Fuel/injector Requirements For E85 - The Theory

[bigkevracer]

I've done a lot of research recently on what I'd need to run my M35 Stagea on E85. I'm looking at upgrading my fuel system knowing it's overkill for the standard turbo, and knowing I'll be upgrading to a highflow or something down the track.

The simple answer is 1000cc injectors and a 255lph+ fuel pump.

Ok so that's that out of the way. I've already got a Deatschwerks 300lph pump to fit, and I'm looking at ordering some ID1000 injectors from PHR, an SAU sponsor who gave me a great price. Job sorted.

But reading through the spec sheet for the ID1000s the other day, it had me thinking (bearing in mind I'm still trying to learn all of this)...

Flow rate is 1015cc/min @ 3.0 Bar (43.5 psi). So lets assume it'll be around that for 100% duty.

6 injectors would therefore require 6090cc/min at 100% duty. That's near enough to 6.09L/min, which works out to 365.4L/hour. But they wouldn't be at 100% duty in theory, they'd want to be at 80-90% depending on the tuner. 90% is 328.86 lph.

According to Deatschwerks, the most their pump will flow is 325lph @ 40PSI. Which isn't enough to feed the ID1000s. But heaps of people are doing just this, and it's working fine.

I also found other calculators online, such as http://www.eastcoastrollingthunder.com/jmiller/finject1.htm, which is more concerned about your horsepower and aspiration.

So I guess the question is, if you really want to be a geek and calculate your fuel requirements down to the last cc, how do you do it? What formulas do you use? Or am I on the right track?

Because if I'm on the right track, that'd make me think that you'd want twin pumps to properly feed 6 x ID1000s E85 at 85%+ duty cycle?

[scotty nm35]

Thats the conclusion I came to, and I didnt think I would be using 80% on the 1000's with such a small turbo either...

[Legionnaire]

If you want to be as accurate as possible, you should start with energy density of the fuel you're going to use, that make a plausible estimation of BSFC in your engine and HP you want your engine to produce. Energy density is rarely used though, you can got to it only if you're really into physics and like calculating stuff. If you don't want to dive this deep, go straight to BSFC [brake specific fuel consuption]. This value varies from engine to engine, and it's not constant in all operating modes either - it changes with load and RPM.

I'll start calculations in imperial units, 'cause imperial figures are easier to find, also garrett comp maps are in lb/min.

I'll be conservative and assume E85 to have BSFC of 0.77 lb/(hp*hour). Now let's say you want 500 flywheel HP. From this you can see that you need 0.77 *500 = 385 lb of E85 in order to make 500hp for an hour or 6.42 lb/min = 2.913 kg/min. Now assuming 6 cylinders with a single injector per cylinder you need to have injectors capable of at least 486 g/min of E85. To convert this to cc/min, you have to divide by fuel specific gravity at certain temperature, I'll use 810 g/litre, this gives roughly 600cc/min injector, and then by maximum injector duty cycle, say, 85%, which results in 705cc/min injector.

For fuel pump requirements do it backwards: 600 cc/min * 6 injectors * 60 min = 216 litre/hour to make 500hp or 705 cc/min * 6 injectors * 60 min = 254 litre/hour to use full injector capacity. Notice that this figure isn't enough to choose a pump yet, we'll add important info below.

Now air requirements. Full power lean AFR for E85 is roughly 8.5:1, meaning you need 8.5 parts of air to burn each part of E85. You remember that you need 6.42 lb/min of E85 to make 500 hp, now multiply this 8.5 by 6.42 lb/min of E85 = 54.57 lb/min of air. GTX3076R provides necessary airflow at as low as ~14psi, but your engine won't consume this much air at this pressure, calcs show that 22-23 psi boost is required to make your engine swallow it (at 7000rpm assuming 102% VE and Tamb 30 deg.C).

Now, knowing your required boost pressure and turbo you'll be using, you're ready to go back to fuel pump selection. All you now have to do is to add maximum boost pressure to base fuel pressure (fixed by FPR used), IDK what's the base presure level on VQ25DET, 43 psi I should think? Then it's 43 + 23 = 66 psi. Therefore you need a pump that provides at least 216 litre/min @66 psi or 4.5 bar.

That's it I guess, I tried to keep it compact, easy and error-free, but if someone notices an error in calculations, please by all means feel free to point it out or correct it asap to avoid confusion.

[scotty nm35]

^ Thats the theory, or you could just buy another pump. lol. Maths was never my strong point.

Are you sure you wont need more fuel flow down the track? These VQ's seem to love using the stuff.

[Legionnaire]

Sure, you can use double pump arrangement, may end up cheaper, in that case you just choose a pump that provides half the necessary amount of fuel, but at the same max pressure as calculated. Downsides could be more current draw, twice as much wires and relays, more pump noise, not enough space inside the tank or pump cradle, and you'll have to make some clever arrangement for the fuel lines.

[scotty nm35]

Thats the main issue anyway. I was maxing the fuel lines running a 33 GTR Tomei pump as the M35 plastic pump cradle only has a 3mm fuel outlet. Two walbro's shouln't draw any more current than one Deatschwerks pump.

I ended up making a new cradle for mine with twin -6 teflon lines. They do sound like they are starving a bit when i'm at 1/4 tank and I go around corners. I have never seen the AFR's affected though, possibly due to the way the stock rails are designed.

[bigkevracer]

Thanks Petros. Theory is great. I'll play with it some more, but it's good to have the formula.

Scotty, I have thought about two pumps, but I just don't need them (yet).

I'd already worked out what I was doing before I started the thread, but I was curious about the maths behind it, especially when I saw maths that didn't add up to me. Petros has cleared that up for me, so thanks!

[Legionnaire]

You're welcome. Any additional questions - feel free to ask.

I don't know what power you're after, but based on the above calculations a package consisting of injector dynamics ID725 and a DW300 series/Aeromotive 340 stealth/Jay racing PT342 pump [the three I believe are identical] should be sufficient to fuel 500bhp @ 23 psi as per calculations above.

Let's check DW301 pump specsheet: at 66 psi it delivers 260-265 litres/hour at 13.5 volts and current draw of 12.3-12.4 amps, which is more than required 216l/h and gives a good safety margin.

Aeromotive 11142 is roughly the same, without magnification of this tiny graph I can't see squat, but enlarged graph shows performance similar to DW301 pump, also at 13.5 volts. Too bad there is to current draw graph

Now Jay racing claims their pump to be higher flowing then the other two, this graph is good because it has some testing details on it

And this graph is good because it's supposed to show the difference between their pump and DW301 pump

but their data for DW301 flow does not match DW's own data. Anyway, if this graph shows the truth, JR PT342 flows roughly 280-285 litres/hour @ 13.8 volts and draws ~14 amps.

Or you can always use twin walbro arrangement (that twin pump cradle of Scotty's is beautiful by the way, terrific job, very compact and tidy), as you can see, two walbros flow 410-420 l/hour at 13.8 volts, which imo is a massive overkill, and draw ~9x2=18 amps.

[Legionnaire]

Just a thought - you can actually do it backwards and calculate maximum power for a given fuel pump operating @ 13.5v and base fuel pressure.

Let's use DW pump as a reference and 43 psi base pressure. Assume E85 fuel and 10lph safety margin.

E85 BSFC 0.77 lb/(hp*hr) = approx. 469 g/(kW*hr)

469 g/(kW*hr) / 810g/litre = 0.579 litre/(kW*hr)

Now we'll just make a table of boost pressures and respective power levels that are achievable by DW301 pump. Remember, this is for 43 psi base pressure, 13.5v pump power supply and 10 lph safety margin.

7 psi boost - 50 psi fuel pressure - 11.6 amps - [295 - 10 safety margin = 285 l/hr] - 285/0.579 = 492 flywheel kW = 660 flywheel hp

12 psi - 55 psi - 11.7 amps - [285-10=275] l/hr - 475 kW = 636 hp

17 psi - 60 psi - 11.9 amps - [275-10=265] l/hr - 458 kW = 613 hp

22 psi - 65 psi - 12.2 amps - [262-10=252] l/hr - 435 kW = 583 hp

27 psi - 70 psi - 12.5 amps - [250-10=240] l/hr - 414 kW = 555 hp

32 psi - 75 psi - 12.9 amps - [240-10=230] l/hr - 397 kW = 532 hp

37 psi - 80 psi - 13.2 amps - [230-10=220] l/hr - 380 kW = 509 hp

42 psi - 85 psi - 13.5 amps - [217-10=207] l/hr - 358 kW = 479 hp

That'll do I think. 42psi is insane boost already, and at this boost level a single DW301will supply enough E85 to make 479fwhp.

Edited November 30, 2011 by Legionnaire