Big Blue's Transformation

[Gary Lewis]

I doubt fuel is dropping out on the floor of the plenum, but it is possible.

The plenum size is definitely camping by pulses the boosters are seeing as individual intakes open.

I'm also not sure how the runner volumes compare to your old intake (which was a Performer IIRC?)

Port velocity is more important for cylinder filling than you might imagine.

I'm thinking metering needles popping up higher (or sooner)

Idle isn't cruise.

With EGR you should be able to go very lean with no load on the engine, but you need fuel on tip in in order to make power to accelerate.

Is it lean at idle, or cruise?

I'm confused.

Yes, the previous intake was a Performer.

As for idle and cruise AFR, they are typically fine. The issue is a very lean mix just off idle. It has been idling at 650 RPM and ~13:1 AFR. But I noticed today that between idle and ~1300 the AFR goes up to 16 - 18:1 and the engine doesn't pull well at all with that mix.

If you give it more throttle the AFR comes back down to ~14:1, but when you are at that low of an RPM you are usually taking it very easy on the throttle. So I'm trying to figure out how to fatten up the off-idle mix. And it isn't just a transition issue, the AFR stays lean when you are in that RPM range.

[ArdWrknTrk]

Yes, the previous intake was a Performer.

As for idle and cruise AFR, they are typically fine. The issue is a very lean mix just off idle. It has been idling at 650 RPM and ~13:1 AFR. But I noticed today that between idle and ~1300 the AFR goes up to 16 - 18:1 and the engine doesn't pull well at all with that mix.

If you give it more throttle the AFR comes back down to ~14:1, but when you are at that low of an RPM you are usually taking it very easy on the throttle. So I'm trying to figure out how to fatten up the off-idle mix. And it isn't just a transition issue, the AFR stays lean when you are in that RPM range.

Well, I know what to do with transition slots and boosters on a Holley.

But again, is it throttle position or rpm related?

I'm assuming throttle position....

[Gary Lewis]

Well, I know what to do with transition slots and boosters on a Holley.

But again, is it throttle position or rpm related?

I'm assuming throttle position....

I can't mod this Eddy since it belongs to Brandon/Bruno2 and is only on loan to me. I wonder if the Street Demon would have this problem? Hmmmm

As for throttle position vs RPM, I think it is throttle. I say that 'cause I lugged it down in 3rd today and got into the dreaded RPM range and then when I opened the throttle enough I saw the AFR come down and the engine perk up.

Which brings me to a thought I had about being jetted too lean. As said, it seems jetted about right at cruise, although it could be a smidgen richer. But I don't think a smidgen is going to solve this off-idle problem.

[ArdWrknTrk]

I can't mod this Eddy since it belongs to Brandon/Bruno2 and is only on loan to me. I wonder if the Street Demon would have this problem? Hmmmm

As for throttle position vs RPM, I think it is throttle. I say that 'cause I lugged it down in 3rd today and got into the dreaded RPM range and then when I opened the throttle enough I saw the AFR come down and the engine perk up.

Which brings me to a thought I had about being jetted too lean. As said, it seems jetted about right at cruise, although it could be a smidgen richer. But I don't think a smidgen is going to solve this off-idle problem.

We have very different expectations from and goals for our 460's.

I would never be trying to lean out my engine chasing 1-2 mpg if my plugs look right and the engine pulls the way I want.

[ArdWrknTrk]

I can't mod this Eddy since it belongs to Brandon/Bruno2 and is only on loan to me. I wonder if the Street Demon would have this problem? Hmmmm

As for throttle position vs RPM, I think it is throttle. I say that 'cause I lugged it down in 3rd today and got into the dreaded RPM range and then when I opened the throttle enough I saw the AFR come down and the engine perk up.

Which brings me to a thought I had about being jetted too lean. As said, it seems jetted about right at cruise, although it could be a smidgen richer. But I don't think a smidgen is going to solve this off-idle problem.

How long until you go to EFI?

You have Eddy jets.

Do you want to play with my 650 AVS?

I replaced the accelerator pump before I put it on the shelf.

[Gary Lewis]

We have very different expectations from and goals for our 460's.

I would never be trying to lean out my engine chasing 1-2 mpg if my plugs look right and the engine pulls the way I want.

I'm not trying to lean it out. I've enrichened the mix from where it was on the old engine and have it running nicely save for this off-idle issue. I'm willing to enrichen it more if that's what is needed, but I'm not sure that making it richer at cruise there will fix this.

[Gary Lewis]

How long until you go to EFI?

You have Eddy jets.

Do you want to play with my 650 AVS?

I replaced the accelerator pump before I put it on the shelf.

EFI is planned for this fall/winter, so I don't think I want to spend a lot of time playing with this. Especially since I'm not convinced that it is just an issue with the carb. But if there's an easy fix with this carb, or if swapping on the Street Demon might do it, I might give that a try.

So, thanks for the offer but I think I'll pass on the AVS.

Watching the AFR as the throttle and load changes has me convinced that EFI is the way to go. Keeping the mix constant has got to make a huge difference.

[ArdWrknTrk]

I'm not trying to lean it out. I've enrichened the mix from where it was on the old engine and have it running nicely save for this off-idle issue. I'm willing to enrichen it more if that's what is needed, but I'm not sure that making it richer at cruise there will fix this.

Well, the carburetor IS too lean, and you can't play with it because it's not yours.

I don't know why this engine is making that carb too lean at that throttle position.

It may be the cam. Maybe it is the intake.

Maybe a little bit of both?

Are you going to ask Scottie's advice?

He'll probably tell you to put a 750 Holley on it.

 

[85lebaront2]

Well, the carburetor IS too lean, and you can't play with it because it's not yours.

I don't know why this engine is making that carb too lean at that throttle position.

It may be the cam. Maybe it is the intake.

Maybe a little bit of both?

Are you going to ask Scottie's advice?

He'll probably tell you to put a 750 Holley on it.

Ok, gentlemen, let me point out a few things:

First, if you look inside a manifold designed to run a carburetor for street or road racing you will notice the inside of the runners have a slight amount of surface roughness. This is by design, fuel in the incoming air stream is actually tiny droplets, even though it is called vapor. These droplets are heavier than air and at low velocity (small throttle openings) will tend to settle out of the air stream. The roughness inside the runners helps to create some turbulence to keep the droplets in suspension.

Second, plenum volume can play a large part in throttle response, the bigger the volume, the greater delay in the air flow moving to the cylinders when the throttle is opened. The reason most street V8 intakes are set up as 180° design is to keep a relatively constant flow through the carburetor. The original open plenum manifolds back in the 70s were designed for high rpm use and work quite well with their relatively short, fairly straight runners at WOT, they were a pain to get good low end response though.

Lets take a look at what happens when the throttle is opened, air flow through the carburetor increases, but, due to the greater inertia of the liquid fuel, it lags, so the accelerator pump is used to alleviate this problem by squirting extra fuel in to make up for the delay. The other issue, when the air fuel mix turns a corner, the fuel, being heavier goes to the outside of the corner, this is where the roughened surface helps, the turbulence at the outer edge keeps the fuel in suspension.

Now that we are beyond that, let's examine the very lean condition at transition. This is the area of throttle between idle and cruise and is addressed in different ways, some carburetors actually have a transition slot or hole above the idle circuits that feeds fuel when the throttle plates are partially open, but before the main circuits take over, another solution is multiple booster venturiis so they main circuit comes in sooner. Another item that can affect the transition is the emulsion tubes, surprisingly, the more holes in them the better the throttle response and they can actually make the mixture too rich by adding air to it, specific example, Mercedes-Benz 2.8L SOHC sixes for the 1972 US market had dual Zenith 35/40 INAT carbs, the primary emulsion tubes had two small holes in them where the air from the primary air jet fed into the main well, these carbs were notorious for sluggish acceleration until the vacuum secondary would open. I took a pair of Holley-Weber emulsion tubes from a Pinto and installed them, I ended up reducing the main jets slightly and enlarging the air jets as it was too rich after improving the mixing in the main wells. Float levels can also affect this transition.

Since that is a borrowed carb, I would suggest installing one you own so you are free to "tweak" it. If you are still getting the super lean condition at low rpm, with the air filter off and the engine warm, observe the main nozzles as you transition, if the lean condition is there before the main nozzles start feeding then you may need to raise the float level slightly. If the lean condition is around the transition point and setting the idle mixture richer helps, then the actual transition jetting may be too lean (very common on Mopar Holleys untill a transition hole was added).

[ArdWrknTrk]

Ok, gentlemen, let me point out a few things:

First, if you look inside a manifold designed to run a carburetor for street or road racing you will notice the inside of the runners have a slight amount of surface roughness. This is by design, fuel in the incoming air stream is actually tiny droplets, even though it is called vapor. These droplets are heavier than air and at low velocity (small throttle openings) will tend to settle out of the air stream. The roughness inside the runners helps to create some turbulence to keep the droplets in suspension.

Second, plenum volume can play a large part in throttle response, the bigger the volume, the greater delay in the air flow moving to the cylinders when the throttle is opened. The reason most street V8 intakes are set up as 180° design is to keep a relatively constant flow through the carburetor. The original open plenum manifolds back in the 70s were designed for high rpm use and work quite well with their relatively short, fairly straight runners at WOT, they were a pain to get good low end response though.

Lets take a look at what happens when the throttle is opened, air flow through the carburetor increases, but, due to the greater inertia of the liquid fuel, it lags, so the accelerator pump is used to alleviate this problem by squirting extra fuel in to make up for the delay. The other issue, when the air fuel mix turns a corner, the fuel, being heavier goes to the outside of the corner, this is where the roughened surface helps, the turbulence at the outer edge keeps the fuel in suspension.

Now that we are beyond that, let's examine the very lean condition at transition. This is the area of throttle between idle and cruise and is addressed in different ways, some carburetors actually have a transition slot or hole above the idle circuits that feeds fuel when the throttle plates are partially open, but before the main circuits take over, another solution is multiple booster venturiis so they main circuit comes in sooner. Another item that can affect the transition is the emulsion tubes, surprisingly, the more holes in them the better the throttle response and they can actually make the mixture too rich by adding air to it, specific example, Mercedes-Benz 2.8L SOHC sixes for the 1972 US market had dual Zenith 35/40 INAT carbs, the primary emulsion tubes had two small holes in them where the air from the primary air jet fed into the main well, these carbs were notorious for sluggish acceleration until the vacuum secondary would open. I took a pair of Holley-Weber emulsion tubes from a Pinto and installed them, I ended up reducing the main jets slightly and enlarging the air jets as it was too rich after improving the mixing in the main wells. Float levels can also affect this transition.

Since that is a borrowed carb, I would suggest installing one you own so you are free to "tweak" it. If you are still getting the super lean condition at low rpm, with the air filter off and the engine warm, observe the main nozzles as you transition, if the lean condition is there before the main nozzles start feeding then you may need to raise the float level slightly. If the lean condition is around the transition point and setting the idle mixture richer helps, then the actual transition jetting may be too lean (very common on Mopar Holleys untill a transition hole was added).

I get how surface roughness affects laminar flow, and that you can keep the flow attached or never let it attach. Just like dimples on a golf ball.

Plenum volume is definitely acting to damp how quickly the carb gets a signal from the engine.

Like I said,do know what to do with a Holley, but not an AFB.

My AVS is a little easier to adjust (from the top) but you are the carb guru.