[kyle_blake]

Hi,

Question, for the engine builders out there. Say you build an engine and you know it will produce say 6.5 Hg or less vacuum. I chose the number 6.5 because most holleys come with a 6.5 PV. A lot of guys just expect a carb to work and throw it on.

Can an engine builder approximate what sorta HG it will make during the build phase?

So many engines are never engine dyno'd or chassis dyno'd.

For the average Joe who throws his new brawler carb on his car and attempts to tune it using a vacuum gauge but doesn't know his cam may not produce enough vacuum anyway.

could he be ->

"fighting a power valve that is essentially open right out of the gate?"

Anyone ever had this happen to them?

How would he even know? Turn in the idle mixture screws and if it doesn't die then that is one test to verify it is running off main jets, and most likely boosters are supplying fuel?

Hopefully not a ton of other variable that one needs to answer the question. Worth a try.

Thanks. just trying to expand my knowledge

[steve25]

Nothing other then a full on race motor ( 6000 rpm and above power band) or a motor with a vacuum leak or mistimed Cam will produce only 6.5” of vacuum .

[SRGN]

The 6.5 PV on a combo that has low vacuum can cause an issue with the carb going into power enrich when it's not needed. This can cause a rich condition, then trying to tune around it without changing the PV will cause tuning issues at other times, often a lean condition when someone tries to jet down to compensate. You generally have to run the engine while monitoring with a wideband to get the tune where it needs to be. PV, jers, air bleeds, and pump shot all have to be in sync. A race only combo is usually easier to tune since part throttle driveability and cruise are irrelevant.

[chiphead]

Power valve lore is one of those things that drive me nuts. The power valve can be open at idle and it won't affect anything. Yeah, I said it.

The fuel metering for idle and transition slot comes from the idle feed restriction, which is after the main jet. You can have a 90 jet and a power valve completely open, and it wont make any difference to idle mixture if the IFR/transfer slot/IAB/idle mixture screw relationship is correct.

Power valve being open only matters when the main boosters are flowing.

Now if you've got a very low vacuum situation at idle, and you've got the primaries cracked so far open that the nozzles are dripping, then maybe the PV being open will make the drip worse.

The whole "power valve should be half the idle vacuum" thing is a starting point. A starting point. One way to tune is to set the HSAB and MJ relationship so it comes on the mains as the transition slot starts to taper off, and it's not too rich or lean at cruise. Then you find a hill, and climb the hill in high gear while adding throttle. You add enough throttle to make it go lean, but not so much that you open the PV or open the secondaries. I've disconnected the secondaries before, just to get the primaries right.

Watch the AFR and vacuum gauges, when the engine starts to go too lean (about 15.5-16.2 AFR) that's the vacuum you want the PV to open. In my case, I have a 10" PV installed, because it wants to go lean at about 8".

A PV does not "snap" open at the setpoint, that's when it starts to open. The full-open vacuum may be 2-3" below the rating on the PV. My 10" PV isn't fully open until about 8". But it closes at 10".

[chiphead]

If you've got some experience, it's possible to estimate the idle vacuum for a particular combo. It comes down to static compression, engine size, intake tract design and camshaft specs.

A 400 at 10:1 with a stock intake and 068 cam is going to have 15" or better vacuum. The compression creates a good pull during the intake stroke, the 068 cam has a early intake closing and a wide LSA, and the stock intake has a small plenum volume that promotes port velocity.

Take the same 400, lower the compression to 9:1, use a single plane intake, change the cam to RAIV spec. It's going to struggle to make 10" at idle, and want a lot more idle fuel. The vacuum is too low and reversion too high to draw fuel through a idle circuit sized to feed the 10:1 engine that is more efficient at breathing.

That's why these new holley carb calibrations are so fubar. Holley expects most knuckleheads to slap it on a low compression chevy 350 with a single plane, and a "super rattler hog momma" cam that's way too big for the application. That carb better dump fuel like a garden sprinkler, or said knucklehead is gonna say the carb is bad and send it back to summit.

[grivera]

“Super rattler hog momma” - that’s gold

[kyle_blake]

Thanks everyone, appreciate learning.

[HWYSTR455]

When choosing a PV, you pick a number that's 1/2 of the vacuum at idle as a starting point.

PVs don't open at idle, unless you have the throttle blades opened too far.

So for 6.5" vacuum at idle, you need a #30 (3.0") PV, or more likely, a 2.5.

Start with whatever the carb came with, get your idle vacuum, and then choose a PV.

You have to have the car somewhat tuned for good idle vacuum, obviously you do that first.


.

[Formulajones]

2 things have to happen before a power valve is able to pass fuel.

Vacuum drops below rating while simultaneously having the throttle blades open far enough to cause an air venturi affect to pull fuel out of the boosters.

In other words the power valve does not dump any fuel at idle if the throttle blades are adjusted properly, making it impossible for a power valve to cause any sort of rich idle condition. The blades have to open with enough air flow to pull fuel out of the bowl through the main circuit and out the boosters

Once you understand that you'll start to understand how Holley carbs work and will be able to make the appropriate tuning changes and understand what those changes will do.

You can have an engine making 6 inches of vacuum, put a 10.5 power valve in it, and it will not affect the idle what so ever. If that's your problem you need to look at why your throttle blades are open too far at idle

[Region Warrior]

Chiphead has good info.

Took 2 hrs on a supposed expert Holley guy set up a 850 for a buddy's 455 with 230/240 something cam with torque 2 intake.
Everything wrong.
Fuel level in bowls 3/4" high.
Pump levers set over tight fuel leak at idle.
Primaries open above idle slots.
Needed 5 jets lower both bowls.
Same guy told him 28* max timing.

[kyle_blake]

That was perfect additional explanation formula jones, thank you sir, you guys really helped advance my knowledge on the subject.

[chiphead]

Originally Posted by Region Warrior

Chiphead has good info.

Took 2 hrs on a supposed expert Holley guy set up a 850 for a buddy's 455 with 230/240 something cam with torque 2 intake.
Everything wrong.
Fuel level in bowls 3/4" high.
Pump levers set over tight fuel leak at idle.
Primaries open above idle slots.
Needed 5 jets lower both bowls.
Same guy told him 28* max timing.

it seems like the guy was trying to get it to provide more fuel. Raising the level richens it up everywhere and helps the mains pull over faster. Too much transfer slot means it needed secondary plates open a smidge and more idle fuel. That would allow closing the primaries down.

This is why big name carb builders charge so much. It seems like regular enthusiasts have no idea how a carb really works. Any time a carb runs rich, the power valve or main jets get blamed every time. If it runs lean, its the accel pump shot. Its nuts.

I found the Holley secret decoder ring on some website in the corner of the internet. I was searching for emulsion bleed tuning and found it. The curves show you what orifice to change to move the fuel curve where you want it. I've tested this and it works. Add the factors of fuel pressure, fuel level and aircleaner restriction and you can make a holley 4150/4160 work how you want.

Check this out.

[HWYSTR455]

The standard 4150s generally come with .097 needle & seats, and will cause lean conditions with more 'thirsty' combos. People throw jets at it and it turns into a pig.

I think the out of box double pumper come with .110 needle and seats, even that can be an issue.

IMO, go right to a .130 or .150, there's no downside really, and since there's more area to seal, it seals better, you can run more psi too.

A lot of people think they need 50cc pumps too, and that's not always the case. Everyone starts playing with cams, diff squirters, and it can quickly get out of hand.

Emulsion bleed tuning really is more useful in drag or circuit racing, when you have some gear more towards one side of the spectrum and specific demands. For 'general' street strip stuff, you can get by with the out of box bleeds.


.

[kyle_blake]

It is hard to learn from a good mentor of carb experts these days. Agreed. I try and focus on one circuit at a time. So far I'm on the idle circuit learning phase with edelbrock's and holley 4150's. Learning the accelerator pump arm clearance setting sure helps with instant throttle response. To be honest having the 02 gauge has been helpful for setting up a carb after rebuild and revving it but obviously there is way more to driving it and getting into the transition slots then part cruise power circuit and then WOT etc. Without a 1/4 miles strip near by and plugs to read it's hard to hone that skill. I'd love a data logger 02 . Then at least I can somewhat have some info to refer back to so i can keep eyes on the road.

[HWYSTR455]

Get a Dragy. The are easy to use, can use it from vehicle to vehicle, and amazingly accurate. Just find an isolated stretch and you're good to go:

https://dragymotorsports.com/product...erformance-box

Don't rely on O2 sensors alone, there's a lot of ways it can fool you, and can get into trouble real quick.

EDIT: Looks like the current version is Dragy Pro, and previous units are sold out:

https://dragymotorsports.com/products/dragy-pro

.

[Formulajones]

I like the dragy. It helps when tweaking things and gives me a baseline 60 foot to work from. The wideband I pay most attention to for wide open throttle to make sure it's safe, the rest of the fuel curve the wide band isn't that useful. In that respect I tune for best drivability part throttle cruise and tip in and don't much care what the wide band says there. Most guys would freak at what the wide band says but it's more about what the engine wants and how the car drives, not what you want to see on a wide band.
9 times out of 10 I've already run it on a dyno anyway and found what the engine wants for WOT AFR and that gets me close. The drivability part typically isn't that hard as long as the timing is dialed in and working as it should with vacuum advance mixed in properly. Most cars I put my hands on with supposed carb issues stem from crappy timing curves. 90 percent dont even have enough idle timing to start with and cause all kinds of tip in issues.

[kyle_blake]

Wow that product is super cool!!! thanks for sharing.... i have yet to to make any changes with part throttle or tip using the 02 as a guide. My next thing to learn is to go from 6.5 PV to 8.5 PV or 10.5 PV when my engine at idle makes 15 Hg. Like you said that is the key what the engine wants , finding that out . Would be neat to hear what you mean by most people would freak out. Learning that with out Vac Advanced hooked up then with it to ported, then with it to manifold . That's what one day I'd like to at least witness.

[chiphead]

Originally Posted by HWYSTR455

Emulsion bleed tuning really is more useful in drag or circuit racing, when you have some gear more towards one side of the spectrum and specific demands. For 'general' street strip stuff, you can get by with the out of box bleeds.


.

I want to mention this part, since it's part of the wackiness we see in modern carbs. For decades, the stock Holley carbs on production cars like Ford 390s had a small number of small emulsion bleeds in the metering block. Those were either 2 vertical bleeds into the main well, or the bleed tubes that are inside the main well. The upper hole right below fuel level controls pullover, and the hole about 1/2 down from that helps shape the fuel curve once the mains get going. Those 2 holes, along with the high speed air bleed and the main jet, control the main circuit fuel curve.

There were a few cases where adding a 3rd hole between the two would add fuel at part throttle, heavy load conditions for racing. Adding emulsion air to the main well lifts fuel and makes it richer, especially at low to moderate venturi airflow. The fuel is lifted by the bubbles, so it takes less venturi signal to flow fuel. This makes the curve richer until venturi draw gets stronger, and then the air through the bleed system starts to displace the fuel and the delivery leans out. The rate of fall in the wells depends on the size of the main jet. Smaller the jet, the faster the fuel falls and uncovers additional bleed(s). This is how the jets, emulsions and the HSAB add or remove fuel from the main circuit.

Then the aftermarket figured more is better, and now you have 3, 4, 5 emulsion holes in the main well of aftermarket billet metering blocks. "Out of the box" bleeds are now out to lunch for a mild street motor.

This usually results in rich fueling from 2300-4000 RPM, when on the mains at part throttle. Then when you got to WOT, the mixture leans out too much as the air displaces the fuel in the main well. You can't add enough jet or power valve to richen it back up at WOT, without rolling coal at 3000 RPM.

The solution is set the top hole and the 3rd hole at .028" and block all the others. That recreates the old-school setup from the 1960s. That usually helps get the main circuit back under control. Then sizing the main jet, HSAB and the fuel level starts the mains at the correct time and keeps it from curving rich/lean at WOT. You see this if you follow the curves from the chart.

The most interesting thing is look at the effect of the HSAB. (Some call it a MAB, I call it a HSAB and I aim to kill yo with it- Karl from Sling Blade)

Conventional wisdom is that the larger the high speed bleed, the later the mains start and leaner it is everywhere. However, a larger bleed will blow air through the emulsion well and the main well at low venturi signals. Those bubbles lift the fuel up to the booster and now its flowing fuel. This will actually start the mains EARLIER and make it RICHER, until the system starts to displace fuel in the wells with bleed air. At that point, it swings violently lean, cuz it taint got no gas in it. Its all about the total amount of air available from the HSAB, the size AND location of the emulsions in relation to the fuel level in the well and how hard the booster is pulling on the well.

[Formulajones]

The most emulsions I've seen lately on aftermarket carbs is 3. I got my hands recently on an ATM and it only has 2.

The problem with that carb is how insanely lean it is on tip in and light throttle to the point the engine gets into a stumble it doesn't want to recover from . Very difficult to just cruise around town.
Im talking AFR's going from 13.0 at idle to pegging the gauge at 20 with any light tip in. It's crazy how much I've fattened all kinds of circuits up to cover that hole. Forget about AFR readings, lol

[chiphead]

Have you relocated the IFRs to the lower positions? That usually helps a lot with the condition you describe.

Most all newer holley clones locate the Idle Feed Restrictions in the high location, above liquid fuel. In a lot of cases, you can tune the idle mixture to be correct, but when you get into the transfer slots, it goes lean or erratic. Moving the IFRs to the lower location under the fuel level really helps stabilize the idle circuit fueling.

I never really tuned a mechanical secondary carb until I bought my current '67. It had a QuickFuel 750-HR double pumper, and it ran like chit. I did the typical stuff but it was still fat and lazy.

I searched the internet high and low, Yellowbullet, The HAMB, Don Terrel's speedtalk, the Mopar boards, racingfuelsystems.com, here on PY, etc trying to figure out how to tune it. I bought a couple of the older Holley tuning books. Nobody covers the HSAB or the emulsions in deep detail. Its nuts how quiet everybody is on those critical parts of a holley carb. Tom V, Kenth and a few of the carb guys here were very helpful on explaining the advanced tuning approaches. Its in a thread somewhere here on PY.

Finally I found posts copied from the old Innovate Forum back in 2011 or so. There were some guys named Klaus and Tuner that explained two-phase flow in detail, and the cutaways on the below website helped me understand it.
Sorry for the long-winded posts, but I tend to brain-dump on carb threads now. Somebody in the future will come across this and it'll save them some time.

https://garage.grumpysperformance.co...59/#post-56233