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DCR Calculation

10K views 25 replies 10 participants last post by  cobalt327 
#1 ·
There is a huge thread on this topic that has been moved to "The Dump". Why?

I didn't have time to go through the entire thread, but I immediately noticed that the duration used for the calc was the wrong duration.

The only way to figure DCR is by using the duration where the intake valve is effectively closed, which is the "seat-to-seat" duration, AKA, "advertised" duration. Comp Cams uses duration at .015" valve lift, and sometimes .012 amd .010 as well. This is known in the SAE as J604d duration, which is duration at .006" valve lift.

Was this ever addressed in the thread?
 
#2 ·
You are pretty much correct on this. The rising piston is applying a lot of pressure to the mixture in a situation not unlike the exhaust side during that cycle. So having the intake closure point at .015 inch and gussing an additional 15 degrees if that's even done is simply not adaquate as the pistong will be forcing much greater reverse flow than one would think if that information was related to intake flow at a depression of 28 inches of water.

Different manufacturer's give absolute degrees in some value other that truely shut valve degrees these are often measured from .001 to .006 inch from the seat. While not "perfect" this is a lot better situation than from .015 inch or guessing that the cam actually seats the valve with an additional 15 degrees as many cams, especially older grinds have extraordinarily long ramps and 15 degrees doesn't get close to true valve closure.

Bogie
 
#5 ·
I can't figure why it was dumped either, I learn a lot by reading through these posts too.

F'BIRD-88, this is for you in regard to you question in the 'dumped' thread. I sent to you PM also, before I found this one.

My aim was to achieve 11:1 SCR, or real close to it. If my heads were as advertised at 64cc , that's right about where I would be. The combustion chambers in the heads were measured accurately with the correct equipment by me and my machinist at his shop.

The block, a 2 bolter from a '73 Impala, was stock everything when I tore it down. Never had the heads of, factory.

Specs are now,
Bore: 4.030"
Stroke: 3.48"
Piston to Deck Clearance: .005"
Head Gasket bore: 4.060"
Head Gasket Compressed Thickness: .030"
Pistons: KB171-020 hypers with compression distance of 1.561", not rebuilders. Valve reliefs are marginal at 1cc, actually not even 1cc but I called it that. My machinist and I fly cut the pistons to give minimum piston to valve clearance as I was chasing compression.
Head Chambers: 61cc

Let's know what you get.

Duke
 
#6 · (Edited)
There is a huge thread on this topic that has been moved to "The Dump". Why?
Here's the explanation by Moderator Sam.....

"Rude personal attacks are not tolerated here and I am not sorting out any ones personal complaints. I am dumping this thread so if you guys wish to discuss this topic you can do so in a polite and courteous way.."

Sam
...................................................................................................................................................................................................................
I personally would like to see this subject continued and I promise to be civil. I have no idea who wrote the straw that broke Sam's back and don't care.

Richard
 
#7 ·
Agreed....I'm completely ignorant when it comes to this stuff and am learning a ton.....and will happily keep any sarcasm in check. If you had a clue how much cash I spent 3 years ago on a build that never overly impressed me, and now feel very silly about. You would laugh your *** off. The difference between then, and now, is this place....and these sometimes ridiculously long winded threads that actually contain alot of good info. And I appreciate it. Nuf said.
 
#13 ·
Here is a good article on dynamic compression ratio:

Classic Inlines - Compression Ratio's

As you can see the dynamic compression is what the engine actually sees in operation. This can be manipulated by valve event timing in other words at what point in the cycle of the cylinder does the valve actually close.

Sam
 
#18 · (Edited)
There are several calculators out there that can calculate DCR for you. I've always heard target values of 8.5:1 for pump gas and 9:1 for race gas, although I'm sure that is pretty generalized and you could probably exceed that a little bit with good engine design.

Dynamic Compression is rooted in the fact that the intake valve closes after bottom dead center. Since your cylinder isn't sealed right away as the volume above the piston begins to decrease, you do not keep all of the air that is in the cylinder at BDC (Bottom Dead Center). What is compressed is what is left in the cylinder after the intake valve closes. This is why Dynamic Compression is always lower than Static Compression.

So one of the important things you need to know to calculate DCR is how long (in degrees of rotation) the intake valve stays open after the piston hits bottom dead center*.

When you are looking at your fuel you will have critical values, such as maximum pressure and temperature the fuel will take before autoignition, which is another thing to take into consideration when you get this intricate into engine design.

*Thanks to techinspector for teaching me how to do this several years ago.
 
#20 ·
These days serious engine builders work everything out on paper before they ever order a part. then they use a scientific rigor to evaluate the results. today we also use computer controlled injection systems and computer controlled ignition systems to get as close as we can to perfect for that build. Setting up design parameters is critical to the success of the build. One of the parameters is picking the design fuel for use in the engine as an example for a street engine we use pump gas as the design fuel and work around that parameter. Power band is another so where do we want the power band to be. Is this a Tow motor or sports motor just what is the intended use of the vehicle..Once we know all of this then we build a motor to suit the intended purpose..No longer do we just throw parts at the build.

Now if you do go to an experienced engine builder most likely he has various builds saved in his data base and can most likely have a parts list for that build. For the guys stepping over the edge the we accept that sacrifices to the gods of speed will be made and we will bust some parts on the dyno or race track at times and this is the price of progress..

Sam
 
#21 ·
I remember a quote from the movie Heart Like A wheel where after in the early days Sherry blew up one of her engines in her FED,Connie telling her she needed to feel the car nosing over long before the engine let go.
I know in real life how true that is.Test the limits of the parts??. We certainly do. But are real logical about how we go about that. Best lap in the 1/8 was 4.56@ 150mph. But we limited the driver to only .002 on the bottle and it was in the middle of the lap.

Oh,he doesn't have any "engine ears" on either,but most certainly a sensitive butt.
 
This post has been deleted
#25 · (Edited)
At the risk of beating a dead horse, I'll just say this:

DCR is one of many tools in the engine builder's chest. And like any tool, it works best when used for what it was intended. Most guys know better than to pound a nail using a torque wrench (although sometimes I wonder); using DCR is no different.

For those interested, here is a different approach to the same concept as DCR. I've not used it yet but I'd be interested in hearing how it compares to DCR calculations made w/the "standard" DCR calculators (three are below), should anyone use it. This might be where some of the somewhat incomplete concepts posted in this thread came from, but either way this is a clearly presented case w/the math included. BTW, he uses "supra" a lot. It means basically "there's more info above".

Wallace Racing DCR calculator
Kelly DCR calculator
KB/Silvolite DCR calculator
 
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