Calculating HP..

[Mirage2LTurbo]

Don't jump my case yet I just got my setup running, wondered if it was possible to calculate a rough HP from injector duty cycles, or any other engine stats. Thanks

 

[1badtsi]

they have this nifty thing called a dyno, it will give you a pretty good idea

 

[Mirage2LTurbo]

Yeah thanks buddy, I plan on using it for a race gas tune since I don't think street tuning would be a good idea on that high of boost. My question still stands, I'm using 45% IDC of 1680cc injectors or roughly 750cc's at 8500rpm, 1 bar.

 

[autronicDSM]

Why won't you hook up Virtual Dash software to SMC. It will calculate HP for you.

*EDIT* I think there is a hp calculator on Autronic's site.

 

[espinelli]

Below is a formula that will give you an idea.

(air flow lbs x 10) x drive train loss = estimated power

FWD Drive Train Loss ~ 15-20%

AWD Drive Train Loss ~ 20-35%??

 

[thegreatms]

First of all is 1680 cc for all your injectors combined? Or is each one 1680 cc's? I'm going to assume it’s the later.

Well I can give you an estimate based on the fuel...but it will be very inaccurate...let’s call it a Fermi order of magnitude approximation.

Some conversions:
756 cc/min * 4 cylinders = 50.4 cc/sec = .0132 gal/sec

Based on estimate that one gallon of gas contains 127 million joules of energy:
(from 48 million joules/kilo heat of combustion)
1676400 joules/sec = 1676400 watts = 2248 horsepower
Add in the thermal efficiency of .20 (this is the biggest guess: it takes into account internal engine heat loss, fuel burning efficiency, and the entire drivetrain loss)
final hp @ wheel = 2248 * .20 = 449.6 wheel hp

Here’s another estimate, it is used to get injector size from desired hp we will use it in reverse:

(((HP/# of injector)*BSFC)/duty cycle)=injector @ lb/hour per inj.

So, using a BSFC (brake specific fuel consumption) of .50 we can start.

1680 cc/min = 153 lb/hour

hp = ((153*.45)/.50)*4 = 551 flywheel hp
then to get is to wheel is another estimate lets say 80%
whp = .80 * 551 =441 whp

This method is really the same as above; it just splits up the efficiency number into a drivetrain efficiency and a BSFC.

We can check these numbers by scaling up linearly from a stock engine.

210 flywheel hp with 450 cc at .70 duty cycle (not to sure about this number, I didn't have a pocket logger when totally stock)
210 fw hp at 315 cc's
with 756 cc we get (756/315)*210 = 504 flywheel horse power = 403 wheel hp


These results are all in the same order of magnitude, so I think we have completed a valid Fermi approximation. Most of the error comes from the efficiency numbers. i.e. BSFC, drivetrain, and thermal efficiency. If these numbers where more accurate then the final answer would be much more reliable.

QED

 

[Mirage2LTurbo]

Yeah, the autronic dash has that.. I'll load it up next

Thanks for the breakdown. Like I said, I'll be at the dyno soon as I get the VCE installed, I just wanted to get a rough guess. Thanks

 

[kpt4321]

Do you know what your AFR is?

If you know your IDC and your AFR, then you know your mass airflow. If you know your mass airflow and you know that your tuning is decent, you can get a good estimate of power.

 

[Mirage2LTurbo]

45% IDC on 1680's, 12:1 a/f. How do you get airflow from those two?

 

[myblack98gst]

COLin - quit bench racing already and run that thang!! :laugh: