Jump to content

Any advantage by switching to a 8 plug head on a Z engine?


Recommended Posts

Would there be any advantages of going from 1 to 2 plugs per cylinder? I remember overhearing a guy with a turbocharged corolla gt and he had switched to an 8 plug head because of more powerful combustion if I heard correctly


I have a Z20E with a turbo conversion, megasquirt and coil on plug conversion and I have wondered if there was something to gain by going to an 8 plug head. 

Link to comment
  • Replies 14
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

The dual plugs would fire large amounts of EGR diluted and hard to light lean mixtures.


An added plus is the need to reduce your timing advance. With two points of ignition the fuel burns sooner. Shorter burn time means lighting it later is possible and still get maximum cylinder pressures just after TDC . This means less time at maximum temps and pressure when NOx is formed. (the real reason Nissan went with it)


Shorter burn time means less time for heat to be transferred to the combustion chamber walls and piston tops and more available for expansion pressure to push the pistons down. This would reduce hot spots and pinging on an already detonation resistant hemi head anyway.





 In N America the early Z20 single or dual plug head is the same as the Z22 heads which were all dual plug. The Z24 heads are the same as the Z22 with these exceptions.... The opening for the mechanical fuel pump is not machined out so no block off plate and the intake ports are squarish with rounded corners rather than round.


All Z20/22/24 combustion chambers are 57cc, all valve the same size and all cams the same.





In '83 when the Z24 came out in the 720 truck a 'Mileage Option' Z20 engine was offered. It has smaller combustion chambers of about 45cc and flat top pistons for 9 to 1 compression (and dual plugs)  Note the small top and bottom quench areas on the W04 casting Mileage Option Z20S head above



Dual plugs would be more detonation resistant and more efficient. On a stock engine the difference would be small. On a turbo high HP engine this would be magnified..

Link to comment

I thought the Z24 head was the better of them? The one that flowed best? 


Over here I've seen few dual plug Z engines if any. And the few D21's that was sold with Z24 where for the most part carbed with mechanical fuel pump in the head. Not like the one you have a picture of.


I found this little study while searching for relevant info, seems there is some advantages to dual plugs, at least with a side mounted spark plug like in a lot of the 2valve heads. http://www.ijsr.net/archive/v2i8/MDIwMTMyNDM=.pdf


I don't have any egr stuff on my engines, not as strict emissions here in europe compared to the us at that time. 

I'm more intrigued in the potential for power, and more so reducing the potential for detonation and reducing combustion heat. Better fuel consumption is also welcomed :P


If you have a 4 plug head, can you drill and tap on the exhaust side for the other set of plugs or do you need the 8 plug head specifically ?

Link to comment


This is my '80 Z20E head, '81s were dual plug. As you can see, and I assume yours is the same, you could with care drill and tap for the extra exhaust side spark plugs.




Z24 head with dual plugs.



Again I only have N American, and Canadian in particular, Z heads. Two Z20E single plugs, Two Z22 dual plug heads from the 720 truck and two Z24 dual plug heads from a 720. All valves are the same size, all cams are the same and other than the intake port shaope they are essentially the same. I cc'd all six twice.... same.




Link to comment

Correct. Both Z20/22 have round intake ports with EFI injector notches. The Z24 s do not. (my picture does because I cut them in)





I see, but everyone says a Z20 head on the Z24 block gives worse performance than the Z24 head on a Z24 block? Might be drifting slightly off topic here now but.


Maybe something is different about the European heads, I can't say. All I know is what I've reported about the three pairs have. The squarish Z24 intake may flow a bit better, but I doubt it's much.

Link to comment

I'm running coil on plugs on my Z20, so I would need four more coils :-P but they would melt, I checked today and they would come dangerously close to the exhaust manifold and the number 1 would interfere with the compressor housing on the turbo.

If I ran ls coils or some motorsport wastespark coils with spark plug wires it could be done.

There's a machine shop not far from me that could probably drill new holes i guess if I wanted to go that route

  • Like 1
Link to comment

As an ignition guy, I don't think you'll gain anything.  Its such a small cylinder that even if you are boosting it up to an equivalent of 16:1 compression, a single spark will do the trick.  If you want more power, increase the flow of the head.  The square port does have a significantly larger cross-section than a round port, so it will flow better.  Dual plugs have nothing to do with a power increase - unless you get to a point where a single spark doesn't have enough time to burn the volume of fuel/air in your cylinder.  Since the chamber is quite small, and only maybe 88mm across, the flame has plenty of time to travel the entire distance before the exhaust valve opens.  Spend your time and money on head porting, or potentially open your plug gap to increase coil output.  Try .050".  

Link to comment

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...

Important Information

By using this site, you agree to our Terms of Use.