Injector duty cycle to calculate rough power figure VS actual Dyno graph

[djvoodoo]

Injector duty cycle to calculate rough power figure VS actual Dyno graph

So, might seem like a dumb question. I know there is a formula for it... Just not sure which is correct. or the math....

If I am seeing a max injector duty cycle of 82% on 700cc injectors (Rb26 - base fuel pressure), in theory, can you compare this calculated HP figure to that of what the Dyno reads out?

Would i say that 1cc = 1HP, so 0.82% of 700CC = 574HP

Is that fairly accurate?

# never was good at math's

Edited October 22, 2016 by djvoodoo

[GTSBoy]

Engine horsepower, yes.  Not chassis dyno horsepower.

FI cars run richer so you may not get quite as much as 1HP per CC.  But good enough for estimating injector requirements.  Maybe slightly less reliable for what you're asking about.

[djvoodoo]

So give or take, we'll say 82% duty cycle on 700cc is roughly 520-550 engine HP. At the wheels you'd be looking at mabee 420ish (or about 315rwkw)

More to the point - What if the dyno reads quite low compared to what you are seeing max injector wise? Can they be compared? I'm guessing fuelling and how much fuel is dialled in at the top end is also a factor which can skew duty?. Lets just say i'm running about 11.5 to 11.8 in the top end AFR's.

In more just curious at how many different ways you can calculate wheel HP other than just a dyno number.

[r32-25t]

I always thought it was 5cc of fuel was required per horse power, so a 550cc injector is good for 110hp per cylinder then multiplied by 6 cylinders was 660hp then times it by .8 if you want to only use 80% duty cycle is 528hp

[Dose Pipe Sutututu]

Drag strip, what you're doing is guessing at best. x amount of fuel injected does not equal y power... period!

 

[r32-25t]

There is never going to be an exact science but obviously there has to be something close otherwise picking the correct injector would be s stab in the dark 

[r32-25t]

Double post

[GTSBoy]

The fairly rough rule, particularly true for RBs, at least RB26s, is that (given 6 injectors) then the cc size of a single injector is the maximum engine power supported by those injectors.  Factory 440cc injectors will run up to about 440HP.  Plus or minus the usual range of variables of course.  It's rough but good enough.

[Dose Pipe Sutututu]

yes rough rule for approximating the size of injector you need.. but the OP wants to determine how much power his car is making based on injector DC.. that's obsurd.. just as obsurd as a moron on a Facebook page telling someone to install a resistor pack on their high impednace injectors because their car won't crank over.

[djr81]

You can actually do a better job if you can measure the airflow rather than the fuel flow.  Do you have afm's? If you can convert the voltage to a flow you can google any number of calculators to convert the air flow to rwhp or engine hp or whatever.

[djvoodoo]

1 hour ago, djr81 said:

You can actually do a better job if you can measure the airflow rather than the fuel flow.  Do you have afm's? If you can convert the voltage to a flow you can google any number of calculators to convert the air flow to rwhp or engine hp or whatever.

I'm using MAP sensor. May as well do a data log session on the Vipec and look at air flow.

[Dose Pipe Sutututu]

you want to work out power, drag strip.

x weight can going so y speed in z time and k distance (factor in some basic air drag) - that is much more accurate then working out how much power a motor makes based on air flow and fuel usage.

You need to remember a motor is not 100% efficient, and x air/fuel does not equate to a direct 100% energy transfer to mechanical energy - there are losses such as heat due to inefficiencies.

[djvoodoo]

1 hour ago, Dose Pipe Sutututu said:

you want to work out power, drag strip.

x weight can going so y speed in z time and k distance (factor in some basic air drag) - that is much more accurate then working out how much power a motor makes based on air flow and fuel usage.

You need to remember a motor is not 100% efficient, and x air/fuel does not equate to a direct 100% energy transfer to mechanical energy - there are losses such as heat due to inefficiencies.

Certainly will do bud. Just have to work out when!!!

[djr81]

5 hours ago, Dose Pipe Sutututu said:

you want to work out power, drag strip.

x weight can going so y speed in z time and k distance (factor in some basic air drag) - that is much more accurate then working out how much power a motor makes based on air flow and fuel usage.

You need to remember a motor is not 100% efficient, and x air/fuel does not equate to a direct 100% energy transfer to mechanical energy - there are losses such as heat due to inefficiencies.

The correlation between air flow and horsepower is much closer than that between terminal speed and horsepower.  You don't, for example need to compensate for such minor things as drag coefficients, frontal area, gear change, traction, engine torque characteristics etc etc. 

[Lithium]

11 hours ago, djr81 said:

The correlation between air flow and horsepower is much closer than that between terminal speed and horsepower.  You don't, for example need to compensate for such minor things as drag coefficients, frontal area, gear change, traction, engine torque characteristics etc etc. 

+1.  And again, anything you use to estimate "power" is going to be "estimated engine horsepower", not hp @ wheels - and I know from experience that often the calculated results often get heavily question because by nature of how crazy low Oz dynos read the crank hp results which get calculated if done correctly are often much higher than people expect from typical @ wheels readings on a Dyno Dynamics.

A thing no one has mentioned here is fuel as well, if you treat the % duty cycle of x size injector = hp described above for 6 cylinder turbos and are running E85 you will get very optimistic results

 

 

[djr81]

If I remember rightly it is about 10 horsepower per pound per minute of air flow on petrol.  NFI what happens with E85.