Bosch Platinum +4 plugs


I figured I would go ahead and change the plugs on the XJ this weekend and decided to try a set of Bosch Platinum +4's on the advice of a very reputable Jag mechanic. They have four ground electrodes which form an open face to the center platinum electrode. The effect is like J-gapping a plug, but four ways so that the spark always has a path.

The idle quality of the Jag is now so smooth you literally cannot feel it running anymore! Previously there was a nice "thrumming" idle, not rough, but definitely an inline six.

No change yet on the trip computer as far as fuel consumption and it's hard to tell a big difference in power, but the plugs really feel like they made a difference.

I'm not sure there's a size to fit the old 200/250, but if there is, they may be worth a try.

I also found these adapters online. This would definitely give you better options for plugs as so few of the newer designs are made in the old 18mm size.
the +4s are good for Normal Aspiration cars. You probably will not notice a difference in fuel economy or power. Iridiums yield a little power increase...but the +4s will not.

:unsure: I have tried the platinum plugs and I found that the stock plug gave me better overall perfomance and milage. Once my car warmed up it was no better than the standard Autolite plug. I was running them in a warmed 302. I don't know if they do make them for my 200's but I wasn't impressed with the plug.
I think the Platinum needs to warm up or something. After the car warmed up and the temp came out of the basement it ran fine. But untill the motor warmed up it was like the timing was off. Of course that's with an eight so it's not relevent on this site but that's my experience.

I'm sceptical of most claims of big improvement from a single item like a plug or filter. But there have been a lot of changes to spark plugs in the last ten years and the Jag is running noticeably nicer than usual (which is to say, very nice).

Platinum really seems the way to go on electronic systems and other high performance environments. The AC ribbed electrode plug, the Bosch plugs, some high-tech versions of Autolite and Champion. I'm sure there may be some incremental advantage to some of those designs, but the old 18mm tapered seat is not widely used so most of those plug designs aren't available.

One thing I've noticed that seems to be consistent is that the electrode size is smaller on the newer design. I suppose that's because it's easier to propogate a spark from a thin electrode.
There is a lot of history and testing behind making a better spark and getting a complete burn.

From what I have read, all of these ignition systems provide only one spark from the center to the side electrode. Even the multiple spark systems produce only one arc of energy at a time, just several arcs’ very fast per cycle. Then there are the different types of alloys the plugs are made from, Platinum ect. The idea behind the softer alloys are that the plug will actually add alloy to the mix during combustion and is supposed to aid in getting a complete burn. This is the idea behind the Champion brand, but this idea wears out the plug quicker.

Remember the US Government award to the company that produced the inline dispenser of platinum directly into the fuel line? This design actually increased mileage and reduced emissions. One problem though, Platinum is expensive and the dispenser needs to be refilled.

I use the Autolite because the alloy used is nice and hard. The edges to the center and side electroid stay sharp longer. The advantages to the multiple side electroids are the additional paths to ground. There are four times the number of sharp edges to ground and the center electrode will now wear evenly all the way around increasing plug life.

There is another cheaper way to get additional ware from your Autolites. Open up the gap so the perpendicular face of the side electrode is on the centerline of the center electrode. The spark will naturally jump from the point of least resistance so the center electrode will ware evenly all the way around. Another trick is to change the face of the side electrode with a grinder to a vee shape this doubles the length of the sharp edge. You can even buy plugs that have a dado type grove down the underside length of the side electrode. This again provides three or 4 times the sharp edge on the side electrode.

There is nothing that replaces the good old tune up. Fortunately with new technology and materials we are able to travel more miles between each one.

My two cents, Ric.
Some people say they can feel a difference (like Jack) and others say they don't feel a thing. I have never tried the +4 but will give them a try. I think choosing a plug brand and type is like choosing oil. Too many choices.
I was told once to leave the gap setting alone on Platinum plugs, that they were set at the factory for the best temp and burn.

Curious, did you gap the plugs or install them as they came?
Every engine reacts differently, but I think that some of the newer technology may work well in the old six. One of the drawbacks of the chamber design is the fact that the plug is so close to the edge of the cylinder and the flame front has to travel all the way across. In newer engines and heads the plugs are located much closer to the center so that the flame front moves faster to the outer edges and gives a more complete burn. Moving spark kernel even slightly could give the little six a better burn.

One technique that seems to benefit some engines is to J-gap the electrode. The ground electrode is cut short and the end is rebent to point directly at the side of the center electrode. That opens the spark directly to the chamber. I think that's the idea behind the +4. Offer the J-gap advantage in a platinum design, but with a more reliable spark propogation using 4 electrodes.

Even with 4 ground electrodes, there will only be one spark. That's why Splitfires are considered snake oil. Splitting the ground electrode doesn't really do anything.
Interesting that you saw such an improvement Jack. I've sold a lot of +4 plugs and sell a lot of the +2 plugs as well. This gives me a little more confidence in questioning whether or not a customer will want a refund after spending over $50 on spark plugs. Of course, they are still cheaper than those AC "Pro-Platinums" GM has been using for the last 5 or 6 years.

The "sharp-edges" bit makes me think of the AC Rapid-Fire plugs. The center electrode is "fluted" like a rifle barrel and tapered to a point. Lot's of sharp edges. J-gapping a set of those might make a nice combination.

I'll have to take a look and see just what plugs are still available for the 18mm tapered seat design. The 300 is the biggest help, being in production so long, and using the same thread depth and heat range as the little six.
Well like I said, every engine is different. The 4.0 Jag AJ engine is a 24 valve pent roof with the coil-on-plug dead in the center of the chamber. It could be that the open center of the +4 is very good for that type of configuration. It could also be that it has no significant effect on a wedge chamber.

I'm just thinking out loud again and think it may be worth a shot to see if it cures any idle misfires or adds power. They are kind of pricey, but you only have to buy six! ;)
One thing that hasn't been mentioned yet about plug positioning is clocking of the side electode. Those of you that have the Dr. Guide have read the procedure.

Note the location of the side electrode and mark the insulator with a perminante marker to referance its position. After the plug is installed make sure the side electorde is to the outer side of the chamber (I try to stay within 15 degree + or -). If it isn't clocked properly try the next plug. I have always been able to get acceptable results without using any washers to make the adjustment. The idea is to rotate the side electrode out of the way giving maximum exposure of the spark to the air/fuel mixture.

Hope this helps, Ric.