Stupid question, air/fuel ratio

Does anybody collect data or have thoughts about a "good" air/fuel ratio for a blown alcohol BBC ?????????

 

my fuel sytem shows launch about 4-1 and top end at 5-1, this is from spud millers tune.

 

So my 3.2 is like WAY fat then ????????

 

Id say a good safe base is around .330 gallons/per 1 lb of boost.

When you get around .305-.300, your getting a little "happier"..

 

Cool, Thanks !

 

The appropriate AFR with methanol completely depends on cylinder pressure. So, the amount of boost you're making, compression ratio, load on the motor, intake valve closing, etc. figure into that.

A motor with 8:1 compression making only a few pounds of boost does just fine with a 5:1 AFR. Something running 11:1 compression and 15 PSI boost will want a richer AFR around 4.4:1 or so. A 13:1 compression motor getting 45 PSI boost needs around 3.5:1.

A motor may not see high boost, but if the static compression ratio is high, the intake valve closes early, heavy car, tall tire, etc. it'll want more to eat.

On the other hand, someone with poor heads/cam may show 30 PSI boost, (stacking in the runners) and it'll be a dog if fed the diet that 30 PSI boost would dictate. The cylinder pressure and heat achieved just won't burn all that.

Spud
www.fuelinjectionent.com

 

Spud,

Thanks for the excellent explanation of the requirements of an alky engine and the factors controlling them.

Richard Gavle

 

I'm going to agree with mike. It is what it is. I've run numerous cars and used the same methods. And not hurt anything. And had success.

Not saying anyone else is wrong or trying to cause an argument, but that's how I do it.

 

My experience (and success) flowing and tuning an extremely diverse group of different combinations has shown me that different setups can require very different AFRs. Cylinder pressure is key and the various factors mentioned contribute to the final result. This is true for supercharged or naturally aspirated applications.

But if you have another way to get there that works for you, that's sure fine with me

Spud
www.fuelinjectionent.com

 

Certainly, the more air in the cylinder the more fuel you need to go with it. There's no doubt about that. But that isn't the only determining factor. Every motor running methanol does not want the same ratio of air to fuel from what I've witnessed. So, I have to disagree with you.

The more cylinder pressure, the higher the heat of combustion and the more fuel the motor will burn (provided it has enough ignition to light it - another cylinder pressure sensitive issue).

Put 20 pounds of boost to a motor with a 13:1 compression ratio and I absolutely guarantee it will burn more fuel per unit of air than the same exact setup with only a 9:1 compression ratio and 20 pounds of boost. We only changed how hard it is squeezing the air and fuel that made it into the cylinder, we didn't change the amount of air getting there. So, to burn more fuel in this case means the AFR is indeed richer. Do we agree?

A blown motor removed from a 1400 pound dragster needed more fuel when made to power a much heavier door car. The load changed a bunch and it ate more fuel per unit of air. The guy had to jet it a couple of steps richer. It ended up being about 12% less main pill area. Not huge, but certainly observable and significant.

A few years ago, a local guy built an injected motor nearly identical to his buddy's that had been running really strong with a .090 pill. Instead of 14:1 compression though, he put some blower pistons in it that he had and ended up at a miserable 9.7:1. Same block, displacement, heads (even ported and flowed by the same guy), cam, rocker ratio, injector, pump flow was within .03 GPM, comparable mag, gears, tires and converter in a dragster that weighed within 100 pounds of his buddy's. The cars and motors were practically twins. All the same stuff, but the difference in compression landed him with a .130 pill. He was about 4 tenths slower too. Cylinder pressure made a huge difference in what it would actually burn. Pretty close to the same air getting into the cylinder, but a big difference in the amount of fuel it wanted. Squeeze was the only significant difference.

I see evidence all the time that different setups dictate a different AFR and that it very closely follows factors that ultimately determine cylinder pressure.

If we don't agree on at least some of this by now, it's probably hopeless I would guess that you're used to mostly playing with monster pro mod or top alcohol motors that are roughly the same in these keys areas. Therefore, the AFRs you run are all pretty close to the same and the fuel-to-boost multiplier mentioned earlier works well for you. However, I bet a tractor puller or a little dragster with a 671 running straight up wouldn't fare as well with that ratio.

Spud
www.fuelinjectionent.com

 

...and I can't disagree with a word you typed.

However...practically speaking, we have to do the math to calculate a tuneup at some point which ultimately is pounds of fuel to pounds of air to derive a ratio. I'm not sure how I'd compensate the calculations for the factors you mentioned except that the practical AFR is indeed richer. We can't really say: "5:1 AFR at the molecular level with 2.2 GPM added for the adiabatic heat of compression" It may be the scientific truth, but it isn't going to do me much good with the calculator. The 5:1 plus the hypothetical 2.2 GPM may calculate out to something like 4.1:1 AFR. And then knowing what a cubic foot of race day air weighs - thanks to my weather station - I know exactly how much fuel to dump in it now to make everything happy.

So, my take on it is that we are indeed in agreement that if a guy sits down with his calculator to figure an AFR (or how much fuel he needs to feed his motor), the PRACTICAL ratio he needs to use is going to be richer as effective cylinder pressure is increased. Some people don't ever calculate an AFR at all, and that works for them. I'm not sure how a person would precisely compensate for weather without doing that in some form or another.

Here's something related...

A puller running a Super Mag II and tons of boost and static compression is getting nice plug color and great performance with a .130 main pill. He borrows his buddy's Pro Mag 44 for the next pass (same density altitude) and has to richen the mixture 5 steps to make the motor happy now (same plug color and EGT). The air going into the motor doesn't change a bit. But he's now running a .105 pill and burning 1.3 GPM more fuel with the same amount of air ingested. His practical, calculated AFR just went from 3.82:1 to 3.43:1. Yet another tuneup consideration, but still somewhat related to cylinder pressure. In a motor NOT making tons of cylinder pressure, a mag change like that usually does not make that large of a difference and the AFR would not make a big shift like that.

Spud
www.fuelinjectionent.com

 

Great post`s Spud and Mike,..

Heavy stuff man,..heavy.....

Thank you both



Vic


.

 

WOW, great stuff, Mike and Spud!! You guys should teach a class, "Advanced Fuel System Calculations"

 

Mike and Spud. This is Moparious Maximus's Dad Great stuff and way over my head. I think you both forgot to add in the effect of the movement of tectonic plates under the track. Thus disrupting the magnetic flux lines in your flame front creating a plasma wave on the leading edge. Damn now thats heavy stuff. SMILE just a little fun. Larry reads all he can on this site and gets good info me I,m just a wrench. Thanks Larry Sr.

 

Larry, you forgot to include that they will need 1.21 gigawatts of electricity (achieved through a nuclear reaction, lightning bolt, or Mr. Fusion device) occuring simultaneously at 88 mph, to achieve the temporal displacement event.

Thanks to Spud and Mike for the great info though.

Chris