How To Choose The Best Head For Your Engine Hard Facts & Figures

[HeadDoctor]

This article is written by Denny At JDS Induction Products
(Posted 10-7-2006)

How Do I Know If The Heads I’m Buying Are Correct For My Engine?

Introduction

You Can’t unless you own a flow bench or have some other way of testing the product, or you’re an engine builder and have used that particular head in the past and had success with it. However, in this article we’re going to show you how to be more confident in your choices.

The 6 ways to increase power are:
Displacement
Compression
Ignition
Intake/Exhaust
Volumetric Efficiency (VE)
Power-Adders

In this article we are primarily concerned with VE. The list says that #6 is a power adder, for me that’s N2O because a blower is a VE enhancer, even though it increases power. Because the internal combustion reciprocating engine is basically an Air-Pump we have to talk about the dynamics and inter-related things that affect the way air enters and exits the heads.

We are not dealing with rocket-Science it’s just elementary physics so books become another important ingredient in becoming an engine wizard.

Science & Technology Encyclopedia ISBN 0226742679
Understanding Physics –Isaac Asimov ISBN 0880292512
Physics For The Rest Of Us – Roger S Jones ISBN 0760712638
Marks Standard Handbook For Mechanical Engineers
High Performance Automotive Fuels & Fluids - Jeff Hartman ISBN 0760300542
Two Stroke Tuners Handbook – Gorden Jennings ISBN 0912656417
Turbochargers – Hugh Mcinnes ISBN 0895861356

Holley Carburator Books:
Holley Carburators Dave Emanuel
Holley Carburators Mike Urich
Super Tuning Holley Carbs – Alex Walordy

With that out of the way lets look at what we’re going to talk about:
1) The correlation of interdependent components
2) How the cam affects the choice of a head & visa versa
3) The rod ratio
4) Displacement concerns
5) Determining the variables within the head

Here’s some Dennisims that will help you to stay within the basics:
The pistons go up and down, The valves open & close, The air/fuel goes in & out, and The spark plug starts the fire.

KISS
Keep
It
Simple
Stupid

Otto invented this thing in 189? And it hasn’t changed its basic principles yet!!

1) The Correlation Of Interdependent Components
A) Rod Ratio
B) The Cam
C) Displacement
D) The Head Itself

A) The Rod Ratio
The first consideration I always take into account is the rod ratio, because it controls the dwell time and the piston acceleration away from TDC and BDC. The rod ratio controls 2 very important things:
1) Piston scuffing
2) Negative Pressure – Its not really vacuum

A vacuum is the “absence of air” We’re concerned with creating a very high negative pressure wave inside the cylinder. Generally speaking the longer the rod – A Higher Ratio – the higher in the RPM band is the power and torque. But because of the characteristics of the rod ratio, it can actually help the head fill the cylinder better. We’ll talk about that in the velocity section concerning the variables within the head, because there is a direct correspondence between Air-Flow, Port Velocity, and the BHP/Torque of the engine, and the rod ratio is part of that equation.

B) The Cam
Another simple device that converts one kind of motion into another – like the crank it converts rotary motion into linear motion, although the cam is a bit more complex in accomplishing the same job. If I can de-mystify this stuff you will be well on your way to understanding engine dynamics. The 3 basic concerns with the cam are:
Lift
Duration
Acceleration

Lift or the vertex of the parabolic curve of the cam profile is the distance the cam opens the valve.

Duration is the amount of time the cam-lobe holds the valve open throughout its event.

Acceleration-Deceleration – this is the cam designers nightmare, because it is a compromise of variables. Here we are concerned with how quickly we can get the valve open. Theoretically we want to start opening the valve at TDC have it open by peak piston acceleration and get it closed by BDC. Of course this is an impossibility with the valve train dynamics that we have to deal with in push-rod engines. Nevertheless we are concerned with “The Area Under The Lift Curve.” Consequently the faster we can accelerate/decelerate this puppy the more area we’re going to have, which is important to cylinder filling, hence the air-flow characteristics of the head and manifolding systems.

What goes up must come down or we’re in a George Carlin event.

The cam designers have to develop a negative acceleration ramp near the top of the cam to keep the lifter from flying off into nowhere land, called valve float. This creates a dynamic problem called “The Jerks.” When you here people talk about harmonics this is what causes the valve-train harmonics, we’ve seen it completely destroy the valve-train when the cam profile is too violent or aggressive. However with modern polyneumerical equations this is probably in the distant past. However you still have to match the valve-train components with the cam-profile or your going to have serious difficulties with valve float.

Basically the more aggressive the cam the less duration there is between the .020 number and the .050 number and then you want to look at the .200 number as well, because they can cheat the .020- 050 numbers. But to be really sure you have to check the entire lobe, and specifically at the valve. Take Note - that the rocker arm ratio modifies this entire dynamic, and the area under the lift curve.

C) Displacement
You might think that finding the right head for the displacement of an engine is obvious but it’s really not. For instance you could put an E-7 head on a 460-cid engine and it will run and do all the things that any engine does except make the power it is supposed to. Therefore we try to pick a head that will at least have the capacity or port volume to handle the displacement of the engine it’s being used on. Another question is how did you arrive at the displacement in question?
Did you bore the block out to the max to achieve it?
Did you stroke the crank to achieve it?
Did you do both?
If you increased the stroke and that altered the rod ratio which way did it go and by how much?

Because the stroke and the rod ratio are interdependent and that affects cylinder filling then we must concern ourselves with those parameters. Even though by increasing the bore, the stroke or both also increases the power potential one affects horsepower more than the other. That’s way small displacement high revving engines generally have high bore to stroke ratios. The reason for this is to keep the piston speed down and the cylinder filling high.

D) The Head Itself
There seems to be a great discussion about which part of the head is most important. Some say that intake is most important because it’s harder to get the air in than it is to get it out. However it is my contention that both the intake and exhaust ports have to be efficient and have a specific flow ratio, but it is the combustion chamber that is most important. The reason being is that this is the last place that the A/F sees before the spark plug ignites the charge. Hence it is the entry into the exhaust bowl area and the areas just beneath the intake valve exit that are just as important. Because it really is harder to get the air in the cylinder; we have to contend ourselves with those extenuating circumstances and the compromises therein.

Each of these components has a direct affect on the charge density inside the cylinder when it is time for the power stroke.

Of course it is the head that becomes the single most important piece of this equation because, with the valves and passages it is responsible for conveying the A/F mixture into the cylinder and removing the waste gasses. Consequently the heads get a lot of attention, and generally to the exclusion of the others or visa versa, where it is just an after thought. Actually the smart engine builder designs the rest of the engine components around the head he wants to use, thus reducing the compromises that may exist.

2) How The Cam Affects That Choice
Once you have some idea of where you want to go with any engine, it primary use and other variables the camshaft selection begins. When the knowledgeable engine builder does this he generally disregards the common cam choices in favor of the profile list. This is better for 2 reasons:
1) You know exactly what you’re wanting to do
2) There is more information

Even if you are not a knowledgeable about cams you will be after you read this and follow through with the projects included. As discussed above we said that the most important parts of the cam are lift and duration. 2 figures are generally given for this, the duration at .020 & .050 and the lobe lift – call this your first indicator.

Subtract the .050 number from the .020 number and that gives you the basic level of acceleration for that cam profile and an indicator of the area under the lift curve. The more degrees of rotation the slower the cam accelerates the lifter, and it will be gentler on the rest of the valve train. Mechanical roller cams in the 33 to 36 degrees of rotation between the 20 & 50 numbers are smooth by today’s standards. Cams with 28 to 32 degrees of rotation are generally very aggressive and hard on parts. With modern cam and spring technology it is possible to run mechanical roller cams on the street, if you’re up to adjusting the valves often.

Hydraulic Rollers
Unfortunately they don’t give you the .020 number on hydraulic cams and this makes the choice somewhat more difficult. Here you are dealing with a fictitious duration number computed at .006 degrees of rotational lift. It’s fictitious because the lifter takes up the normal clearance that would be in a mechanical cam. Because this can vary from I lifter type to another the cam companies arbitrarily decided to use the .005 or .006 number.

Armed with just these 2 pieces of information you can begin to look at heads in a different way. Consider the duration at .020 verses the .050 and what is most apparent?

The .050 number is always smaller that the .020 number, hence we can assume that all the numbers are diminishing as the cam approaches max lift. Therefore the cam holds the valve open longer between .100 and .500 the most – depending upon the actual lobe profile and its design criteria. If the cam your looking at has a gross lift of .500 then you have to decide for yourself where it is holding the valve open the longest (.500 divided by rocker ratio of 1.6 = .3125 lobe lift).

This is not a what if determination!!!

Now you must find out the “Flow At Lift” of the head you want to put on this engine. Remember the more low lift flow you have the better off your going to be. With this determination you’re still at the 1 indicator factor, so the next thing your looking for is the runner volume. Although this is not the best way to determine the correct port it is better than nothing, if you can get the Velocities At Cross-Sectional Area the cc volume becomes meaningless.
Now - Armed with just these 4 pieces of informational indicators you have reduced the mystery of “Which Head” into the realm of this can work correctly.

To be sure beyond reasonable doubt you have to get a “Cam-Doctor” report. This generally costs about 30 to 50 bucks. But you could build own mechanical “Cam Doctor” if you want, which is what I did before there was such a thing. The cam doctor didn’t arrive on the scene until 1989 at the Daytona Trade Show, along with the first ever fully CNC ported head by Kenny Weld.

To accomplish this you’re going to need a few items, several 1’” travel dial indicators, a block, a head with 1 cylinder assembled. Creak rod and piston for #1 cyl, timing set, flat tappet & roller lifters, adjustable push rods, rocker arms etc.

Once you have the block assembled, with the cam for #1 cyl. Find TDC install the degree wheel and install a permanent pointer. Now put the head on and the rest of the valve gear. Use the adjustible push rods to make sure that your rocker arms are “Dead On Correct.”

Next you have to fabricate a plate to hold the dial indicators for the intake and exhaust valves. This sits on the exhaust side of the valve cover rail, and you can use magnetic base holders if you like. But you may have to put a foot on the dial indicator to get to clear the rocker arm.

Yes you’re degreeing the cam – yes I know that you already know how to do this!

1) Now you’re all set up – turn the engine over and measure the gross lift and write that down.
2) Get your graph paper out and create a lift VS duration chart with room to put the duration numbers and lift numbers. Find the half-way point and draw a line so that you know which is the opening side and the closing side.

I do this every .025 of lift all the way around the cam, however this is a bit tedious, so try it at .025 for the fist points up to .100 and then go to .100 the rest of the way, its up to you. After you have the this information written on the graph paper draw horizontal lines across the paper at the lift points on the flow chart. I use .200 through .800 on my flow charts, (don't forget to divide the valve lift by the rocker ratio so the points on the cam are correct) but with low lift hydraulic cams you may want closer numbers, so specify to your head porter what you want – of course there is an added charge for that!

When we send charts to Dema Elgin he always asks for every .025 on the lift curve, and this is true for most cam grinders.

Now you can visibly see the area under the lift curve and compare the flow numbers of your various heads to this chart. Pick out the heads that have the best low-lift flow and your on your way to having an efficient engine combo.
BUT!
Suppose you already have a set of heads that doesn’t have good low-lift flow numbers as compared to others of its type, size all things the same etc? What can you do?

Here’s where this procedure comes in handy – cams cost less than heads – so how many cams can you buy for 1 set of heads?

Considering a decent pair of after-market heads is going to cost you $2000.00 and up.

Since most cam grinders have cam-doctors you can call them and try to find a cam profile that has higher acceleration ramps that are going to give you more area under the lift curve. You could switch from a hydraulic cam to a mechanical street roller. The valve train components are more expensive but – but – but !! How much will it cost to tune up the heads you already have? Then what if the low-lift flow can’t be improved because of the way the heads have been ported? A bird in the hand is better than the two or three you see across the street.

3) The Rod Ratio
The rod ratio is that dimension between the stroke length and the rod length or 5.700/3=1.9
It’s more than just the dwell time because it also determines the acceleration of the piston away from TDC and BDC. When you understand that the intake valve chases the piston down the cylinder, you also understand how important it is to keep this relationship as close as mechanically possible without interference. When you can accomplish this it assists the incoming A/F mixture into the cylinder. Another thing we have learned, because of the Wet-flow technology, is that flat valves work better than dished valves so we have to find a different way to keep them light as possible.

Now then because the rod-ratio modifies the piston acceleration it also determines where the piston achieves max-acceleration. To determine the best cam timing events you have to know where this happens.

Since you already have your test stand built you can put a rod and piston assm. In the #6 on a Chevy or #7 on a Ford. The crank causes the piston to achieve max acceleration when it is at 90 degrees in its travel up or down the cyl. So you have to know where that is with your particular rod/stroke combination.

Once you know this location in crank degrees you can adjust the timing events to maximize the head-flow characteristics. This will be true for both the intake and exhaust valves. Also you will want to know exactly where the intake valve is in relation to the piston as it moves down the cylinder. Is it open at peak flow by the 90-degree point?

Now – for you turbo guys – it’s very important to know exactly where the exhaust valve closes, because you want to keep the pressure as high as possible for as long as possible, before the intake valve bleeds the cylinder down. In other words how late can we close the valve and still have adequate overlap? Ok – We have to be a bit realistic here – in 8 & 9000 RPM drag-racing engines we have discovered that somewhere between 40 & 50 degrees of overlap at .050 is about all a turbo engine can stand and still have boost with low lag during launch conditions. Unfortunately I can't divulge the cam specs publicly.

Here’s what I did to remedy this. I rummaged around in John’s old drag racing junk and found three old aluminum rods. From those pieces I made one adjustable rod with a bolt in it so I could slide it up and down to change the length. Then I called Sparky at forgedtrue pistons and had him make me a special piston with no rings and .002 clearance and the pin high up in the skirt. Now I can visibly see what is going on between the piston and the valve timing events. Yes there is a 5 inch dial indicator on the piston in #6 hole – yes it’s a Chevy block. Hay we were into NASCAR at the time – it’s all relative anyway!

The engine doesn’t know the difference – right?

I know you, wiz-kids out there can mathematically calculate all this stuff, but that gives you no dynamic feeling for what’s really going on, nor does it give any real conceptual knowledge. However you can successfully reduce the time by calculating this mathematically and just use the test stand as a visual reference.

3) Displacement Considerations
Does the displacement of the engine determine the size of the cam? NO!
The cam is going to do whatever it does regardless of the size of your engine!
It's the head that determines how much air and fuel gets into the cylinders.
So there is a correlation between the runner volume and displacement. Now – What most people don’t understand is that it’s the stroke length that determines this volume to volume ratio.

Hence the large bore short stroke engine will use one size head while the same size engine with the smaller bore and longer stroke will require a different size head. When you add the rod-ratio into that mix you can see why you have to look at all these variables first.

Generally speaking the large bore short stroke high rod-ratio engine will make its power peak higher than the same size engine with a smaller bore longer stroke and lower rod-ratio. The latter generally makes lots of torque and less BHP and does it at a lower RPM. Because of these variables the way you choose the head for one engine configuration verses the other is way different, now it is possible to arrive at the same power to RPM with either engine when you understand these variables.

Do you realize that the rod-ratio is not relative, any specific rod-ratio will accomplish the same things regardless of the engine size? So a 1.9 rod-ratio does the same thing in 3-inch stroke engine as it would in a 4-inch stroke engine.

I want you to start looking at the calculated BHP in a new way – so:
BHP = Torque X RPM / 5250

What that means is at 5250 RPM the Torque and BHP are equal. So 450 BHP equals 450 foot lbs of Torque. But you can’t calculate torque, it has to be measured on the dyno and the dyno has to be able to Load the engine in order to accomplish that.

5) Determining The Variables Within The Head
Up to this point you’ve learned the importance of matching the air-flow to the cam for a given size engine by using the area under the lift curve. Congruent to that you’ve learned how to use the rod-ratio to either enhance that combination or rectify an undesirable head configuration. The only thing we haven’t discussed is velocity, and this is a very misunderstood part of the engine’s ability to make power.
The determining factor in all racing engines is the overall velocity through the intake ports and where is it.
Maximum torque or BMEP should occur at 185 to 205 FPS
Maximum BHP should occur at 220 to 240 FPS but not to exceed 260 FPS at the valve.
I’ve looked at hundreds of formulas and these seem to consistently work the best:
Velocity = Volume of 1 cyl in cc’s X RPM
Diameter^2 X 4630

Optimum Dia = Cylinder Volume X RPM
Gas Velocity X 2264
Pipe Length = V X T
RPM
V= Velocity T= Time
Exhaust V = 1700 FPS – T = 120 Degrees
Intake V = 1100 FPS – T = 85 Degrees
BMEP = Displacement in Cu. In
Torque Ft LBS X 150.8
That should keep you guy’s busy for a while!!!

With this info you should be able to determine the affects of velocity on the power curve. Generally speaking the power curve of the engine is directly related to the velocity through the intake ports. Decide at what RPM you want peak BHP and find the valve size that creates that gas-speed shown on the chart above – as not to exceed 260 FPS. Once you have computed that diameter then it generally takes 70 to 80% of that area as far away from the valve as possible, preferably where the intake bolts on to the head or just inside of that at the push-rod bump. Make the height times width calculation at that smallest point.
Example:
Valve head dia = 2.00 area = 2 * 2 * .7854 = 3.1416 * 80% = 2.51328/.7854 √ = Dia 1.7888544

Therefore you want to have peak efficiency then you want peak torque or BMEP at 185 to 205 FPS and peak BHP at 220 to 240 FPS but not to exceed 260 FPS at the valve. Then the cross-sectional area at the push-rod bump will be around 80% of the head diameter of the valve. If you can get these dimensions correct then the CC runner volume is irrelevant, because your ideal shape is the controlling factor.

Note: Most of this does not work for the exhaust! Getting the exhaust to work properly is a matter of cut and try – all we can do is to produce the most efficient port configuration possible, however once you understand what it takes most of the time the same things work on all heads. To produce the best results of intake flow to exhaust flow use the 80% ratio, at every lift point. Sometimes this is not possible do to the way the head is cast. The only thing you can do then is to fix it with the cam.

Finding the cross-sectional area:
The best way to do this is to have cold bond silicone rubber, just pore it into the port, and when it cures you can pull it out and measure the cross-sectional areas. The other solution is to make tools from inside calipers that will fit into the ports so that you can measure them. OR – You could cut the head into various pieces and measure it that way.

Matching Port Configuration To Valve Size:
Try to match the intake valve size to the where you want the peak BHP for the displacement you have, not for peak torque. Then if you can follow the 80% rule the peak torque will take care of itself and you are well on your way to an efficient engine combination.

Combustion Chamber Shape:
Most of you are going to be concerned with the “Wedge-Shape” chamber, and in-line valves. Some of you may be looking at the compound angle head like a Cleveland, 429/460 or other such engines. In the late 70ies & 80ies there was a lot of Hoop-La about swirl, let me straighten that out for you. In low RPM Street (every-day drivers/ grocery-getters) this is probably a must-do because of the emissions controls the govt puts on the manufacturers.

You know if you have a tach in the car that the engine is only turning about 1500 to 2000 RPM at highway speeds, so the only way they can get the mileage up is to increase swirl at these low RPM’s. Racing engines never see that because their RPM range is between 4500 and 8500 RPM. Then if you put a turbo on the engine it will assist in mixing the fuel/air charge so again we’re not concerned with the swirl, and in fact it could hurt the performance of our high-efficiency engine.
In chamber shape the first thing I look at is – where is the spark plug?
Because – How am I going to get the compressed air/fuel charge directed at the plug?
Next- How much area can I get around the intake and exhaust valves?
Then – How do I squeeze all that into the exhaust bowl area most efficiently?
Now in a wedge-head above 3500 RPM the swirl takes care of itself so we don’t have to try and increase it, but we do have to manage the swirl so it doesn’t put the spark out.
Hemi-Heads are an entirely different game so we are not talking about hemi’s.

OK – there are 2 sides to the wedge chamber the spark plug side – I call the squish side – and the opposite side I call the quench side. The quench side usually has the most area and I think that it is responsible for pushing the A/F mix at the plug, therefore I tend to reduce the area and shape of the squish so that it assists this process. In doing this we have to pay close attention to where and how the Wet-Charge moves around inside of the chamber area, because if it gathers too much in the area near the plug it can wet the plug sufficiently to cause it to mis-fire or foul out. When this becomes severe you’re in real trouble, in the less severe case it’s a matter of ignition-timing advance and heat-range.

You have to get the plug exposed to the A/F mixture so what I said above is critical!!!

Exhaust Port Dynamics:
Without getting into a deep discussion about exhaust port configuration, which would take up a whole book, just remember that the waste gas is moving out of the bowl area under the valve at above the seed of sound, so it doesn’t like sharp edges, consequently smooth is really important, so is the finish. I’ve found that mirror finish tends to have a lasting effect on the valve job and the ports ability to reject carbon build up. Unfortunately doing the mirror-finish is time consuming and most people can’t afford that at 70 bucks an hour.

6) Conclusion
Hopefully there is enough information there to get you on the right track to making substantial gains in power with your engine combinations. So review what I taught you:
1) Pay attention to basics
2) Pay attention to details
3) Don’t sdkip over essentials
4) Never say “That will take care of itself.”
5) Be meticulus
6) Always have fun at whatever you are doing

Some other books:
The Design And Tuning Of Competition Engines By Philip H Smith
ISBN 0837601401
Tuning For Speed And Economy By Philip H Smith ISBN 0837600057
Theory Of Tuning By A. Graham Bell ISBN 0854292756

This Article Written By Denny Schmidt @ JDS Induction Products

[HeadDoctor]

You guys can't find anything to say about this article?

What about making it a sticky?

[Unit 5302]

Didn't you know? The best head is always the biggest head money can buy. Ask any 1970 Chevy 350 owner! Okay, that's not really true. It's more like the biggest cam money can buy on the 2V 350 heads, haha.

Seriously though, I wish more people out there cared about the dynamics behind how an engine works. There would certainly be a lot fewer "Bubba's" just slapping combos together for 10 years and then talking about how much they know from how many unnecessary failures they've had.

[1989GT]

Lots of great info here, now it's a sticky.

[HeadDoctor]

Thanks for making this article a sticky hope everyone enjoys reading it and learning from it.

Denny

[88workcar]

Choosing a set of heads should be a decision that goes along with what you want from the car. Rest of the package has a serious % of the decision. Once one decides, on what the cars purpose is, the rest falls into place. The info provided is wonderful, we all need to be educated. I love to see that stuff.

[Rev]

Thanks for the low down on head choices. I do doubt that many of us will build a mule engine to determine the best head/cam/rod ratio for a specific application. I think many of us might rely on software from Engine Analyser or Dyno Sym or from some other program. From everything I've been able to gather, rod ratio makes only a small difference in expected peak power or torque for most street driven engines.

Don't get me wrong here. I appreciate your expert input on these questions. It makes all of us think about how the ICE works.

Rev