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 Stagger jetting single-plane intakes (firing order) 
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Joined: Wed Dec 18, 2013 2:23 pm
Posts: 102
One of the sections in the latest David Vizard carb tuning book that caught my attention was his observations on stagger jetting single-plane intakes to offset the side effects of the ignition firing order.

He gave a couple of examples of what he did for the standard Chevy & Mopar 18436572 order, indicating that the #7 firing after the #5 caused a mixture imbalance in the plenum (although he seemed to describe different "fixes" for the same issue, depending on the example).

Has anyone done any tuning along these lines? Given a certain firing order, etc., are there any reasonable "rules of thumb" for jetting changes to compensate for this type of condition?

If you don't have O2 sensors for each cylinder, it seems to me that it could be really hit-or-miss about whether any changes correct the bias w/ the expected lean vs. rich cylinders.

Just kind of thinking out loud here... - Brad


Thu Apr 24, 2014 4:37 pm
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Joined: Fri Feb 08, 2013 2:37 pm
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So many things affect distribution I don't see how any generalization can be made for firing order or anything else. I have seen the rich-lean cylinders move from one to another simply changing to a different intake manifold of the same part number on the same engine with no other changes, same carb, heads, cam, headers, exhaust system, just happened to pick a different intake manifold from the stash, in this case a cast-iron BowTie “marine” intake on a SBC circle track engine, 14.8/1, BowTie iron heads 750 and 390 Holley carbs. I have seen the lean corners change on BBC oval and square port, changing from one brand of single plain to a different brand, Team-G to Vic Jr. I have seen the lean hole move from changing header pipe diameter up 1/4”. If you remove the choke butterfly it will affect it.

You need to treat each engine as an individual. Without O2 or EGT you need to read plugs. Look for the relative height of the fuel ring on the plug ceramic, add fuel to move it up and take it away to move it down. I would do that even with O2 and EGT.

In the days when the factories made thousands of identical engines the distribution fixes like trim tabs and stagger jetting were effective because the mass produced engines were (nearly) identical. There is a late 60's, early 70's, 413 Dodge HD truck engine Holley that has four different jets and woe is the person that gets them mixed up because it doesn't just run bad, it burns the valves in just a few hundred miles of heavy use when the holes that needed the large jets got the smaller ones.


Thu Apr 24, 2014 7:17 pm
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Joined: Wed Dec 18, 2013 2:23 pm
Posts: 102
You mean it's not as easy as the book makes it look? Imagine that... :O


Fri Apr 25, 2014 1:48 pm
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Joined: Fri Feb 08, 2013 5:50 pm
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You can look at plugs and get an idea on what to stagger, the only thing is like Tuner said you can't arbitrarily do it this amount because you use this intake with this engine. And how well the carb atomizes fuel has an impact, usually the better it does the less stagger you will need.


Sat Apr 26, 2014 4:50 pm
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