Injector selection

A key factor in getting a fuel injected engine to run properly is choosing the right injector. There is a great temptation to go for something huge to ‘future proof’ the installation but life is never that simple. At that time I was running a readily available Bosch injector with an output of about 550cc per minute. There is a very simple equation that basically put, states that for every horsepower your engine makes you need to be able to deliver 0.45 to 0.5 pounds of fuel.

Injector flow rates are usually stated in cc’s per minute so a requirement in lbs/hour is not very useful. It is however fairly straightforward to convert from one unit system to the other.

0.50 lbs/hr is equivalent to 5.41cc/min per horsepower
0.45 lbs/hr is equivalent to 4.87 cc/min per horsepower
0.40 lbs/hr is equivalent to 4.33 cc/min per horsepower

I have given three figures here because the amount of fuel you need is very dependant on the design of the engine, some engines use more fuel than others depending on cylinder head design, camshaft profiles etc. If you are running a turbo you should probably use a figure nearer to 0.6 lb/hr (6.5cc/min per horsepower).

For more information on injector sizing there is a useful worksheet on RC injection’s web site:

I was aiming to build an engine capable of delivering up to 90 hp with a single injector. Using the figure of 5.41cc/min I needed a flow rate of 487.8 cc/min my 550cc injector looked about right.

The ECU has various modes for testing all of its critical functions. One of them allows you to drive the injectors and fuel pump. I had the throttle body off the engine and programmed an injector opening time of 1.5ms (about right for idle). I was expecting a finely atomised spray of fuel from the injector. What I got was a jet that went 6 feet across my workshop before hitting the wall. Bearing in mind the vertical section of my inlet manifold was about 100mm this didn’t seem to be a good thing. Even bearing in mind I had no significant airflow in my test I now understood why so much of the fuel was hitting the manifold wall.

I swapped the injector for a Siemens unit that claimed to have a wider (hence shorter) spray pattern. This improved the idle but it was still not as good as I wanted. One evening I came across an injector that looked like it might offer an improvement. Where most injectors have 1, 4 or 6 holes in their tip this one had at least 12. The flow rate was only 375cc/minute so it looked a bit small for my application but it wasn’t expensive so I bought one to give it a try. Testing the injector showed that it did indeed have the finely atomised spray pattern I was looking for. The difference was instant, finally I had a good idle and great throttle response. Clearly there is more to choosing the right injector than just finding one that can flow enough fuel.

There’s a really important characteristic with all injectors that often seems to get overlooked. It seems to be particularly important on older vehicles due to their design of alternator. An injector is a digital device, it is either open or closed but it takes time to open and to close. This is termed ‘injector dead time’ the really difficult thing is that the dead time varies with the voltage applied to the injector. In modern cars the voltage generated by the alternator is pretty much constant. When the alternator is operating within its optimum range it should produce between 13.8 and 14.0 Volts. In our older vehicles it can drop to as little as 12.5 volts at idle. this might not sound like much but a difference of 1.5 Volts has a significant impact on the injector dead-time. For each of the injectors I’ve tested I’ve used the ECU test mode to characterise them between 8 Volts and 14.5 Volts. Battery voltage should only drop below 10 Volts during cranking but it should never really get anywhere near 8 Volts. At 8 volts dead-time can be as much as 2.5mS, dropping to as little as 0.8 mS at 14.5 volts. It takes three times as long to open the injector at 8 volts as it does at normal operating voltage. Normal opening time for the injector at idle is around 1.1mS if this figure were used when the battery voltage drops below 10 volts (usually during engine cranking) no fuel would come out of the injector at all. To overcome this the ECU measures battery voltage and uses the figures I’ve measured for injector dead-time and adjusts the time it opens the injector for accordingly. Nobody ever said this was going to be easy.

Imagine the situation where you have 4 injectors, one for each cylinder. Each would have a slightly different characteristic of dead-time. Do you take an average or program a different characteristic for each. Is it any wonder that so many cars fitted with aftermarket fuel injection idle at over 1000 RPM.

Bosch 550cc injector

Siemens injector

Bosch 375cc injector

Now I had a good idle I had a system I thought was good enough to take for a rolling road session. The engine I took (installed in the 1965 camper) Had the following specification,

Cylinder heads: standard 35.5mm inlet x 32mm exhaust valves, Valves modified for improved air-flow
Crank: standard 69mm, non-counterweighted
Flywheel: standard
Camshaft: standard
Throttle body: one off prototype fitted with 40mm throttle
Injector Bosch 375cc at 3 Bar fuel pressure
Fuel pressure regulator: Bosch, 3 bar

I took a 3.8 Bar pressure regulator just in case the injector turned out to be too small.

So, very standard engine with just the square edges removed from the inlet and exhaust valves. This session was intended as a baseline for all the engines that would follow.

After a long (and very cold) day of mapping we finally ready to do some power runs. With a throttle opening of 40% we had 48 horsepower, not bad considering this spec of engine should give around 50hp but we weren’t finished yet. By 60% opening we had 63.5 hp but then that was all we got. Opening the throttle to 70, 80, 90 and 100% didn’t give any more power. We were all slightly disappointed but then it was essentially a standard engine, 13 horsepower just for an hours work on the valves and a better induction system was a good result.

What was clear is that a 40mm throttle is too big for this specification of engine. Fitting a throttle that is too large can give the impression of more power as the power tends to come all at once but drivability will definitely be compromised. I’ve driven cars where I constantly have to adjust the throttle to maintain a constant speed as you always seem to have either too much or too little air flow.

On the plus side during a 10 hour day of testing I didn’t need to lift the engine lid once. My setup worked perfectly throughout the session. The only comments I got from the tuners were that my ECU had solved all the problems they had seen on Megasquirt based systems before, and that my injector was probably a bit big. That was a surprise given my flow rate calculations. I wasn’t too concerned as my plan was always to produce an engine with a lot more power.

Driving the vehicle you would never believe it only had 63.5 hp it feels like a lot more. Power is there all the way through the rev range not just when the carburettor hits a sweet spot with the camshaft, and of course it is always using the optimum amount of fuel. The running temperature (cylinder head temperature close to number 1 cylinder) never exceeded 120 degrees Celsius even on the hottest day, occasionally towing a trailer.