Upgrading Z31 Fuel Injectors on Your 300ZX Turbo

By: Jason84NA2T

Turbo Z31's come with 259cc/min fuel injectors (at 37psi working pressure) from the factory. There are two styles of fuel injectors to go along with the stock rails used on the VG30ET. The early engines (84-85T and 84-86NA) had top-feed fuel injectors, and the later engines (86-89T and 87-89NA) had dual/side feed style injectors.  These injectors vary in type over the years, but the important thing to consider is the injector rail type. To make it easy I will just say that the side/dual feed injectors cannot be upgraded without locating the top-feed style rails found on 84-85 turbo and 84-86 non-turbo cars. The part numbers are 17521-V5220 for the left fuel rail and 17522-V5220 for the right fuel rail. This will convert your side/dual feed injector (2 hoses to each injector) system to accept the top feed injectors. The truth is you can find replacement side feed injectors that are larger, but you will spend a year looking for them and then probably have to buy them new for $90 each.

The most popular method of upgrade is to use the early (top feed) fuel rails with higher flow top-feed injectors from another car with the same style injector (type 5, hose barb fitting). The most popular replacement injector are those from a 1975 Mercedes Benz 450SEL, among other cars. They are rated at 420cc/min flow rate at 37PSI fuel pressure and are sold as Bosch: 280150036, Standard Ignition: fj118, and Borg-Warner: 57512. You will need to shave the injector connectors down slightly to get the stock connectors to fit; be sure the connectors are held securely to the injector. These injectors will get you to well over 400rwhp before they are maxed out.

Fuel rails can be made from billet fuel rail stock, a reamer, a 6AN tap, AN fittings, a drill press and the appropriate hardware. Obviously, the size of the o-ring on the injectors used will need to match the rails. (pre-made fuel rails  will soon be available from Afterdark Racing).

Fuel injectors rarely go bad. In some, old "junk" injectors just need service to be restored to like-new functionality. There are many places you can have fuel injectors serviced, and typical cost is between $20 and $30 per injector for complete service (clean, replace screen, new o-ring, etc) and flow testing.

Looking for used fuel injectors from other cars is a good way to find larger injectors much cheaper than buying them new. Unfortunately, there are a very few types of larger fuel injectors available that are the hose barb type like the stock top-feed Z31 injectors. There are also many different styles of injector spray metering (pattern and flow control type) including but not limited to:

The annular orifice Bosch pintle type injector is still the most common type, and fortunately most pintle caps are around the same diameter and length. Remember that the spray pattern of the injector was tailored into the design of the intake runner and port. Don't simply assume that using another type of injector is going to give you better (or for that matter worse) results without first taking a close look at the spray pattern vs a stock fuel injector. I select RX-7 injectors for use in many cars because they are almost exactly the same spray pattern and physical dimensions as the stock injectors, but offer much higher flow rates and different impedances depending on what year/model car they came from.

There are many vehicles with the o-ring type of injector that can be used as shown below. DISCLAIMER: This method is not technically a 100% "correct" way to install fuel injectors. I make no claims to this being 100% safe, but it has been working fine on numerous cars in the past and on my car for over two years. Be careful working around gas; do NOT light yourself on fire. I will take no responsibility whatsoever if you screw something up and burn your car to the ground.

The Z31 used hose-barb injectors, and as their nickname insinuates, they are made to be fed directly from a hose. Most other injectors are o-ring type, and are made in many different sizes. Pay attention to the feed type at the top of the picture below. Also, the pintle cap at the bottom/end of the injector can vary in size as well. If the pintle caps on the "new" injectors will not fit your stock seals, you should buy a set of new seals and ream them out slightly using a fine rat-tail file.


Top feed Z31 fuel injectors vs. RX-7 injector (typical import o-ring type)

From left to right: RX-7 o-ring injector, stock top-feed fuel rail hose barb, 3/8" hose, 5/16" hose. Note that 3/8" hose will fit quite snugly on the fuel rail barb if hose clamps are used.

The 5/16 diameter high-pressure fuel injection hose crammed on, use 3/8 hose instead.

Same type of injector with a 3/8 hose installed.

O-ring injector installed on Z31 top-feed fuel rail.


I was asked how you retain the injectors on the lower intake manifold. It's quite simple actually. Because the injectors are the same shape and size, you can swap the stock retainers off of your stock top-feed injectors onto the new injectors being used. Many bosch injectors are the same dimensions, so don't limit yourself to searching for RX-7 injectors. Swapping the stock retainers is as easy as removing the snap-ring keeping them secured on the injector.

Note the size of the injector itself is almost exactly the same.

Replacement injector on right fitted with stock retainer.

Some tuning for larger injectors info.

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