Fuel System

The fuel delivery system has to be customized for this swap. The reason is that the CRX fuel delivery system is set up as a return-type fuel system while the K series engine is setup for a returnless system. Honda changed over to returnless fuel delivery systems around 2001 and most modern cars use this type. The difference is that the return-type is set up in a loop that provides more fuel to the engine than needed and then a pressure regulator maintains constant pressure at the injectors relative to the manifold pressure via a diaphragm type regulator. In a returnless system the fuel pressure is regulated at the tank so you have constant fuel line pressure and demand is satisfied either by compensating with the injection time via the ECU or, in some systems, adjusting the pump line pressure at the tank.

The change over involves installing a suitable fuel rail at the engine, a pressure regulator in the fuel loop, and either adapting the existing or using an aftermarket fuel filter. Naturally this also involves a bunch of plumbing as well. There are a number of manufacturers to choose from, we wound up going with an AEM fuel rail, AEM universal adjustable fuel pressure regulator, and and AEM high volume fuel filter. If we were to do it again we would probably not use the AEM filter as it was harder to mount and uses a 14X1.5 bottom fitting which is not that common a size. But it is a nice unit with high flow and uses standard NAPA or FRAM replaceable filters. You either fabricate a bunch of hoses using AN type fittings or get someone to do it for you. If you are not familiar with AN, it is a standard developed for fluid systems by the US military and jointly agreed upon by the Army and Navy, hence "AN". The fittings are generally anodized aluminum (in cool colors) and use a 37° flare. The fittings are designed to work with "hard" tubing or flexible hose, usually a rubber (nitrile) hose with a braided stainless steel cover to add strength, and bling. They are not that hard to put together if you are careful, and there are some reasonably priced assembly tools to make it easier (I used the Koul tools unit) but you have to be very careful and check your work and clean the hoses well. The best way to check the hoses you make is to pressure test the entire assembly. If you are not careful about cleaning you have the possibility of crudding up the injectors with perhaps disastrous effects to your engine, so be anal. And finally, you don't want a leaky fuel system, no need to say why.

Another issue with fuel system plumbing is you get conflicting opinions circling around NPT fittings. There are a number of NPT fittings on the fuel rail and the pressure regulator design primarily to fit gauges which generally use NPT fittings. NPT are the tapered threads you commonly see in general plumbing. When you screw an NPT fitting together, the taper causes interference and holds the fitting in place. I am sure everyone remembers trying to figure out how to position a fitting without over or under tightening it. Normally the fitting will leak without some type of sealant, common sealants are teflon tape, plumbers pipe thread sealant, and teflon based sealants such as High Temperature Permatex. Teflon tape is a definite no-no in fuel systems because of the possibility of small particles breaking off and clogging the injectors, very bad. Normal pipe thread sealant is not fuel proof so that is a bad idea. That leaves the teflon pastes but many people say don't use that either (e.g. AEM). Well, you have to use something or it will leak. The solution seems to be some of the specialized thread sealant pastes many of which are approved for aviation applications which are the most stringent. The paste must be suitable for use with fuel. I wound up using the Permatex High Temperature Thread Sealant. It does contain teflon but does not shred like tape. When you apply thread sealant, leave the first turn of thread clean to assure nothing gets in the fuel line. There are other products as well, it is too bad that information on this topic is so random and conflicting.

One of the real bitches we ran into was connecting the Honda fuel line to a standard AN fitting. You can go the route of using the stock filter although you should relocate it away from the header, but it is an ugly solution. The normal CRX fuel hardline has a flare fitting and nut which is a non-standard fitting. So how do you get a standard AN fitting on it. Remember, this is the high pressure side so rubber hose clamps are not a good idea. There are a lot of proposed solutions out there, the one that I used is a special hardline adapter made by Goodridge that converts a straight 8mm steel hardline to a -6AN female. This does mean that you have to cut the flare off the end of the tube, and there not much room to work with, and leave enough tube to thread the adapter on. A good thread on the topic can be found here. Once this is done, you are home free. The return line is on the low pressure side so you can use a conventional hose clamp here. My layout is illustrated below. One thing to keep in mind is to carefully route the fuel lines and clamp them appropriately so they do not rub on anything.


You also need to worry about the emissions of fuel vapor. Not just for environmental reasons but also because you do not want your ride smelling like an open gas can. Remember, the fuel system is a sealed system to prevent evaporation of fuel into the atmosphere. But we all know as gas flows from the tank air needs to replace it, or if the temperature changes the tank needs to vent. So what happens is there is a vent line that runs from the tank to the engine compartment that is connected to a charcoal canister. The bottom of the canister is vented to atmosphere but the charcoal adsorbs the gasoline vapors to prevent them from escaping. But what happens when the charcoal gets saturated? Well, a purge line connected to the manifold has a purge valve which opens periodically and draws air through the canister. The fuel is pulled off the charcoal and into the manifold where it gets burned through the engine.

The original CRX canister had some very complicated valving which is really not necessary, so you can throw it all out. We just sourced a canister from an EK DX and fabricated a mounting bracket (you can see a lot of custom brackets in this fuel setup in the pictures). Brian at Karcepts steered us to this suitable canister, thanks Brian.



Evap canister and vent and purge lines
Fuel setup close up
Fuel setup