skotx

I was researching injector duty for another project and came across some info on max horsepower for injector combinations and its related uses. As I studied the formulas, it became apparent that we have just about everything we need to come up with theoretical numbers for horsepower, and thus, torque.

In a nutshell, an injector setup is capable of a theoretical max hp at 100% injector duty. For an FD with 2 x 550 and 2 x 850 injectors, this max is about 420 hp. I'll show you how we arrive at this number.

The first step is to calculate the flowrate in cc/min:

2 x 550 = 1100 cc
2 x 850 = 1700 cc
1100 + 1700 = 2800 cc/min

divide this by 10.5 to get lbs/hr: 2800 / 10.5 = 266.7 lbs/hr

The only variable we have to guess about is something called BSFC, which I won't try to explain (see sources). Suffice it to say that the recommendation is to use a BSFC of 0.60 or higher for a turbocharged car. The fuel calculator on rx7.com's site uses a BSFC of about 0.63, so we'll steal that.

To get HP, we use the following formula: flowrate (in lbs/hr) * inj. duty / BSFC

So, for our above 100% duty example: 266.7 * 100% / 0.63 = 423.3 hp

Note, this is theoretical flywheel horsepower. To get the equivalent torque, we use the following: 5252 * HP / RPM

Aha, now you're thinking what I'm thinking right? What if we applied this to every line of our datalog, factoring in our injector duty? Well, if I'm not way out in left field, then you should get a decent, albeit completely theoretical, dyno run.

I've attached a sample excel spreadsheet to show you my first attempt at this. I would love to see how a log that's more "dyno run-like" would turn out. Make sure to re-calculate the lbs/hr figure if you are not running the stock injectors.

Enjoy.

-Scott


Sources:

http://www.rceng.com/technical.htm
http://www.rx7.com/cgi-local/3catalog.cgi?cat=11
http://toyotaperformance.com/airflow.htm

Wargasm

Scott, it's good to see ya working on this stuff but I think there is a bug in your logic here...

Your program thinks my car makes peak torque at about 3200 rpm with my single turbo and it says I have around 425 ft*lbs which is a bit optimistic

I think it might be getting confused by the duty cycles due to the staging of the injectors. Remember that the primaries operate first, then the secondaries kick in later. So you end up with your injector duties rising up... then when the secondaries phase in, the INJ% drops suddenly because now you have ALL of the injectors working.

Perhaps you could write a spreadsheet that used the primary and secondary injector "on times" measured in milliseconds to figure it out.

Another question... the spreadsheet should have a place to put in your 0 boost fuel pressure shouldn't it? I -think- most injectors are tested at something like 40 or 43 PSI, but many people run a different base rail pressure. (I run 36 for instance). This affects fuel flow and therefore HP.

And a final note... do these calculations account for 1:1 boost compensation of fuel pressure or not?

Regards,
Brian

twokrx7

iTrader: (1)

Suggest you consider using the time data on the log and calculate how much hp/tq is needed to accel the car from the speed at one data sample to the next. You may need to skip a few samples to get decent data.

Using injector duty cycles will net bogus data. All one would need do is up the fuel to get a calculated higher hp. How does one account for the AFR, leaner usually means higher hp but would result in the opposite effect by your calcs.

skotx

Damn, looks like I jumped the gun, as usual. Oh well.
That's a good idea.
I just looked at the rc engineering site and they do have a formula for recalculating cc/min when changing fuel pressure, so it looks like it could be done.

As for your second question, I have no idea.

Wargasm

I kinda think that using the change in speed could be another good way to do it, but now you have to deal with people having different diameters of tire setups as well as the somewhat tricky to estimate drag on the car too...

Brian