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Discussion Starter · #1 ·
Advice needed for beginner, achieving high rpm seems to be one question on my mind. I understand lighter valvetrain, and rotating assembly along with forced induction, long rods, moderately wide lsa, and single plane manifold along with other factors properly synchronized together will produce fairly high rpm. Now the real question is how does d.c.r. factor in to all this. Is there any certain ratio that people aim for when building these motors?
Is a low compression motor going to have a better chance of achieving higher rpm than a high compression motor?
 

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Advice needed for beginner, achieving high rpm seems to be one question on my mind. I understand lighter valvetrain, and rotating assembly along with forced induction, long rods, moderately wide lsa, and single plane manifold along with other factors properly synchronized together will produce fairly high rpm. Now the real question is how does d.c.r. factor in to all this. Is there any certain ratio that people aim for when building these motors?
Is a low compression motor going to have a better chance of achieving higher rpm than a high compression motor?
The higher the compression the better off you are.
 

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Advice needed for beginner, achieving high rpm seems to be one question on my mind. I understand lighter valvetrain, and rotating assembly along with forced induction, long rods, moderately wide lsa, and single plane manifold along with other factors properly synchronized together will produce fairly high rpm. Now the real question is how does d.c.r. factor in to all this. Is there any certain ratio that people aim for when building these motors?
Is a low compression motor going to have a better chance of achieving higher rpm than a high compression motor?
You don't need forced induction to rpm a motor.
You don't need long rods to rpm a motor.
You don't need high or low DCR to rpm a motor
Referring to DCR, you figure this when you're planning the motor, before you ever buy a part. It is DCR that determines the fuel you will have to use to prevent detonation. Another way of looking at DCR is cylinder pressure. If you're gonna make a lot of cylinder pressure (high DCR), then you will need to use a fuel that will be resistant to detonation such as racing gasoline or alcohol. If you use a lower cylinder pressure (low DCR), the motor may run on unleaded regular gasoline without a whimper.

Problem is, there are many different calculators to figure DCR and all of them are a little different. You need to zero in on one calculator and use it all the time so that you know the results are valid. I have used the calc on Keith Black Pistons site for years and know that for a pump gas motor, I will want somewhere between 8.0:1 and 8.5:1 DCR. I adjust it by altering the static compression ratio (SCR) and the intake closing point of the camshaft.
 

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You don't need long rods to rpm a motor.
You don't need high or low DCR to rpm a motor
Depends on how high you want to spin it, long rods (ie lighter pistons), and high compression come into play rather quickly after about 6500 RPM. Even at low RPM they are a benefit but become a necessity the higher you turn.

It really comes down to, "how high is high rpm?"
 

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Birkey,what do you call "high" rpm? 5500 rpm and an old v-8 is not a problem with little more than a cam and kit,by the time you reach 7500 rpm(still using an old v-8) you have incorporated roller rockers,forged pistons,steel crank,solid lifter cam,better induction and exhaust. when you go from 8-9,000 rpm,you have the best of everything,you have had to do a piston speed calulation(pistons have a limit to how many feet per second before coming apart), very special cam grinds,extreme valve spring pressures,extreme lengths to stabilize rocker geometry,custom induction and exhaust that is very specific to application. after 9,000 rpm you better know some engineering basics and metalurgy basics and understand flame propagation/combustion chamber design and fuel burn speeds. at this point you need extremely deep pockets for the exotic (short lived) parts
 

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Discussion Starter · #11 ·
As a matter of fact I do. What is the difference in timing, between a low compression motor (8:1) vs a high compression motor (10:1) obviously there is a change in timing when compression is changed. I just want to get an idea of the range.
 

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Discussion Starter · #13 ·
I knoow there are many factors that change timing. But bare with me and say you run 91 octane. I'm not.looking for a specific answer just wondering how big the gap is. Is it just a few degrees or much more?
 

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also have to consider combustion chamber shape,camshaft profile,fuel metering system,ignition system controller,,, look up the new mazda economy engine that burns gasoline and has 14:1 compression.lots of high tech euro engines have 12:1 cr.,,,cooling system makes a difference,metal used in cylinder head,like aluminum or cast iron
 

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As a matter of fact I do. What is the difference in timing, between a low compression motor (8:1) vs a high compression motor (10:1) obviously there is a change in timing when compression is changed. I just want to get an idea of the range.
Indirectly, there is a correlation between SCR and ignition timing. The higher the SCR, the more cam you have to use and the more initial ignition timing you have to use. Here are charts from Barry Grant that may be useful to some of you fellows.....
Demon Fuel Systems
Click on the tabs "Performance Cam Profile" and "Radical Cam Profile" to see the entire range of duration/spark timing.....
 

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QUOTE=birkey;1596389]Advice needed for beginner, achieving high rpm seems to be one question on my mind. I understand lighter valvetrain, and rotating assembly along with forced induction, long rods, moderately wide lsa, and single plane manifold along with other factors properly synchronized together will produce fairly high rpm. Now the real question is how does d.c.r. factor in to all this. Is there any certain ratio that people aim for when building these motors?
Is a low compression motor going to have a better chance of achieving higher rpm than a high compression motor?[/QUOTE]

The answer to your last question is NO. The effort to overcome increased compression pressure by the crankshaft is minimal compared to the energy released by burning the mixture at higher pressures.

The Dynamic Compression Ratio is tied to the cam timing. Specifically it computes the actual compression lost to reverse pumping action of the piston as it rises up the bore at the point where the intake valve is closed. (The effect is that the stroke appears shorter than it actually is at lower RPMs) The problem DCR is attempting to correct is one of the engine's performance with a late closing intake which is typical of a radically timed cam. With such a cam the intake is held off its seat till late in the compression stroke. The advantage of this is strictly to greatly improve high RPM, high power output. But the bottom and mid RPMs suffer pretty large torque losses as a result. The reason all revolves around the intake mixture velocity which is used to generate mass inertia to force feed the cylinder. This only happens at high RPMs where the mixture velocity generates enough inertia to overcome the reverse pumping of the rising piston this permits holding the intake open late into this cycle. The downside is that when the mixture velocity is low the rising piston does pump mixture back into the intake. This results with the cylinder pressure being quite a bit lower than the Static Compression Ratio (SCR) would suggest. Therefore low and mid RPM power is significantly reduced.

The DCR is a calculation tool used to modify the SCR to derive a compression ratio that increases the absolute cylinder pressure in the low through mid RPM bands in-order to recover power lost to reverse pumping the intake mixtures. A rough rule of thumb is that the DCR should be about 8.5 to one with an allowance of about .5 ratio lower for open chamber, iron heads and regular fuel to about .5 ratio higher for a tight chamber, aluminum head, and premium fuel. There is quite a bit of rattle space in this simplified recommendation for different operating and induction temps; type, strength, and amounts of ignition advance; type of fuel, mixture ratio and how it's delivered carb/TBI, port injection, direct injection; vehicle weigh, gearing, transmission type, converter stall; excreta.

A caution is that you need to be careful in relating power and efficiency as related to compression ratio. There are plenty of misunderstood reports that assert that there isn't a lot of power to be gained by compression ratio increases. These reports tend to not make clear that this is true only where the increase in compression ratio is not accompanied with increases to the cam events. When the cam events at the valve are increased; a great amount of lower through the mid RPM power that could be gained will be lost if the DCR (thus the SCR) is not high enough. While the affect of the added compression is fairly low at high RPM, the gains in the middle of the power band are quite large when the compression and cam events are properly correlated.

Bogie
 
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