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Old 10-05-2012, 05:13 PM
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rpm vs compression

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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Old 10-05-2012, 05:41 PM
How fast is fast enough?
 
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Quote:
Originally Posted by birkey View Post
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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Old 10-05-2012, 08:22 PM
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The higher cylinder pressures wouldn't slow the piston when nearing tdc?
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Old 10-05-2012, 08:47 PM
How fast is fast enough?
 
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Originally Posted by birkey View Post
The higher cylinder pressures wouldn't slow the piston when nearing tdc?
Seriously?

No, not even close.
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Old 10-05-2012, 09:05 PM
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Quote:
Originally Posted by birkey View Post
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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Old 10-05-2012, 09:14 PM
How fast is fast enough?
 
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Originally Posted by techinspector1 View Post
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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Old 10-05-2012, 10:49 PM
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Tech

Really glad to see you. More to come I hope.

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Old 10-05-2012, 11:40 PM
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Old 10-06-2012, 12:54 PM
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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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Old 10-06-2012, 02:14 PM
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It's great to see you back Tech1


To the OP, you've gotten some good answers here, any other questions
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Old 10-07-2012, 03:05 PM
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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.
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Old 10-07-2012, 03:18 PM
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compression by itself is not what determines static or total timing.
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Old 10-07-2012, 03:28 PM
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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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Old 10-07-2012, 03:45 PM
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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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Old 10-08-2012, 02:38 AM
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Quote:
Originally Posted by birkey View Post
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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