Reher-Morrison Racing Engines' David Reher and Darin Morgan answer your questions about cylinder heads or anything else concerning your racing engine building problems. Remember who you're are dealing with so no soft-ball questions.
8/8/06
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Joe,
When we invest large amounts of our time and money into a project, the last thing we want to find out is that we made a mistake. Its human nature to rationalize a situation and put a positive spin on the end result even though the result was somewhat unfavorable.
Let’s look at the facts of the case. You changed two things and the car slowed down. That pretty much sums it up doesn’t it? The engine did not respond favorably to the changes so the changes, of and by them selves, are probably at fault. That’s not to say that there may be some tuning changes that would help the situation. In this case I am afraid there might not be any. From the changes you have made it looks as though you have changed the average velocity profile of the induction system along with the pulse tuning and the consequence of this are extremely poor cylinder fill and a corresponding drop in power. A four tenths drop in ET is not only significant it can be bad news! You could be putting the engine in a lean condition that could hurt components over time. We are talking about a significant drop in power here! The MPH verses the ET looks as though the low end recovery characteristics are worse than the top end power. Now what could be the cause?
Who designed the sheet metal manifold? Did he do the calculations correctly in order to tune the high amplitude wave pulsations? Did he do all the calculations to optimize the mean and instantaneous velocities over the intake tract? Did he design the plenum so as to minimize air fuel distribution problems? If not, I am afraid you may be in trouble. All too often people think that just because the manifold is made of sheet metal and looks pretty and shiny that it will somehow be better. I have built well over a hundred sheet metal manifolds and I can tell you from experience that small variations in design can have profound effects on power. Worse yet, when someone builds a sheet metal manifold on a wing and a prayer with total disregard for the physics involved, it never works out and is almost always a total disaster. I am not saying that this is true in your case, I am simply stating that these are things you need to take into account when diagnosing a problem with a new intake manifold.. The one BIG mistake people make when they first start building manifolds is that they make the runners too big. The second mistake is making them too short for the engines respective power band. From the information you provided, I have a strange feeling your manifold may have both faults.
A 500 cid engine at 7200rpm max would have peak power at 6800rpm and peak Torque would have to be at about 5200rpm. You have to think of a manifold like a tuning fork. Its internal vibrations (Pulsations) must be at the same frequency as the engines power band or they will be out of phase and not work with each other. As a matter of fact, if they are to far out of phase they will have a negative effect on power production by actually trying to pump air out of the cylinder before the intake valve can close. There are a multitude of equations we could use with varying degrees of accuracy and complexity, but for the sake simplicity, we will use the old 1965 Chrysler engineers equation that corresponds to peak TQ. It’s a little vague as to where peak power will occur but it is some what accurate in regards to peak Torque. Please note that this equation is only an example and is used for demonstration purposes only and will not be accurate in all cases.
84000 \ Runner length = RPM at Peak TQ
- Runner length = length of the entire intake tract from valve seat to plenum opening.
- 84000 = constant derived from intake valve closing in crank shaft degrees and sonic wave speed in an air fuel mixture at about 14:1 air fuel ratio.
Your peak TQ should be at about 5200rpm so we reverse the equation.
84000 / Peak TQ RPM = Length
84000 / 5200 = 16.15”
Check yourself by doing the equation the other way.
84000 /16.15 = 5200rpm
This exercise shows how the power band corresponds to changes in runner lengths and it is used to demonstrate how shortening the runner will raise the power band just as lengthening it will lower the power band. There is no way to properly fit 16” long runners under that hood with stage 6 heads without making a cross ram because the manifold would be to high and the corresponding angle intersects to the cylinder head would be horrendous. That is why the Chrysler engineers designed the Cross Ram. Because it’s the best overall compromise in order to fit all that runner length under the hood. There are other equations to make the runners fit another tuning range such as the second or third order pulsations, but those calculations are complex and you should consult a professional in manifold development to see what situation would fit your particular needs.
Another thing to consider is that you can band-aid an out of order situation with air speed. The induction system air speed can fix or “band-aid” many problems. It is the great equalizer! Air speeds even more so than runner length have a profound effect on over all volumetric efficiency. Too big and slow and you are DEAD. A little too small and fast, and your probably ok. If your runners are too short you can help the situation with manipulating either the average or instantaneous velocities in the system to increase inertial ram effects, but it will never be optimized doing it this way. I can not adequately answer your question without having all the data in front of me. I would need a long list of information such as valve size, cylinder head flow, cam specs, Timing requirements for max power and a host of other variables pertaining to the taper and size of the runners on the manifold as it is now and a host of others in order to give you the specific cause to your dilemma. It sounds like your runners are too short and too large for the power band, but I can’t be sure without more information.
I can’t specifically answer your question because I don’t have enough information to do so, but then again, I didn’t choose this question in order to answer it. Now I got you mad at me didn’t I? Actually, I chose this question so I could enlighten people to the fact that the induction system is often far more complex than they think. Everything looks easy from the outside but when you look under the hood, you find a maze of complexity and the answer to the questions you seek are seldom simple or easy.
Call me, I will help you in any way I can.
David Reher asked Darin Morgan to answer this question because it is his area of expertise.
Reher-Morrison Racing Engines
1120 Enterprise Place
Arlington Texas 76001
817-467-7171
FAX-468-3147
Visit our web site at http://www.rehermorrison.com
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