AFR

[topcatcustom]

Thought this may be handy as its own thread, I have a wideband AFR sensor and gauge fitted which I am sooooo glad of, would never set the carb up without it! Question is- I know 14.7 is optimum afr for emissions etc, but what should I be aiming to see on the gauge in terms of range?

At the moment it sits around the 14.0-14.8 mark at idle, but when driving it will vary between 10 and 17 (the extremes), obviously it will change when you take you foot off the go pedal and still moving quickly, and when you slam it to the ground due to the accelerator pump giving a squirt etc, but is there a rule as to what exactly to aim for or should I just try to get it as close to 14.7 as possible as much of the time?

Also just how much adjustment is possible with the eddy 4bbl? The rods only have 2 steps in them so not exactly what you would call infinite adjustment (though I know you can tweek the rods etc by hand)...

[sidecar]

I can't my engine to run that lean on cruise, it goes all flat and 'orrible!
The tickover starts to go unstable if I go too lean as well.
My mates 7.0 SBC is the same, it won't go any leaner than my setup on cruise (He uses a 800 Eddy, I use a 500)

My AFR's are Idle=12.2:1 Cruise=13.5-14:1 Accel=13:1 WOT=12.5:1

To get the above settings I run the following setup:-

Primaries = 86
Rods = 67-47 (one off's and I might go to 67-45 to richen up the acceleration a touch) If I do this mod I may have to go slightly smaller on the secondary jets.
Springs are silver.
Pilot screws out 2+1/4
Accel pump in middle hole.
Secondaries 66 (one off's as the smallest you can get are 77's)

Don't pay any attention to the 14.7:1 figure, that is just figure if you burn't petrol in a laboratory, in an engine the mixture is not homogeneous so it is not the same.

Set the pilots up for the best tickover, I start with them both out quite rich, maybe 2+3/4 turns, slowly turn them in 1/8 of a turn going from one to the other the revs may rise as you start turning the screws in. At some point the revs will just start to drop VERY slightly. Stop turning the screws in at that point and go out 1/4 of a turn on each screw. Then check out what they are set to, if one is 1/8 different you can tweak it to the same as the other. Set the cruise as lean as you can go before the response goes bad. Set the acceleration to around 12.5:1 and the WOT to 13-13.5:1 (You can see from the figures above my WOT is very slightly too rich, but not much).

Of course with a blown lump you may want to go a touch richer just to keep things cool.

There is a good book in the Speed Pro range on these carbs, it mentions using a special plate under the carb on blown engines so that the rod pistons sense the engine vacuum, not the blower vacuum.


Pete

[kiwicar]

Hi Tom
with the blower between the carb and the engine you should be able to run closer to 14.7 than without, you get a nicely churned up mixture with little fuel dropping out of suspension, this is particularly true with a 4 barrel which normally delivers a very badly atomised mixture. You will benefit most from a run on a dyno with this engine as the blower will to an extent will enable you to run a less than ideal mixture, as I said earlier the blower really churns up the inlet charge very well.
With 14.7:1 ratio fully burnt you get most energy of the fuel as the petrol fully burns to water and co2. With richer mixture some of the fuel only burns to water and CO which gives out about 1/10th of the energy.
I would start by reducing the amount of fuel pumped in by the accelerator pumps so that you run in the range of about 12.5:1 to 13:1 as you press the throttle (judge it by making it as lean as possible without it hesitating when you push the throttle, start lean, rich-en it as you go until it "softens" then come back to your previous setting) 14.5 to 14.7 under light cruse and 13.5 to 14 to1 under hard acceleration, then take it to a dyno and tweek it from there for maximum output (I don't think it will be far out). With the blower set up the tuning is allot more subtle than without as the fuel isn't sitting on the walls of the manifold it goes into the engine and burns!
Best regards
Mike

[DaveEFI]

Thought this may be handy as its own thread, I have a wideband AFR sensor and gauge fitted which I am sooooo glad of, would never set the carb up without it! Question is- I know 14.7 is optimum afr for emissions etc, but what should I be aiming to see on the gauge in terms of range?

At the moment it sits around the 14.0-14.8 mark at idle, but when driving it will vary between 10 and 17 (the extremes), obviously it will change when you take you foot off the go pedal and still moving quickly, and when you slam it to the ground due to the accelerator pump giving a squirt etc, but is there a rule as to what exactly to aim for or should I just try to get it as close to 14.7 as possible as much of the time?

Also just how much adjustment is possible with the eddy 4bbl? The rods only have 2 steps in them so not exactly what you would call infinite adjustment (though I know you can tweek the rods etc by hand)...

This might be some use:-

http://www.pumaracing.co.uk/combust.htm

[topcatcustom]

Cheers people, after a change of rods I am pretty much in the 13's and 14's all the time now, will do some more driving and see how it goes when I start putting my foot down more!

[bones]

just out of interest, how can a wide band afr do both banks as it only reads once the gases are in the single part of exhaust. So could 1 bank be different in set up to the other, but then i dont know how they work rich

[sidecar]

just out of interest, how can a wide band afr do both banks as it only reads once the gases are in the single part of exhaust. So could 1 bank be different in set up to the other, but then i dont know how they work rich

A wideband probe won't do both banks unless you put a boss in both collectors. Having said that there is not much point anyway as you can not adjust a AFB carb on a cylinder by cylinder basis anyway. (You could get clever with fuel dams and fuel slots but you would have to be pretty darn keen to put it mildly!)

[bones]

so using 2 narrow band with 1 in each manifold would be a better choice ?? rich

[mgbv8]

Rich!
Narrow bands need to be referenced on a dyno or with a co gauge really as they are not that accurate . When I set mine up I used an external gas analyser at idle. When I got idle right I marked the narrow band gauges. Both were reading half way into the rich band. I used this as my ideal indication.

[bones]

found these at not to bad a price, http://prosportgauges.com/digital-air-f ... gauge.aspx . would change to use just as a fuel meter, it works out at £115 for both posted to me and thought digital read ut would be more accurate, rich

[DaveEFI]

so using 2 narrow band with 1 in each manifold would be a better choice ?? rich

Narrow band isn't much use as only cars with cats run at 14.7 AFR most of the time. A wideband one will read AFM outside the normal limits of any engine - ie from about 9:1 to 19:1.
I use and like the Tech Edge unit which is available as a DIY kit. Building it yourself means it can be repaired at component level rather than just replacing the lot if there are problems. I've had mine for over a year without any.

[topcatcustom]

My wideband sensor is slap bang in the middle of the X pipe so measures all cylinders- ok I cant fault find which bank is rich or lean but I have an open manifold anyway so shouldn't vary.

[Ian Anderson]

Tom

Surely from top to bottom you have

Air filter
Carb
Supercharger
Manifold
Heads

In which case the supercharger will be mixing whichever fuel air mix it gets and distributing it to all the cylinders

If you have
Air filter
Supercharger
Carb
Manifold
Heads

As the pressure builds up it will lean out (unless you also pressurise the fuel bowls)

One way your engine will melt the other it should be good!

Ian

[bones]

How much difference does it make with the edelbrock if side 1 is 14.9 and side 2 is 14.0 as example with a dual plain inlet. And what difference at wot, or would it equal itself out ??? rich

[topcatcustom]

Ian you're right, the rich/lean thing was just an example of something I can't do with my sensor measuring all cylinders. Though still pretty pointless- the only way it would be really handy to fault find would be to have a lambda on every primary but that would just be daft!!!