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Beating Vacuum Rules


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this is a copy of a post from www.4m.net about 4 cyl mustangs, But the same stuff will work on any engine 4 cyl or V8, after reading this you will see that a lot of this stuff will only work IF a engine has a good modified exhaust system such as headers and big pipes, With AC V8 lead sleds and spartan V8 pure stock cars having a restricted exhaust system of a single 2 inch outlet no duals allowed, much of the stuff talked about here will not work, but some stuff WILL )



JBV-HEADS Beating Vacuum rules #1

Armchair Crew Chief




2006/5/8 22:51

From Twin Cities, NorCal

Posts: 401

I've been asked this enough again and some tracks are thinking about vacuum rules again. I'll put it back out there and hopefully racers will have an arguement to present to the track to keep it from being adopted. This is about 2 years old but was effective in our area. A racer took this to the promoter and the rule was dropped. Of course the promoter was not happy with my description of promoters that use this rule. But if it looks like a duck, walks like a duck, and quacks like a duck, by golly it's a duck. Use it to your advantage.


Beating Vacuum Rules




I’m sorry it has taken me so long to write this, but I’ve been tied up with CNC literature, programming, and salesmen.

Vacuum rules are based on the false pretense that a powerful motor has low vacuum at idle. When in fact, only motors with large overlaps in timing, or those that have mechanical problems, have low vacuum. If you build your engine using techniques to build power that do not require using large overlap periods, you will have avoided the trap the lazy track promoter used. The reason overlap periods affect vacuum is because the exhaust invades the intake manifold at low RPM. At mid range, the velocity of the intake limits this contamination and the overlap actually scavenges the cylinder better, creating a vacuum that pulls the intake charge in better. At high RPM, this overlap vacuum, because of the length of time, fades quickly.

What you need to do is build your motor with only the necessary overlap needed and limit the intake contamination if you do use more overlap in your buildup. How do you do this should be your next question. You want mid-range power but you don’t want lower idle vacuum.


Limiting the effects of overlap;


LONG RODS- use the longest rods you can. Long rods leave the piston near the top of the cylinder longer. The piston moves less for a given amount of duration. If the piston isn’t pushing air up, it can’t push air into the intake. This will allow you to use more overlap in your cam. This is also the main reason Toyotas do so good in vacuum classes. Their opposing valves also help, but to a much lesser extent.


AR cones- Anti-reversion cones- these cones were never designed for this type of racing, but one side effect they have is, they create vacuum behind the exhaust valve. The vacuum pulls the exhaust gases out the exhaust port, away from the intake, and also pulls on the intake charge. A little back door super charging goes on. This will also let you add a little more overlap for mid-range power.


AR valve treatments- if we are limiting gases from entering the intake port. Let’s make it hard for it to do it. Round the chamber face and margin on the exhaust valve. It makes it easier for the gases to enter the exhaust port this way. Now lets put a 45 degree bevel around the intake valve on the chamber side. Any gases headed toward the intake valve are now bounced away from the intake port. This small step is a giant multiplier to everything you do. These 3 items will turn a 9 in motor into a 14 in motor at 1000 rpm.


Super advanced timing- we would put a MSD timing advance module under the dash. You pull into tech and turn the nob all the way up to additional 15 degrees. Don’t race this way, but for tech it’s worth close to an inch. You can also set a MSD digital 7 programmable for additional 15 degrees between 900-1100 rpm. This is probably not possible in an entry level class do to cost.


Variable cam duration


Since we can use a little more overlap in our mid range, but not at idle, let’s make dual durations of cam timing. This is for fords. Take a compensator and gut it. Take the shell to a high end machine shop and have them EDM or lazer a small hole in the bottom. Don’t try to drill it as it is as hard as a diamond. Clean and put the compensator back together again. Take a small chisel and put a small crease into the ring at the bottom of the compensator boss. This will allow the compensator to leak down at idle, but pump up at racing rpms. If you make the crease to big. You can put a jet into the leak down hole that comes from the bottom up toward the valve cover. Just call it a valve spring oiler. We would put a valve on the oil line to the head that would drain straight to the pan. Turn it on and circle the track once before going in to tech. Just tell people it goes to your oil pressure guage. Which ours did. Valve just happened to be downstream.


For vacuum classes, you should use lobe separations of 114 or more. If you use all these items 114 will work. If not, you need to be closer to 120. Durations should be 240 @ .050 or less. 240 if you use these items and 220 if you don’t. This is based on 15 inches at 1000 rpm.


Power adders that don’t need overlap;


Nitromethane, nitropropane, propylene oxide, methanol, mtbe and oxygenated gas. You would be surprised at how many tracks don’t address these. Some from ignorance but most because they can’t test for it. After all they have a vacuum rule and don’t use calipers to check cams for lift.


Some people think this might be cheating. But I have never seen the first 3 addressed in any rules. The fourth, I have seen a track have people take the timing additions off because it had gotten out of hand. The variable cam duration has never been addressed. Power adders are only on about a third and only once was it semi enforced. It was when I handed tech a hydrometer and said,”here is the test for nitro’, at the racers meeting. Three cars decided to change the foam in their gas tanks after the meeting.


There are many other ways that are out and out cheating. Number 1 I’m sure is an electric door lock pump tied into the power brakes. Power brakes on a mini (red flag). I won’t go into this any farther as I am not interested in telling racers how to cheat. I am only interested in telling racers who don’t have the extra money to pay high end shops for this type of information. I would like all racers to have an equal chance and this rule, which favors the well to do, banished.

Remember, your goal is to increase HP without increasing overlap, or to limit overlap's effects on the intake port.

Good racing and Good luck ,



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