60 RPM drop after changing W8BC plugs to W8AC

[sagaboy]

I have a 40 year old 1500 stock vw bug using W8BC Bosch plugs with idling set to 950 RPM and when I change the plugs to W8AC as originally recommended in the manual I notice that the RPM has dropped by 60 RPM

(a) Is the W8BC plug more efficient?
(b) Since the RPM is higher does it mean I will be using less (or more) gas?
(c) Does it also mean that the engine head will be running cooler?

Notes: It’s 2 weeks since changing to W8AC plugs and the color is classic “light grayish/brown” upon removing it for inspection (so was the W8BC before). I am thinking of going back to W8BC plugs. Any suggestions appreciated

 

[Matt_S]

An RPM drop from just the plugs? The only thing I can figure is that maybe the new plugs aren't firing the mixture completely and the engine is producing less power. I'd think you'd certainly be able to feel that power drop, though, and you should have seen fouling on the plugs too.

I think we need some more info. Besides the lower idle, how does the engine run with the new plugs? How is smoothness, power and fuel efficiency compared to the old plugs? Did you do anything else at all? Could you have bumped something and thrown the idle off in another way?

 

[sagaboy]

100% certain that nothing else was touched and during 2 weeks of driving didn't notice anything unusual or loss of power with the W8AC plugs in-fact seem to consume less fuel but not conclusive on just 2week/2 tank full

I can simply change the plugs back to W8BC and report back.

 

[Tosh]

Perhaps a plug wire got damaged or made a bad connection? Wrong sparkplug gap? Which of the two plugs is 'cooler'?

 

[Tree Hugger]

I would suspect a difference in plug gap changing the timing...

 

[ediamiam]

Are the W8AC recessed tips ? Are the tips visually identical? I remember using W5AC that had recessed tips.

 

[1 FMF]

spark plug gap does not affect ignition timing. If you think that, no matter how theoretical you want to get, it's ridiculuos. Guys at iboats talk like that, then guys post questions like why does my motor not run, i'm on my 3rd rebuild in 2 seasons...

how are you determine the 60 rpm drop? That's kinda small. Since it's 40 years old, it's got a carb right? What about ignition, breaker points?
If both are the case, cooler weather less humidity, angle you park the car at could all be possibilities.

but if you can easily swap plugs quick without distrurbing anything else other than spark plug wires, go for it.

bosch numbering scheme:
http://www.boschusa.com/AutoParts/FAQs/SparkPlugs/

W = M14x1.25 thread size and seat, gasket seat?
8 = heat range
*A = reach is 12.7mm (1/2inch) and electrode extends 1mm from base of plug
*B = reach still 12.7mm but electrode extends 3mm from base of plug
C = copper electrode

Looks like the W8BC are projected tip, and projected tip plugs typically are better than non projected tip in every application if there is no contact with the piston. So it looks like the plugs may be the cause, examine the two carefully next to each other, is there a difference in how the center electrode extends out like in the bosch diagram?

 

[mechtech]

Use the projected tip plugs. They put the spark farther into the C.C., and have a slightly wider heat range.

 

[sagaboy]

Upon measuring the W8BC plug the tip gap it turned out to be 0.7 mm whereas the W8AC is 0.6 MM and I was able to use the W8BC plug my bug is using a higher energy coil with electronic contact point.

Since the W8BC plug has a slightly extended (3mm) tip plus 0.7 mm tip gap which improve engine performance as well as let the spark out into the open rather than being within the plug tip make the spark/charge burn faster to a more central position in the chamber that cause better engine detonation which may be the reason why my engine idling speed drop by 60 RPM when I changed from W8BC to W8AC plugs.

OK, I have gone back to W8BC and the engine idling speed went up by 60 RPM as well

 

[sagaboy]

I am using a bosch techdwell meter.

Hey guys, thanks for all the suggestions that allowed me to figure out the problem.

 

[ediamiam]

Yep. Usually the last thing you swap out causes your problem. Unless youse gets ambitious and do multiple changes to get into your dilemna..

 

[andyd]

when I saw the header for this thread, the plugs sounded familiar.

Thats what I always ran in my Bugs. I saw a good looking red 68 Bug the other day. They are rare in these parts now adays. . I miss their simplicity. Keep # 3 cylinder cool and the engine will last a lot longer.

 

[Curious Kid]

Just a thought - I wonder if the W8BC plugs have a greater displacement effect on the chamber's volume, thus slightly bumping-up the compression ratio?