AFR calculations

[Storer]

I thought I would try some maths before building my fuel system, but I need some information from those with far more knowledge than I.

What is the air to fuel ratio of an LS7 at wide open throttle. I know it will vary depending on tune and is adjusted by the ECU according to various parameters but there must be a known range.

Does anybody know what a standard LS7 and a tuned 700hp LS7 AFR are at WOT?

When doing my calculations I will be assuming an air consumption of between 25,000 l/m and 30,000 l/m. Are these reasonable?



Paul

[738 driver]

Paul, the GM factory ECU defaults to pig-rich at wot....with .82 - .87 Lambda usually seen on untouched dyno runs.

Edited by 738 driver on Friday 14th October 14:53

[macgtech]

738 driver said:

Paul, the factroy ECU defaults to pig-rich at wot....with .82 - .87 usually seen on untouched dyno runs.

I presume the factory ECU uses throttle as load, but does it have a correction in for the MAP sensor?

[harry b]

An engine is basically an airpump. So volume of air displaced multiplied by the VE gives an aproach figure. If tuned right, VE will be somewhere around 1.15.
Fuelratio is 1:12.5 for max power. 1:14.1 for efficient.
If your EGT goes up too much you can use more fuel to cool, but it won't give more power.

If you have a new engine, don't use 1:12.5 ratio's to avoid the risk of cylinderwash, but I presume you know. I wonder you didn't know these numbers too, or I fully misunderstood your question.

Modern engines are different in being able to run leaner, so don't use these basics on these.

[Storer]

Well, at a ratio of 1:13 (fuel to air) that is 1923 litres of fuel/min assuming 25,000 lit air / min or 2300lit at 30,000.

Surely that can't be right!!!!

I know these engines are thirsty but that is impossible. I assume I have got it completely wrong.

Idiot's guide please.


Paul

[anonymous-user]

You need to us "mass" not "volume".

I.E. Stoichiometery is achieved at a 14.7:1 ratio of mass !!!! (hint, air is pretty light, fuel is relatively heavy ;-)

[738 driver]

...........for starters 1225 gms per cu mtr at 15 deg C

Edited by 738 driver on Friday 14th October 22:29

[Storer]

Ok, I will make it clear.

I am a total numpty in this area.


Air weighs 1.2kgs/1000lit at 20 deg, so 30 to 36 kgs air/min so fuel usage is 2.14 to 2.6 kgs / min (all approx figures).

Petrol weighs approx 0.71kgs/litre so we would need 3.0lit to 3.66lit per min.


Am I getting warmer?????


Paul

If I have cocked up somewhere it could be due to the bottle of wine I have had this evening.

Edited by Storer on Friday 14th October 22:59

[Storer]

OK a bit more on this.

My Aeroquip A1000 hp pump will deliver 383 litres/hour at 60 psi (600lbs).

The engine will use between 180 and 220 litres/hour AT MAX REVS.

I know we need excess fuel deliverable at all times but do we need nearly double the demand at WOT????

Aeroquip do state that their A1000 pump will support a 1300hp NA engine so this supports my calculations.

I do not want to return fuel to the swirl pot from the HP side (going back to the tanks) so it will be very important that the LP pump can over deliver to keep the HP pump supplied.

Can't find the make of my LP pump. Only numbers on it are 19375-117 and 1437A. Could the 117 be gallons/hour?

Paul

[spatz]

these whole calculations make not much sense, since you never run the car at full wot for more than let us say 1 minute, unless you go to NARDO maybe.
So in general your catch tank must be able to deliver this amount of fuel that you need in the worst secenario plus the amount of fuel your LP pump can deliver at that time. I have a facet pump that does about 170/liters per hour you can fill up the catch tank at a rate of 3 liters/minute. If your catch tank holds 2 liters, you can take 5 liters of fuel/min before your hp pumps would start to run dry. Then your engine will eat 300 lites/hour which is probably a very safe figure.

Edited by spatz on Monday 17th October 15:12