<TECH ONLY> DCR affects with LSA discussion
04-07-2007 | 12:03 PM
#21
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From: Decatur, AL
There are just too many variables. That chart is conservative for sure. According to that chart I should be running 97-98 octane with my 8.8:1 DCR and 185* coolant temp. But If I had a BBC or a gen1 SBC with a crappy combustion chamber design and the piston .025" down in the hole that chart may be dead on. 
04-07-2007 | 12:08 PM
#22
Originally Posted by TurboGibbs
There are just too many variables. That chart is conservative for sure. According to that chart I should be running 97-98 octane with my 8.8:1 DCR and 185* coolant temp. But If I had a BBC or a gen1 SBC with a crappy combustion chamber design and the piston .025" down in the hole that chart may be dead on.
It also leaves our timing, which is an integral part of avoiding detonation (the *real* limiting factor of DCR).
01-01-2012 | 07:42 PM
#23
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bringing this back from the dead with some questions hoping someone may have some ideas on.
Now for my questions....
Wouldn't DCR increase with rpm due to piston velocity drawing more past the intake valve and threw the intake runner?
The next question....I dont see how that calculater would work with forced induction as the calculator is designed aound vacuum. You can't just add the lbs of boost to it then devide it. Theoretically it works out mathmatically to give you an area that close to NA DCR. I think there are way to many variables with weather, density and altitude. Also the amount of boost in the intake is not always the same as it is on the back of the valve.
Now for my questions....
Wouldn't DCR increase with rpm due to piston velocity drawing more past the intake valve and threw the intake runner?
The next question....I dont see how that calculater would work with forced induction as the calculator is designed aound vacuum. You can't just add the lbs of boost to it then devide it. Theoretically it works out mathmatically to give you an area that close to NA DCR. I think there are way to many variables with weather, density and altitude. Also the amount of boost in the intake is not always the same as it is on the back of the valve.
01-02-2012 | 08:16 AM
#25
You might be confusing it with volumetric efficiency, but I wouldn't say VE is governed by piston velocity. Its a measure of the "cylinder filling" ability of the engine and mainly governed by factors such as valve area and port design.
01-02-2012 | 08:21 AM
#26
I had wondered this myself.... Turns out Wikipedia has an answer:
So you could actually derive this exponent for your conditions by measuring the cranking pressure pressure of all cylinders and then comparing that to your DCR.
The absolute cylinder pressure is the result of an exponent of the dynamic compression ratio. This exponent is a polytropic value for the ratio of variable heats for air and similar gases at the temperatures present. This compensates for the temperature rise caused by compression, as well as heat lost to the cylinder. Under ideal (adiabatic) conditions, the exponent would be 1.4, but a lower value, generally between 1.2 and 1.3 is used, since the amount of heat lost will vary among engines based on design, size and materials used, but provides useful results for purposes of comparison. For example, if the static compression ratio is 10:1, and the dynamic compression ratio is 7.5:1, a useful value for cylinder pressure would be (7.5)^1.3 × atmospheric pressure, or 13.7 bar. (× 14.7 psi at sea level = 201.8 psi. The pressure shown on a gauge would be the absolute pressure less atmospheric pressure, or 187.1 psi.)
01-02-2012 | 03:19 PM
#27
Nope, you don't, that just increases overlap as long as duration of I/E is consistent, all that occurs is a lower LSA as long as ICL stays the same. That will affect boost though, as there is a longer amount of time the Intake and Exhaust valves are open at the same time allowing intake charge to "bleed down" in a FI engine.
Last edited by ForcedTQ; 01-02-2012 at 05:21 PM.
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