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MPG for aggressive street port 12A

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Old 08-29-17, 03:33 PM
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MPG for aggressive street port 12A

Considering an aggressive street port on 12A would run with "fat Nikki" carb.


My current stock port with "fat Nikki" gets around 22 MPG on highway.


Would not change exhaust, currently run RB street port system.


Wondering what my gas mileage would be with aggressive street port?
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Old 08-29-17, 08:41 PM
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I'd recommend 74 spec ports. Aggressive streetports are really over-rated.

You MPG is largely contingent on air&fuel in vs power out. A port job can let more air in. More air draws in more fuel. More power is created. That much is obvious. But with bigger ports come less drivability at low RPM, thus requiring more technique aka revs revs and more REVs than a stockport, thus FAR LESS MPG in stop and go driving.

On the highway the MPG should stay nearly the same with the same size carb. It's not like you have to keep your foot into it at normal highways speeds. Follow forum user peejay had some things to say about this very subject and maybe he'll see this and chime in.
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Old 08-30-17, 08:50 AM
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Not sure what the 74 port spec is Jeff, but if I ever build a ported 12A the primaries
would stay stock with some minor cleanup but keeping the interior surface as is to
stimulate flow turbulence and I would port the secondaries fairly aggressively and
then tune the Nikki secondaries to take full advantage of it. I know I'm glossing over
some other tuning issues like AP but that would be my basic plan.

My Fat Nikki gets 24 MPG running @80 MPH consistently. Drops to 17 in the city and
drops a little bit more on mountain runs. If you don't mess with the primaries much
and keep the nikki primary tuning the same you should still see your 22 MPG or close
to it.
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Old 08-30-17, 02:09 PM
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Don't worry yourself about gas mileage if you want to do an aggressive street port in a 12a. I'd guesstimate 20 highway, 15-17 city - somewhere in that range depending on supporting mods and such.

It's all about the SPG (Smiles Per Gallon )!
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Old 08-30-17, 06:05 PM
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Well the semi pp Nikki is getting 24 mpg [email protected] soooo
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Old 09-04-17, 04:44 PM
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Old 09-05-17, 07:39 AM
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I typically get 13 MPG in a mix of highway/around town. 83 GSL, stock Nikki, TFIDFS (tg's link), stock exhaust. Is that really low and something's out of adjustment?
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Old 09-05-17, 04:47 PM
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Originally Posted by Toruki View Post
I typically get 13 MPG in a mix of highway/around town. 83 GSL, stock Nikki, TFIDFS (tg's link), stock exhaust. Is that really low and something's out of adjustment?
Kind of low. Especially for a stock nikki.
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Old 09-05-17, 05:03 PM
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I used to get 24-27 with a stock Nikki and a fairly large street port.

I found that fuel economy improved every time I added power. Key with making the Nikki work is pulling fuel pressure away, NOT making it stupid-rich. It's already stupid-rich from the factory. Just pulling fuel pressure down from 4ish PSI to 1.5-2.5psi makes an enormous difference in feel on the highway. In truth I could never tell what the pressure was, I would just tweak it lower until it started to feel a lot "cleaner" at cruise. And more often than not, it would also make the most WOT power at that level as well.

Lower pressure makes the twin changes of lowering float level in the bowl, and reduces aeration as the fuel goes past the needle and seat. Which is why I always saw a fuel economy gain from getting rid of those damn Facet style rattlebox pumps and going to a Holley type vane pump. Fuel flow was a lot better behaved and this resulted in better fuel "quality" in the bowls.
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Old 09-05-17, 06:41 PM
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Originally Posted by t_g_farrell View Post
Not sure what the 74 port spec is Jeff, but if I ever build a ported 12A the primaries
would stay stock with some minor cleanup but keeping the interior surface as is to
stimulate flow turbulence and I would port the secondaries fairly aggressively and
then tune the Nikki secondaries to take full advantage of it. I know I'm glossing over
some other tuning issues like AP but that would be my basic plan.

My Fat Nikki gets 24 MPG running @80 MPH consistently. Drops to 17 in the city and
drops a little bit more on mountain runs. If you don't mess with the primaries much
and keep the nikki primary tuning the same you should still see your 22 MPG or close
to it.
not quite sure of the port timings, but i do know that the 74 has the tall ports in the center housing.
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Old 09-05-17, 07:32 PM
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How "tall" (in mm, inches, microfurlongs) are the '74 primary ports?

I only ask because I have yet to see a 12A that didn't have very tall ports in the intermediate housing.... certainly a lot taller than any EFI 13B that I've had, and I've had four different series of injected 13B (S3, S4 N/A, S4 Turbo, S5 N/A). I assume all four port engines are the same 13B and 12A, year for year...
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Old 09-05-17, 08:56 PM
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74 spec, as the name implies, came stock on 74-75 engines. Then Mazda made the ports smaller for 76-85.

If you are a numbers guy, I can tell you the port closing timings in degrees, just to give you a concrete idea.

76-85 spec: 40 degrees
74 spec: 50 degrees
mild streetport: 55 degrees
aggressive streetport: 60 degrees

Or if you are a weights and measures guy, I can tell you that a 74 spec port closes 6mm later than a stockport. The mild streetport closes 2.5mm later than 74 spec. The aggressive streetport was 5mm later than 74 spec so that's 11mm total.

The reason I call it an aggressive streetport is because it also opens earlier than our 74 through 85 ports. This can also cause a low RPM or idle issue sometimes.

I hope this shed some light on the subject and corrected any preconceived notions.
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Old 09-05-17, 09:00 PM
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peejay, genuine 74-75 primary ports had short runners and look similar to an injected 13B in runner area/size. Look for a "3B" in raised letters and no nitriding at all. No injector bungs either, obviously.
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Old 09-05-17, 09:08 PM
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Interesting. So it sounds like they were the same height as the GSL-SE intermediate housings... but with a lot more port timing.

It would certainly explain why Racing Beat made the four port 13B Holley manifold with ports that almost exactly matched GSL-SE primaries. I didn't think they did it for us pervs making 4 port 13Bs with 12A end housings on GSL-SE engines
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Old 09-05-17, 11:13 PM
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Exactly. Or you can compare the 1757 intake gasket on Mazdatrix to the N304 one. Like super mario twins, they look the same (at the intermediate ports, not in color as both are green).
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Old 09-06-17, 07:22 AM
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Originally Posted by t_g_farrell View Post
Kind of low. Especially for a stock nikki.
Originally Posted by peejay View Post
I used to get 24-27 with a stock Nikki and a fairly large street port.

I found that fuel economy improved every time I added power.

<SNIP>

Fuel flow was a lot better behaved and this resulted in better fuel "quality" in the bowls.
Thanks guys. Off to the search button.
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