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Brake Specific Fuel Consumption

EFI Tuning Fundamentals

Relevant Module: Fundamental Engine Principles > Air Fuel Ratio > Brake Specific Fuel Consumption

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Discussion and questions related to the course EFI Tuning Fundamentals

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Hi there, Im trying to do some calcualtions but I cant get good numbers. Example:

Fuel Flow In LB/HR: 30 lb/ hr is equal to 306cc inyectors

30 lb/hr / 250hp : 0,12 lb/hr/hp

60 lb/ hr / 450hp : 0,13 lb/hr/hp Im sure Im doing something wrong but I cant figure what is it , I tried to do this a lot of ways different, searching in yt also but I cant .Help please

Hello Luis! I don't understand your question... what calculation do you want to do?

I think the basic mistake is confusing the fuelling of ONE injector and applying that to multiple cylinders, without multiplying it be the number of cylinders?

The second thing is the lbs/hr per hp. It's been a while but the general rule of thumb was ~0.5 lbs/hp/hour. A little less for efficient N/A engines, a bit more for turbo's, and a bit more again for superchargers.

However, as Rodrigo observed, you don't seem to be making your actual question(s) clear?

Below is a good article to read about BSFC


Hi there, I was investigating more in depth about B.S.F.C, and my question wasn"t clear at all I wanted to calculate B.S.F.C of my car, this is a N/A 2.0 DOHC.

It has 230hp and has 350cc inyectors (Gasoline) . The Fórmula says Inyector Fuel Flow in lb/hr / hp so 350cc/ 10,2 : 34,3 lb/hr per cylinder, everything good until here, so 34,3 x 4 cylinders: 137,25 lb/hr now I have to divide this by the hp : 137,25/ 230hp : 0,59 B.S.F.C

It looks a little high, anyways I bought the car with these inyectors, maybe I only need 250cc inyectors, but at the end The thing I wasnt doing was to multiplicate the amount of cylinders.

I have done many examples of different cars with different setups but the Fórmula Andrew showed here to calculate inyectors (hp x B.S.F.C) /(N°Cylinders x Max duty) Should I multiplicate the result x 30% more if Im using E85? Asumming e85 consumes more fuel?

If you just want to calculate fuel injector size for e85 there are plenty online fuel injector size calculators. They already have appropriate bsfc included in the formulas as well as e85 volume factor.


As far as your calculation goes- you took injector fow at 100 percent duty cycle which isn't the case in a production car. 80 percent duty cycle is the maximum you should take. So your 0,59 figure should be 0.47 the most which is not bad at wot for NA engine.

Shota is correct, you need to take the duty cycle into account - 100% is the theoretical power the injectors could support.

You also need to consider the nominal flow rating will be a specific fuel pressure (or more accurately, the delta between rail and manifold pressures), and less pressure will reduce the flow, more will increase it.

Estimation is a wonderful tool when you have not already tuned the engine, and don't already know how much fuel it requires.

This module includes the info you are asking about:


As the other folks have mentioned there are also online calculators, but I think it's a good learning experience to do yourself. If you make it into a spreadsheet you can always go back and do it quickly next time, just like an online calculator, but you'll know yours is always there and working.

I'm not clear if you've tuned the car for gas already or not, but just in case and as general info someone else may find useful...if you've already tuned an engine on gasoline, you can do the following to determine injector sizing needs for E85.

Current injector flow rate at the differential fuel pressure you are operating the engine * maximum injector duty cycle = current max flow required on gas

Let's say it's 350cc/min * 90% max duty cycle = 315cc/min

Some injector manufacturers state flow rates at many pressures, some only state flow at a single pressure. If you're operating your engine at 60 psi differential fuel pressure and looking at injectors which are only rated at 3 bar (43.5 psi), you will have to account for this. For example, if you need 569.5cc/min at 60 psi, an injector rated at 550cc at 3 bar, will actually be able to flow enough at 60 psi differential fuel pressure. Because the relationship between flow and pressure is a bit more complex I won't dive into that here.

To convert 315cc/min usage on one fuel to needs on E85 another, you'd add a percentage based on the stoich point change in the fuel.

Old fuel stoich ratio / new fuel stoich ratio = multiplier

Example if you're on E10 gasoline, switching to 85% ethanol: 14.131 / 9.862 = 1.433

If your local E85 is significantly less than 85% ethanol, you can calculate injector needs as if it's 85% ethanol and have a bit of headroom, or you could use a multiplier that represents the stoich point based on actual ethanol content, but in most cases it makes sense to be prepared in case you get 85% ethanol.

Then using my example value, you'd do 315cc/min * 1.433 = about 450cc/min needed for new fuel, at similar max duty cycle and differential fuel pressure IF you will run the same lambda target at high engine load.

If your lambda targets at high engine load where you have high fuel demand will change, that will also impact your fuel injector requirements. You can also apply correction for that. You can do something similar, old lambda / new lambda, to come up with a multiplier to account for this i.e. 0.9 / 0.8 = 1.125, then 450cc * 1.125 = 506.25cc/min. Your target lambda at WOT will more likely be about the same on gas and E85, so I've just made up some numbers to give an example of how you'd do it IF target lambda will be significantly different.

If your current duty cycle is rather high, you should also add a percentage to your final calculated new injector size to give yourself appropriate headroom. For example if you're at 90% max on the old injectors, want to be at 80% max on the new injectors, you could again do old / new, 90 / 80, and then multiply by the result 1.125, so 506.25cc/min * 1.125 = 569.5cc/min.

When it comes to tuning and parts selection, I love seeing people calculating things and knowing the outcome. It's a great mental exercise, and practically useful in many cases. I do also like to remind people that in some cases a more simple pragmatic approach may make sense. For example, I use OE injectors when they are original to the vehicle, occasionally use OE injectors from another engine if I have access to all the injector characterization data, but in the aftermarket I generally use Injector Dynamics injectors so I will have proper data. I know their smallest flow rate is 1065cc/min (ID1050xds), so I know off the top of my head it's plenty big for your needs and I don't need to do that math, I'd just buy those.

I would just add to Mike's comments that having an approximation figured out beforehand is also very useful if the test results/data is rather different from expected.

Usually the actual test is more accurate, but if you find it's double, or half, what was expected, it's a good idea to just stop and figure out where the discrepancy lies - you may actually have a problem that could cause serious issues.

Another good point Gord.

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