my static compression ration is 10.07:1, and according to the wallace racing calculators my dynamic compression ration is 7.82:1

i know about SCR, but i dont know much about DCR could someone explain a little bit to me, what DCR ranges should i be looking for when building an engine and picking a cam

DCR is the application of the effects that cam timing and piston movements have on compression pressure expressed as a compression ratio. It is the measure of mixture density in the cylinder as a function of volumetric efficiency.

The static compression ratio is simply a measure of all cylinder volumes divided by the volume remaining above the piston when it's at TDC. In a dynamic sense, it assumes the pressure would be representative of a volumetric efficiency at 100%

In the real world of operation a 100% filling of the cylinder is very rare without a super charger of some sort. So what you'll see is the DCR is almost always lower than the SCR. Also, keep in mind that the true DCR is variable with throttle position and RPM so what these calculators that deliver a number are doing is computing some sort of an average. The DCR will be very low at idle, will probably be at its best with full throttle opening as the RPMs pass over the torque peak, and will begin to diminish again as the the RPMs approach the horsepower peak where the engine just can't get a breath in the time available for the intake to be open.

The dynamic compression ratio is a formula that is backward calculated by motoring an engine and watching, with a pressure measuring instrument, what sort of cylinder compression pressures are made from different combinations of manifolds, ports, valve sizes and shapes, cam timing, connecting rod lengths, throttle opening, RPMs, etc. This is also the heart of the equations that are running inside electronic fuel injection computers.

The desired one ratio fits all is 8.5:1 DCR. What your 7.8 is saying is that for your cam and maybe rod combination, if it asked that question, is that you could stand to raise the SCR a bit higher. On the other side of the coin, it could say you need a milder cam for your chosen SCR. Ying and Yang at work sort of a thing.


Bogie

well i dont have the cam, but yes it asked the bore, stroke, rod, SCR, inlet valve closes ABDC, boost pressure in PSI (i dont know what to put here, it's a NA motor), target altitude

im still kinda stuck between two cams one gives me a DCR of 7.82:1 and the other gives me 9.05:1, but the later cam wants 10.5cr or better, which i dont have so im a little leary on buying it, both cams are very similar with RPM band @ or very close to 2200-6600 but im running this in a 385 so i figure those # will go dwon to 1800-6000 in this area which is what im looking for, in trying to build power am i looking to get my DCR as close to my SCR as possible? what do you think about the the cam that wants the higher CR could i get away with it @ 10:1 or should i scrap the idea

9.05 would be too much for the street, to pull that off would take a lite car, big gears, and racing fuel. Even though it's probably a milder cam than what youre proposing it shows the SCR is too high for the cam timing.

Your question might be better served with defining the parts selection to us. The Devil is in the details and the combination of cylinder head and piston crown style is most important to how far the compression ratio can be pushed. This is something that these quick and dirty equations don't take into account. Yet we know from experience that chamber shape closed or tight, versus open or large, versus Vortec/Fastburn versus not and piston selection of flat top versus circular dish, versus D dish, versus domed, heads of aluminum versus cast iron are significant decisions affecting the tolerable limits of combustion ratio for a given octane fuel. Vehicle characteristics are also important, weight, gearing, transmission type, torque converter stall speed if applicable, accessories all come into play with the engine design process.

So I think you'd be better served with asking the question around this is the vehicle I have and the engine I'm thinking of, what do you guys think?

Bogie

i have the short block and the car

assembled short block:
Eagle cast steel crank= 3.75
Eagle SIR 5.7 rod's
KB Hyper pistons w/ 18cc D-dish 4.040
0 deck
all ARP fasteners
heads:
DART Iron Eagle Platinum-200cc/ 2.02/1.60 64cc
cam/ valvetrain
284/296 240/246 .507/.510 110lca 106ic hyd-flat tap
+100 pushrods
1.5 roller tip rockers
performer RPM intake
holley 4160 750cfm w/ vac sec, elc choke
700R4 w/ 3000 stall
4.10 gears with 28" tires
car weighs aprox 3450 with me in it

i would really like to go with a .020 head gasket to get the compression to 10:1 but im not really sure what my quench is and i know i dont want to jack that up

so what do you guys think

i think you would be ok with the .020 head gasket. i used a .019 with a set of .100 dome pistons and those older dart heads your aware of. i had no problems. you 18cc dish's should be fine. even if they are the stepped type.

do you know the compression hight?. and if the block has been decked at all?
(3.75/2)+5.7+(compression hight)= the sum. your deck hight(-)the sum= your cleanance (-) quench hight.

i think thats right. and if not, some one please correct me.

87Z, if you do have a true zero deck as you listed, there is no way you can run the .020" head gasket.(unless you plan to stack two of them per side ). Minimum piston to head is about .035" and that is getting into danger territory if you don't know what you are doing.

X2...........

ive always concidered .035 the "proper quench"

with deep enough valve reliefs(sp) this would be ok right?

dont you want your quench as small as possable with interferance?

please correct me and tell em why not...

wrong.

Valve reliefes have little to do with it, the quench is there to insure the piston doesn't come into contact with the "quench pad" of the head, not the valve. the factors that go into deciding quench are rod quality/deisgn/material/length, anticipated RPM, piston bore clearance/comp ht/skirt length/material, stroke, and a few other thngs.

I'm guessing that the piston is the KB 135, in which case its crown height is 1.43 inch which is a zero with 1/2 the 3.75 stroke and a 5.7 inch rod in a 9 inch block. With a .020 gasket it is in a mechanically risky interference zone to where you have a high probability of getting pistons and heads meeting.

If these assumptions are true, you're in the range of 10.8 static compression with a .040 gasket which is right on the minimum clearance you can get away with using a steel rod. I didn't compute for the true diameter of the head gasket, just used the bore dia. so the real static could be a few tenth's lower. Getting this close dimensionally using arithmetic, I'd want actual measures as this assumes all the machining is on the mid point of tolerance, that usually isn't the case.

How was the .1 inch longer push rod arrived at, by head manufacturers suggestion, or by sweep measurement between the valve stem and rocker.

I'm concerned that your assemblage of parts are not dimensionally compatible.

An aside to the engine. The 700R4 with it's extra deep low gear, a 3000 stall converter and a 4.10 axle is likely to be a real exercise in wheel spin when launching this.

Bogie

yes they are kb135040's
you have a 10.8:1 with 64cc heads? with a .040 gasket i come up with 9.69:1 and with the .020 gasket it comes up with 10.07:1 with 0 deck

i made an assumption about quench with the 18cc dished pistons, and i see where that assumption got me, that is why im on here asking questions, i dont know everything wish i did

which head gasket should i use with the piston zero'd at the deck, i guess the .040's but what about bore the cyl. is over @ .040, im going to make another assumption and say 4.100

the motor is not 100% brand new, i got it out of a friend of mines 71 chevelle right after he bought it, he is against sbc's, he put a 461 in it and sold me this motor, for a killer price, i have all the recipts from the original builder, he is the one who went with the +.100 pushrods with a set of over worked 305 heads, that flow garbage, for what i paid for this motor i am 100% happy with redoing it as a shortblock (- RPM intake) the motor has an about 5k miles on it, and as i pulled the heads off, the valve train lookes emaculate along with the pistion tops and cyl walls

for gears im willing to go as high as 3.73 but no more, i know ill have to beef up the rear suspension to get it to hook, that is already in the plans

With a .041" x 4.166" gasket(Felpro 1003) and 18cc dish, I got 9.65-1 for compression, and that is all it can be without milling the head to reduce the chamber size or changing pistons. If you have the piston zero decked like you said, you cannot use a .020" gasket without crashing the piston into the head with any kind or rpm over 4000.

Have you actually measured the deck height on the assembled short block or are you just "eyeballing" it?

eyeballing
what would be my best way of getting the measurement, one person has told me a set of feeler gauges, what would you suggest

would you suggest milling the heads to get the higher CR or should i just stick with the 64cc, i want to use these heads but im sure i cant get away with the 50cc heads that would gve me an unstreetable CR i would emagine

Feeler guages along with a good straight edge can be used in a pinch. Calipers depth rod also works, best is dial indicator or a depth micrometer.

I'd stick with the compression in the mid 9's so you don't have to be perfect everyday on the tune to avoid detonation.