Spec Miata Valve Job

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HDBD
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Spec Miata Valve Job

Post by HDBD » Sat Sep 08, 2018 9:15 am

Can somebody give some information on what was a very effective valve job, intake and exhaust on these heads?
Thanks

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Re: Spec Miata Valve Job

Post by Frankshaft » Sat Sep 08, 2018 11:52 am

Never worked on those, suprised, a head I haven't worked on. But, if I had one come in, I would likely do a 35-45-60, and if I could fit it, a 75 under that. On the exhaust, I would likely do a 35-45, 12 degree radius. I wouldn't try and get cute with it. I know the above works well on a lot of 16 valve heads.

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Re: Spec Miata Valve Job

Post by jed » Sat Sep 08, 2018 11:57 am

When u speak of spec Miata I assume u are talking SCCA. Unless the rules have changed the valve job has to be like
The factory manual states and that is a 45' seat with a flat top cut and a bottom cut of a angle I have forgotten.
Maybe a 70' bottom cut.
When I was doing them I used a single angle 38' seat, topped with a 90' and a 70' bottom to narrow the 38' seat.
I refaced the valves to a 38' angle.
If I rember right the flow bench showed 1.5/2.5 CFM increase per valve.
Exhaust was a factory 45' seat narrowed as per factory spec.

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Re: Spec Miata Valve Job

Post by modok » Sat Sep 08, 2018 6:19 pm

I expect it would be important to know what head does the maita has? And is that the right choice?
The early head IMO it appears the intakes could be unshrouded further, but the factory valve job is pretty darn good.
The later head, much smaller ports, should make just as much hp and more torque.
Glen Urban

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Re: Spec Miata Valve Job

Post by mag2555 » Sun Sep 09, 2018 6:46 am

As posted those valve seat profiles are very well sicenced out!
I would bet that within the rules you may be subject to that just adding a back cut to the Intake and Exh valve will tack on more overall flow then reworking the seats!
Also keep in mind that adding more of a radiused way around the Intake seat / valve by means of more angles does nothing to help any balled up or stream like wet fuel issues , and can even make them!
The Intake side wants sharp defined angles !

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Re: Spec Miata Valve Job

Post by midnightbluS10 » Fri Sep 14, 2018 3:35 am

Seems that spec miata valve jobs are pretty set in stone. They go over everything that is or isn't allowed. Plunge cuts, etc... This article goes over quite a bit. It doesn't give a defined list of specs but when you read it all, they go over a lot of it. Things like
there must be a sharp edge where the valve relief cut meets the chamber. That edge must be present and unmodified


https://nasaspeed.news/tech/engine/flow ... nder-heads


link to rules/specs

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Re: Spec Miata Valve Job

Post by SMITHBERGRACING » Wed Sep 19, 2018 8:41 am

They busted a ton of people cheating a few years ago. They want OEM specific parts used and there is a specific cutter number to be used. I don't have the current info but I'd get in contact with the racing organization.

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Re: Spec Miata Valve Job

Post by mag2555 » Wed Sep 19, 2018 12:33 pm

By far and in general the biggest Exh flow gain where it really counts ( at low lift!) can be had by back cutting the valve itself if it's not already of a pretty good tulip shape , but not too tall in that regard!
In short some flow bench time is needed!

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Re: Spec Miata Valve Job

Post by randy331 » Thu Sep 20, 2018 10:34 am

mag2555 wrote:
Wed Sep 19, 2018 12:33 pm
By far and in general the biggest Exh flow gain where it really counts ( at low lift!)
You need to get a new sorce of information.

Randy

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Re: Spec Miata Valve Job

Post by David Redszus » Thu Sep 20, 2018 11:19 am

jed wrote:
Sat Sep 08, 2018 11:57 am
When u speak of spec Miata I assume u are talking SCCA. Unless the rules have changed the valve job has to be like
The factory manual states and that is a 45' seat with a flat top cut and a bottom cut of a angle I have forgotten.
Maybe a 70' bottom cut.
When I was doing them I used a single angle 38' seat, topped with a 90' and a 70' bottom to narrow the 38' seat.
I refaced the valves to a 38' angle.
If I rember right the flow bench showed 1.5/2.5 CFM increase per valve.
Exhaust was a factory 45' seat narrowed as per factory spec.
Spec Miata rules for SCCA competition are very explicit and limiting.

Valve seat angles permitted are: 30 - 45 - 70 degrees.
The valves themselves cannot be machined in any way.

Recently completed several Miata SCCA engines and think their many rules are often insignificant
and unrelated to actual performance. Yet they will, with great delight, DQ any competitor who mis-steps even
slightly or in error.

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Re: Spec Miata Valve Job

Post by iadr » Sat Sep 22, 2018 12:19 pm

randy331 wrote:
Thu Sep 20, 2018 10:34 am
mag2555 wrote:
Wed Sep 19, 2018 12:33 pm
By far and in general the biggest Exh flow gain where it really counts ( at low lift!)
You need to get a new sorce of information.

Randy
Stock cams.

That would make your implication(s) erroneous. ;)

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Re: Spec Miata Valve Job

Post by midnightbluS10 » Sun Sep 23, 2018 11:56 am

SMITHBERGRACING wrote:
Wed Sep 19, 2018 8:41 am
They busted a ton of people cheating a few years ago. They want OEM specific parts used and there is a specific cutter number to be used. I don't have the current info but I'd get in contact with the racing organization.
Am I on Global Ignore or something? The specs are directly above your post. All 16 pages. Or do you guys just automatically ignore my posts because you believe I, have nothing to add?

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Re: Spec Miata Valve Job

Post by HDBD » Sun Sep 23, 2018 1:30 pm

So hypothetically if valve job was the only allowable change on these heads how would you change the angles and perhaps add radius? Valve back or front cuts?

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Re: Spec Miata Valve Job

Post by modok » Sun Sep 23, 2018 4:15 pm

How about you tell us what year miata engine you have already.
Glen Urban

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