MomsDuster
Well-Known Member
Gotcha, I’ll play with this over the weekend and we’ll see where it ends up.
Thanks.
(2)450 DP's vs (2)750 DP's
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Thanks.
It's possible that you're feeling the missmatch between the timing and the AFR.
If you can get a bit more advance at that rpm I think it would help.
But also agree it seems lean for the conditions based on the WBO2
Sidenote - Not sure I entirely believe it (the WBO2) since you observed fuel burning in the exhaust. The WBO2 interpretation to AFR depends on the burn being within the expected norms....
MAB can effect start up of the mains, but I'd want to know what and where the emulsion bleeds are first.
Might try both changes but one at a time. I would do mains first.
That's interesting. I see it the other way. Idle is about normal (for pre-smog) AFR but by 30 mph its going lean and going leaner. I'd be looking to richen or extend the idle-transition so it doesn't go away so quick.
MomsDuster
Well-Known Member
emulsion is a 3 hole stack with the middles blocked and the top and bottom.028
A side note: I did find an exhaust leak on the drivers side. Opposite side of the O2. That seems like that may have taken care of the “pop” I was hearing. But it still doesn’t sound right in those higher AFR readings.
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MomsDuster
Well-Known Member
If they are more efficent, the burn will need less lead time.
OK. lets discount the pop as representing anything other than a bit of air getting in the exhaust.
With 4 primary barrels I think making sure the boosters start carrying fuel as soon as possible won't hurt.
Could increasing the PMAB do this? Maybe.
IF they are too small.
Let's look at Obert's diagram and highlight what the MAB will do if too small and what it may effect on the start up.
If the MAB is already correct, will cause the top end to go lean (less of an issue with 8 bbls). There's probably a little window where a slightly larger MAB may help initiate without noticibley impacting the high rpm or causing a noticible rich - lean - initiation.
I think two .028" e-bleeds is probably as large as we normally want to go. Larger or more and I suspect we would see the rich-lean- and then recovery if we were logging a drag run.
One issue with e-holes too large is that the fuel air in the carb circuit isn't as well mixed and the so the results in out testing aren't as nice and neat as the graph above.
OK. lets discount the pop as representing anything other than a bit of air getting in the exhaust.
With 4 primary barrels I think making sure the boosters start carrying fuel as soon as possible won't hurt.
Could increasing the PMAB do this? Maybe.
IF they are too small.
Let's look at Obert's diagram and highlight what the MAB will do if too small and what it may effect on the start up.
If the MAB is already correct, will cause the top end to go lean (less of an issue with 8 bbls). There's probably a little window where a slightly larger MAB may help initiate without noticibley impacting the high rpm or causing a noticible rich - lean - initiation.
I think two .028" e-bleeds is probably as large as we normally want to go. Larger or more and I suspect we would see the rich-lean- and then recovery if we were logging a drag run.
One issue with e-holes too large is that the fuel air in the carb circuit isn't as well mixed and the so the results in out testing aren't as nice and neat as the graph above.
MomsDuster
Well-Known Member
Ok, this tune seems to have broadened the lean area. The previous tune had .035 MAB’s and .070 IAB’s Still had a lean area but it seemed smaller, like 45-50 mph, maybe as high as 55mph. My initial thought was to go back to the .035’s and maybe upping the IAB’s to say .072 and see if that might extend the transition a little. But that was just my first thought. And correct me if I’m wrong, but I can make all these changes on the primary side and if it gets out of shape as the secondaries start coming in or @ WOT I can work on the secondary side separately.
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In my experiance the idle circuit doen't initiate like a velocity based circuit. ( I guess that's because it isn't. )
Applying that to the graph, modifies like so.
I know I've told the story of modifying my first 3310 and not being able to drive home until we put wires in the IABs. My first and most enduring lesson idle slot exposure, adding air at idle to close the throttles more, and IABs.
Applying that to the graph, modifies like so.
I know I've told the story of modifying my first 3310 and not being able to drive home until we put wires in the IABs. My first and most enduring lesson idle slot exposure, adding air at idle to close the throttles more, and IABs.
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Something to consider:
Shrinker on the "Kill Bleed"
G'morning Shrinker,
So what happens with the opposite extremes? One case being too large of an air bleed (always atmospheric in the air well), and too large of a 'kill bleed' and/or e-holes above fuel level. The other case also being too large of an air bleed, but no kill bleed or upper e-holes.
Shrinker:
The Kill bleed is very important. It causes flow through the HSAB to occur BEFORE the booster vacuum is enough to lift the fuel to the outlet level of the metering block. Without the kill bleed you wont get any booster fuel until the vacuum is able to lift the fuel as per normal but once it starts it will come in real strong. The kill bleed flow is there to reduce the pressure in the main well and to make the pressure in the well transition to sequential jetting flow rules once the top e-bleed functions. The air pressure in the well is reduced by air flowing out of the well, and this is how it works--- the HSAB is the first jet in the air well system, the kill bleed is the second so the flow rate from it is sequential to the HSAB and then the top e-bleed gets uncovered and the area of flow is now doubled (it depends on the size of the e-bleed of course this is just an example). Now you have LESS sequential jetting effect. Once the next e-bleeds further down the block come in to operation you keep adding outlet area to the main air well so the sequential jetting rules may eventually not apply (depending upon sizing's etc).
Eventually you may reach the only jet being the HSAB itself.
The sequential jetting rule is this-- flow is restricted unless a downstream jet is greater than 4 times the area of the upstream jet.
Think about that for a bit.
Shrinker on the "Kill Bleed"
G'morning Shrinker,
So what happens with the opposite extremes? One case being too large of an air bleed (always atmospheric in the air well), and too large of a 'kill bleed' and/or e-holes above fuel level. The other case also being too large of an air bleed, but no kill bleed or upper e-holes.
Shrinker:
The Kill bleed is very important. It causes flow through the HSAB to occur BEFORE the booster vacuum is enough to lift the fuel to the outlet level of the metering block. Without the kill bleed you wont get any booster fuel until the vacuum is able to lift the fuel as per normal but once it starts it will come in real strong. The kill bleed flow is there to reduce the pressure in the main well and to make the pressure in the well transition to sequential jetting flow rules once the top e-bleed functions. The air pressure in the well is reduced by air flowing out of the well, and this is how it works--- the HSAB is the first jet in the air well system, the kill bleed is the second so the flow rate from it is sequential to the HSAB and then the top e-bleed gets uncovered and the area of flow is now doubled (it depends on the size of the e-bleed of course this is just an example). Now you have LESS sequential jetting effect. Once the next e-bleeds further down the block come in to operation you keep adding outlet area to the main air well so the sequential jetting rules may eventually not apply (depending upon sizing's etc).
Eventually you may reach the only jet being the HSAB itself.
The sequential jetting rule is this-- flow is restricted unless a downstream jet is greater than 4 times the area of the upstream jet.
Think about that for a bit.
TT5.9mag
Two atmospheres are better than one
Absolutely no idea. I have one and only one experience with stealth heads and it’s on @Turbo440Dart Chris’ car. I was there when it was tuned on the dyno but have no first hand knowledge of the timing curve it left with. My guess would be they are possibly a bit better than stock 915/906 chambers but it’s a guess at best. What does the strap on your spark plugs look like at your current wot timing?
TT5.9mag
Two atmospheres are better than one
I aim for mid to high 14s for idle afr, if I saw his ~12.5 numbers I’d be leaning it out.
Turbo440Dart
crank runner over-er
It left with 35° and pulling 6° total. As to how they compare to a cast head I can’t say.
MomsDuster
Well-Known Member
To be honest, I don’t have a place close by I can really run that type of test. No tracks close by and the closest doesn’t have a test and tune time that I’m able to attend at this point. Next time I make a wot run I’ll see if I can scout out a decent location to do it.
That's even leaner than late 60s CAP/CAS targets! LOL
IIRC '68-'69 targets were 14.1 and idled around 700 rpm for most engines
TT5.9mag
Two atmospheres are better than one
That will tell you if the chamber is happy or not at your current timing. If you have ngk plugs in it, you can make 2 degree swings starting at like 28deg and watch the mark move around the bend of the ground strap.
MomsDuster
Well-Known Member
TT5.9mag
Two atmospheres are better than one
Yea. I’m spoiled by looking at lots of EFI data from my truck and watching a dead steady o2 on that. I strive to make my carbureted stuff run similar to that. Lean idle and cruise, slightly leaner light acceleration and fat wot.
MomsDuster
Well-Known Member
I figured that
I think Mark's 2x4bbl will be fine at 12.5
There is a lot going on there (at idle) that could be tested and trialed.
My recollection (it could be wrong) was that 12" Hg in drive is pretty decent with the cam etc.
On the other hand, I'd love to see less rpm drop.
Anyway that's its own rabbit hole - although not entirely seperate from what happens in the transistion circuit.
Shrinker was taking the extreme examples (which unfortunately we've seen in some of the the 'new and improved' as well as occassionaly in older models) of too large an MAB or too many holes above fuel level.
I assume the fuel levels are about here.
You can check by marking with a pencil on th eoutisde of the block and then when you have it apart take a ruler or square to it. Those holes look to be .028 or bigger but realize thats a WAG from looking at a photo.
Here's an example of the the fuel line and FWIW these bleeds into the angle channle are drilled to .021, but .026 is more typical. I just wanted to start small.
They have a couple of functions, one is as a siphon break (when we don't fuel any more) and to let out vapor.
The other function is the one we're talking about.
First it allows an easy path for a bit of air to flow from the MAB to the booster. This can slightly delay the start of fuel flow. However once fuel begins to flow it provides another boost to fuel moving. Imagine the wind blowing a spray off the top the ocean when you're at the beach.
Just enough air moving past the venturi to cause a small reduction of pressure in the venturi.
When there is enough pressure difference any fuel above the upper e-bleed gets pushed or blown into the booster. If the fuel level is right at the e-hole level the fuel gets blown.
With a bleed into the angle passage this effect is increased once there is enough pressure difference to get the fuel moving.
Tuner on Kill Bleeds
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TT5.9mag
Two atmospheres are better than one
I think his will be fine there also but if it were mine (and if what I’m thinking is correct) I’d try to kill two birds with one stone and lean out the bottom and richen the middle.I figured that
I think Mark's 2x4bbl will be fine at 12.5
There is a lot going on there (at idle) that could be tested and trialed.
My recollection (it could be wrong) was that 12" Hg in drive is pretty decent with the cam etc.
On the other hand, I'd love to see less rpm drop.
Anyway that's its own rabbit hole - although not entirely seperate from what happens in the transistion circuit.
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MomsDuster
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To throw a little more of a twist in the situation. I’ve brought up the stance of the car. I have a 4.2* rearward slant on the carbs on a level surface. (Pic 1)
Giving me an approximate fuel level somewhere in this (pic 2) area. Will the difference in fuel levels on each emulsion stack create different timing in the booster flow?
Giving me an approximate fuel level somewhere in this (pic 2) area. Will the difference in fuel levels on each emulsion stack create different timing in the booster flow?
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