Spec Miata head worth the time/effort to swap? - Miata Turbo Forum - Boost cars, acquire cats.

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Old 05-19-2011, 01:35 AM   #1
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Default Spec Miata head worth the time/effort to swap?

I have a 91 with a stock 1.6. I have a turbo set up with DIY MS ready to go on. However I recently aquired a spec miata head that was run for three races several years ago until the car was totaled and the head sat for a while. It was a full rebuild from Rebello so I know it is very high quality, but it is still a spec head. So the question is, will I get enough gains from this head to be worth the effort of changing it out on a non tracked car?
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Old 05-19-2011, 01:38 AM   #2
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If it's a shiny new part, why the hell not?

No one can decide if it's worth it or not except for you

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Old 05-19-2011, 01:41 AM   #3
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Well, technically the dyno can.
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Old 05-19-2011, 01:42 AM   #4
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Ok, well I guess what i'm really asking is what kind of HP / Torque gains will I see by swapping in a Spec head on a stock miata, and will those gains be even larger when the turbo goes on? When building a spec head the components a stronger and the tollerances tighter, but is the flow really that much better if they can't port it?
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Old 05-19-2011, 01:47 AM   #5
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spec head maybe and probably is milled down to legal limit for spec miata, you would get bump in compression
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Old 05-19-2011, 08:29 AM   #6
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Go download a copy of the SCCA GCR and read the rules. When you do you will see just how LITTLE can be done to a Spec Miata engine. When you do you will realize that unless you are trying to stay within the limits of those rules a Spec Miata head is of little performance benefit to anyone other than a Spec Miata racer. They can't port it, they can't change valve angles, they have to use all factory Mazda supplied parts, they can't mill it to increase the compression ratio beyond what the factory spec was (9.4:1 on a 1.6). So what you have is a VERY well built stock head that depending on the condition of your factory head could give you a max hp gain in the 3-5 hp range. If you aren't worried about rules I would treat it like a core, have it ported by someone that knows what they are doing, have the valves back cut or consider +1mm valves, 5 angle valve job, have the valves un-shrouded, polish the combustion chamber and look at aftermarket cams. Then it for sure would be worth it.
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Old 05-19-2011, 01:09 PM   #7
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It also matters if its a legal spec miata head. There are definitely many spec miatas out there with worked heads not getting caught because they only race the regional level.

I'd say it's probably not worth your time/effort unless you know the head is 100%. Also, Rebello's motor quality has decreased significantly since Mike Haag left and started his own shop (http://www.haagperformance.com/). Just noticed I'm in a picture on top, silver car on the inside trying to get by P1 and P2.

Edit: and it seems photos are always changing now that I visit link again.

Last edited by crashnscar; 05-19-2011 at 02:21 PM.
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Old 05-19-2011, 01:13 PM   #8
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Quote:
Originally Posted by crashnscar View Post
It also matters if its a legal spec miata head. There are definitely many spec miatas out there with worked heads not getting caught because they only race the regional level.
I would imagine that would be hard to determine without having the head mapped. Like I said the original owner said it was a rebello head so I would think it was legal.

TNTUBA - I was thinking of going this route, I just don't have the cash for it ATM.

I guess for now I'll skip the head and start with the turbo install...
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Old 05-19-2011, 01:16 PM   #9
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Quote:
Originally Posted by crashnscar View Post
It also matters if its a legal spec miata head. There are definitely many spec miatas out there with worked heads not getting caught because they only race the regional level.
Exactly. From what I understand there are casting differences in the early model years that are better. The early heads on the short nose crank motors were smoother and had less inperfections in the intake/exhaust ports than the later years. Some spec guys do way more than allowed and as long as it is not obvious and they can get it past the tech shed, then they don't care.
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