Onwards to nozzles. The injection process uses a pump to put fuel into the motor. The pump spins up as the engine spins, giving more fuel as the engine need it. In fact, the pump provides a little too much fuel, so some is sent back to the tank by the primary bypass. The primary bypass has a “pill” in it, which is just a small piece of brass with a hole in it. You change the size of the pill (bigger or smaller hole) to tune how much fuel the car gets, and how much goes back to the tank.
The pill works in conjunction with the nozzles, which are the squirters that spray fuel into the engine. The nozzles have a fine hole in them too, and can limit flow. Normally you don’t change the nozzles to turn the car – they stay fixed, and you tune the pill instead. Nozzles are sized by how much they flow, with every nozzle manufacturer having a different view. They are often referenced back to the Hilborn scale, which tests nozzles at 30psi. The table below shows the Hilborn sizes (#4a through to #20A) and how much they flow (in both GPM and lb/minute of methanol).
- Hilborn nozzle sizes.png (10.34 KiB) Viewed 690 times
So how big should the nozzles be? In practice, about big enough to give 100psi of fuel pressure.
Take as an example a car running as per the image below. The car is running (imagine the banshee wail), spinning the pump up fast enough to deliver 10gpm. The engine only needs 8gpm, so the pill is selected to make 2gpm flow back to the tank. Because the nozzles are a little big, the fuel pressure only gets to 50psi. It’s not the end of the world, but 50psi doesn’t give great vapourisation out the nozzles.
- Oversized nozzle.png (10.9 KiB) Viewed 690 times
We could swap in smaller nozzles, like the image below. The car is still running at the same revs, the pump still puts out 10gpm, and the engine still only needs 8gpm. The smaller nozzles make it more difficult for the pump to put 2gpm into the engine, so the fuel pressure ends up higher (in this case 100psi). The pill will also be a little smaller too. With 100psi the fuel is better vapourised.
- Correct sized nozzle.png (13.27 KiB) Viewed 690 times
We could go too far though, and put in tiny nozzles like the image below. Fuel pressure comes up to 150psi, making the pump work hard (more load on the gears and drive, more leakage past the seals). The pill would need to be much smaller too, with the tiny pill sizes being more expensive. There is less room to move tuning with tiny pill sizes, making life harder at the track.
- Undersized nozzle.png (14.73 KiB) Viewed 690 times
So how big a nozzle do I need? This is often experienced based. Ken Lowe’s book reckons that an 150-180ci engine (like mine at 155ci) needs the equivalent of a Hilborn #10A nozzle set. I can get other opinions, but for now that will do as a starting point for me.
The injection set came with McGee “18X” nozzles. The nozzles are 90-degree (three left, three right) with a -3AN fuel inlet fitting. The thread that screws into the injector manifold is 1/8-27NPT, and they have two 0.134” external vents for aeration. I had these flow tested, and they run to 0.38lb/min@30psi (22.8pph) and 0.51lb/min@50psi (30.6pph). This is close to a Hilborn #5.75 (not quite a #6), and a little too small for what I am doing (it would drive fuel pressure waaaay over 100psi).
So on to Plan B. The injector set also came with six spare straight nozzles. These have a 0.121” internal bore (massive compared to the 0.018” 18X nozzles), and two external vent holes (0.093” and 0.063”). These don’t have an AN connection, just NPT, and need small inserts to mate them to either AN or SAE hoses (more info on those inserts in my McGee thread:
viewtopic.php?t=23332). I suspect the inserts needed have a very fine hole to allow the nozzles to be controlled. I borrowed an insert from another set I have with 0.034” holes, and got the nozzles to flow 1.37lb/min@30psi (82.2pph) and 1.80lb/min@50psi (108pph). This is close to a Hilborn #20+… waaaay too big for me. It would make fuel pressure way too far below 100psi, and likely would dribble instead of vapourise. To use the Plan B nozzles I would need new inserts made up, with holes smaller than 0.034” and larger than 0.018”. This is fiddly work… it would be easier to source new nozzles.
So I’m on the hunt for some Hilborn 10A nozzles. Hilborn no longer make them, but I can get equivalent Enderle nozzles provided I am willing to sell a kidney to import some. I think I have a lead on some through one of the Repco head gentlemen… will post back here if I am successful.
Cheers,
Harv