Puma Race Engines - Calculating Air Demand and Filter Size

Efficient air filtration is an essential part of any good engine installation both to produce the maximum possible horsepower and keep dust and debris out of the engine. To estimate the required filter size for a given engine it's helpful to start by knowing how much air an engine actually uses to produce a given amount of power. This can be done by direct measurement with a flow meter as part of an engine dyno setup but in the absence of that we can deduce a lot from the fuel consumption and air/fuel ratios that engines commonly require.

The Brake Specific Fuel Consumption (BSFC) of an engine is the mass of fuel per horsepower per hour it uses. Traditionally it's been measured in lbs per bhp per hour although those Johny Foreigners are apparently now doing it in grams per kWhr which of course being proper chaps we'll ignore completely and stay resolutely fixed in the previous century. There is a wealth of data on BSFC out there in technical books and on the Intergoogles. Modern petrol engines, regardless of design, tend to operate in a fairly narrow band of BSFC figures so we can take generic data and apply it to most situations.

Petrol engines being air supply throttled operate much less efficiently at low throttle openings when the cylinders are not filling completely. However we are primarily concerned with full throttle operation and peak power so we can ignore that. Efficiency tends to peak around the same revs as peak torque and good engines can see BSFC figures there around 0.42 to 0.43 lbs per hp per hour. As revs rise so do internal frictional losses and efficiency drops i.e. the engine needs more air per flywheel bhp but this is partially offset by the air/fuel ratio richening as the engine approaches peak power rpm. At peak power rpm BSFC figures tend to be between 0.5 and 0.55 and above this they worsen even further as power drops and frictional losses continue to increase. The worst case scenario with race engines that need to go right to the rev limiter all the time would be about 0.6.

At anything below peak power and full throttle opening modern cat equipped engines are constrained to run stoichiometric air/fuel ratio of about 14.7 lbs of air per lb of fuel. At peak power road engines are calibrated for about 13:1 and race engines ideally a bit richer at 12.6:1 for optimum power output. The last thing we need to know is that air weighs about 1 lb per 13.1 cubic feet. We can now start putting the above data together to calculate air and fuel requirements.

At peak torque an engine achieving a BSFC of 0.43 is using 43 lbs of fuel per hour for every 100 horsepower it produces. At the stoichiometric A/F ratio of 14.7 it's therefore using 632 lbs of air per hour, i.e. 10.5 lbs per minute which times 13.1 equates to about 140 cubic feet of air per minute (CFM).

At peak power a road engine achieving a BSFC of 0.53 on an A/F ratio of 13 is therefore using 53 / 60 x 13 x 13.1 = 150 CFM per 100 bhp.

Worst case scenario of a race engine revving well past peak power is 60 / 60 x 12.6 x 13.1 = 165 CFM per 100 bhp.

The last step is to translate air consumption in CFM into required filter size and for high efficiency paper or cotton filter elements it's reckoned that you need about 1 square inch of filter area for every 6 CFM to achieve good filtration without causing a power loss. Foam elements are less efficient so you'll need a somewhat bigger filter for a given power output. For road engines we therefore require about 150 CFM / 6 = 25 square inches of filter area for every 100 bhp the engine produces. For race engines where every last bhp counts you'd want to go about 10% bigger and for severe use in really dusty and dirty conditions like deserts and off-roading it pays to go bigger still, say an extra 50% to be on the safe side. There is of course no downside to using a filter bigger than necessary so if you have the space fit the biggest one you can afford. It'll require cleaning or replacing less frequently as a bonus.

As far as fuel requirement goes we can also use the above to calculate the pump size required. Worst case scenario of an engine achieving a BSFC of 0.6 is 60 lbs of fuel per hour for every 100 bhp which is 1 lb per minute which equates to just over a pint. To be on the safe side and allow for wear and tear on the pump and flow losses in the fuel piping if you fit a pump with a rating of 1.25 to 1.5 pints per minute for every 100 bhp the engine produces you'll be fine. Most OE equipment road car EFI fuel pumps flow about 3 pints per minute so they're good for at least 250 bhp when they're new.

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