333pg333

Ahh, ok thanks Stephen. Hmm, I wonder if they will be big enough for my application? I assume there is a formula for working out these things.

eniac

Yeah there is:

Injector Size (lbs/hr)= (Crank HP x BSFC) /(total number of injectors x duty cycle )

BSFC for turbod cars is generally .60-.65

lbs/hr x 10.5 = cc/min

The following is from www.witchhunter.com:

* Brake Specific Fuel Consumption: is the amount of fuel consumed (in lbs. per hour) for each horsepower made. This should be .45 to .50 for naturally aspirated engines, .55 to .60 for supercharged engines, and .60 to .65 for turbocharged engines.
** Maximum Injector Duty Cycle should be around 80% max for safe operation. This allows for some future horsepower increases.

333pg333

So until we get on a dyno and determine hp at the crank we can't work out the best sized inj?

eniac

Calculate for what hp your shooting for and use a 80% duty cycle so if you happen to get lucky and go over your goal for hp, you will have some extra room for that.

Mikes3.0cabturbo

whats your crank HP goal Patrick...

A.Wayne

Eniac
That formula you have choosen is about as accurate as a lucky guess, It does not account for RPM
500 bhp at 9000 rpm requires bigger injectors than 500 bhp @ 6000 rpm and turbo engines are classified as being exhaust driven superchargers as opposed to belt driven superchargers ( whipple ) a belt driver positive displacement supercharger requires bigger injectors than an exhaust driven turbocharged engine of the same power, not the other way around as you have explained .....

eniac

A.Wayne

Yes yes, but it is a rough calculation and you nor anyone else have yet to provide anything better.

Using an HP value in this equation does account for RPM.

How so?

HP = Torque * RPM / 5252

So just substitute "hp" in the equation for "tq * RPM/5252"

Personally I found it much easier to just type "hp".

If you have something more accurate to post then please do, it would be a great help to those asking.

A.Wayne

Nice try Stephen , LOL

I won't then attempt to mention the effects of Static FP, Fuel System GPH and max boost desired vs RPM .... or the 16 big ones used to fuel one of these. With supercharged engines you really need to measure the BSFC on a dyno. A little DA, this motor produced 289ft.tq @ 1100 rpm , what size injectors would your calculations say i need MR

333pg333

So really you are saying that you have to come up with a preordained hp figure and work backwards when specc'ing the components? I can see why, it just seems a bit backwards, but maybe that's really forwards instead of arriving at an hp outcome based on what goes into the engine and just hoping for the best. Long gone are the days of paying the bills and just getting in the car and driving it. Luckily there are those that know what they're doing and I don't count myself in that tidy little group. lol

reyk

Hi Patrick,

Best explanation for injector size I've found is in How to Tune and Modify Engine Management Systems by Jeff Hartman ISBN 0-7603-1582-5. In chapter 13, EMS Tuning 101, there are ~ 30 pages of factiods, including the calculation for injector size, and lots of other useful considerations.

His basic calculation determines the amount of air going thru the motor and divides it by the target air/fuel ratio to get amount of fuel. Here's a sample assuming 100% volumetric efficiency - at the end we should cut it back to reflect actual 80 - 100%.

3L motor = 183 cubic inches = .1059 cubic feet
Air weighs .07651 lbs/cubic foot, so every 2 revs (intake stroke all 4 cyl) at atmospheric pressure

0.1059 cubic feet x .07651 lbs/cu ft = .008106 lbs air thru motor
So for 1 cylinder = .008106 / 4 = .002026 lbs air
At 2 atmospheres (~ 1 bar boost) it's twice that = .004053 lbs air

At 11.5/1 AFR, need .004053/11.5 = .0003524 lbs fuel per 2 revs per cylinder. This is what injector must deliver every 2 revs. If running batch fire injector fires every rev, so cut injector requirement in half.

.0003524/2 = 0001762 lbs fuel/rev/cyl

An 80 lb/hr injector delivers 80/3600000 = .00002222 lbs fuel/millisecond, so injection time is

.000176/00002222 = 7.9 milliseconds

6000 RPM is 9.76 msec/rev, so the injector is on about 81% of the time, max, about right.

Anyway, it's a bit tedious but not rocket science. I used this to get to 80lbs/hr injectors, and my A/F ratios are nice and the injectors don't get much past 80% duty cycle. My understanding is injectors are rated at 85% duty cycle @ 6000RPM.

Lots of other factiods and rules of thumb in the tuning book - very helpful.

Reyk

333pg333

To be honest I don't know Mike. I guess each build has it's own idiosyncrasies based on all manner of things. Capacity, c/r, ems, VE, turbo, exhaust, i/c & pipes, octane rating etc....I don't know really what to expect. I'm not shooting for crazy numbers on this build. It has to last me for the season unlike last season's mess. So judging by similar builds I'd guess somewhere in the 450-500 crank will be fine. I'll leave that 600hp for you mate. If we pursue the E85 on this build then that may change.

gt37vgt

stagged is the only way to go pat
here are 3 reasons
1 your old injectors don't get wasted and are used as primaries you can get secondaries (1100cc/min) as big as you want and not stress about idle .
And third the secondaries can be placed up the runner providing more cooling for when you inevitably map for E85

A.Wayne

Much better approach and getting closer to the gold , The error being a 4 stroke engine has 20 ms @ 6000 rpm making the 9.76 calculation closer to 50 % duty cycle ( seq. operation is good to 70 % ) unless you are referring to a 2 stroke engine, then you half.



Yes 2win stage is very good and is worth as much as 10 % mor epower in some applications, very difficult to get right as the displacement drop is hard on the Fuel system and of course injector placement hasto be in the correct flow path in the runners not to get fuel drop ... but highly recommended , better to keep both stages the same size.....
Look at this video of and F1 engine on the dyno , look at the fuel vapor cloud , imagine how important it is for proper placement with say 1600 cc injectors !http://www.youtube.com/watch?v=cU2KIaUu9ms

eniac

Well he's not building one of "these" and you have yet to post anything more usefull. This was not my personal calculation that I made up. It's what is used and widely accepted as a baseline of what injectors to use on a new motor. This calculation is used by every injector retailer that I can think of on the web right now. On our motors it really is pretty close if you have any common sense at all. On my own car it was within 5% accuracy and why I picked 83# injectors and not the 75# I was going to get initially. I never said it was exact, but it will get you close on most street motors.

Injectors are rated at 3bar or 3.5bar of pressure...static pressure which would be static fuel system GPH. The engine you posted doesn't even use the same type of "duty cycle" injectors so it in no way applies to this formula. You could also account for altitude change, global warming, people starving in Africa, and men horny in Canada but there's a point when a basic formula is enough to start with.

A.Wayne

Touchey!

One of "these" uses the same injector as you or your internet ignorance calculator attempts to size matey! based on your response , well it must be your milky time ,sorry to interrupt GN!