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The mkiv Supra Owners Club

Injector & fuel pressure upgrades


Guest Ash
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A few members contemplating turbo upgrades have been enquiring about fuelling. So I thought I'd publish a few handy calculations to give you an idea of what's what.

 

The main question has been along the lines of, "I want to upgrade to xxx horsepower, will my stock injectors be adequate or not, and if not then what size injectors should I use?"

 

The other question has been about increasing the system fuel-pressure and how much extra flow does one get for a particular increase in fuel pressure.

 

Okay, first you need to work out your Brake Specific Fuel Consumption (abbreviated BSFC). BSFC is simply a measure of how much fuel is being consumed per horsepower per hour. Fortunately, this figure can generally be estimated.

 

A normally aspirated engine will have a BSFC of 0.50 give or take. Turbocharged engines will run at around 0.60 to 0.65. Personally, I use 0.65 as it gives a bit more fuelling headroom.  

 

Also, you do not want the injector to reach a condition where it is open all the time. The generally accepted maximum duty cycle should not exceed 80%. Though for a road engine, that would only remain at full throttle for short bursts, then this figure could creep to 85% without problems. But certainly no more than that.  

 

The formula for calculating the size of injectors related to engine power is:

 

(Horsepower x BSFC) / (number of injectors x max duty cycle) = lbs of fuel per hour per injector

 

Assuming 500 horsepower, at a BSFC of 0.65 with a maximum injector duty cycle of 80% and 6 injectors......

 

(500 x 0.65) / (6 x 0.8)

 

= 325 / 4.8

 

= 67.71 lbs of fuel per hour per injector

 

Convert to cc per minute multiply by 10.5........

 

67.71 x 10.5 = 710.955 cc per minute of fuel flow.  

 

The nearest common sized injector is 720cc. Therefore, if you are looking to develop 500 horsepower from your MKIV, 720cc injectors would be the proper injector choice.

 

To find out how much horsepower you can make from any particular size of injector, the formula is.........

 

(Injector size in lbs per hour x max duty cycle) / BSFC

 

So with 6 x 720cc injectors the maximum power you could properly make is.........

 

First convert injector flow from cc/min to lbs/hour, i.e. divide by 10.5........

 

720 / 10.5 = 68.57 lbs/hr

 

So it's........

 

(68.57 x 0.80) / 0.65

 

54.856 / 0.65 = 84.393 hp per injector

 

We have 6 injectors so the maximum power we can properly make with 720cc injectors is........

 

6 x 84.393 = 506.358 hp

 

To find the new fuel-flow rate after a fuel pressure change, the formula is..........

 

Square-root of the new pressure divided by the old pressure x original injector flow rate.

 

So say the stock J-spec 440cc injectors are used with a new system pressure of 46 psi, an increase of 10 psi above the stock pressure of 36 psi (from memory I'm sure its 36 psi but in any event the formula is the same).

 

The new flow rate would be.........

 

46 / 36 = 1.278

 

The square root of 1.278 = 1.1305

 

So now we multiply the old flow by the square root number........

 

44 x 1.1305 = 497.42 cc/min

 

So if the stock fuel-pressure is raised from 36psi to 46 psi, each injector will flow an extra 57.42 cc per minute of fuel.  

 

To compare relative maximum safe horsepower levels, first we calculate stock conditions......

 

440 cc/min = 41.905 lbs per hour

 

Using the same horsepower-per-injector calculation above........

 

(41.905 x 0.80) / 0.65 = 51.575 hp/inj

 

Times by 6 injectors.........

 

51.575 x 6 = 309.45 hp

 

Calculate increased hp following 10 psi fuel pressure increase...........

 

Higher pressure injector rating, as calculated above, is 497.42 cc/min = 47.373 lbs per hour

 

Using the same horsepower-per-injector calculation above........

 

(47.373 x 0.80) / 0.65 = 58.305 hp/inj

 

Times by 6 injectors.........

 

58.305 x 6 = 349.83 hp

 

From the above it is shown that the stock injectors are adequate up to around 310 hp. If fuel pressure is increased by 10 psi over stock then the maximum safe power that can be developed is in the region of 350 hp.

 

A common injector swap is to fit the 550 cc injectors from the UK-spec car. The maximum safe power using 550cc injectors is...........

 

First convert to lbs/hr......

 

550 / 10.5 = 52.381 lbs per hour

 

Using the same horsepower-per-injector calculation above........

 

(52.381 x 0.80) / 0.65 = 64.469 hp/inj

 

Times by 6 injectors.........

 

64.469 x 6 = 386.814 hp

 

Therefore, swapping the J-spec injectors with the UK injectors allows you to safely up the power from a maximum of about 310 hp to around 390 hp.

 

HTH

 

Yours,

J

 

 

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OK looking at those figures, it follows that almost ANY mod to a stock Jap-spec will push the "safe" BHP over your estimate of 310bhp ?

I think a lot of the potential variability is in the estimate for the BSFC figure.  

Why doesn't someone simply derive this figure by using a mkiv of KNOWN bhp (i.e. say a stock UK = 326bhp) and then run it on a dyno for a fixed amount of time with an exact amount of fuel in the tank, and then see exactly how much fuel has been consumed.  

OK so i can see that running it at max.power is not likely so why not run it at reduced revs, say 4000rpm, get the BHP figure from the dyno and then calculate fuel consumed per hour at that BHP.

Just a thought.   might be talking crud !

 

If 310bhp is approx. max. BHP for a Jap-spec then almost all the Jap-specs out there are runnning lean at max.power cos almost everyone's Jap-spec on this list is running more than that.

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And that's the big mystery I am still trying to solve.

 

I also note you use the word "estimate" of 310 hp. This is not an estimate but a calculated fact.

 

A particular sized injector will flow a calculated amount of fuel at a particular duty cycle.

 

I used to run my MKIV when it had turbos with a duff wastegate and it would run mad rich at WOT.

 

Even if you say an injector duty cycle of 99% at a BSFC of 0.6 then the maximum power is 415 hp.

 

Fact is, you can't bend the laws of physics. So something has got to give. That's why I reckon 99.9% of those published power figures are porky pies.  

 

Also, BSFC estimated to between 0.6 to 0.65 is a close enough figure. It's rather like when you estimate volumetric efficiency of a 4-valve per cylinder engine of 80%. It may be 78 or 81 but such a variation doesn't make all that much difference.

 

In the same vein, I always call 1 Bar = 15 psi. Some people call it 14.7, others 14.5. But there is no real difference in typical automotive calculations. After all, this is not rocket science.

 

Yours,

J  

 

(Edited by Ash at 7:28 pm on Aug. 5, 2001)

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Yeah sorry should have clarified - when i said "estimate" I wasn't taking issue with the calcualtion at all but what i was implying with any calcualtion is that the input value assumptions accurately reflect the real values.   as you say, the 2 variables that are may be open to slight debate (hey, you're 10 times more techie on this fuellying stuff than me so i take your word for it !) are the duty cycle of 80% and the BSFC of 65%.    The actual value of either one of these could vary up or down by a few percent and that would affect the end result of 310bhp.    

i quite like all this theory stuff.   It's good to know what actually happens inside an internal combustion engine.  A little bit of knowledge goes a long way!

 

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No problem.

 

I just fitted a set of stock turbos to Justin's car and I'm going to get to the bottom of the whole fuelling cum power equation using his car as a test bed. That is, before he decides to have me fit uprated turbos.

 

It'll be an interesting exercise comparing the theory to practice.

 

Yours,

J

 

 

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We put Justin's car back to bog stock so we can run some tests. I'm gonna find out what duty-cycle the injectors are running at WOT so I can calculate how much fuel is being injected. I can also measure the tail-pipe air ratio at the same time. So from those two measurements I will be able to estimate the air flow and the power.

 

I think it's about time someone made some meaningful tests on a J-spec car to find out exactly what the car *truly* makes at various level of boost.

 

Yours,

J

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  • 5 years later...
Guest majid

not correct. mate il tell u something about the injector power ratings of the japanees 440 cc high impednace injectors. the max support of the japanees 440 cc injectors in the jap spec supra is around 420-430 rwhp ( on dynojet)

 

my bpu supra has 440 cc injectors with raised base fuel pressure of 55 psi and an inline high pressure fuel pump ( walbor or bosch 044) and i am generating 430 rwhp:D :D :D on 15 psi boost level using jap ceramic turbos and HKS 272 s.

 

the rule of thumb is that

440 cc = 440 rwhp support

550 = 550 rwhp support and so on

 

cheers [GRIN][/GRIN]

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