(2)450 DP's vs (2)750 DP's

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Quick update for those still following. I just wrapped up replacing intake gaskets. Noticed the vacuum gauge fluctuating a bit more than normal at idle after the last update, that’s what led me to check for a vacuum leak and found the intake leaking. I also blocked the middle emulsion holes and dropped the secondary jets. Will check the timing and give her another shake down cruise tomorrow to note any changes.
 
MomsDuster said:
Ok, for those interested. Took her out for a “data” session and here are the results. (See attached photo’s)

Would like to lean it out just a touch everywhere. May try to lower the floats 1 flat and see what happens. Also need to get smaller jets for the secondaries, it’s got 66’s in there now and the next smaller size I have is 60’s. Thinking that’s probably a bit too small. New PB’s on all Dragy data points. Not super dramatic but improvements are always good! As always, all comments are welcomed.

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Let me ask are your TSR really .088 ? And PVR .038 is that stock QF450 block size?
 
66Valiant528 said:
Let me ask are your TSR really .088 ? And PVR .038 is that stock QF450 block size?
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They didn’t have TSR’s. The holes were larger than that, so I tapped them and threw in the .098’s. Closest to the original size of the passage. And yes, the PVR’s are .038 and from the factory.
 
MomsDuster said:
They didn’t have TSR’s. The holes were larger than that, so I tapped them and threw in the .098’s. Closest to the original size of the passage. And yes, the PVR’s are .038 and from the factory.
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On a tunnel ram your TSR’s are probably .020 bigger than the need to be.

I start at .074 on single 4 stuff and usually go down from there.
 
Ok it’s been a minute since last update. A bout of diverticulitis and the weather have kept me out of the garage. So I’ve plugged the middle emulsion holes and have the following tune

IFR .032 in the lower position.
IAB .070
Main Jets 55/62
MAB 35
PV 9.5
PVCR .082

Took wifey out for a little put around town so I don’t really have any numbers to share yet. I will note that it ran really well. A few things I want to address though.
Under light acceleration it gets a little lean. I can hear and feel it (just a little flat). It’s not super bad 15-15.5 AFR but would like to look into it. (IFR,TSR?)
Also at say 45mph+ cruise it trends upwards a little lean for my liking. Believe that’s about where the transition is tapering off before the mains come in. (IAB,MAB,IFR,TSR?)
And at a steady cruise anytime after 40mph I can hear an exhaust pop. I can’t feel it or see it in any of the gauges and I haven’t pinpointed any particular rpm, but it is kind of annoying to listen to. (Timing maybe?)

Seems it might be fat when the PV comes in. That will be determined after getting some #’s.

At any rate going to get it out soon, get some numbers and check the timing curve (when I can get a helper in the garage).

Feel free to post up some ideas for me to kick around.
 
It seems you are pretty smart on this carburetor thing,,,,,you really do good analysis on the tune .
I was pleased to see you have annular boosters on your 750 s,,,,very good work .

I hope the others reading this thread remember that you are using a Tunnel Ram,,,,,and that’s a whole different animal .
Also,,,,it’s difficult to tune it perfectly for street,,,,,because it’s made to run WFO !

Man,,,,I love a Tunnel Ram on the street,,,,,had one on my Challenger since 1987 .
It really hits hard about 2800 on up,,,,although I guess that rpm range is also the cam coming in.
My 452 liked 7600 as the max RPM,,,,screaming like an animal ,,,lol .

Tommy
 
Tommy, @Desoto291hemi thanks. Smart? I wouldn’t say that, but I know enough to fake it through most of it LOL. I do enjoy the process of working out the tune for sure.
 
MomsDuster said:
IFR .032 in the lower position.
IAB .070
Main Jets 55/62
MAB 35
PV 9.5
PVCR .082
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MomsDuster said:
Under light acceleration it gets a little lean. I can hear and feel it (just a little flat). It’s not super bad 15-15.5 AFR but would like to look into it. (IFR,TSR?)
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It sounds like the TSR is too big to be doing any restriction. You set it up in case its needed, so no issues there. I'd leave it alone until the other possibilities have been explored.
If it feels flat and you think its lean, stick some wires in the IABs. Change nothing else.
See what it does. ABAB or ABBA to be sure it wasn't a difference of cold vs warmed up.

MomsDuster said:
Also at say 45mph+ cruise it trends upwards a little lean for my liking. Believe that’s about where the transition is tapering off before the mains come in. (IAB,MAB,IFR,TSR?)
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Is it pulling? is lightly fluttering?
Might as well check the 45+mph cruise with the restricted IABs while they are in there.
At 65 mph, if it was the idle/transisition, then a change in IAB should have no effect.
MomsDuster said:
And at a steady cruise anytime after 40mph I can hear an exhaust pop. I can’t feel it or see it in any of the gauges and I haven’t pinpointed any particular rpm, but it is kind of annoying to listen to. (Timing maybe?)
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Maybe timing or could be the mixture. Did you make any changes since 2021?
Doesn't sound like a backfire right at the exhaust valve.
There's unburned HC in the exhaust, and someplace in the exhaust is hot enough to set it off when a bit of O2 becomes available.
In theory steady cruise should be low density and relatively cool chamber so look at vacuum advance plus the mechanical timing.

What I'm saying is yes it could be too rich, or it could be correct but not reasonably completing combustion.
I'd work on solving the idle/transition first and then come back to this.


Maybe an Aside, but my gut feeling from working with single 4 bbls is .035 MAB is large for a 55 MJ
So that would be a second path to explore.
(Even though you know I prefer making MAB adjustments only after seeing the AFR shape for the top half of 3rd or 4th gear at full throttle)
 
Your tuning approach looks good. Those 450’s I believe flow even less then what they are rated at. IMO those are a bit small for a 440. I think a 600-650 range carb would be more appropriate. 750’s could be a tad on the large size given the combo is very mild. I personally like to keep the MAB smaller and adjust around that.
 
Thanks for the input Matt @Mattax

I’ll change the IAB’s first and go from there. I have plenty of air bleeds and plenty of blanks. You think taking a bigger swing at it, 65’s?

As for the 45 cruise, it’s not giving any fits whatsoever, might start sounding a bit flat but not surging or anything. It just bugs me seeing those 15+ afr’s. Probably just need to cover the gauge! LOL

I haven’t changed anything in the distributor in a long while. I did remove it when I did the intake. Makes it a lot easier to put that tunnel ram with carbs by myself. I haven’t been able to find my latest cure notes. So I just need to double check it.

I agree with you on the MAB’s going to hold off there until I get the rest sorted out.

Think what I’ll do is get it out as is. Get my numbers then change the IAB’s and go right back out and retest.

Thanks again for the input
 
68 HEMI GTS said:
Your tuning approach looks good. Those 450’s I believe flow even less then what they are rated at. IMO those are a bit small for a 440. I think a 600-650 range carb would be more appropriate. 750’s could be a tad on the large size given the combo is very mild. I personally like to keep the MAB smaller and adjust around that.
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Thanks for the input, always appreciated!
 
Ok latest tune and results. I went from .070 to .067 IAB’s, and from .035 to .033 MAB’s. Runs really well. It still sounds like a miss or light exhaust pop at the end of the transition slots, 40-55 mph. Checked the timing curve and I don’t think it’s a timing issue. Thinking it’s not liking the leaner conditions in that range. Will likely increase the IFR’s to give myself more fuel to work with in the transition area. Will probably increase the secondary jets a number or two also. Attached is a photo of my notes. Curious to the FABO tuning guru’s thoughts and comments.

image.jpg
 
MomsDuster said:
Ok latest tune and results. I went from .070 to .067 IAB’s, and from .035 to .033 MAB’s. Runs really well. It still sounds like a miss or light exhaust pop at the end of the transition slots, 40-55 mph. Checked the timing curve and I don’t think it’s a timing issue. Thinking it’s not liking the leaner conditions in that range. Will likely increase the IFR’s to give myself more fuel to work with in the transition area. Will probably increase the secondary jets a number or two also. Attached is a photo of my notes. Curious to the FABO tuning guru’s thoughts and comments.

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I think you are about 7 thou too big on the MAB.

That gets on the booster quicker and it changes your PV timing and T slot sizing.

And of course main jet sizing.
 
Newbomb Turk said:
I think you are about 7 thou too big on the MAB.

That gets on the booster quicker and it changes your PV timing and T slot sizing.

And of course main jet sizing.
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I really appreciate you chiming in on this!
I’m following what you’re saying, and I like that idea. Easy enough to do.
The one question I have is, how does that affect the PV timing? I understand it will affect the PV restrictions, and they will likely need to be reduced. But I’m not following the PV timing aspect.
 
MomsDuster said:
I really appreciate you chiming in on this!
I’m following what you’re saying, and I like that idea. Easy enough to do.
The one question I have is, how does that affect the PV timing? I understand it will affect the PV restrictions, and they will likely need to be reduced. But I’m not following the PV timing aspect.
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The smaller the main bleed the LATER the booster starts.

The larger the main bleed the SOONER the booster starts.

So…if you go from your .033 MAB down to an .026 you will start the booster later.

At that point you start getting a bigger gap in the time between the near (later) end of the T slot and getting on the booster. Under certain conditions you may have to start the PV a bit sooner (higher number) so when the booster does come on and the load is high enough you have PV fuel there.

And it may require some tuning on the accelerator pump circuit.


EDIT: ignore the PV timing. I just came off a 3.5 hour nasty drive and I lost my mind.

Until it’s on the booster the PV opening point won’t matter. Unless I’m thinking wacky again.

I’ll sleep on it.
 
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You need to figure out what circuit you are running on when you feel the miss. are the primaries flowing or are you still on the idle/t slot circuit?
 
mderoy340 said:
You need to figure out what circuit you are running on when you feel the miss. are the primaries flowing or are you still on the idle/t slot circuit?
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I believe that this is the area where the transition is tapering off and the mains coming in. This last change of bleeds seems to maybe have broadened that area. I may just go back to the previous tune and retest.
 
MomsDuster said:
I believe that this is the area where the transition is tapering off and the mains coming in. This last change of bleeds seems to maybe have broadened that area. I may just go back to the previous tune and retest.
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As a test, try more main jet
It's simple to do. See how this affects the driveabilty first then look at your numbers. It seems the changes you're making are not changing the a/f ratio much in the area you notice the problem. Take a bigger swing and try to move the a/f a whole number just to see if the motor likes it. You can figure out what change you want to incorporate to get the resulting a/f ratio after you determine that is what the engine wants in the area where the problem exist.
 
92b said:
As a test, try more main jet
It's simple to do. See how this affects the driveabilty first then look at your numbers. It seems the changes you're making are not changing the a/f ratio much in the area you notice the problem. Take a bigger swing and try to move the a/f a whole number just to see if the motor likes it. You can figure out what change you want to incorporate to get the resulting a/f ratio after you determine that is what the engine wants in the area where the problem exist.
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I like that idea, Thanks!
 
Sounds to me like you have a lot of transfer fuel and ( [email protected]) and the transition to the mains (off the transfers) is too large of an afr gap. I’d drop the tsr and up the main jet. As a test. What’s the cruise (~2600 rpm) AFR?

Edited to add;
Looking back at your chart I see 14.9 AFR at 2900 and highest vacuum at 17in. You’re definitely on the mains there and lean, I’d step up 4 numbers on the mains and retest.
 
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Newbomb Turk said:
The smaller the main bleed the LATER the booster starts.

The larger the main bleed the SOONER the booster starts.

So…if you go from your .033 MAB down to an .026 you will start the booster later.

At that point you start getting a bigger gap in the time between the near (later) end of the T slot and getting on the booster. Under certain conditions you may have to start the PV a bit sooner (higher number) so when the booster does come on and the load is high enough you have PV fuel there.

And it may require some tuning on the accelerator pump circuit.


EDIT: ignore the PV timing. I just came off a 3.5 hour nasty drive and I lost my mind.

Until it’s on the booster the PV opening point won’t matter. Unless I’m thinking wacky again.

I’ll sleep on it.
Click to expand...
Now you got me thinking about PV timing and narrowing the gap between coming on the booster, and adding PV fuel. Never thought about that aspect of PV tuning before.
 
TT5.9mag said:
Sounds to me like you have a lot of transfer fuel and ( [email protected]) and the transition to the mains (off the transfers) is too large of an afr gap. I’d drop the tsr and up the main jet. As a test. What’s the cruise (~2600 rpm) AFR?

Edited to add;
Looking back at your chart I see 14.9 AFR at 2900 and highest vacuum at 17in. You’re definitely on the mains there and lean, I’d step up 4 numbers on the mains and retest.
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I’m really liking the increase the mains thought. What size TSR are you thinking? .065 ish? Also, by reducing the TSR won’t that cause an even leaner issue? Or will the increase in the mains increase the need for smaller TSR’s? Just trying to follow the thought process here.
 
The increase in the mains fuel does two things, it fattens up the cruise afr (youre fairly lean there and have lots of timing) and closes the gap between mains and transfer slot. The transfer fuel is tricky, if you can stand a lower idle, you can kill some transfer fuel by closing the throttle blades slightly and re adjusting the mixture needles. (If you don’t like the lower idle, crack the secondary side open a touch) Although that’s more of a fine adjustment. .078 too .065 is a big jump and I like making big initial changes but I don’t like giving numbers when referencing carb tuning. Every single one I’ve ever tuned is different and don’t want people seeing numbers on the internet saying “it works for him”, that kind of thing.
 
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MomsDuster said:
Also, by reducing the TSR won’t that cause an even leaner issue?
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Yes it will, but look how rich you are at low rpm, so lean out that range, and fatten up the range (1500-3000) above it. That’s the goal.
 
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