AF lean out on 2-3 change

[Chargermal]

They are 500 cfm AVS/Thunder secondaries - I can hear them opening when I rug it...in fact I ditched the AFBs because the air doors were opening too quickly. - Even the spring loaded air doors are set fairly tight to avoid bogging.

And yes, float levels ade correct and I’m running the hi-flow seats - Ive been tuning Eddies for 30 years.

I’d respectfully suggest that if this configuration works on a Chev 350 .....my 6.1 G3 Hemi should have no issues.

[amcenthusiast]

My first thought was valve springs, not GPM fuel related.

Suggesting comparison to NASCAR engines turning 10K with only single carb and restrictor plate.

http://www.epi-eng.com/piston_engine_te ... ciency.htm

[Geoff2]

Loosen a/valve spring 1/4-1/2 turn & increase sec jets 2 sizes.

[Tuner]

Something to consider related to A/F change after a shift:

Assuming the A/F exiting the carburetor remains the same or nearly so regardless of WOT air flow change, what will be the effect of intake port air velocity change on ratio of puddle to vapor?

As mean port velocity is related to mean piston speed and absolute pressure in port is related to air velocity, what will be the effect of RPM drop on percentage of fuel in liquid form on the walls (AKA "the puddle") vs. vapor in the air stream and subsequent effect on A/F reaching the cylinder?

[steve cowan]

Something to consider related to A/F change after a shift:

Assuming the A/F exiting the carburetor remains the same or nearly so regardless of WOT air flow change, what will be the effect of intake port air velocity change on ratio of puddle to vapor?

As mean port velocity is related to mean piston speed and absolute pressure in port is related to air velocity, what will be the effect of RPM drop on percentage of fuel in liquid form on the walls (AKA "the puddle") vs. vapor in the air stream and subsequent effect on A/F reaching the cylinder?

Interesting question Tuner but I doubt there would be a one size fits all answer alot of variables-
Application
Induction tract CSA and length
Bore/ stroke / intake valve
Max piston speed
# of carbs and size
Efficiency of engine / pumping losses
how the cylinder fills after BDC / VE related etc etc
Hopefully you can educate us Tuner

[Tuner]

Something to consider related to A/F change after a shift:

Assuming the A/F exiting the carburetor remains the same or nearly so regardless of WOT air flow change, what will be the effect of intake port air velocity change on ratio of puddle to vapor?

As mean port velocity is related to mean piston speed and absolute pressure in port is related to air velocity, what will be the effect of RPM drop on percentage of fuel in liquid form on the walls (AKA "the puddle") vs. vapor in the air stream and subsequent effect on A/F reaching the cylinder?

Interesting question Tuner but I doubt there would be a one size fits all answer alot of variables-
Application
Induction tract CSA and length
Bore/ stroke / intake valve
Max piston speed
# of carbs and size
Efficiency of engine / pumping losses
how the cylinder fills after BDC / VE related etc etc
Hopefully you can educate us Tuner

You are overthinking this. The answer is in the question.

The engine details you mentioned are ordinarily important but not in this very basic case which applies to all engines regardless of configuration. Disregard all that usual stuff and simply visualize the basic physics of port velocity, pressure, and fuel vapor to liquid ratio. Consider port velocity to directly correlate with piston speed, piston speed changes, air velocity changes.

Though it isn't necessary to grasp the basic premise, also to consider is the distillation curve of the fuel and relationship of boiling points of fuel constituents at various absolute pressures (which in this case of discussing engines we ordinarily call "vacuum" ) and how the ratio of vapor to air changes when velocity and pressure change in the port runner, valve bowl and through the intake valve curtain.

[Rick!]

Loosen a/valve spring 1/4-1/2 turn & increase sec jets 2 sizes.

I'd also change the plugs before the next asphalt session. With no data (especially rpm trace) or time slips or plug readings or even fuel level in the tank, might as well go full old school on fixin' it with a screwdriver and an handful of brass. Since 2x4 426 Hemis had 3/8" fuel lines back in the day, I'm thinking that the pump feed line isn't the issue either unless the Carter vane pump truly can't suck.
This appears to be the combination, engine-wise, but we still don't know what car, what rear gear, weight, fuel tank setup and the configuration of the cold air setup. The 2x4 dual plane setup falls down up top and makes less hp than a single plane with a 4150 sized throttle body.
https://www.hotrod.com/articles/test-ed ... en-3-hemi/

[BradH]

Something to consider related to A/F change after a shift:

Assuming the A/F exiting the carburetor remains the same or nearly so regardless of WOT air flow change, what will be the effect of intake port air velocity change on ratio of puddle to vapor?

As mean port velocity is related to mean piston speed and absolute pressure in port is related to air velocity, what will be the effect of RPM drop on percentage of fuel in liquid form on the walls (AKA "the puddle") vs. vapor in the air stream and subsequent effect on A/F reaching the cylinder?

This made me wonder what I would have seen from testing in the past had I owned a wideband 02 at the time. Two carbs that showed virtually the same HP and torque curves on an engine dyno, but one was .2 quicker and 2 MPH faster on the track. Since the quicker carb had smaller venturi and the improvements were believed to be the results of better shift recovery, how would the AFR readings compared between the two carbs (especially after the 2-3 shift)?

[Tuner]

Some Edelbrock carb secondary venturi clusters have a "pinched" emulsion tube that has a .073" orifice in the bottom of the tube which the fuel passes through to enter the tube, so regardless the secondary main jet size the fuel flow to the discharge nozzle is restricted by the .073" orifice. Further, the secondary main air bleed is also .073" which is ridiculously large. The combination of the pinched main well tube and huge air bleed renders the carbs with these secondary clusters unable to be jetted rich enough for a performance engine. If you have these secondary clusters that is the likely reason the power is lacking at high RPM. That, and it is likely some arrangement of stagger jetting is necessary. If you have read the carb section in a Mopar engine book you know what that could entail.

[steve cowan]

Something to consider related to A/F change after a shift:

Assuming the A/F exiting the carburetor remains the same or nearly so regardless of WOT air flow change, what will be the effect of intake port air velocity change on ratio of puddle to vapor?

As mean port velocity is related to mean piston speed and absolute pressure in port is related to air velocity, what will be the effect of RPM drop on percentage of fuel in liquid form on the walls (AKA "the puddle") vs. vapor in the air stream and subsequent effect on A/F reaching the cylinder?

Interesting question Tuner but I doubt there would be a one size fits all answer alot of variables-
Application
Induction tract CSA and length
Bore/ stroke / intake valve
Max piston speed
# of carbs and size
Efficiency of engine / pumping losses
how the cylinder fills after BDC / VE related etc etc
Hopefully you can educate us Tuner

You are overthinking this. The answer is in the question.

The engine details you mentioned are ordinarily important but not in this very basic case which applies to all engines regardless of configuration. Disregard all that usual stuff and simply visualize the basic physics of port velocity, pressure, and fuel vapor to liquid ratio. Consider port velocity to directly correlate with piston speed, piston speed changes, air velocity changes.

Though it isn't necessary to grasp the basic premise, also to consider is the distillation curve of the fuel and relationship of boiling points of fuel constituents at various absolute pressures (which in this case of discussing engines we ordinarily call "vacuum" ) and how the ratio of vapor to air changes when velocity and pressure change in the port runner, valve bowl and through the intake valve curtain.

In theory the velocity/vapour / puddle should remain constant ratio between themselves,
I have read where if you are using a rough plenum floor finish liquid fuel can sit in these divets and spin in a circle no matter how much load or rpm is put on the engine,all to do with vacuum under the throttle blades I guess.

[Chargermal]

The car does not noticeably ‘nose over’, but had the motor running 116mph in a previous car of similar weight (3650lbs with driver) with no lean out - so the current 113 mph with better headers and more comp is telliing me something.

There are far too many unchecked variables to start ‘internet tuning’ unfo - Timing curve, spacers etc.

Timing curve has not been dyno checked - I WAS running a 4 hole and a 1” open spacer on the old build - and did notice a BIG improvement. Im only running 4 holes atm, so maybe plenum volume could be an issue?

The valve springs are PSI 1511 ML- cam is .565 lift Hyd Roller. Springs are two years old , 6.1 SRT heads flowed 312cfm at .650 (picked up 14 cfm @ .400 with a seat cut).

Gears are 4.10, 60ft doesn’t bare repeating because of old tires and zero track prep. I change at 6600rpm. AF is rock steady at 12.7 till I shift 2-3.

Main thing is I posted to deal with the AF lean out....the tuning I can chip away at.

[rebelrouser]

I have been racing NSS, using 2, 750, federal mogul AFB's for almost 30 years. I have O2's on my car and have had a couple different engine combinations on the dyno. My car now runs 9.70's 139 mph. All the AFB's I have ever used lean out at high RPM's. To help with this issue I have done the following which made it better. First AFB's have a very small float bowl, if you take the top off the carb, and just lay a float in the chamber you will see what I am talking about. My son is a mechanical engineer, and in discussing this issue with him, his work had a computer program for calculating flow and other variables in hydraulic systems. I gave him the specific gravity of the fuel I was using, and all the dimensions of the fuel system, and the demand for fuel we used was .5 lbs per horsepower. His results, said the needle and seat orifice and the orifice in the pressure regulator were the restrictions in the system. I changed out the pressure regulator, got new large orifice needles and seats, and drilled the carb bodies for dual feed. It still leaned on the top end but it was an improvement. I did notice that it became very consistent after the modifications, most of the time slips were nearly identical, unless the driver screwed up.
There are some older threads about the newer AFB clone carbs and the boosters etc. on speed talk, look them up, lot of good info. My next step was to put the carbs on a flow bench and use the velocity probe in the jet to measure how hard the venturi pulls on the jet. By polishing and trimming on the boosters, I got all eight to pull the same amount. Next step was to lighten the secondary weights, and measure how much vacuum it took to open them, and again grinding and swapping spare parts until I got exactly equal secondary opening. Also on the secondary opening, the amount the blades open can really affect the amount of pull on the jets, if the blade opens too far and goes slightly over center it really messes with the booster signal. I say this because lots of guys are cutting off choke air horns for more flow, and are letting the secondary blade open too far because they cut off the material that used to stop it. I also changed the way I cut off AFB airhorns, because of the turbulence you get on a flow bench around the air bleeds and top of the boosters if you cut them too short.
Then in reading the older speed talk posts on AFB's I made some thin sleeves to extend the primary boosters into the narrowest part of the venture. That also gave it another step in the booster which my idea was to improve fuel atomization. Not sure what helped the most, or if I even did everything right, but those carbs are worth 35 HP on my dyno over my old ones. And guess what, they still lean out some on the big end, so I am chalking it up to the small float bowls.
And just on my car, I have found it likes more ignition timing at the launch, and less on the big end, so I have an old MSD retard that works off rpm, to retard the timing at 5,000 rpm. When I first put it on, it was worth a tenth. I need to see if it still works with my current combo, that was 4 engines ago.
I know guys that are running NSS cars into the 8's with AFB's and I have seen their carbs apart and they don't have a ton of modifications, I am sure they lean out as well, I don't see how they could not. My car runs through 1st and 2nd right at 12.5, then in 3rd , it leans to 13 to 1 as I cross the finish line, if I richen it to run 12.5 across the line it slows down, and plugs read rich. On the dyno the air fuel remained pretty constant, but the length of the pull was not as long as a run, and of course the load on the engine is different than a pass down the dragstrip.

[Tuner]

Then in reading the older speed talk posts on AFB's I made some thin sleeves to extend the primary boosters into the narrowest part of the venturi. That also gave it another step in the booster which my idea was to improve fuel atomization. Not sure what helped the most, or if I even did everything right, but those carbs are worth 35 HP on my dyno over my old ones.

What was that? Did F-BIRD'88 vapor lock?

[Tuner]

I have been racing NSS, using 2, 750, federal mogul AFB's for almost 30 years. I have O2's on my car and have had a couple different engine combinations on the dyno. My car now runs 9.70's 139 mph. All the AFB's I have ever used lean out at high RPM's. To help with this issue I have done the following which made it better. First AFB's have a very small float bowl, if you take the top off the carb, and just lay a float in the chamber you will see what I am talking about. My son is a mechanical engineer, and in discussing this issue with him, his work had a computer program for calculating flow and other variables in hydraulic systems. I gave him the specific gravity of the fuel I was using, and all the dimensions of the fuel system, and the demand for fuel we used was .5 lbs per horsepower. His results, said the needle and seat orifice and the orifice in the pressure regulator were the restrictions in the system. I changed out the pressure regulator, got new large orifice needles and seats, and drilled the carb bodies for dual feed. It still leaned on the top end but it was an improvement. I did notice that it became very consistent after the modifications, most of the time slips were nearly identical, unless the driver screwed up.
There are some older threads about the newer AFB clone carbs and the boosters etc. on speed talk, look them up, lot of good info. My next step was to put the carbs on a flow bench and use the velocity probe in the jet to measure how hard the venturi pulls on the jet. By polishing and trimming on the boosters, I got all eight to pull the same amount. Next step was to lighten the secondary weights, and measure how much vacuum it took to open them, and again grinding and swapping spare parts until I got exactly equal secondary opening. Also on the secondary opening, the amount the blades open can really affect the amount of pull on the jets, if the blade opens too far and goes slightly over center it really messes with the booster signal. I say this because lots of guys are cutting off choke air horns for more flow, and are letting the secondary blade open too far because they cut off the material that used to stop it. I also changed the way I cut off AFB airhorns, because of the turbulence you get on a flow bench around the air bleeds and top of the boosters if you cut them too short.
Then in reading the older speed talk posts on AFB's I made some thin sleeves to extend the primary boosters into the narrowest part of the venture. That also gave it another step in the booster which my idea was to improve fuel atomization. Not sure what helped the most, or if I even did everything right, but those carbs are worth 35 HP on my dyno over my old ones. And guess what, they still lean out some on the big end, so I am chalking it up to the small float bowls.
And just on my car, I have found it likes more ignition timing at the launch, and less on the big end, so I have an old MSD retard that works off rpm, to retard the timing at 5,000 rpm. When I first put it on, it was worth a tenth. I need to see if it still works with my current combo, that was 4 engines ago.
I know guys that are running NSS cars into the 8's with AFB's and I have seen their carbs apart and they don't have a ton of modifications, I am sure they lean out as well, I don't see how they could not. My car runs through 1st and 2nd right at 12.5, then in 3rd , it leans to 13 to 1 as I cross the finish line, if I richen it to run 12.5 across the line it slows down, and plugs read rich. On the dyno the air fuel remained pretty constant, but the length of the pull was not as long as a run, and of course the load on the engine is different than a pass down the dragstrip.

Very impressive effort and real results to show for it. Have you used smaller main air bleeds? Have you made new emulsion tubes with fewer and smaller E-bleed holes? You must have a set of original 409 carbs to study? Notice how simple the primary booster E-tube is and that there is no tube in the secondary boosters? Did you notice the discharge nozzles are larger than the FM and Edelbrock versions ?

[Chargermal]

I changed mine ages ago to the AVS/Thunder design because of the air doors pulling open too soon.

I may be making too much if the lean out....especially as I have not resolved the timing curve yet.

I know another friend with a 600HP G3 Hemi 390 cube stroker who made 35rwhp with an xtra 2 deg timing - these motors don’t need much but respond big time to minor timing adjustments - his is EFI so its easier to tune at the track with the Holley software.

Anyhow, Ive already pulled the trigger on the Tanks Inc 67 Belvedere EFI tank and pump - so if the problem continues I’ll have somewhere else to look.