Which plugs ? High CR 3.2

[Alan.UK]

Hi all,

I am just about to order some spark plugs for a twin plugged 10.5:1 CR, with 98mm J&E jugs.

I am told to buy the NGK racings plugs but was asked if I wanted the deep or shallow ones.

Does anyone know which ones I need?

Thanks in advance

Alan.UK

[KobaltBlau]

One thing to keep in mind is that stock single plugs are 14mm and usually the second set of plugs on a twin plug is 12mm. Someone correct me if I'm wrong. Other than that I don't know the answer.

[Alan.UK]

Thanks Andy. I will try and find this info out today.


Alan

[emcon5]

Not always. Mine are both the stock size.

FWIW, I use cheapo NGKs, BP7ES.

Tom

[Alan.UK]

Hi Tom,

What set up are you running with the twin plugs?

Thanks

[KobaltBlau]

so Tom yours are both 14mm? or both 12mm?

[emcon5]

Originally posted by Alan.UK
Hi Tom,

What set up are you running with the twin plugs?

Thanks

Rennsport 964 distributor conversion, using an SC trigger and 2 stock Bosch CDIs and coils.

Originally posted by KobaltBlau
so Tom yours are both 14mm? or both 12mm?

Standard size, so 14mm I guess. The only problem I am aware of with this setup is you can't use the stock barrel head nut on one lower stud per cylinder, it gets in the way for the plug wrench. You replace the 6 offending barrel nuts with a stock VW bug head nut (I think it is an M10).

Tom

[KobaltBlau]

Originally posted by emcon5
You replace the 6 offending barrel nuts with a stock VW bug head nut (I think it is an M10).

clever! thanks tom! I was confused because competition engineering's page lists the following machining options:

Modify 911 heads for dual spark plugs (includes drilling lower valve covers) $ 400.00
Weld up original 14mm plug holes and machine for dual 12mm spark plugs $ 750.00

of course there's more than one way to skin a cat...

[Paul Thomas]

I have 14mm top plugs and 12mm bottom plugs in my motor. For the top i use NGK BP6E-Y and DP6EA-9 in the bottom. The bottom plug is one that NGK makes for Honda motorcycles.

Paul

[MotoSook]

OK! Time to wake this thread...

Has anyone had interference issues with the 14mm plugs using ARP head studs? It seems that's the only reason one would want to go with a 12mm plug.

[David]

Using 12mm plugs for both will decrease the combustion chamber volume. I've got ARP studs with 14mm top and 12mm bottom.

[MotoSook]

Sorry, I should have clarified. The machinist said he's always use the larger plug for ease of sourcing of the plug, and for more available heat range. I'm just concerned about the bottom plug w/ regard to interference. But since I'm using ARP head studs, I shouldn't have interference with the barrel nuts (not using them).

[Carrera3.5L]

Originally posted by Souk
Has anyone had interference issues with the 14mm plugs using ARP head studs? It seems that's the only reason one would want to go with a 12mm plug.

No problems here, 14mm WR7DC's top and bottom with ARP studs.

Ralph

[MotoSook]

Excellent Ralph! I'd rather use the stock plug size.

Thanks guys.

BTW, Ralph, you jugs will be loosened up soon for another engine I've been too busy to work on it, but soon...soon.