1. I'm working towards XXX RWHP. When do I need to upgrade my fuel system?

Somewhere around 350-400 RWHP if you plan to go beyond it or your daily driving setup is around that level.

More specifically, you'll need to start upgrading when your stock fuel injectors are hitting 100% duty and your car can't maintain a proper air/fuel ratio.

Many experts and fuel injection companies recommend keeping injector duty cycles below 85% or so for reliability reasons. However, the stock FD injectors seem to continue to add fuel up into the 90% duty range, and they also seem to have no problem running at 100% duty. There are many people who run their stock injectors to 100% duty regularly and for long periods with no damage to the injectors. So far, I haven't met anyone who damaged his injectors by running them like this.

Part of the reason 100% duty doesn't seem to bother anything may be because it usually only happens at very high RPM. Even if driving hard on a track for extended periods, it would be difficult to stay near 100% duty for long.

I would still recommend upgrading the fuel system if you regularly see over 85% injector duty.


2. I've heard the stock pump "isn't good for XX psi of turbo boost." Is this true?

No. The stock fuel pump is adequate as long as the rest of the fuel system remains stock. The stock fuel pump doesn't flow enough once larger or additional injectors are added, or the fuel pressure is raised intentionally (with an aftermarket pressure regulator).

My tests have shown the stock fuel pump can maintain pressure for up to 15psi of boost. It can probably flow enough for beyond 15psi, but the fuel injectors will normally go static before the fuel pump runs out of flow (and drops fuel pressure). Likewise, the fuel injectors or fuel pressure will need to be raised to maintain the proper a/f ratio before the actual pump limits are found.

The bottom line is with a stock fuel system the injectors will be maxxed out before the fuel pump becomes inadequate.


3. What are the available injector upgrade options?

Stock primary injectors are 550cc/min and secondaries are 850cc/min.
Most popular upgrade options:

 

4. What about additional injectors?

Additional injectors can work, but they aren't ideal for FDs. More later.


5. What are some of the popular fuel pump upgrades and how do they perform?

PLEASE NOTE, RC ENGINEERING NOW PERFORMS A FUEL PUMP FLOW MEASURING SERVICE. IT HAS BEEN FOUND THAT THE NIPPONDENSO COSMO FUEL PUMP AND THE WALBRO GSS-341 FLOW APPROXIMATELY THE SAME (THE COSMO ACTUALLY FLOWS A BIT MORE). IT WAS ALSO FOUND THAT THE COSMO PUMP HAS A MUCH HIGHER CURRENT DRAW THAN THE WALBRO OR BOSCH PUMPS. BECAUSE OF THIS, I SUSPECT THE POOR PERFORMANCE OF THE COSMO PUMP IN RX-7S MIGHT BE EXPLAINED BY INSUFFICIENT STOCK WIRING (WHICH IS SUFFICIENT FOR THE BOSCH AND WALBRO PUMPS).

SO PLEASE DISREGARD THE FOLLOWING COMMENT THAT THE COSMO FLOWS SIGNIFICANTLY LESS THAN THE WALBRO AND BOSCH, BECAUSE IT APPEARS TO BE INCORRECT. I WILL TRY TO UPDATE THIS WEB PAGE AFTER GATHERING MORE INFORMATION FROM RC ENGINEERING FUEL PUMP TESTS.

I APOLOGIZE FOR ANY INCONVENIENCE THIS MAY HAVE CAUSED.


 

6. Are the stock fuel lines sufficient for XXX horsepower?

Yes - there is really no reason to install larger fuel lines from a performance standpoint. The stock fuel lines combined with larger injectors and a good pump (or two) can provide enough fuel for well over 500 RWHP.


7. Briefly, what parts need upgraded in what order?

1) The pump needs upgraded the moment you upgrade something else, so you might as well do it first. Please note that installing an upgraded fuel pump alone provides no performance benefit compared to stock.
2) Larger injectors and/or an adjustable or rising rate fuel pressure regulator.


8. What about people getting more fuel with a rising-rate regulator?

This is very limited and would only be useful to relatively few people. More later.


9. How do I know when my fuel pump needs upgrading?

You won't benefit from an upgraded fuel pump unless it is dropping fuel pressure (usually occurs at very high RPMs and maximum boost).

AF Ratio

Every car is a little different so there are no guarantees that every car will be just as rich, even with the same modifications. That said, I think that most similarly equipped cars can use the same ECU programs. The most likely reason a given program wouldn't work on another similar car is if one of the cars has a problem such as low fuel pressure, clogged injectors, clogged fuel pump, etc. If that is the case, the smart thing to do would be fix the problem, not upgrade things to work around the problem.

I'm pretty sure the stock ECU drives the fuel injectors over 90% at very high RPMS, about 10psi of boost, and in 3rd or 4th gear. I'm in the process of finding proof of this and trying to get a real maximum (100%?). If this turns out to be true, the stock ECU will in fact support far more horsepower than most of us think (around 300 RWHP with stock fuel system, or around 350 RWHP with a little more fuel pressure).

I've tested a few cars and I would like to test even more with a variety of modifications. My conclusion is as long as a the car has nothing wrong with it, there should be plenty of fuel with the stock ECU to support *several* modifications while still keeping a relatively conservative a:f ratio, as long as boost is kept within the fuel cut (or about 10psi). Those who want to run slightly more boost (such as 12-14 psi) could probably get away with using only a Fuel Cut Defender. I'd like to test this too.


Here is what I've found so far:

 

Fuel Injector upgrade

1. Why would I want to use 850cc primary injectors?

Obviously, to increase your fuel capacity. 850cc primary injectors will give 25% more possible fuel delivery than the stock injector configuration.

Why use 850cc's instead of getting your stock 850cc injectors enlarged? Total fuel flow with stock 550cc primaries and 1200cc secondaries is 3500cc/min. Installing 850cc secondaries in the primary rail gives 3400cc/min. The cost to get your pair of 850cc injectors enlarged is between $350-400, and you have to provide the injector cores (can't sell your old 850's). 850cc secondaries are often for sale, usually for < $100 each, and you can sell your own 550cc injectors to help offset the cost. You might even be able to upgrade at no cost at all. I paid $100 for a pair of 850cc secondary injectors with an extra secondary fuel rail and fuel pressure regulator.

If you have a programmable computer capable of handling larger injectors, there should be no difference in drivability between enlarged secondaries and enlarged primaries.


2. What is needed to use 850cc secondary injectors in the primary rail?

I found that some extra o-rings were needed to keep excess fuel from finding its way into the engine.

Click here for a picture of an 850cc injector with an extra "spacer" o-ring to force the original o-ring to rest as close possible to the injector tip.

Click here for a picture of the fuel-safe o-ring not installed.

Another alternative would be to grind/mill/file about 1/8" from the top of the primary injector rail. This would allow the 850cc injectors to rest a little deeper in the rail and make a good seal at the tip o-ring.


3. What about drivability or idle problems?

With a computer that is capable of fuel adjustments, this should be a non-issue.

I have no idle problems whatsoever. It purrs happily! Low speed drivability is also perfect.

At first, I thought it was possible the 850's were just too larger for primary use, that the duty cycles would have to be so low the fuel couldn't be metered accurately. I dialed my fuel back the appropriate, calculated amount before starting the car for the first time with 850cc primaries, and it started right up the first time and has given me no problems with idle or low speed drivability.

This is just my experience, I have heard others complain of 850cc primaries not working well for them. I have some possible explanations of why 850's wouldn't work, but I can't be sure if any of those possibilities have ever happened.


4. The secondary injectors I have look much different than my 550's. Are you sure they will fit?

Obviously, I can't guarantee that all the injectors Mazda used throughout FD production are the same. That said, I've only seen one type of primary and one type of secondary injector from Mazda, and I've have 3 different sets of secondaries in my possession.

The 550's are a little bit longer than the 850's, so the tips of the 850's will be further up into the rail than the 550's. This doesn't seem to be a problem.

The problem that I noticed and was concerned about was the lower (closest to the injector tip) o-rings on the 850's are closer to the top of the injector than they are on the 550's. If this lower o-ring doesn't seal properly, extra fuel will leak around the injector and into the combustion chamber when the fuel system is pressurized. This could lead to hard starts, intermittent rich running, flooding, etc.

I decided there was just enough seat available in the rail for the 850's o-rings, however, it is *critical* for the o-ring to rest as close to the injector tip as possible. On all the 850 cc injectors I had, the lower o-ring could move up and down quite a bit. In the stock secondary fuel rail, this wouldn't be a problem, since the side seal of the o-ring is all that matters. But in the primary injector rail, the seat for the o-ring is limited, and if the o-ring slid up the injector (very likely to happen when the injector is installed), the seal wouldn't be good.

My solution to this problem was to add a fuel-safe o-ring of smaller diameter and thickness behind the stock o-ring in the o-ring seat of the injector (just above the stock o-ring). See pictures under question #2 above. This forces the stock o-ring to stay close to the tip of the injector. I can probably get the exact size and type of o-ring I used if someone is really interested. I just picked them out from my local industrial rubber company (I took the injector with me and told them it needed to be safe for gasoline - I think the term is "BUNA-N").

I pressurized my fuel system with this setup before bolting the stock primary rail onto the block, and no fuel leaked. So far, it is working well, and I have no signs of fuel leaks or flooding.


5. How much of an increase in fuel flow will I get with 850cc primaries?

Stock primary injectors are 550cc and the secondaries are 850cc. The following table shows fuel flow in cc/min for different injector sizes, assuming stock fuel pressure is not altered.
 

6. I have a PMS and want to use 850cc primaries. Do you have any base maps that will help?

Not for piggyback mode. I never could get the standard PMS to work well with large injectors because the transition from primary to primary+secondary didn't fall exactly on the border of a load/RPM cell, and it moved depending on load/RPM, water and air temp.


7. What ECUs will work with 850cc primaries?

Most ECUs that support adjustable maps will get you reasonable drivability. Any standalone ECU (such as the Apex'i Power FC and Haltech E6K) should support a variety of injector sizes and staging methods, and should have no problem with 850cc primaries, provided someone knows how to tune it.

I think that a reprogrammed stock ECU and some piggyback units will not be useful. For instance, using a stock ECU or reprogrammed ECU will cause idle and low load to be SO rich that the car will probably have difficult running at all, much less run WELL.

The PMS with InterACQ for Windows will allow you to get it "almost" right. Currently (6/5/2001), with a couple of hours of effort, I have my car running perfectly at WOT, transition from low load to high load or WOT, and lower RPMs (< ~3500 RPM). I am having some difficulty getting good drivability in the 3500-4500 RPM range with very light load (higher load is okay). This problem stems from the fact that the stock ECU performs the switch from primary-only to primary+secondary injectors at an RPM that falls in the center of a PMS RPM band/cell, and the transition changes depending on water and air temps. The end result is the a/f ratio is either too rich or too lean in a few cells. This might be acceptable to some people but I wouldn't recommend going this route.

I think the PMS with only a keypad would work horribly since it has even less resolution than the Windows InterACQ version.

The only workaround that I know of that should work well with a standard PMS would be to use 850cc primaries and 1200cc or 1300cc secondaries. This would keep the ratio between primary and secondary injectors close to stock. A large subtraction of fuel across the board should be enough to make the car run normally. This method might work even with only a PMS and keypad.