or you could change over to Champion. Try a C59C or YC plug. Best the check if they are still available, as these plugs are discontinued almost every day.


Good luck.

[David]

I think a NGK '9' heat range is too cold unless you're racing. They might also make cold starting difficult. I'm pretty sure I'm running BPR8ES on top and DPR8EA on bottom.

[Raceboy]

9 is way too cold, for 911 air cooled 8 is ok or even 7.

And with inductive systems I exclusively use (most of the time VAG COP) I never had good experience with iridium plugs, best results have been with plain jane regular thick electrode ones (like BP7ES etc).

For my 911 second set of plugs I used motorcycle ones with 7 heat range and 12mm thread.
You would want to find plugs with everything regular but 16mm hex.

[993Speedster]

Originally Posted by Neil Harvey

Try a R5671A-9 but this is a non resister. or try R 7433-9, which has a resister, but be careful as this may run hot in a turbo engine. Without knowing your engines' performance one cannot recommend a plug.

I came across R5671A-10 yesterday in my searching. It fit my plug "size" but was too cold. I didn't recognize the naming protocol for these plugs either so didn't know what to change.

As of today though I notice R5672A is same but with projected tip like stock plug. I can also get R5672A-9 for $1.99 each which is quite nice.

R7433-9 are discontinued, $40 a plug where available, and iridium it seems.

Originally Posted by David

I think a NGK '9' heat range is too cold unless you're racing. They might also make cold starting difficult. I'm pretty sure I'm running BPR8ES on top and DPR8EA on bottom.

Well that's two people saying 9 is too cold no so maybe then...

Originally Posted by Raceboy

9 is way too cold, for 911 air cooled 8 is ok or even 7.

And with inductive systems I exclusively use (most of the time VAG COP) I never had good experience with iridium plugs, best results have been with plain jane regular thick electrode ones (like BP7ES etc).

For my 911 second set of plugs I used motorcycle ones with 7 heat range and 12mm thread.
You would want to find plugs with everything regular but 16mm hex.

I was thinking about asking you what plugs you recommend. But then I figured I've been bugging you enough lately

So non-resistor plugs are OK?

[Neil Harvey]

993 speedster,

Be careful here. I don't subscribe to what some have stated here that 9's are too cold. Maybe they are, but you should always start cold and work up from there. As long as the AFR's are good and idle is ok with a slightly leaner number and the idle is smooth, you should be Ok. The issue could be the ignition system. If the plugs do foul, and you are using Inductive, you will never clean the plugs. Changing over to CDI will not be an issue. CDI will fire a fouled plug, and clean it, as it has way more energy.

[Neil Harvey]

FYI, the plugs heat range has zero effect on starting. The ignition system controls this. Fouled plugs will, but this is a separate issue.

[993Speedster]

That was kind of my original thought process to be honest... start with a “too cold” plug. Also the R5672A-9’s are $1.99 so at $25 for a full set of 12 I can foul them out weekly. And every 75-100hp increase 1 step colder? I definitely plan to add 150-200 ballpark.

I think start with 9’s and read the first set and see how it goes. I’m looking for the first 2-3 threads of the shell to show gray/heat given all other plug vitals look in order?

[Raceboy]

If you have to switch to CDI due to fouled plug then you have either too cold plugs or totally wrong mixture or both. And more often than not it is both and this is done by well known "tuners"..
I tune cars for living for 15 years and I do that in a climate where you have to start cars in temperatures barely above freezing and yet extremely moist and you don't want to start changing plugs early in the race morning.
I have always said that no amount of parts replaces bad tune.

[993Speedster]

I meant to message you about this and wish you a Merry Christmas, but also wanted to leave you alone over the holiday. I see you're on here anyways just like the rest of us.

1.) Do you keep with the projected tip or do you get a non-projected style plug to resist spark blow-out?

2.) I've read that the stock ign. wires and boots have a bout 2-4k ohm resistance on their own without resistor plugs. Is that sufficient EMF suppression for VEMS to operate smoothly? No bluetooth interuption, etc...

3.) Is the 8 heat range what you use on supercharged 993?