Topic: Subsection: Other:

Let's dive right in to the stock Nikki so we can get familiar with the base jetting that gives us the stock performance.  The Nikki carburetor jets are all drilled in metric measurements. Unfortunately here in The States, we've decided not to join the rest of the entire world in adopting the metric system. I for one would embrace the simplicity of the metric system, but as it stands now we'll need to do a bit of converting to get the stock Nikki jetting into terms most of us are more familiar with. All we're going to concern ourselves with here are the fuel jets and the emulsion tube (air) jets. A more complete chart of North American production Mazda Rx-7 Nikkis can be found here , with the jet sizes also converted to SAE. The primary fuel jets range from .91mm to .95mm, which converts to .036 - .037 inch.  While it is true that jets manufactured in increments of 1/100 millimeter  offer more of a range than jets manufactured in increments of 1/1000 inch, for the purposes of performance tuning the 1/1000 inch jets really are just fine. I doubt any appreciable gains could be had by using "in between" sizes measured in 1/10,000 inch. The chart below shows the main fuel & air jets for all common North American Rx-7 Nikkis. Notice what sizes we're dealing with as a base for jetting: .036, .037 for primary fuel; .063 for secondary fuel; .035, and down to .024 for primary air, and either .055 or .063 for secondary air. Once the conversion from metric to SAE is made for people that are used to dealing with SAE measured jets, the stock Nikki takes on a different light. Immediately obvious is the very large gap between the primary size fuel jet and the secondary. The configuration is good for the stock maximum flow of 313 cfm, but it won't meet the fuel demands of a Nikki modified to flow 465 cfm. There's lots of room for some improvement, but where?   Model Rx-7 Primary Main Fuel Primary Main Air Secondary Main Fuel Secondary Main Air All '79 .037 .035 .063 .055 '80 Man. US .037 .035 .063 .063 '80 Man Canada .037 .035 .063 .055 '80 Auto US .037 .035 .063 .063 '80 Auto Canada .037 .035 .063 .055 '81 - '85 Man .036 .028 .063 .055 '81 - '85 Auto .036 .024 .063 .055

There are a few different flavors of performance tuning. We can jet the modified Nikki moderately in the primary circuit and rich up top. That will give us great performance at WOT, and will conserve fuel when we're cruising along with the primaries just cracked open. We'll be lacking in the acceleration department, but we can compensate for this somewhat by adjusting the accelerator pump. This choice is a fair compromise between fuel conservation and performance and probably the best to use on the street if excessive fuel consumption is a concern.

We could jet the primary circuit rich for great acceleration, instead. this will get us into the secondary stage and into WOT faster, with less bog, too. No need to fatten up that secondary main too much because the primary will be supplying a rich mixture even up top. The draw back is, of course, power at the expense of a noticeable drop in fuel conservation. This is because even when you're just cruising with the primaries only cracked open a bit, you're running fairly rich.

This is the best jetting for track and auto-cross cars, without a doubt. They need that low to mid-range "power on tap" acceleration.

We could just fatten up all of it so it's supplying a rich power mixture all the way through, right up to WOT; - a real fuel pig! But even the drag engine can get drowned by a mixture that's too rich. Fact is, the jetting for a drag Nikki wouldn't be all that radical compared to the ideal configuration for a track Nikki.

 

	Primary Main Fuel	Primary Main Air	Secondary Main Fuel	Secondary Main Air
Economy Tuning	.032 ( smallest )	.027	.052 ( smallest )	.048	Configured for fuel economy. Very little power, and presents the danger of running lean (hot) @ WOT.
Stock	.037	.030	.063	.057 ( largest )	The average of jet sizes for all models. Configured for complete combustion of the fuel (emissions standards).
Street Performance	.048	.043 ( largest )	.065	.052	Configured for high-end performance and low end fuel conservation. Cruising mixture is set for less than 2K RPM.
Track Performance	.055	.027	.063	.045	Configured for full range performance with an emphasis on acceleration. Cruising mixture set above 5K rpm.
Drag Racing performance	.058 ( largest )	.023 ( smallest )	.067 ( largest )	.043 ( smallest )	Configured for power under load at any RPM over 5500. Very rich, and virtually useless for track or street.

In the chart above, I've laid out some very basic configurations to illustrate both the progression of jet sizes as performance increases, as well as the full range of jets we would ever really need for the modified Nikki. Obviously each configuration needs adjustments to suit application, environment, priority and driving habits, but the largest fuel jet (.067) and the smallest air jet (.023) on the chart makes up the full range. No smaller air jet will help, and even the ported 13b rotary is not going to require anything larger than an .067 secondary fuel jet unless the primary fuel jet choice is radically small for some unique application.

I'm going to concentrate most of my discussion on jetting configurations for street performance and track performance since that comprises by far the largest percentage of my customers. I refer to economy tuning and drag race tuning as "special application" tuning, but I consider them no less important in the discussion.

In the Sterling Nikki, the emulsion tubes are modified to receive the same Holley jets that work as Nikki fuel jet replacements. So for the full range of tunability of the Sterling for all applications, you'll need pairs of jets .023 through .067. Jets can start to add up as an expense, so it's best in my opinion to define your application, and then purchase jets according to the chart so that you have pairs that are a size over and under each one suggested.  With the Sterling Nikki, since the Holley fuel jets are interchangeable with the air jets, 8 pairs of jets, carefully chosen, can give you a very good range for fine tuning

Look at the size of the air bleed relative to the fuel jet for each tuning application in the chart. Notice the corresponding air bleed gets even smaller than it's respective fuel jet as performance is increased in each jet configuration. This is because air / fuel ratio needs to be a little richer for performance.

The fuel jet simply limits the amount of fuel that will enter the circuit when the air velocity reaches it's peak. With the same sized air bleeds, an .055 primary main is going to deliver the same air / fuel mixture as an .042 at partial throttle, running at 3000 RPM. This is because at that RPM, the carb is only going to have enough velocity running through it for a certain strength fuel signal. As the velocity increases the subsequent fuel signal increase will soon start to meet the flow capabilities of the .042 jet, and it will start to lean out. If the air jet is too small, the fuel signal would max out the air jet flow capabilities, and the mixture wouldn't lean out as much.

Below we see air / fuel charts from dyno run data demonstrating how small jet changes effect the A/F mixture at RPM increments of 500, going from 2000 RPM to 7500 RPM. We can learn a few important things right off the bat just by looking at the first four runs where there were no jet changes made at all. First and most important is that, even in as controlled an environment as we could get, there's a lot of room for leeway in what the air/fuel gauge hooked to the tail pipe is reading. A/F sensors are inherently finicky, and any sensor clipped to the very end of the exhaust system is going to have a few milliseconds of delayed reading, as well as a less than perfect accuracy of the A/F for any given RPM unless the RPM is sustained for an appreciable amount of time. In my opinion, true A/F readings should be averaged from several track runs. However, for our purposes of simply observing the effects of jet changes relative to A/F mixture, the following chart will suffice.

The second thing we need to realise is that any single slice of data presented here is as useless by itself as a quote from a politician taken out of context. The "whole" of the data must be consumed to fully appreciate how the jet changes effect the A/F mixture, for the reasons given above. As bad as singling out one piece of data from the whole compilation is not taking into account the conditions under which that data was collected.

These dyno runs were done with a prototype manifold, on a stock port with stock ignition, running a Sterling Nikki. The peak rear wheel horse power numbers are just under "flattering", in my opinion, and I think we could have done much better with a few other tweaks, such as advanced ignition timing. The following "mixture data / RPM" was taken during acceleration only. It's easy to see exactly where the accelerator finishes it's travel on the Sterling Nikki under load during these tests, as the mixture is consistently, dramatically effected at 4000 RPM.

In every test run, the mixture begins to richen significantly after 6500 RPM, and by 7500 it's in a steady accelerated decline towards pig-rich. The power begins to decline after 7000 RPMs, as well. This is due to the restriction of the stock manifold, and is unfortunate on so many levels. Obviously, the Nikki, and the Sterling Nikki, of course, would be much better contenders against the likes of the Racing beat Holley and the Weber & Dellorto carburetors if the manifold wasn't such a bottleneck. But even for the purposes of trying to demonstrate jet change results, had the manifold not begun to choke the intake at 6800 RPM, we would have a wider range of changes to work with.  We also were running the fuel pressure too high, but unfortunately didn't have the means to change it at the time. The results demonstrate how important fuel pressure tuning really is. Because of our limited time at the dyno, and the fact that the closest one to me is a 3 hour drive, we couldn't do a full tuning like I would have liked. Chances are, had the fuel pressure been properly tuned on the test car, the A/F ratios would have changed a bit more dramatically, as the emulsion system would have been aerating the fuel better, and the high end mixture wouldn't have been so rich. However, the data still serves as a useful demonstration of what effects jet changes have on the A/F.

As it stands now, one has to disregard the A/F readings at 2000 RPM due to the fact that every run seems to start off with the A/F wildly off course, and we have to pretty much negate the 7000 & 7500 RPM readings due to the manifold breathing limitations.

  Sterling Nikki Dynomometer Air / Fuel Ratio Readings

( prototype ported plenum stock manifold, Racing beat headers, stock timing )

 

Run	Pri. Fuel	Pri. Air	Sec. Fuel	Sec. Air		RW HP		Air - Fuel Mixture / RPM
								2000	2500	3000	3500	4000	4500	5000	5500	6000	6500	7000	7500
1	.053	.028	.063	.043		102.7		12.4	13.1	12.8	12.6	13.5	13.0	13.1	13.3	13.4	13.6	13.2	12.9
2	.053	.028	.063	.043		102.2		13.2	12.6	12.5	12.9	12.8	12.0	13.7	14.0	13.8	13.7	13.5	13.2
3	.053	.028	.063	.043		103.9		13.1	13.0	13.0	12.6	12.7	12.1	12.9	12.8	13.0	13.1	12.8	12.5
4	.053	.028	.063	.043		103.1		13.7	12.7	12.3	12.6	12.1	12.6	12.9	13.0	13.0	12.9	12.6	12.4
5	.053	.028	.065	.043		102.8		19.4	12.8	11.9	12.6	12.2	12.7	12.8	13.0	13.1	13.0	12.7	12.3
6	.055	.028	.063	.043		105.5		13.3	12.5	12.2	12.1	12.0	13.6	13.0	13.3	13.3	13.3	13.2	12.5
7	.055	.028	.063	.043		104.9		13.0	12.3	11.8	12.2	12.7	12.4	13.0	13.2	13.1	13.2	13.1	12.4
8	.055	.023	.063	.043		105.2		12.9	11.9	11.9	12.0	12.0	12.7	12.8	12.9	13.1	13.2	12.8	12.4
9	.055	.023	.063	.036		103.6		12.7	11.7	11.9	11.7	11.2	11.3	12.7	12.3	12.7	12.6	12.4	12.0
10	.055	.023	.063	.036		101.6		12.7	11.6	11.5	11.6	13.1	12.4	12.5	12.4	12.7	12.7	12.4	12.0
11	.055	.023	.063	.049		105.5		12.2	12.1	11.9	11.5	12.0	13.5	13.1	13.3	13.4	13.3	13.0	12.6
12	.055	.036	.063	.049		105.4		13.3	13.2	13.0	12.8	13.0	14.1	13.6	14.0	13.9	13.6	13.3	13.1
13	.055	.043	.063	.049		103.1		14.1	13.7	13.3	13.1	13.0	13.0	14.1	14.3	14.2	14.2	13.8	13.3
14	.055	.028	.063	.049		103.0		12.3	11.8	12.1	12.0	12.1	13.3	13.5	13.5	13.6	13.4	13.1	12.7
15	.055	.028	.063	.049		103.1		13.1	12.4	12.2	12.1	12.2	13.0	13.1	13.6	13.4	13.5	13.2	13.0
Stock	.037	.035	.063	.055

 

Run #1
Pri. Fuel: .053       Pri. Air: .028      Sec. Fuel: .063       Sec Air: .043
7500
7000
6500
6000
5500
5000
4500
4000
3500
3000
2500
2000
RPM
	11					.5					12					.5					13					.5					14					.5					15
Air   to   Fuel    Ratio    ( mass )

 

Run #2
Pri. Fuel: .053       Pri. Air: .028      Sec. Fuel: .063       Sec Air: .043
7500
7000
6500
6000
5500
5000