Gallons Per Minute based on Engine Displacement

[ServoWizard]

Hello,

I'm trying to calculate the GPH of fuel required to maintain a 6.5:1 Air to Fuel ratio. My engine displaces 400 cubic inches, but being a 4 cycle it actually pumps 200 cubic inches per revolution. The engine will operate at a maximum RPM of 8000.

200 * 8000 = 1,600,000 cubic inches pumped per minute.

1 Gallon = 231 cubic inches.

So here I multiply the single unit by the AFR

231 * 6.5 = 1501.5 (6.5 Units)

1501.5 + 231 = 1732.5 (Total Mixure or 6.5 parts Air + 1 part Fuel)

231 / 1732.5 = .1333 (231 represents 13.3% of Total Mixure)

Now using the same logic with the exception that I reduce the total cubic inches down to 1 unit.

1,600,000 / 6.5 = 246153.8462 (1 Unit = 246153.8462)

6.5 Units + 1 Unit = 1846153.8462 (Total Mixure or 6.5 parts Air + 1 part Fuel)

246153.8462 / 1846153.8462 = .1333 (246153.8462 represents 13.3% of Total Mixure))

246153.8462 / 231 = 1065.601 Gallons Per Minute = WRONG!

I'm obviously in over my head. I would appreciate some help.

Thanks,
Ron

 

[patprimmer]

a:f is measured in weight, not volume. Obviously gallons is volume as is CFM or CIM. You need to convert everything to weight.

Regards
Pat
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[ServoWizard]

Pat,

Thank you for your input. I had done some research in an effort to factor by weight, but the weight of 1 cubic foot of air was something like 0.0807 lbs. Air is a rather ambigous gas so I was uncertain as to whether you take it at face value or break it down to oxygen content.

1,600,000 / 1728 = 925.9259 CFM (pumped displacement / 12^3)

925.9259 CFM x .0807 = 74.7222 lbs.

74.7222 lbs. / 6.5 = 11.4957 lbs (1 Unit weighs 11.4957)

11.4957 / 6.6 (weight of 1 gal of methanol alcohol) = 1.7418 GPM

1.7418 GPM * 60 = 104.508 GPH = CLOSE, BUT LEAN

OK, that's a more realist number. One factor that's not included in the calculation is the forced induction and the effect that it would have on the 1,600,000 cubic inches of pumped displacement.

Thanks,
Ron

 

[pontiacjack]

You've assumed a volumetric efficiency of 100%.

 

[WGJ]

Just about to say that, pontiacjack !

Servowizard - you need to read up on volumetric efficiency and, probably, air standard efficiency.
A practical engine will never displace its 'displacement'

Bill

 

[patprimmer]

The terminology is a:f not o:f which kinda makes it pretty obvious.

If your trying to set up an MFI alcohol engine, do the sums at 5.5:1 then add about an extra 10% of fuel to engine, do some short full power bursts with very conservative spark.

Look for heat on the plug and methanol building in the oil.

Remove fuel until you just start to see some cad plating burn of the plug, then increase the distance at WOT progressively until you get the plating burning off about 1 to 2 threads up the plug at the point where it shows the most heat, then bring the spark up until you see heat discolouration to about the turn on the electrode, while still watching the heat on the ring and threads.

If it gains a lot of oil level from methanol in the oil, and shows no heat even on the ring of the plug, take a bunch of fuel out. 5% is a buntch.

Regards
Pat
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[ServoWizard]

To All,

Back in the late 70's I ran a blown alcohol 364ci small block Chevrolet in Pro Comp dragster. My career best was 6.29 @226 mph. I always consider myself to be capable of a good tuneup. Back then I was young and lived for nothing other than drag racing. When it came time to purchase the fuel injection sysytem I just picked up the phone and called Hilborn without any regards to what the cost would be.

http://i843.photobucket.com/albums/zz354/CNC4Cheap/MyDragster.jpg

Today I'm an old man that's looking for a reason to live and money does not flow quite as well as it did when I was younger. For that reason when it comes time to purchase a fuel injection system then I will just go to eBay and buy it piece by piece at a price that I can afford.

My reason for attempting to calculate the required GPM/GPH was for two reasons.

1) There are other sources for pumps and the price is more in line with my budget.

2) I enjoy using my head and hope that by doing so it will extend my ability to use it. I'm not a rocket scientist and do not pretend to be one. That is why I posted in the forum for help.

As for factoring at 5.5:

If I NEW? what I was doing when it comes to calculating the GPM/GPH then I certainly would not choose a pump that was only capable of the calculated GPM/GPH. Naturally l would go over by a 150% to 200% of the calculated GPM/GPH depending on the type of pump. It's all about volume while maintaining the proper delivery pressure.

One of the smaller fuel injection companies started out using a pump that is most commonly used to pump the carbonate water in to foutain soft drinks. The pump is manufactured by PROCON and it is a vane type pump. PROCON pumps have also been used in midget racing. The up side of the vane pump is that they are inexpensive, available in stainless steel, have strong low speed efficiency and don't require a hi speed lean out. The down side is that they tend to falls off at high speed. Hence no hi speed lean out required. The gear pumps are far more expensive but they have a linear flow curve and require a hi speed lean out.

As for the volumetric efficiency (VE):

I stated in my second post that I'm using a supercharger. It is a Don Hampton 8-71 and it will be FORCING air/fuel through a pair of AFR 245cc Eliminator series cylinder heads. I will guarantee you that the engine will displace 200ci+ per revolution. However in a N/A'd configuration with the right camshaft and intake/carburetor combination it would quite possibly achieve 90%+ VE. Whether it's 80% or 120% VE I'm trying to calculate the pump volume for a baseline. I cannot go with 150% to 200% of nothing.

I'm building the engine for my Anglia and it will never be raced. The car weighs 2400 lbs. and I doubt that I will ever push the engine to the extent I would risk burning it down. I just love the crisp sound of a blown alcohol engine with 12:1 compression. If I was a rich man then I would be looking at an EFI setup with an electric fuel pump.

A person that pretends to know everything usually ends up knowing nothing. For that reason I openly admit that this is over my head. However that doesn't mean that I'm not willing and capable of learning. If my calculation is wrong then please work with me to correct it such as patprimmer did.

Thanks,
Ron

 

[patprimmer]

Ron

If it's blown with a roots blower, then you have a positive displacement pump in the blower. The amount of asir the engine consumes depends on how much the blower displaces. If it is on a little engine or poor heads it just builds boost until the boost is enough to flow the mass of air it has displaced. The tune is based on the mass of air the blower sucks in, less any leakage via the rotor clearances.

I am pretty sure an 8:71 displaces 436CI per turn of the blower so you probably need enough fuel for about 90% of that air at ambient temperature and pressure.

Regards
Pat
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[ServoWizard]

Pat,

Another very good point. I had not considered the volume of the blower. I will call Don Hampton and get an exact number. My engine is 400ci and I'm using Air Flow Research 23 degree heads and they will flow 350 CFM @.750 lift.

Thanks,
Ron

 

[pontiacjack]

Comments:

> Flowbench data isn't much help, since it's a measurement of static flow volume at a steady-state pressure differential- probably 28 in.water, which isn't order-of-magnitude close to actual conditions under forced induction. And, of course, pressure values are anything but static in a running engine.

> Your original post stated that you [merely] need to know GPH of a pump. Depending on what the pump will feed (mechanical injection, carburetor, EFI, whatever), a number of pump attributes could be required beyond simply a flow capability [at a particular pressure].

 

[pontiacjack]

Comment I forgot:

> Displacement of the blower, by itself, won't help you- way to many other variables. Even if you had enough data to make use of a compressor map, you're out-of-luck; a Roots blower is not a compressor. (Incidentally- yes, an 8-71 displaces in the ballpark of 436 c.i., don't remember exactly)

 

[patprimmer]

MFI invariably uses a positive displacement gear or gerotor pump engine driven as the fuel pump.

They are mostly, but not always cam driven at 1/2 engine speed and rated at gallons per minute at 4000 pump rpm.

Count on a brake specific fuel consumption of 1 lb/hr/hp or an a:f of 5.5 for initial calculations and you will be close.

To get part throttle working you need to keep filing the slot in the barrel valve, but you will probably never get it better that it kinda runs and guzzles fuel and the exhaust fumes burn your eyes from the formaldehyde.

Regards
Pat
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[ServoWizard]

patprimmer,
pontiacjack,

Several of the old mechanical fuel injection companies are moving to electronical systems, but they are extremely expensive. There's a fellow in Canada that's converting old mechanical fuel injection systems to EFI and his price is more affordable. I will probably purchase an Enderle injection hat and send it to him for the conversion. The EFIsystem can handle the constant fuel pressure from an electric pump so that eliminates the camshaft driven fuel pump.

My Anglia will driven on the street and will seldom be operated at MAX engine speed. The mechanical FI would be a major pain in the rear so the fuel delivery system will be either carburetors or EFI.

Blower Facts:

The GM Detroit diesel engine is a two cycle engine so therefore it cannot use the piston down travel as a means of drawing the air/fuel charge into the cylinder. The roots type air pump was used for that purpose and that purpose only. The first number in the model number represents the number of cylinders and the second number represents the cylinder displacement in cubic inches. The 4 cylinder GM Detroit diesel engine used a 4-71 roots air pump that was designed to pump 4 x 71 or 284 cubic inches per revolution at MAX engine operating speed. Therefore the 6-71 roots air pump will pump 6 x 71 or 426 cubic inches per revolution at MAX engine operating speed. I believe the MAX engine operating speed was governed at 1800 RPM.

The formula below gives you the blower CFM by using the boost. It is a formula that is primarily used to determine the proper carburetors CFM rating. I don't exactly comprehend how you would know how much boost you had without first having the correct CFM carburetors.

A four cycle engine theoretically pumps 1/2 of the engine displacement per revolution. Therefore you must use 1/2 of the displacement when calculating Cubic Feet per Minute.

((((cid/2) x rpm) / 12^3) x ((boost/14.7)+1)) = blower CFM

Example:
Engine displacement = 401 cubic inches
Engine speed = 8000 RPM
Projected boost = 6.5 lbs.

401/2 = 200.5 x 8000 = 1604000 / 1728 = 928.2407 CFM
6.5/14.7 = 0.4422 + 1 = 1.4422
928.2407 * 1.4422 = 1338.7087 CFM * 1728 = 2313288.6985 CIM

My engine theoretically pumps 928.2407 CFM at 8000 RPM and the roots air pump is pumping 1338.7087 CFM at 6.5 lbs. of boost.

Ron

 

[pontiacjack]

As said two posts above this, your best bet is to estimate peak power and work backward, using the one pound per horsepower-hour figure. Then keep in mind that it's merely an estimate, and size your pump generously over that value.

 

[patprimmer]

Ron

I am pretty sure your blower facts are not facts. You are correct as to how the naming system matches the blower in question to a particular engine, however I think they are not driven at 1:1.

Typical diesel engine speed would be a very low speed for those blowers which I think are rated to about 12,000 rpm.

There are any number of EFI systems that can be used with a hat or with stacks. Some are not even that expensive up to a certain power level.

The problem with EFI and blown alky is that the injectors and pumps are not big enough so you need probably 16 injectors and 3 or 4 fuel pumps. That is when it starts to get expensive.

Regards
Pat
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[ServoWizard]

pontiacjack,

The original GM Detroit 71 series diesel engine drove the roots type air pump at 1:1 crankshaft speed. That resulted in the air induction being slightly higher than atmosphereic pressure. In later years the 71 series engine was offered in a turbocharged version. However the roots type air pump was retained for starting and low speed operation.

Source:
Central Detroit Diesel Power
316 943 1231
Bruce

Ron

 

[pwildfire]

While the method you are using, once adjusted for weight rather than volume, and including a 'typical' VE for your engine, which, if it is boosted, will actually be MORE than 100%, should work OK, it is not how I would do it.

Typically this estimation is calculated based on expected power rather than displacement. Most people know within a few percent how much power their build is expected to make. Based on this and an estimate of BSFC (brake specific fuel consumption), you calculate how much fuel is required for that power level. Airflow is irrelevant to this calculation.

You should be able to find a close estimate of the BSFC of your particular setup online pretty easily. This will be expressed in lb/hp-hr.

As an example, if you are tuning for 500BHP, with a BSFC of .75, then you will need 500*.75=375 lb/hr of fuel flow capability. If you are using EFI, you need to derate this to 80% injector duty cycle, so you would want 375/.8=469 lb/hr of flow capability. Fuel pumps also should be operated at 80% of rated capacity or less.

 

[patprimmer]

Sizes and capacity are:-

6:71 small diameter
Rotor dia=5.505", length=14.975", displacement per full turn of rotor=339CI.

6:71 big diameter
Rotor dia=5.778", length=14.975", displacement per full turn of rotor=411CI.

8:71
Rotor dia=5.778", length=15.905", displacement per full turn of rotor=436CI.

10:71
Rotor dia=5.778", length=17.000", displacement per full turn of rotor=466CI.

14:71
Rotor dia=5.778", length=19.000", displacement per full turn of rotor=521CI

This is theoretical displacement.

Retro or high helix will change this considerably. I think in the order of plus 15%.

A worn blower will reduce this a little.

A very good tight new blower and a Teflon stripped blower should be about the same.

Regards
Pat
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[pontiacjack]

Pat- For what it's worth:
Neither 'retro' nor high-helix attributes change the fundamental displacement. They both [typically] improve the air-movement capability by creating a "system" that behaves more like a compressor than does the Roots blower by itself. The 'retro' system depends on the small pie-shaped orifice to accomplish the entry of somewhat-pressurized air into the manifold. The high-helix, in conjunction with an airflow path off the ends of the rotors, behaves much like a screw blower (which is a good ocmpressor).

 

[SomptingGuy]

Why is this whole thread based on volumetric air flow rate? The engine will flow exactly its displacement (excluding overlap effects). But that volumetric flow rate give different mass flow rates depending on VE and charging.

- Steve