It's all about the FUEL

General engine tech -- Drag Racing to Circle Track

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RDY4WAR
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Re: It's all about the FUEL

Post by RDY4WAR »

nitro2 wrote: Sat Oct 02, 2021 11:41 am If you were to run 20% through an EFI there should be no reason to blow down before restarting as the EFI should be shutting off the fuel when the ignition is shut off so the crankshaft spins X number of times with no fuel going in before it comes to a stop, and really there should be next to nothing unburnt in there anyway at idle with EFI.
Bringing this thread back up because I noticed this while rereading through it.

What about with a spark shutoff 2-step?
hoodeng
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Re: It's all about the FUEL

Post by hoodeng »

I can only speak with knowledge of higher percentages of fuel. You shut the fuel off at engine shutdown at the end of a pass or running in the pits, you do not cut the ignition, plug leads are then pulled, and only then the ignition is turned of. Some may have a different procedure.

The reason is that some ignitions can make a stray spark when power is cut when the engine is still rotating and the ignition is activated.

Some have pointed out a confidence in different procedures when running an amount of percentage they are familiar with.

Cheers.
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Re: It's all about the FUEL

Post by nitro2 »

20% is basically nothing compared to high percentages, as far as nitro volume is concerned. If you do the math you'll see what I mean.

Engines that operate at high percentages, when they are at idle on nitro, the plug fires every time as it should, BUT the nitro only lights about every 3rd or 4th plug firing and since there is very little airflow at idle, well you can see how fuel accumulation could be a problem if the shutdown is not done in a very specific manner. However, 20% with EFI is nothing like that scenario.
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RDY4WAR
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Re: It's all about the FUEL

Post by RDY4WAR »

nitro2 wrote: Fri Jan 21, 2022 1:43 am 20% is basically nothing compared to high percentages, as far as nitro volume is concerned. If you do the math you'll see what I mean.

Engines that operate at high percentages, when they are at idle on nitro, the plug fires every time as it should, BUT the nitro only lights about every 3rd or 4th plug firing and since there is very little airflow at idle, well you can see how fuel accumulation could be a problem if the shutdown is not done in a very specific manner. However, 20% with EFI is nothing like that scenario.
Noted.

How do you go about determining the amount of nitromethane needed to reach a certain air/fuel ratio?

For example, you have an EFI NA setup, 11:1 compression, already tuned for E85. You take an E10 93 octane pump gas at ~14.1 stoich and you want to add nitromethane until you reach the same ~9.8:1 stoich you had with E85 so fuel tables can remain about the same. I'm sure it's not as simple as just adding the stoich numbers together and dividing according to the blend ratio. Would you calculate the fuel flow required for each fuel if it was the sole fuel based on recorded CFM and VE data and then do a add/divide by ratio from there?
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Re: It's all about the FUEL

Post by RDY4WAR »

In an effort to answer my own question and typing this out for the world to see as a peer review.

A 350ci engine at 100% VE at 6500 rpm.

350 cubic inches / 1728 = 0.2025 cubic feet

0.2025 x (6500 / 2) = cubic feet per min
0.2025 x 3250 = 658.125 CFM

1 ft^3 of sea level air = 0.0807 lbs

658.125 * 0.0807 = 53.11 lbs per minute air flow

E10 93 pump fuel = ~0.745 SpG or ~6.21 lbs/gal and 14.1:1 stoich AFR

53.11 / 14.1 = 3.77 lbs/min
3.77 / 6.21 lbs/gal = 0.61 gal/min fuel flow

Nitromethane = ~1.139 SpG or ~9.50 lbs/gal and 1.7:1 stoich AFR

53.11 / 1.7 = 31.24 lbs/min
31.24 / 9.50 lbs/gal = 3.29 gal/min fuel flow

If mixing 10% nitromethane by volume....

((0.61 x 9) + 3.29) / 10 = 0.878 gal/min

((6.21 x 9) + 9.50) / 10 = 6.54 lbs/gal

So a blend of 90% E10 93 pump gas with 10% nitromethane would want 0.878 gallons per minute fuel flow for stoich with a SpG of 0.784 (6.54 lbs/gal).

Reverse calculating for air/fuel ratio...

0.878 gal/min * 6.54 lbs/gal = 5.74 lbs/min
53.11 lbs of air / 5.74 lbs of fuel = 9.25:1 air/fuel ratio

We know E85 is ~9.8:1 air/fuel ratio at ~0.79 SpG or ~6.59 lbs/gal which gives us...

53.11 / 9.8 = 5.42 lbs/min
5.42 / 6.59 = 0.823 gal/min fuel flow

5.74 / 5.42 = 1.059

0.878 / 0.823 = 1.067

So a blend of 90% E10 93 with 10% nitromethane would want ~6.7% more fuel flow by volume (~5.9% more fuel by weight) compared to E85 at stoich value.

Am I on point with this or way off the mark?
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