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

 

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.

 

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...

 

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 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'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 :spank:
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.

 

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.

Could someone elaborate a little more on this post? Is .041 the ideal quench area for all engines if it can be achieved? Is there any reason the quench would be less or more? If the quench of an engine would be better higher or lower that .041 would someone explain how you would determine this?

 

Could someone elaborate a little more on this post? Is .041 the ideal quench area for all engines if it can be achieved? Is there any reason the quench would be less or more? If the quench of an engine would be better higher or lower that .041 would someone explain how you would determine this?

I'll weigh in with what little I know.
The tightest squish figure you can hope to achieve is one that will allow the piston to come within maybe one-ten-thousandth of an inch of the underside of the cylinder head under max power conditions. Will you benefit most from a squish that tight? I don't know. What if it were found out later that the best benefit would be achieved with the piston being 5 thousandths from the head or 8 1/2 thousandths or whatever. The current thinking is that on a small block Chevy, the ideal squish is 0.035" to 0.045". David Vizard has said that the tighter he goes on the squish, the more power he finds on the dyno, down to the limit where he had a piston/head kiss at, I think, 0.026". So, on that particular combination, the tightest squish you could run would be just over 0.026".

It would be a completely different dimension on another type of motor that might use heavier pistons or more piston/wall clearance (allowing the piston to rock in the bore) or a different piston alloy that allowed the piston to grow more from the centerline of the pin to the crown, or a heavier crank that would flex more or a heavier rod that might stretch a little more and allow the piston to come up a little higher in the bore or any number of different things that could affect the outcome of a certain squish dimension. Even the air/fuel ratio could affect the squish. A leaner mixture would run hotter, expanding the piston more than a fatter mixture.

Anyway, this is how my mind works, looking at all the different variables and taking all of them into consideration.

 

Could someone elaborate a little more on this post? Is .041 the ideal quench area for all engines if it can be achieved? Is there any reason the quench would be less or more? If the quench of an engine would be better higher or lower that .041 would someone explain how you would determine this?

Engines on pump gas or ethanol:
For a street engine I like to hit between .040 and .060 inch. This is driven mostly by the DCR but head material and the percent of chamber area the squish/quench deck occupies also are considerations.

For a competition engine the end use and frequency the engine will be open are added to the considerations, I keep these between .030 to .040 inch. Tighter for engines that see frequent tear downs and looser fro those that go a season without looking inside. This to a large extend is to insure that the piston and squish/quench deck aren’t colliding or that carbon build up isn’t getting crushed between them.

Heads and pistons that form a large squish/quench area will tolerate more clearance as the surface area functionally makes up for a lack of clearance. Aluminum is more tolerant of a wider clearance as the heat transfer rate is more tolerant of less squish/quench. In either case that’s not to say that running to the lower side isn’t better, it’s that a highly acceptable solution can be found with trades of surface area and material type are available, this gives some play with fuel grade selection and head gasket choice.


Engines on methanol and or nitro methane:

For these the same considerations go in but I keep them on the wider limits because there is so much liquid going into the engine that there is a real chance of achiving hydraulic lock if these clearances are so tight that the materials can’t be ejected from the squish/quench zone fast enough as the piston closes the gap at high RPM. However, the use Singh grooves gives me some confidence that a tighter gap can be used in these cases.

Bogie

 

so if I had my compression ratio where I wanted it at about 9.5:1 on a 383 stroker, would .053 be in a ball park for dynamic compression ratio?

 

so if I had my compression ratio where I wanted it at about 9.5:1 on a 383 stroker, would .053 be in a ball park for dynamic compression ratio?

Yes I think so but we're talking related but different things. First you need to look at the dynamic compression ratio (DCR). Then the shapes of the combustion chamber and where the volumes and spark plug are located. And the squish/quench clearance.

1. Dynamic Compression Ratio. The DCR is the compression ratio that really does the work, although this can get to be a nastier concept when one envisions how the cylinder filling density changes with the speed/inertia of the incoming mixture, but even the rocket scientists among us don't take that one on. So in simple terms the DCR is an adjustment applied to the computed volumes of the Static Compression Ratio (SCR) for what amounts to a loss of stroke for where the intake valve closes in crankshaft degrees. Go here for a calculator http://www.kb-silvolite.com/calc.php I have my own but it runs in EXCEL so I can't upload it to the forum, otherwise I would. My calculator is not a "Black Box" so it makes apparent what's going on to reduce the SCR to the DCR, always a reduction. This is really the same formulas and calculations used to compute the effects of rod length changes, it computes piston movement and speed for degrees of crankshaft rotation. In the case of the DCR it is used to compute what is called effective stroke for degree of crankshaft rotation to where the valve closes. The critical values you need to run this are the degrees where the intake seats, or at least a darn good guess, so you need cam data plus the stroke and rod length. The DCR (I need to make a table of this) should be in the high 7:1s to low 8:1s for a street engine running regular to mid grade. This is also good for a blower motor around 6 pounds of boost on premium. The mid 8:1s to the upper 8:1s are good for cast iron heads on a hot street engine with street gears on the lower end and aluminum heads and street gears on the upper. The upper 8s to low 9:1s can be used in a competition engine with premium grade pump gas at the lower end and leaded race track fuel at the upper and using competition type gear ratios. Gear ratio provides leverage or the lack there of in high ratios. This and vehicle weight and aero and rolling resistance determines how hard the engine has to work, harder begets greater detonation issues.

2. Combustion chamber shape including that of the piston crown have a lot to do with detonation resistance. For a selected DCR, open chamber heads and circular dish pistons are more sensitive to detonation than are tight chambers and D-dish to flat top pistons. The reason is that tight chamber heads have more surface area of the squish/quench step as a total of the chamber surface area than large chamber heads, they also tend to place the spark plug closer to the center of the chamber. For the squish/quench function, the squish pushes the mixture into the valve pocket and in-front of the plug. This gives the mixture one hell of a stir and packs it in front of the plug increasing density on that side of the chamber which reduces miss-fires and increases burn speed. Half the trick to minimizing detonation is to get the burn over with before it can explode on the far side of the chamber from the plug. It's the collision of the pressure waves when detonation or preignition happens that does the damage, not a fast burn rate from one side to the other. The quench function is where the large surface to volume ratio of the far side of the chamber sucks heat out of the burn keeping the far side temperature and pressure from becoming so high that the unburnt mixture ahead of the flame front suddenly explodes from spontaneous combustion or flash-over if you're a fireman and understand what that term means. Now the race guys writing to the popular press espouse keeping the squish/quench gap around .040, they point out grater power when pushing that tighter, actually the greater power comes from keeping the cylinders under the detonation limit longer, it's not a magic thing like increasing compression ratio for more power. The problem with all this is keeping enough clearance to keep from banging the piston into the head. It doesn't ride straight in the bore, the thrust forces alone rotate it about the pin taking up the wall clearance. Since, like rods and stroke we're dealing with triangulation of dimensions this results in a vertical movement of the piston crown in a greater distance proportional to the clearance. Add to that going over TDC on exhaust there is scant downward force on the piston so it can loft using up all the available bearing clearance. Then there's the expansion with heat that will lengthen the piston faster and greater than the cast iron block. So all these things together can lead it down the path to banging on the head which would be an expensive thing to happen. racers are out there for the glory and the money so getting under .040 is a risk they take, not one they always get away with, but the benefits in the winners circle can make it a risk worth taking. For a street engine I find that squish/quench clearances in the range of .040 to .060 are just fine and a damn site tighter than the what you get from a factory style deep dish pistons. In fact Chevy High Performance Magazine many years ago built a 400 plus horse 350 on a stock, except for cam, Goodwrench bottom end. They used a Comp 272 cam, TFS 23 degree heads, Edlebrock Performer RPM intake and a 750 Holley. With the the stock .025 deck clearance and .015 shim gasket (not a gasket I'd recommend with an aluminum head) for a total of .040 inch squish/quench clearance. But the motor had a stock .080 inch deep dish piston so much of the squish/quench area had a clearance of .120 inch. Still this delivered over 400 horse on 92 octane. The SCR was 9 to 1 which certainly isn't getting all they could out of the fuel. They could have pushed this into the upper 9s or lower 10s SCR with a better piston offering more squish/quench and probably squeezed another 40 horses from it before it pinged.

3. Spark plug location. The closer to the geographic center of the chamber the faster the burn gets over with as the distance the burn has to travel is reduced in each direction. A plug that favors the exhaust valve side seems to also help in wedge chambers.

Well that's a long way of saying that your .053 should be fine.

Bogie

 

ok, thanks bogie :thumbup:

 

I am looking at these pistons also http://www.summitracing.com/parts/UEM-KB142KTM-030/ now I don't know if the information is wrong but if I have a 5.7" rod, 3.75" stroke, and a 1.561" compression distance on the piston that would put my piston height at 9.139", correct? This would be above the deck by .114", correct? So how would that work?

 

I am looking at these pistons also http://www.summitracing.com/parts/UEM-KB142KTM-030/ now I don't know if the information is wrong but if I have a 5.7" rod, 3.75" stroke, and a 1.561" compression distance on the piston that would put my piston height at 9.139", correct? This would be above the deck by .114", correct? So how would that work?

Pistons with ~1.560" compression height are meant to work with a 5.703" rod and a 3.480" stroke.
Pistons with a ~1.425" compression height are meant to work with a 5.703" rod and a 3.750" stroke.
You can figure up any stack you want to just by adding half the stroke, the rod length and the piston compression height and making the stack fit into a ~9.025" block deck height.

For instance, half the stroke on a 350 would be 1.740" (3.480 times .5). Add 1.740, 5.703 and 1.560 and find 9.003", leaving the piston crown down in the bore (piston deck height) by 0.022".