DCOE 50f8 too lean / 55f8 too rich - what next?

[man_mark_7]

Looking forward to putting all of these suggestions to the test - Many thanks!

 

One additional piece of the puzzle is my air filter setup. Does anyone have definitive info about how having the bowl vent open within the plenum of the air filter will affect AFR?

 

I plan to rig up a setup to see how pressure (vacuum) in the bowl varies over RPM. I am guessing ideally it would stay around atmospheric pressure. If it doesn't stay around atmospheric I was planning to punch a hole in the top of the air filter lid and stick a piece of pipe through the lid into the bowl vent. I imagine that, with the current setup, as RPM rises and volumetric flow into the engine is increasing the pressure in the air filter, and therefore in the bowl, are falling. I can't quite rationalize how this would affect AFR. I suppose it would prevent the air correctors from seeing full differential pressure and therefore it could be causing the richness @ higher RPMs?

 

Love this stuff.

Edited November 17, 2020 by man_mark_7
Added picture

[2002Scoob]

What brake booster is that? I thought mine was big!

 

Bowl vents with my car are uncovered (I don't know why you'd want them otherwise?) and are open to air inside the filter. I don't think there's any rationale for doing otherwise, but someone correct me if I'm wrong. 

 

Are you running trumpets inside those little pancake filters? 

 

Ideally you want the longest unobstructed runner length you can manage, and trumpets are a part of that recipe. You also want as large of a still air volume to pull from for DCOE's to be happy. 

 

I always get concerned seeing people running pancakes, because they never appear to have enough length to run trumpets, and the filter area/air volume doesn't appear to be much... But it's honestly unfounded, I've got no experience with them otherwise. So maybe it's all good

[man_mark_7]

6 hours ago, 2002Scoob said:

What brake booster is that? I thought mine was big!

 

Bowl vents with my car are uncovered (I don't know why you'd want them otherwise?) and are open to air inside the filter. I don't think there's any rationale for doing otherwise, but someone correct me if I'm wrong. 

 

Are you running trumpets inside those little pancake filters? 

 

Ideally you want the longest unobstructed runner length you can manage, and trumpets are a part of that recipe. You also want as large of a still air volume to pull from for DCOE's to be happy. 

 

I always get concerned seeing people running pancakes, because they never appear to have enough length to run trumpets, and the filter area/air volume doesn't appear to be much... But it's honestly unfounded, I've got no experience with them otherwise. So maybe it's all good

It's a standard Tii booster. It is probably just the camera angle that makes it look big.

 

I think I'm confusing you. The bowl vents are open to the air inside the pancake filters, but I have read (and it makes sense) that they are then seeing lower-than-atmospheric pressure, especially as RPM rises.

 

No trumpets inside the filters. I do have trumpets, but in this case keeping dirt out of the intake / engine won out over my appreciation of the beauty (and function) of vel stacks.

 

I am with you - I have heard that you want long smooth airspace around carb intake for the best performance and that vel stack length tuning is an important parameter, but I haven't ever seen any dyno data to show just how much the improvement is. Is it chasing fractions of an HP or is it much more?

[uai]

on my setup I can plug or unplug the connection between airbox and bowl. Makes a little difference

[2002Scoob]

3 hours ago, uai said:

on my setup I can plug or unplug the connection between airbox and bowl. Makes a little difference

 

In what way? 

 

Just trying to work out in my head what the effect would be and can't decide if it would enrich, or lean things out. It's the first I've heard of using it as a tuning feature. 

 

Tell me everything...

[2002Scoob]

3 hours ago, uai said:

on my setup I can plug or unplug the connection between airbox and bowl. Makes a little difference

 

In what way? 

 

Just trying to work out in my head what the effect would be and can't decide if it would enrich, or lean things out. It's the first I've heard of using it as a tuning feature. 

 

Tell me everything...

3 hours ago, man_mark_7 said:

I think I'm confusing you.

And don't worry. Happens allot, and is easy to do.

[uai]

Connected is leaner. I was hoping to use my airbox setup with ram air but that didn‘t work well no partial throttle only all or nothing.

[uai]

Forgot the Photo. The default downdraft has the same it's used / intended for compensating for a clogged up Airfilter. On Turbocharged Carburetted cars you need an additional Fuel pressure regulator.

Think of the Fuel pressure regulator on injected cars, that has a link to the intake as the Fuel pressure is always in relation to the MAP. nothing fancy all regular stuff.

Edited November 17, 2020 by uai

[TobyB]

Quote

I think I'm confusing you. The bowl vents are open to the air inside the pancake filters, but I have read (and it makes sense) that they are then seeing lower-than-atmospheric pressure, especially as RPM rises.

 

Yes, but crucially so are the carb throats so that there's no differential across the jet stacks.  If the pressure across

the stacks changes, that will affect how much air is pulled by the air correctors.  Thinking it through, having positive 

pressure in the float bowl area relative to the throat would push more air into the mixture, thus, as UAI says, it's the same

effect as having a larger air corrector- running leaner when the air corrector is effective.

 

I tie them to the intake plenum- that way, both parts of the carb see the same relative pressure, and it's one less

thing to tune around if, say, the filter starts to lose efficiency...

 

t

 

[man_mark_7]

5 hours ago, TobyB said:

 

Yes, but crucially so are the carb throats so that there's no differential across the jet stacks.  If the pressure across

the stacks changes, that will affect how much air is pulled by the air correctors.  Thinking it through, having positive 

pressure in the float bowl area relative to the throat would push more air into the mixture, thus, as UAI says, it's the same

effect as having a larger air corrector- running leaner when the air corrector is effective.

 

I tie them to the intake plenum- that way, both parts of the carb see the same relative pressure, and it's one less

thing to tune around if, say, the filter starts to lose efficiency...

 

t

 

Interesting mental model puzzle. I'm not sure I believe there is a positive pressure in the float bowl though - if the bowl is plumbed into the air filter. I think the pressure is higher than in the carb throat, because of the venturi effect in the throat, but if the carb is pulling air into the filter, then the filter (and therefore the float bowl) must be seeing a pressure below atmospheric.

 

 

That being said, the float bowl pressure (small amount of vacuum) is certainly higher than the throat pressure (greater amount of vacuum) when the bowl is plumbed into the air filter. I would expect the float bowl pressure to be even more different than the throat pressure if the bowl is vented to atmosphere. 

 

Either way, I think your conclusion is right - plumbing the bowl vent to either the air filter, or the intake plenum, reduces the pressure differential across the jet stacks and leans out the mixture. I will have to try some experiments to confirm if this true!

 

I just got a bunch of jets delivered today but I had a bunch of things to do and it was near freezing here. Tomorrow is a bit more open and is going to be warmer so great day to test!

 

[man_mark_7]

Just now, man_mark_7 said:

relative to the throat

OH - just noticed this bit. The "relative" changes everything. My bad.

[Son of Marty]

On the other hand having a higher pressure in the float bowl will increase atomization of the fuel and the finer fuel droplets will lead to better combustion. 

[man_mark_7]

46 minutes ago, Son of Marty said:

On the other hand having a higher pressure in the float bowl will increase atomization of the fuel and the finer fuel droplets will lead to better combustion. 

Ouch - my head is starting to hurt. So, to get the highest pressure possible in the float bowl (short of actually pressurizing the bowl), you would vent to atmosphere.

 

I guess at the end of the day, plumbing the bowl into the air filter or the manifold plenum is probably similar to going down a size or two (or 3 or 4) on the air corrector.

 

Now I need to dig for the pic I've seen on this forum before that shows a Ferrari(?) with 6" long vel stacks on the carb throats, + 1" stacks on the float bowl vents. ?

 

Edited November 19, 2020 by man_mark_7
added info

[2002Scoob]

I've got 55f8's in now...think I'm gunna throw my 50f9's back in again. Cuz why not. I'm sure I had a good reason/logic going for why I am where I am...

 

But since my brain is a blank slate at the moment to why... I might as well start with that

[Stevenc22]

12-13 is perfect for carbs.  If you were shooting for 14.7, that is what modern cars use for emissions and doesn't apply to us.