Spark Plug Recommendation for Carbed M30B35?

sreams

sreams

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I'm about to put a 1991 M30B35 motor into my '72 3.0CS. In order to keep things simple initially, I'm sticking with the dual Weber DGAVs that were on the 3.0 motor. I'm reusing the distributor from the old motor as well (with adapter nut). Should I use the same resistor plugs that are recommended for the stock 3.0cs motor? Should I go with a colder plug to account for the increase in compression? Thanks ahead for any advice.

Also... I'm noticing that the intake ports on the 3.5 head are about 1/4 inch wider than the ports on the old 3.0 head. Is it a good idea to enlarge the ports on the intake manifolds to match?

-Scott
 
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Carbs and 3.5

Scott,

My recommendation would be to blend the manifold ports to the head ports an inch or so into the manifolds to ease the transition.

On the plugs- concentrate first of the mixture of the carbs by determining the jet sizes. There are a few owners that run carbs on the 3.5 and a search can tell you much about what worked for them. Echappe comes to mind a year or so ago on the webers.

I then would check all my ignition and finally watch the plugs for any fouling after several hundred miles keeping in mind that the chokes and idle have an effect on the plugs that can be misleading.

Good luck and post your jet sizes if you get the chance.
 
As an adjunct to 61Porsche's always sound advice, conventional wisdom is that increased compression may indicate the use of colder spark plugs; however, much depends upon the engine's overall condition, the true compression increase over the baseline, and your intended driving plans. In other words, unless the compression increase is significant and/or the majority of your driving will be on the autostrada, there may be no need to alter plugs from whatever you have been using (assuming your were satisfied with them).

As a minor aside, your original '72 engine was, in all likelihood, not originally equipped with resistor plugs. The practical differences between non-resistor and resistor plugs (5000 ohms) for an engine in good shape, are negligible. Most of the engines, including those with slightly higher compression e.g., 9.5:1, were equipped with the equivalent of Bosch W8DC (formerly W175T30). Bosch has since renumbered its current offerings. If you read through this forum, many contributors seem to favor NGK's BP6ES and/or BP5ES plugs. Obviously, there are as many choices as there are manufacturers.

hth

http://www.ngksparkplugs.com/tech_support/spark_plugs/p2.asp?mode=nml

http://www.ngksparkplugs.com/tech_support/spark_plugs/p4.asp?mode=nml

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A lot of my peers in the biz are refusing to use Bosch plugs...
They come stamped with :
Russia
Brazil
India
on them and have been nothing but trouble.
Outsourced to little factories in emerging countries with zero experience in producing quality products.
NGK is my choice now.
 
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