Holley Carb - IFR Top or Bottom?

[USMC_Spike]

Okay, I'm at the point where
I can go forward or stop circle the wagons then move on

What have been your observations of:

Holley carb with the Idle Feed Restrictors (IFR) at the top?


Holley type carb with the IFR at the bottom?

Cheers,

Spike

[levisnteeshirt]

what application ??? do you want to see epic dyno print outs , or throttle response ?

if its anything other than a drag car leaving off a brake and a 2 step when its held wide open or not on a dyno when the motor is not throttled and loaded from an idle position , , but brought to a state in its range and then sent to WOT and loaded ,, I like 'em at the top

the crate late I'm doing testing on was fastest in practice this weekend at a 3000 purse event ,, soo ,, every color of the rainbow carbs were there ,, black , red , green ,, my basic holley color carb busted their asses ,

[Holley Carb Guy]

Honestly, I haven't seen a difference and I get really good drivability and throttle response. Sometimes its all in the way you skin the cat and there are a lot of ways to skin it

[jmarkaudio]

Every one we change improves idle and transition quality. And too many overlook the importance of that even in a race engine.

[levisnteeshirt]

this will prolly turn into a chicken or the egg type of thing ,, I doubt there is any serious HP or torque gain that you'll ever feel by switching them ,, but throttle repsonse always gets better when i move them up ,, just what I see

for a circle track or street car ,, there is no way I would put them on the bottom

if MPG is your goal ,, i might leave them on the bottom

[Tuner]

If moving the idle jet to the top of the block improves throttle response it indicates there was a need for a more aggressive accelerator pump.

Anybody who has the opinion a high idle jet is good is not working with any data systems which supply accurate AFR information or they don’t understand the following information.

Locating the Idle Jet below float level is more important than many people realize because cruising at a low RPM and steady speed the fuel delivery is nearly 100% through the idle circuit (T-slot), and, because intake vacuum oscillates rapidly over a wide range, with the jet above float level the AFR oscillates a lot, as much as 2 AFR or more. Smooth idle, clean sparkplugs and oil, and good fuel economy, all depend on a consistent idle-low speed circuit.

An un-damped vacuum gauge will clearly show engine manifold pressure (vacuum) oscillates constantly and as valve timing becomes more radical the increased overlap causes larger pressure swings and more intense oscillation.

An un-damped gauge clearly shows engines are not a steady flow device. The intake pressure (vacuum) is oscillating with each intake suction stroke and overlap reversion pulse. Because passages and cavities in carbs and manifolds have individual resonances at particular frequencies, the resultant effects are more pronounced at some engine speeds than others. Because air is compressible, it is elastic, it can rebound like a spring and as a result it bounces and reverses direction in small passages in carbs.

With a high (above float level) idle jet, air can easily get upstream behind the jet when the idle fuel is pushed backwards by the pulsing.

When the jet is submerged, the air cannot get back upstream of it as easily because the much greater viscosity of the liquid fuel (than air) on both sides of the jet impedes the backflow and the air cannot get back through the jet unless the flow is reversed for a longer period of time than the frequency of the pulsations.

Another undesirable phenomenon caused by the high idle jet is the larger oscillations in the idle circuit causes a pumping effect on the main well that results in the booster nozzle dripping in spurts and blobs before it would normally start to flow if the venturi air flow were the only excitation and the idle circuit flow was more regular. With a submerged jet the metering reversion isn’t as severe or does not occur at all unless the pressure reverses for a longer time period than the manifold pressure (vacuum) pulses at a cruise.

This submerged idle jet phenomena was thoroughly studied and well understood 95 years ago during aircraft carburetor research for World War One.

[raceczar]

If you are using OE Holley blocks put the IFR in the bottom. If ou have billet blocks, move it to the top. I don't understand why anyone would ever fool with OE Holley metering blocks if you don't need to.

[Tuner]

If you have billet blocks, move it to the bottom. I don’t understand why anybody would fool with a high idle jet if you don’t have to.

Billet blocks have no advantage over (old school) OE Holley blocks. Billet blocks with a high idle jet have a disadvantage until the jet is moved to the bottom of the idle well.

It doesn’t matter what kind of block - or carburetor, Holley or otherwise - an idle jet above float level is discharging into air and is more subject to the pulsation of the idle and transition orifices exposed to intake pulsing. Why is it so hard to understand that a carburetor is not a steady flow device because it is attached to an engine which is not a steady flow device …?

[lee_beazley]

Re: Holley Carb - IFR Top or Bottom?

Post by Tuner » Wed Sep 04, 2013 2:04 pm

If you have billet blocks, move it to the bottom. I don’t understand why anybody would fool with a high idle jet if you don’t have to.

Billet blocks have no advantage over (old school) OE Holley blocks. Billet blocks with a high idle jet have a disadvantage until the jet is moved to the bottom of the idle well.

It doesn’t matter what kind of block - or carburetor, Holley or otherwise - an idle jet above float level is discharging into air and is more subject to the pulsation of the idle and transition orifices exposed to intake pulsing. Why is it so hard to understand that a carburetor is not a steady flow device because it is attached to an engine which is not a steady flow device …?

No need to sugar coat it, Tuner, set em straight.

[raceczar]

If you have billet blocks, move it to the bottom. I don’t understand why anybody would fool with a high idle jet if you don’t have to.

Billet blocks have no advantage over (old school) OE Holley blocks. Billet blocks with a high idle jet have a disadvantage until the jet is moved to the bottom of the idle well.

It doesn’t matter what kind of block - or carburetor, Holley or otherwise - an idle jet above float level is discharging into air and is more subject to the pulsation of the idle and transition orifices exposed to intake pulsing. Why is it so hard to understand that a carburetor is not a steady flow device because it is attached to an engine which is not a steady flow device …?

The billet blocks I use not only have more options, like 5 emulsion holes instead of 2-3. Also, they have lowered the holes and relocated the PVCR holes. This all affects fuel curve shape and tuneability.

As far as where the IFR's go...you can ask 10 carb guys and get 17 different answers. I do know, without a doubt, that at least 2 very prominate carb builders suggest IFR's at the top.

Ultimately, you must tune your own carb and test and try all the variables. If you go by what is said here and test all the options, you are probably leaving something on the table.

[jmarkaudio]

I'm beginning to wonder about the onslaught of new members (or the same person) that keeps arguing the point.

[colebalster]

I'm beginning to wonder about the onslaught of new members (or the same person) that keeps arguing the point.

I was thinking the same and the metaphors look familiar.

Cole

[Holley Carb Guy]

I'm beginning to wonder about the onslaught of new members (or the same person) that keeps arguing the point.

I was thinking the same and the metaphors look familiar.

Cole

What metaphor is this?

[Holley Carb Guy]

If moving the idle jet to the top of the block improves throttle response it indicates there was a need for a more aggressive accelerator pump.

Anybody who has the opinion a high idle jet is good is not working with any data systems which supply accurate AFR information or they don’t understand the following information.

Locating the Idle Jet below float level is more important than many people realize because cruising at a low RPM and steady speed the fuel delivery is nearly 100% through the idle circuit (T-slot), and, because intake vacuum oscillates rapidly over a wide range, with the jet above float level the AFR oscillates a lot, as much as 2 AFR or more. Smooth idle, clean sparkplugs and oil, and good fuel economy, all depend on a consistent idle-low speed circuit.

An un-damped vacuum gauge will clearly show engine manifold pressure (vacuum) oscillates constantly and as valve timing becomes more radical the increased overlap causes larger pressure swings and more intense oscillation.

An un-damped gauge clearly shows engines are not a steady flow device. The intake pressure (vacuum) is oscillating with each intake suction stroke and overlap reversion pulse. Because passages and cavities in carbs and manifolds have individual resonances at particular frequencies, the resultant effects are more pronounced at some engine speeds than others. Because air is compressible, it is elastic, it can rebound like a spring and as a result it bounces and reverses direction in small passages in carbs.

With a high (above float level) idle jet, air can easily get upstream behind the jet when the idle fuel is pushed backwards by the pulsing.

When the jet is submerged, the air cannot get back upstream of it as easily because the much greater viscosity of the liquid fuel (than air) on both sides of the jet impedes the backflow and the air cannot get back through the jet unless the flow is reversed for a longer period of time than the frequency of the pulsations.

Another undesirable phenomenon caused by the high idle jet is the larger oscillations in the idle circuit causes a pumping effect on the main well that results in the booster nozzle dripping in spurts and blobs before it would normally start to flow if the venturi air flow were the only excitation and the idle circuit flow was more regular. With a submerged jet the metering reversion isn’t as severe or does not occur at all unless the pressure reverses for a longer time period than the manifold pressure (vacuum) pulses at a cruise.

This submerged idle jet phenomena was thoroughly studied and well understood 95 years ago during aircraft carburetor research for World War One.

So you think you got it all figured out

[Holley Carb Guy]

If you have billet blocks, move it to the bottom. I don’t understand why anybody would fool with a high idle jet if you don’t have to.

Billet blocks have no advantage over (old school) OE Holley blocks. Billet blocks with a high idle jet have a disadvantage until the jet is moved to the bottom of the idle well.

It doesn’t matter what kind of block - or carburetor, Holley or otherwise - an idle jet above float level is discharging into air and is more subject to the pulsation of the idle and transition orifices exposed to intake pulsing. Why is it so hard to understand that a carburetor is not a steady flow device because it is attached to an engine which is not a steady flow device …?

The billet blocks I use not only have more options, like 5 emulsion holes instead of 2-3. Also, they have lowered the holes and relocated the PVCR holes. This all affects fuel curve shape and tuneability.

As far as where the IFR's go...you can ask 10 carb guys and get 17 different answers. I do know, without a doubt, that at least 2 very prominate carb builders suggest IFR's at the top.

Ultimately, you must tune your own carb and test and try all the variables. If you go by what is said here and test all the options, you are probably leaving something on the table.

Most engines dont need 5 emulsion bleeds and by far most guys and gals dont know what to do with them any way. Lower power valve channels may or may not improve performance or the fuel curve, in most cases it doesnt.

You can get 100% of the performance on 98% of the applications with a cast block