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PostPosted: Tue May 29, 2018 11:38 am 
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Location: Stranded in Iowa - Better get the Breakdown squad out
I never paid too much attention to idle and when I did it was just twisting idle screws and changing bleeds but I need a rough idea of how much air flow is required at engine idle. I searched the subject and found a couple threads that say gasoline engines typically require .08-.1 gal/hr of fuel at 650-700rpm for every liter of engine displacement and fuel consumption changes fairly linearly with idle speed.

When I crunched the numbers at 14.7:1 AFR, I got 2.57 scfm of air for a 5 liter engine at the above idle speeds. It’s a very small number but the valve and seats on idle air control valves are very dinky too though they usually have a strong vacuum signal to promote flow. I remember drilling very small holes in throttle plates of other engines (motor cycles, go-carts, outboard marine etc) with carbs that had crude idle circuits to get them to idle better. I recall it used to be a common thing to do on 4brl carbs. Anyone ever drill a couple holes in the throttle plates for idle air and if so, how big were they and what was the engine displacement and idle speed?

Brent, do you ever see figures for fuel consumption and air flow data at idle on your dyno?

Best,
Kelly


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PostPosted: Tue May 29, 2018 3:51 pm 
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I've got carbs stacked everywhere from 600 CFM to 800+ both double pumper and vacuum secondary. They have been on engines from 250HP to near 600 and displacements from 289 to 393 CID. None, zip, zero have the butterflies drilled for idle quality. If idle is a bit too fat on some carb for some application I would use wires in the IFRs (Holley) to lean it out. Also, never had any idle at 14.7:1. More like 11.x~12.x - maybe 13 on something really lean. Too lean IMHO causes idle drift or misfire. e.g. - carbs of the 1970s fuel crisis in the USA. All in that day leaned out lumps of aluminum that drove us nuts.

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PostPosted: Tue May 29, 2018 7:56 pm 
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Most of the time the reason you drill holes in the throttle plates of a carburetor is not to work on the idle mixture but to help with the part throttle transition. Having a hole say 1/8 inch through the primary throttle bores allows the throttle to be closed more at idle and thus expose less of the transition slot. This saves more of the slot above the throttle which can help greatly when running at part throttle before the main metering circuit starts to function.

Most of the time on a Holley once you've got it idling as good as possible if you take the carburetor off of the engine and observe the position of the throttle plates you should see the exposed portion of the transition slot looking like a square(as tall as it is wide). If this isn't the case and the throttle is open too much the slot will look more rectangular(tall) and either drilling a hole through the throttle plate or opening the secondary a little more will be helpful.

In my experience most of the time the engine likes to idle richer than you'd think. It likes to be closer to 12.5:1 vs 14:1 to get the best possible idle quality irregardless of the cam etc.


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PostPosted: Wed May 30, 2018 6:51 am 
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Location: Stranded in Iowa - Better get the Breakdown squad out
These are Inline Autolite carbs being used in IR. In stock configuration they have idle transfer slots and curb idle adjustment but they have large 2 ¼” butterflies and no means of indexing or synchronizing the individual barrels like you would on a Weber IDA. The OE linkage leaves plenty to be desired as far as precision. Consequently they don’t return to a repeatable position nor idle well and the large throttle plates demand higher precision for idle control where there is little. I can modify the linkage and carb but they are collector’s items and many of the folks that own them don’t want me to do so.....make'em work but don't modify them....uh, ok.

I can fairly easily make the throttle plates all shut down well in full closed position and repeat without modification. I started plumbing a vacuum line from each cylinder to a small common plenum and metering air into that common plenum like you would in EFI with an idle air control valve only manually setting the inlet air, then just metering fuel with the curb idle adjustment and take what you get from the transfer slot. I routed the inlet to the idle air plenum back into the air cleaner with a hose so the air source was filtered. Although it can cause some other unwanted off idle transitional affects, this is more easily dealt with and it works well at idle, but I can’t trim individual cylinders without a lot of fussing around because the idle air plumbing is fixed.

I don’t intend to drill holes throttle plates. Since I usually end up making intake manifolds for them anyway, I was just going incorporate the plumbing features into the manifold and introduce the idle air in the manifold right under the transfer slot and curb idle discharge port. I have a scheme to vary and tune the idle air orifice area to each runner that also allows me to trim each cylinder individually which can be very handy in IR set up.

I don’t intend to set the idle up at 14.7 AFR. I just used that as max condition for sizing air flow in the idle circuit, thus my question about the hole in the butterfly….just to gauge required air flow as a starting point and make sure I have adequate air supply as bigger plumbing gets more difficult to package. Usually in this scenario you just set high idle and call it a day which on track cars is ok but these are being run on street and show cars and getting street manners and performance out of big IR carbs isn’t easy, but doable along with custom cam grinds.

Best,
Kelly


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PostPosted: Wed May 30, 2018 8:59 am 
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I read about a test that Iskenderian did one time where they were going to try to even out the air/fuel distribution in the intake manifold which in this case was one with a common plenum by drilling holes and installing a valve in each port right at the head surface. The though was that they could just give one or another ports a small air leak to lean that port's mixture. What they found though instead was that opening any of the valves even just the slightest bit leaned all of the cylinders. I just wonder if this would also be the case and more air would simply be drawn into all of the cylinders if you give one a "leak". It might be easy enough to test your theory by drilling a hole in each intake runner and then screwing in say a 10-32 set screw which could be drilled with your choice of hole size or plugged if it didn't work out. This could be used to meter your filtered air source to each cylinder individually while still having an overall idle control.

I thought that was a great oversight on those Autolite carburetors by not having a good solid stop for the throttles.


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PostPosted: Wed May 30, 2018 9:01 am 
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Quote:
Most of the time on a Holley once you've got it idling as good as possible if you take the carburetor off of the engine and observe the position of the throttle plates you should see the exposed portion of the transition slot looking like a square(as tall as it is wide). If this isn't the case and the throttle is open too much the slot will look more rectangular(tall) and either drilling a hole through the throttle plate or opening the secondary a little more will be helpful.


A key tuning point, well written. I usually use the secondarys as the tweaker.

Quote:
I started plumbing a vacuum line from each cylinder to a small common plenum and metering air into that common plenum like you would in EFI with an idle air control valve only manually setting the inlet air, then just metering fuel with the curb idle adjustment and take what you get from the transfer slot.


IIRC - Sometime before BG hit the wall, they came out with a modification on their carbs where they basically put a tunable air leak in the space below where the air cleaner stud threads. Another way to skin that particular cat. :)

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PostPosted: Wed May 30, 2018 10:11 am 
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Falcon67 wrote:
Quote:
Most of the time on a Holley once you've got it idling as good as possible if you take the carburetor off of the engine and observe the position of the throttle plates you should see the exposed portion of the transition slot looking like a square(as tall as it is wide). If this isn't the case and the throttle is open too much the slot will look more rectangular(tall) and either drilling a hole through the throttle plate or opening the secondary a little more will be helpful.


A key tuning point, well written. I usually use the secondarys as the tweaker.

Quote:
I started plumbing a vacuum line from each cylinder to a small common plenum and metering air into that common plenum like you would in EFI with an idle air control valve only manually setting the inlet air, then just metering fuel with the curb idle adjustment and take what you get from the transfer slot.


IIRC - Sometime before BG hit the wall, they came out with a modification on their carbs where they basically put a tunable air leak in the space below where the air cleaner stud threads. Another way to skin that particular cat. :)


A lot of Dominator carburetor packages have that feature too where a couple of little interchangeable air bleeds are drilled through the top of the carburetor and into the chamber that houses the linkage right up by the filter stud as shown on my well worn 1050. They can be handy to save a little transition slot which can be really important on a Dominator in particular so that the intermediate circuit doesn't have to be too rich.

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1050 Dominator Air Bleed.jpg [118.22 KiB]
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PostPosted: Thu May 31, 2018 11:39 am 
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Location: Stranded in Iowa - Better get the Breakdown squad out
DaveMcLain wrote:
I thought that was a great oversight on those Autolite carburetors by not having a good solid stop for the throttles.


For the smaller version of the Inline carbs typically used on common plenum intakes, it’s still a PITA but not as big of a deal because the common plenum tends to dampen any bore to bore differences but still can cause some low speed fuel distribution problems. IR is a different matter and usually has ~3x more butterfly area than most 4150s on common plenum intakes, so a little rotation (or position error) means a lot, not to mention 4brls have primaries/secondaries halving the active throttle plates whereas IR must rely on progressive linkage for off idle throttle resolution.

DaveMcLain wrote:
.............It might be easy enough to test your theory by drilling a hole in each intake runner and then screwing in say a 10-32 set screw which could be drilled with your choice of hole size or plugged if it didn't work out. This could be used to meter your filtered air source to each cylinder individually while still having an overall idle control.


I was thinking of just using a carb jet. I have plenty of those. They are cheap and precise. I can mount them so one side is exposed to the runners and so the other side is exposed to the a plenum, then I think they can all be tuned by metering the air leak into that idle air plenum. You could change a jet size to trim out an individual cylinders.

On a somewhat related subject, a lot of people running IR have found the use of relatively small communication ports between each isolated runner produces better overall performance. Of course they aren't completely isolated runners at that point. We’re talking much larger than needed just for idle air….like 3/8” ID hose but still pretty small potential for flow at any significant power level. It was years ago but I stumbled onto it by accident trying to provide a vacuum tap for a remote power brake vacuum reservoir. Systems that had previously refused to idle suddenly would and also perform better just about everywhere. Sparked quite a bit of debate about why it works but these were typically in engines with quite a bit of int/exh overlap. The reversion pulse seemed to be the culprit and really wreak havoc with the booster. At low speed I suspect benefit may be flow related and at higher speeds, the port may disrupt the pressure pulse and related harmonics, like uncovering a different hole on a flute or brass musical instrument. It’s a similar thing to cutting small notches in dual plane intake dividers. Any thoughts on that?

Best,
Kelly


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PostPosted: Thu May 31, 2018 12:33 pm 
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I think that the smallest jet you can get from Holley is about a 40 and that's probably too big for metering on one cylinder. That's why I'd think that the ones which are used for idle feed restrictions and in the emulsion leg of a billet block might work better and they are easy to drill if you make a fixture out of a block of aluminum.

Probably what tends to happen with the communication ports is that it tends to dampen the very strong pulses in the runner. While having strong "signal" at the carburetor is usually a good thing in this case the pressure jumps up and down wildly so much so that it probably swamps out any signal that's being produced by the venturi(what's actually supposed to be used to meter and feed the fuel to the engine). This probably makes the fuel curve look like the Rocky Mtns at certain engine speeds and loads.

It might be really interesting to use hoses for the communication ports and then cross them over/share them between running mates. 1 and 6 etc. Then if that's better put a T in the lines and hook them together so that they share in 180 degree increments like a dual plane intake.


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PostPosted: Thu May 31, 2018 4:00 pm 
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DaveMcLain wrote:
I think that the smallest jet you can get from Holley is about a 40 and that's probably too big for metering on one cylinder. That's why I'd think that the ones which are used for idle feed restrictions and in the emulsion leg of a billet block might work better and they are easy to drill if you make a fixture out of a block of aluminum.


Understand, but I think that may only be the case if the jets are vented to atmospheric pressure. Metering the inlet air to the idle air plenum should make it operate at an intermediate level of vacuum making the pressure drop across the orifice to the runner smaller and with less flow for a given orifice size. This is only the case at/near idle where there is a strong vacuum signal as it will quickly go to zip if the throttles are open for any significant amount of time. The size of the plenum can matter too. Too large and it takes too long to pump it back down returning to idle. Too small and you have a spikey signal.......this is the way it was with the speed density MAP plenum on IR.

DaveMcLain wrote:
It might be really interesting to use hoses for the communication ports and then cross them over/share them between running mates. 1 and 6 etc. Then if that's better put a T in the lines and hook them together so that they share in 180 degree increments like a dual plane intake.


That is an interesting idea. I'll have to give that some thought. I'll have to send you a picture of what I'm working on. It's a great test platform for such a set up.

So does your dyno display fuel consumption in lb/hr at idle?

Best,
Kelly


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