Stagger jetting question

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66Valiant528

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Anybody doing the staggered jetting on there open plenum style intake as per Vizards book? And why add 2 jet sizes to the pass rear of the carb when cylinders 5 and 7 are on the driver's side? Open plenum M1 max wedge port 4150 950 ultra hp carb

20211006_163626.jpg
 
Never done it myself but I do know that Chevy did it on the 427 COPO
 
Vizard is a good writer. However I learned the hard way to be very very careful in assessing whether specific modifications can be applied from his tests or experience to whatever it is I'm working on. All too often generalizations fail when based on too small of a data pool. He is also limited to the format of the SA publications so stuff gets left out.
Differences in exhaust arrangements, stroke/rod, valve timing, etc can effect the intake and fuel distribution.

I do not have experience with your combo. But I would suggest basing any stagger jetting on observations of your combo after a pass. Compare plugs, then make a change and compare plugs. Of course note if the mph changed with the jet change.

Direct Connection recommended stagger jetting on a number of specific race combinations based on their and their sponsered racers experiences.
For example in this 1977 bulletin: #34 Racing Carburetor and Manifold Calibrations (scans at Moparts)

They also made use of spivies and notches to redirect the fuel from richer to leaner cylinders.
You'll find a few factory high perfomance carburetors did so as well.
My point is that is another tool in your bag that you can make use of if you see a need to redirect fuel.

Spivies: Spivy
(sorry tapatalk lost the photos when they took over but the text is all there)

List-4668 List-4668

Notching: Booster notching
 
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I was thinking about throwing a little more jet at the engine as the dyno sheet shows 13.2 a/f ratio. I would assume I'd see any improvement in mph and having just read that section yesterday I figured I'd see if any other members/racers had done that. The bottom of the M1 intake has no dams . Flat so I'm thinking it may suffer a bit from the 5-7 issue described by Vixard. I doubt it'll hurt to try. And 4.5 bore 4.15 stroke flat top diamonds 13.8 308 comp solid roller
 
I staggered the primary jetting between my front and rear carburetors...
If you were to buy a dual quad Edelbrock carburetor set up they'd be jetted and set up different..
I would say carburetors are very motor specific.. and getting them right takes experimentation...
 
Generally almost never does a carburetor jump out of a box and onto a car and run perfect...
But never say never LOL...
It actually happened to me yesterday for the first time in my life...
We broke in a mid-60s 327 for a customer's 65 Chevelle on an engine stand with a known carburetor. Yesterday we got it ready for a test drive and after setting the timing the dual AF gages were reading just about perfect... I checked this brand new 1406 carburetor that we pulled out of the box and it was out 3/4 of a turn on both adjustment screws. I had to turn the idle up... Afterwards I tried to make some adjustments but nothing made it run better so I left it where it was...
I really think that these kind of 350 327 Chevys were what these were built around anyway so I guess it is possible I just never seen it before.
 
Maybe my "lack of knowledge mind" doesn't understand, but on a single 4 bbl sitting on a open plenum intake, I cannot see staggering jet sizes from driver's to passenger side. I've never done it. I guess I don't know anyone that does do it.
 
Maybe my "lack of knowledge mind" doesn't understand, but on a single 4 bbl sitting on a open plenum intake, I cannot see staggering jet sizes from driver's to passenger side. I've never done it. I guess I don't know anyone that does do it.
I would think just like a lot of carburetors come in with two stages of primary and secondary it's making it more like it has three stages and comes in more gradual and not overloading all at once...?..
 
I would think just like a lot of carburetors come in with two stages of primary and secondary it's making it more like it has three stages and comes in more gradual and not overloading all at once...?..
My way of thinking, if a guy has an open plenum, all cylinders are pulling from the throttle plates so staggering isn't helping one individual cylinder. Again, I've not chased down that avenue, nor have I felt the need to. Somebody that's doing it can share their benefits. Waiting to see or hear.
 
Anybody doing the staggered jetting on there open plenum style intake as per Vizards book? And why add 2 jet sizes to the pass rear of the carb when cylinders 5 and 7 are on the driver's side? Open plenum M1 max wedge port 4150 950 ultra hp carb

View attachment 1715802096


I've stagger jetter dual planes, never needed to on a single plane intake.

The indy dual plane is the worst i've ever seen.
 
You could try it and see if the car reacts positively. The 5/7 firing signal drags volume from that quadrant of the intake plenum. Makes sense, but, test to verify the need.
Best if you had individual AFR for all cylinders. That's an eye opener sometimes regarding poor fuel distribution.
 
I was thinking about throwing a little more jet at the engine as the dyno sheet shows 13.2 a/f ratio. I would assume I'd see any improvement in mph and having just read that section yesterday I figured I'd see if any other members/racers had done that. The bottom of the M1 intake has no dams . Flat so I'm thinking it may suffer a bit from the 5-7 issue described by Vixard. I doubt it'll hurt to try. And 4.5 bore 4.15 stroke flat top diamonds 13.8 308 comp solid roller


I would make a jetting change based solely on an O2 reading. They are really combustion meters so you have to use them correctly. I’d look at the plugs very close and compare that to the O2 and then make a change.
 
There's a few engine masters episodes that get into stagger jetting to help with distribution. What I found interesting is that often AFR changes showed up in cylinders FARTHEST from the changed corner, and sometimes not - it seemed rather random at times, and was VERY dependent on the combo and particular parts in use.
 
Unfortunately my drag car does not have an 02 sensor set up. A/F ratio was from dyno tune on the engine.
And.....my other car, dual quad street car has the 02 sensor stuff in case anyone is confusing the two cars.
I am inquiring for my race only Valiant.
 
My way of thinking, if a guy has an open plenum, all cylinders are pulling from the throttle plates so staggering isn't helping one individual cylinder. Again, I've not chased down that avenue, nor have I felt the need to. Somebody that's doing it can share their benefits. Waiting to see or hear.


It happens more than you think. Just because you have a plenum doesn’t mean you won’t have distribution issues. Some of them occur because the booster design is wrong. Sometimes it’s because of the position of the carb over the plenum, even on a single 4. There are issues caused by big overlap cams disrupting air movement in the plenum and also cause a rich mixture, which requires more heat to vaporize the fuel. I’ve seen several times where getting the fuel curve cleaned up also cleaned up distribution issues.
 
It happens more than you think. Just because you have a plenum doesn’t mean you won’t have distribution issues. Some of them occur because the booster design is wrong. Sometimes it’s because of the position of the carb over the plenum, even on a single 4. There are issues caused by big overlap cams disrupting air movement in the plenum and also cause a rich mixture, which requires more heat to vaporize the fuel. I’ve seen several times where getting the fuel curve cleaned up also cleaned up distribution issues.
ok, but how do you tell #7 cylinder to pull from the 76 jet rear throttle plate on the drivers side and #5 cylinder to pull from the 78 jet rear throttle plate on the passenger side on an open plenum ?? I'm not being sarcastic at all, but I don't understand.
 
ok, but how do you tell #7 cylinder to pull from the 76 jet rear throttle plate on the drivers side and #5 cylinder to pull from the 78 jet rear throttle plate on the passenger side on an open plenum ?? I'm not being sarcastic at all, but I don't understand.

I don’t have an answer for that except to say what happens in a plenum is almost an uncontrolled chaos. There are two reasons why the firing order gets changed. One is because the 4-7 swap significantly reduces the torsional loads on the crank and the engine is noticeably smoother. If you do the the other swap which I think is 4-7-5-2 but I just don’t remember the other two cylinders for sure, you get an even smoother engine. The other thing that happens with firing order swaps is it changes the distribution in the plenum. Even at very high rpm there is air and fuel moving towards and away from the valve. And, even if you have the A/F mixture as good as you can get it, and it comes out of the booster clean and you get good shear off the throttle blades the fuel will still be heavier than the air and it is less responsive to changes in direction. When that happens the fuel falls out of the air and hits the floor and the walls. Then it runs into the cylinder. But not always the cylinder that it was headed to. That’s the best guess I can give you, because that’s what my limited experience allow me to understand. I do know at one point in pro stock they were moving the carbs forward as much as .600 to help control fuel with a 3g launch. I only know that because I had a friend that was working with an engine builder doing some of that stuff and he told me. Otherwise I wouldn’t know that. That’s with carbs. With EFI and the injectors at the valve cover rail it’s a bit different. I just saw the EFI guys are starting to move the injectors up. Doing that will make tuning EFI more like tuning with carbs as far as distribution and such is concerned. I’ve only read and never witnessed that some are putting the injectors right in the plenum and they are making more power. In my opinion that means that anything you would do with carbs as far as distribution is concerned you’d be doing with EFI.
 
I don’t have an answer for that except to say what happens in a plenum is almost an uncontrolled chaos. There are two reasons why the firing order gets changed. One is because the 4-7 swap significantly reduces the torsional loads on the crank and the engine is noticeably smoother. If you do the the other swap which I think is 4-7-5-2 but I just don’t remember the other two cylinders for sure, you get an even smoother engine. The other thing that happens with firing order swaps is it changes the distribution in the plenum. Even at very high rpm there is air and fuel moving towards and away from the valve. And, even if you have the A/F mixture as good as you can get it, and it comes out of the booster clean and you get good shear off the throttle blades the fuel will still be heavier than the air and it is less responsive to changes in direction. When that happens the fuel falls out of the air and hits the floor and the walls. Then it runs into the cylinder. But not always the cylinder that it was headed to. That’s the best guess I can give you, because that’s what my limited experience allow me to understand. I do know at one point in pro stock they were moving the carbs forward as much as .600 to help control fuel with a 3g launch. I only know that because I had a friend that was working with an engine builder doing some of that stuff and he told me. Otherwise I wouldn’t know that. That’s with carbs. With EFI and the injectors at the valve cover rail it’s a bit different. I just saw the EFI guys are starting to move the injectors up. Doing that will make tuning EFI more like tuning with carbs as far as distribution and such is concerned. I’ve only read and never witnessed that some are putting the injectors right in the plenum and they are making more power. In my opinion that means that anything you would do with carbs as far as distribution is concerned you’d be doing with EFI.
well, changing the firing order is a complete different subject. But for the topic at hand by the op, I can't see staggering jets from passenger side to drivers side on an open plenum intake will cure the fuel distribution for cylinders on the same bank. I'm lost there...
 
All of my engines get broke in and tuned on a crank dyno. Individual EGT probes on each cylinder, AFR on each bank.

Unless you can actually monitor each cylinder I wouldn't be stagger jetting.

IF you have an AFR in each bank and a single plane, maybe you could stagger jet side to side. I really think that is splitting hairs. Very few people are heads up racing here. If it feels fine on the street, you're good. At the track you're looking for consistency.
 
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:popcorn: even pulling from an open plenum or single plane it seems there could be small gains made by staggered jetting. But the plugs would have to be showing a fairly significant burn difference on certain cylinders.
 
Yes as the OP I was confused by Vizards suggestion to jet up drives front pass rear. How does that help the 5/7 issue? When I get home I'll post his illustration.
 
The inner runners on a single plane have a shorter and more direct line to the ports which could cause uneven distribution. But with readings from wach cylinder via O2 or plug reading you will never know .
 
When my Indy headed small block was on the dyno the afr’s were about two points off side to side. A 1” Super sucker brought the afr’s to a couple tenths of each other. And it made 13 more hp, no other changes.
 
When my Indy headed small block was on the dyno the afr’s were about two points off side to side. A 1” Super sucker brought the afr’s to a couple tenths of each other. And it made 13 more hp, no other changes.

I had nearly the same experience. I've used super suckers ever since
 
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