I was reading through some post, and I see some of them are talking about DCR. I don't really understand what this means. Could someone please explain this to me? Also explain how this relates to pump gas motors.And does DCR effect power, torque.
I saw before where someone posted a link to a sight that figures your compression, and DCR.What is a a good DCR?
I am trying to figure out the info for my motor.
350 0.030 bore
64 cc chamber heads aluminum trick flow twisted wedge heads
flat top four valve relief heads
0.038 compressed head gasket
My block has never been decked
I figured the comp. to be about 9.8 to 1
I am running a 268 xtreme cam.
5.7 rods
I know this is long, I am just trying to understand. You know, Expand my Mind.

Here is some very good information from Pat Kelly. Maybe this will help clear your memory: http://members.uia.net/pkelley2/DynamicCR.html

Ditto DTL504,

Pat's site is great. I have read it several times and always get more good info from it each time.. pdq67

I will try to over simplify this so if I state something wrong straighten me out guys.

Picture just a single cylinder with a head on it and with the piston at bottom dead center (BDC) If you fill this cylinder with something like water or air-fuel mixture so it's full you can measure the volume of that cylinder. Obviously we don't run water through our engines but that should help you grasp cylinder volume. Now since that piston is at the bottom you know it can only move up in the cylinder. This is where compression comes in. The static compression is how much what ever is is the cylinder gets compressed when the piston reaches top dead center (TDC). Static compression assumes that both valves are closed the entire time the pistion is moving from BDC to TDC so nothing escapes it's just compressed. To find the static compression you need to know the bore, stroke, head chamber and plus or minus for pop up or dished pistons as well as head gasket thickness.

Dynamic compression takes valve timing into all this. Since it takes time for valves to open and close the piston will start moving from BDC to TDC before the valves are closed on the compression stroke. (to those that know this remember I'm keeping it simple and don't want to get off into valve timing) some of the fuel and air mixture (what's really being compressed not water as in earlier example) will sneak out before the valves close so the amount or volume being compressed is actually less than in the static example.

That's the "what is" part of your question. If that made sense then check out Pat's site for the why and what good DCR is.

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...Dennis
"The '69 (http://chevelles.com/showroom/ww.jpg), the '96 (http://www.camaroslimited.com/graphics/memcars/96ss.jpg) our local club (http://camaroslimited.com/)"
and the "daily driver" (http://chevelles.com/showroom/DjD/dsc00016.jpg)

simply put:

Static Compression Ratio (SCR) is a ratio of the entire cylinder volume(also called sweep volume) plus the combustion chamber volume(head volume and misceleanous volumes like head gasket and deck height)
to combustion chamber volume

Dynamic Compression Ratio (DCR) is a similar ratio to SCR, but it takes the closing point of the Intake Valve of your unique camshaft into consideration along with rod length, stoke, ect. Since the the intake valve does not close until after the piston is already on it's way up in the cylinder the complete sweep volume is not attained because the piston is above Bottom Dead Center(BDC) when the cylinder is finally sealed. The sweep volume is the value that you have to change to determine the DCR. If you have Excel you can check out my DCR calculator here (http://www.eden.rutgers.edu/~vath/327/compression.xls). just change the blue text to match what yer set up is.

i guess there really is no simple way to say it... think of this as looking at a piston straight on from the front of the engine.
left is at BDC, right is at intake valve closing
http://www.eden.rutgers.edu/~vath/327/Image2.jpg
the bottom circle is the crank swing
stroke=3.25" (for a 327)
intake valve closes at 67 degrees after BDC
2.460/3.25=75.69% trapped cylinder volume
static compression ratio uses 100% trapped cylinder volume, that's why dynamic is always lower than static
2.460 is the dynamic stroke that is used to calculate the new sweep volume which you replace in the SCR calculation to get the dynamic compression ratio
there is of course a 3rd dimension to all this, but it's easy to draw it this way
(new sweep volume + chamber volume) / chamber volume = DCR
there's more but this is the jist of it.

[ 05-21-2003, 11:24 PM: Message edited by: JIM68 ]

Damn, you guys are good!

If you can make it so I can even sorta understand it, then most 5 years old will pick this right up!

Thanks a bunch!
Bill C.


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Bill C.
Colorado Springs, CO

68 Camaro... someday!
71 Chevelle SS
70 Nova

how do i figure my intake valve closing point? the cam card says nothing about that..
it is the GMPP HOT cam

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1971 Nova(looks like 69 camaro from underneath!)
355sb, vortec heads, HOT cam,T-10 tranny, 3.70 gears 16X8" IROC wheels. 12" Corvette brakes on the way.
see dirty pics here of a car that gets driven http://community.webshots.com/user/novaderrik

Novaderrik,

To calculate IVO and IVC do the following:

IVO = 1/2 duration - ICL

IVC = 1/2 duration + ICL - 180

You subtract the 180* during the IVC calc because IVC is in degrees After Bottom Dead Center instead of degrees Before Top Dead Center like IVO.

Bink,

It's just like Djd and the others explained - compression doesn't REALLY start to occur in an engine until all the valves are closed right? Air can espcape from the cylinders until all the valves are closed during the compression stroke (remember - intake, compression, power, exhaust is the order of events in a 4 cycle engine).

The last valve to close is the intake valve during the compression stroke as the piston starts to move upward in the cylinder. All the cam timing stuff is just used to figure out how far that piston is from top dead center when the intake valve closes. I mean if the intake didn't close until the piston was a 1/2 inch from the top there wouldn't be much compression would there? Or if the intake valve closed all the way at the bottom (just as the cylinder starts to move upward) you would have a LOT of compression.

So how does all this effect how an engine runs? Well, in general the more compression you have the more heat is produced when the compressed air/fuel mix is burned. The more heat you produce during the burn the more pressure you produce (heat and pressure always go together - when heat increases so does pressure, when pressure increases so does heat). The more pressure you put on the piston the more force you put on the crankshaft through the connecting rod and the more torque you make.

Sounds great, right? Just make the max pressure you can and you make a bizzillion ft/lbs of TQ right? wrong. there is a limit to how much pressure you can build with an air/fuel mix in a cylinder before it explodes on its own. When A/F mix explodes it is called detonation, and detonation produces unusable and extreme pressure inside the cylinder potentially damaging parts.

So the whole idea behind DCR is finding the point where you can build the maximum pressure possible in cylinder without detonation. You can reach this point by changing the intake valve closing point (cam timing) or by changing the volume of the combustion chamber.

For iron heads about 7.8 - 8.0:1 is about the max DCR you can get without detonation on pump premium gas. With aluminum heads max is somewhere around 8.5:1, maybe a little higher. Aluminum can tolerate a higher DCR and greater cylinder pressure because aluminum removes heat (absorbs heat and transfers it to the coolant) faster than cast iron.

Here's another twist - you can make the same DCR a few different ways. If you have a relatively large combustion chamber (a low static compression ratio) and a small cam that has an early intake close point you can reach your desired DCR of say 7.8:1. You can also do this with a high compression engine (relatively small combustion chamber) and a large cam that has a very late intake close point.

The difference between the two engines and how they will run is this: the small cammed engine will make good TQ because of the high cylinder pressures, but will "run out of steam" at higher RPMs because the intake valve and exhaust valve aren't open long enough to pack a lot of air/fuel mix in the cylinder - this engine just takes "quick gulps" of air. The high compression / big cam engine will take in bigger pulls of air/fuel so it will be able to rev higher.

The larger cam engine also has another advantage - scavenging. For a brief period of time at the end of the exhaust stroke and beginning of the intake stroke when the piston is at top dead center, BOTH intake and exhaust valves are open at the same time. When this happens, the high velocity exhaust gasses in the header tubes actually PULL fresh air/fuel mix through the intake valve and into the cylinder. With big cams some of this freash air/fuel mix actually gets pulled straight through the cylinder and is wasted in the exhaust.

The down side of having the intake and exhaust valve open for long periods of time is that at low RPM there is not enough velocity in the exhaust pipes to pull fresh air/fuel into the cylinder. Instead exhaust gas "leaks" backward into the intake. This is called reversion. The exhaust gasses in the intake dilute the fresh air/fuel mix in the intake and doesn't burn well when the "contaminated" mix is pulled back into the cylinder.

Reversion creates a rough idle in an engine and is why "big cam" engines idle so rough. You have to increase idle RPM enough to create more exhaust velocity to compensate and reduce reversion.

Sorry so long winded, just happen to like this subject http://www.camaros.net/forum/biggrin.gif

[This message has been edited by Eric68 (edited 01-22-2003).]

[This message has been edited by Eric68 (edited 01-22-2003).]

Damn that was good Eric! Amen, Brother! Let me get you a drink, I know your mouth is dry, and BREATH!

And the same to the rest of you, Dennis, Jim68, and especially to DTL504 for the site.

New food for thought......

The only problem is some of the cam manufacturer's don't give adv duration. But I'm sure one can figure it out, A cam card would be great!

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Everett "OBJECTS IN THE MIRROR APPEAR QUICKLY UPON RAPID DECELERATION"

[This message has been edited by Everett#2390 (edited 01-22-2003).]

Great Info!!

How do you figure out the deck height(My 383 is zero decked), piston to cylinder wall clearance, top ring land height and piston dome/dish(I have flat tops, do you have to figure the valve relief)?

Great explanation on DCR Eric. There are some other factors that should be looked at to prevent detonation. Car weight and gearing, environmental changes, etc.

Say you have a 3000 LB. car with 3.73 gearing and the motor has a 8.5 DCR. Take that same motor and put it in a 4000 LB. car with 3.73 gearing and it's PING PING PING.

A car in say Denver Co. at 5000 FT and 70º air temp. has a motor with an 8.5 DCR. Take that same car to sea level and a 95º air temp. and it's PING PING PING.

Calculating an optimum DCR is not an exact science, there are just too many variables involved. Combustion chamber shapes, piston designs (a domed piston will detonate before a flat top will) cylinder head temps, cam timing, (cams are like snowflakes no two are ground exactly the same)

While most of us like to get every smidgen of power available, building an engine to the extreme is likely to give you a lot of brain cramps. It,s best to play it safe. Lowering a 8.5 DCR to 8.0 is about a 0.7 drop in the SCR. Which would be maybe 10 less HP and 8 FT LBS of TQ. Thats a small price to pay to have the peace of mind of knowing the motor won't self destruct. What works for some people may not necessarily work for others. I hope this sheds some more light on this subject. George

[This message has been edited by run-a-way-69 (edited 01-22-2003).]

I just wanted to thank everyone for all the responses. I have been busy reading. Thanks again for the help.

paulm,

You have a zero deck height engine so you will have to use just the headgasket to create the quench dimension you want. I figure a standard .038" or .040" composition headgasket should do you just fine....

BTW,, GREAT INFO, EVERYBODY!!!

Guy's, I'm gonna go over the whole thing real slow later to make sure I get all the little gems of wisdom...

THANK'S!!!! pdq67