Injector sizing guide.


New Member
Authorized Seller
Aug 21, 2009
Lima, Ohio, United States
Wasnt sure if this had ever been posted here. Sorry if it had.

As the title says, this should be the end all to injector sizing questions. I'll try to explain the math behind it all also so you can try it out yourself. This is only a guide and should not be taken as written in stone. Alot of things can change the outcome such as atmosphere (altitude, humidity, temp) and fuel types. But this WILL allow for some of these changes such as lean spikes and the likes.

First, to explain some of the things involved. I took into account A/F ratios, BSFC, VE, injector duty cycles, and a few other things. I used a common 13:1 A/F ratio, a .5 BSFC (I'll explain this in a minute), 95% VE (this too) and 80% duty cycles for common injector sizes.

Most know what A/F ratios are. It's the amount of air per unit of fuel. So 13 cups of air to 1 cup of fuel so to say. If you richen it up a bit to say 12:1 or 11:1 you might say that it will help but you will actually raise the BSFC makeing less power per unit of fuel.

BSFC (Brake Specific Fuel Consumption) is a bit tricky. It's kind of like HP measurements as it doesn't really fit anywhere but to refrence something. This something is a ratio of fuel consumption to HP produced. Not all fuel is turned into mechanical power (driving the piston down) but turned into heat and "wasted" energy. This ratio shows how much fuel is being used to make mechanical power. Side note, most forced induced motors have a slightly lower BSFC .40-.45 meaning more fuel is turned into more mechanical energy. This also realates to VE and A/F ratio. You really only see this number on a dyno.

VE, or Volumetric Efficeincy, is how much fresh air and fuel actually get into the cylinder on each intake stroke. 2 valve carb engines are around 85%, 4 valve carb - 88-90% 2 valve injection - 90% 4 valve injection - 92-95% and 4 valve force induced (us) are from 95-100%, sometimes 105-110%! This can happen from the sheer volume of air being forced (but will create heat) or, more preferably, intake/exhaust manifold design and valve size and timing.

Injector duty cycles are just how hard/long the injectors work per stroke.

The math behind it. You can skip down if you haven't already lol. Simple algerbra really with some key conversion numbers.

The equations:
Air flow - (((x*A/F*BSFC)*60)/A/F)*VE ***(VE needs to be intverted. 95% would be 1.05, 90% would be 1.10 etc.)***
Fuel - ((x*10.5092)*6)*.8 "x" = injector size 6 = number of cylinders .8 = duty cycle

First is this little equation - x*13*(.5/60) where (x) is the desired HP goal (flywheel HP), 13 is the A/F ratio, and .5/60 is the BSFC converted from hours to minutes. This will give you the flow rate in lbs/min of air.

Next, we convert this to lbs/hr by multiplying by 60 which is 3,250lbs/hr.

Now to take our desired injector and convert it from cc/m (cubic centimeters per min) to lbs/hr. It uses this equation: x/10.5092. 10.5092 is one of those key numbers I was talking about to take cc/m straight to lbs/hr.

So the common 550cc injector flows 59.58lbs/hr. Now we need to multiply this by 6 to show the amount of fuel for the whole engine which is 314lbs/hr. Now to imply our duty cycle or 80% - 251lbs/hr

Next we imply the VE and A/F ratio. x/13*1.05. "x" is the 3250lbs/hr. The reason I multiplied the 1.05 instead of .95 for our 95% VE compensates for the fuel requirements in the charts. This equation gives us the individual unit of air so we can see how much fuel we would need. 3250/13*1.05=262.5lbs/hr

Take the two numbers in red. 251 is the amount of fuel at 80% the 550cc injectors can supply. The 262.5 is the amount of fuel needed to make 500HP. Now to show that 550cc's are not recommended for 500HP. This HP figure is FLYWHEEL HP. Manual transmission rob 5-10% power where autos can take up to 25%!!! If you have a manual, the most you could put down, SAFELY, is around 425-450WHP. Ouch.

Here are some charts. First one is MaxHP by injector size along with required/supplied fuel.

[table][tr][td]Injector Size[/td][td]Max Power[/td][td]required/supplied fuel[/td][/tr][tr][td]550cc[/td][td]475 BHP[/td][td]249/251[/td][/tr][tr][td]650cc[/td][td]560 BHP[/td][td]294/296[/td][/tr][tr][td]680cc[/td][td]590 BHP[/td][td]309/310[/td][/tr][tr][td]750cc[/td][td]650 BHP[/td][td]341/342[/td][/tr][tr][td]780cc[/td][td]675 BHP[/td][td]354/356[/td][/tr][tr][td]1000cc[/td][td]865 BHP[/td][td]454/456[/td][/tr][tr][td]1200cc[/td][td]1040 BHP[/td][td]546/548[/td][/tr][tr][td]1600cc[/td][td]1390 BHP[/td][td]729/730[/td][/tr][/table]

You can see how close some of them are to 80%. The reason they say not to go past this, IMO, is for a lean spike. 80% pretty much guarantees you're ok up to 17:1 A/F. Please don't do that. Heat, things melt, fly apart, boom, people's just not good.

Next is the HP with min and max injectors. Anything under 50% duty I think is a waste soo...
[table][tr][td]Horsepower Level[/td][td]Min Injector Size[/td][td]Max Injector Size[/td][/tr][tr][td]400[/td][td]550[/td][td]680[/td][/tr][tr][td]425[/td][td]550[/td][td]750[/td][/tr][tr][td]450[/td][td]550[/td][td]780[/td][/tr][tr][td]475[/td][td]550[/td][td]780[/td][/tr][tr][td]500[/td][td]650[/td][td]780[/td][/tr][tr][td]525[/td][td]650[/td][td]780[/td][/tr][tr][td]550[/td][td]650[/td][td]1000[/td][/tr][tr][td]575[/td][td]680[/td][td]1000[/td][/tr][tr][td]600[/td][td]750[/td][td]1000[/td][/tr][tr][td]625[/td][td]750[/td][td]1000[/td][/tr][tr][td]650[/td][td]750[/td][td]1000[/td][/tr][tr][td]675[/td][td]780[/td][td]1200[/td][/tr][tr][td]700[/td][td]1000[/td][td]1200[/td][/tr][tr][td]725[/td][td]1000[/td][td]1200[/td][/tr][tr][td]750[/td][td]1000[/td][td]1200[/td][/tr][tr][td]775[/td][td]1000[/td][td]1200[/td][/tr][tr][td]800[/td][td]1000[/td][td]1200[/td][/tr][tr][td]825[/td][td]1000[/td][td]1200[/td][/tr][tr][td]850[/td][td]1000[/td][td]1200[/td][/tr][tr][td]875[/td][td]1200[/td][td]1600[/td][/tr][tr][td]900[/td][td]1200[/td][td]1600[/td][/tr][tr][td]925[/td][td]1200[/td][td]1600[/td][/tr][tr][td]950[/td][td]1200[/td][td]1600[/td][/tr][tr][td]975[/td][td]1200[/td][td]1600[/td][/tr][tr][td]1000[/td][td]1200[/td][td]1600[/td][/tr][tr][td]1025[/td][td]1200[/td][td]1600[/td][/tr][tr][td]1050[/td][td]1600[/td][td]1600[/td][/tr][tr][td]1075[/td][td]1600[/td][td]1600[/td][/tr][tr][td]1100[/td][td]1600[/td][td]1600[/td][/tr][tr][td]1125[/td][td]1600[/td][td]1600[/td][/tr][tr][td]1150[/td][td]1600[/td][td]1600[/td][/tr][tr][td]1175[/td][td]1600[/td][td]1600[/td][/tr][tr][td]1200[/td][td]1600[/td][td]1600[/td][/tr][tr][td]1225[/td][td]1600[/td][td]1600[/td][/tr][tr][td]1250[/td][td]1600[/td][td]1600[/td][/tr][tr][td]1275[/td][td]1600[/td][td]1600[/td][/tr][tr][td]1300[/td][td]1600[/td][td]1600[/td][/tr][tr][td]1325[/td][td]1600[/td][td]1600[/td][/tr][tr][td]1350[/td][td]1600[/td][td]1600[/td][/tr][tr][td]1375[/td][td]1600[/td][td]1600[/td][/tr][tr][td]1400[/td][td]Yeah Right!!![/td][/tr][/table]

Good Luck, Have Fun, Be Safe.

All credit goes to Matt, aka, bassjunkiens5 off of SF.
Last edited by a moderator: