TPI_ElCamino

I posed a question to my auto instructors today and spent about an hour researching it. Two of the four have doctorates in Motor Works and granted this is engine theory but tell me what you think about this:
Q: How large of injectors(lbsperhour) would you have to have to maintain stoichiometric at the maximum flow of the throttle body(cubicfeetperminute)?
Background: I used stock stuff, 52mm=790cfm, and 22lbsperhour injectors.
A:Ratio is 15:1(simplified),
15:1 = 1 gallon of gas:9000 gallons of air
Gas weighs 6lbs per gallon, so 30 gallons per hour (all 8) = .5 gallon per minute.
790 cubic feet per minute = 5909 gallons per minute.
With .5 gallons per minute we need 4500 gallons of air per minute, we are supplied with 5909 gallons of air per minute with a 790CFM throttle body.
My new question is where am I wrong, or why would any body need a larger throttle body?
Please give me your educated responses.
TPI_ElCamino (edited October 31, 2001).]

[This message has been edited by TPI_ElCamino (edited October 31, 2001).]

TPI_ElCamino

I thought i would run the numbers on other combos:

58mm = 1000 CFM = 7480 GPM
52mm = 790? CFM = 5909 GPM
48mm = 640? CFM = 4787 GPM

24lbs = .533 GPM
22lbs = .488 GPM
19lbs = .422 GPM
It looks as though 24lbs injectors are a match made in heaven for a 48mm TB???

The biggest injectors i could find in jegs were accel 30 lbs/hr for 89-92 5.7 TPI:
30lbs = .666 GPM
@ 15:1/9000:1 that injector requires 5994 GPM from the Throttle Body. The TPIS air foil claims 36CFM additions so a 52mm plus air foil = 826CFM = 6178GPM, which seems to me more than enough for 30lbs/hr injectors.


[This message has been edited by TPI_ElCamino (edited October 31, 2001).]

[This message has been edited by TPI_ElCamino (edited October 31, 2001).]

kdrolt

388 cfm is 220 g/sec for air, approximately.
So 790 cfm is 448 g/sec air.

Assuming 15 parts air for 1 part fuel, then the fuel needed is 448/15 g/sec = 30 g/sec.

453.8 g equates to 1 lb, so 30 g/sec is 6.58e-2 lb/sec, which is 236.9 lb/hr total. Assuming 8 injectors, that would be 236.9/8 = 29.61 lb/hr per injector. 22 lb/hr is too small. So is 24 lb/hr. You'd need 30 lb/hr and that's with no margin for duty cycle.

Also at WOT you won't be using stoic; the air:fuel ratio will be more like 12:1 rather than 15:1; That in turn means you'll need injectors larger by the ratio of 15/12, or 1.25... or 25% larger. That's 37 lb/hr instead of 30.

FWIW these numbers are high because you let the airflow be huge: 790 cfm. At WOT this would correspond to around 530 fw hp with a 12:1 air:fuel ratio, so it's not surprising that you need a LOT of fuel and big injectors (and/or fuel pressure).

HTH and I better not be doing YOUR homework. <g> - Ken