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Compression Ratio FAQ

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Flash
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Compression Ratio FAQ

Post by Flash » February 23rd, 2008, 2:36 pm

Since this is in stroker basic...........I will try and do just that.......BUT...........HOLD ON FOR A LONG POST

To understand quench or "squish" we need to First, understand compression ratio. Both, Static Compression Ratio "SCR" as well as Dynamic Compression Ratio "DCR"

SCR: is Bore X Bore X stroke + Volume above Piston......Divided by volume above piston.
So here is my formula to figure it out
.7854 X Bore 3.895 X Bore 3.895 X stroke 3.875=46.1718 Cubic Inches X 16.387, converts to Cubic Centimeters "CC"=756.61853
Now we just need to know the volume above the piston
Head volume ..........................................................57.00cc
Head gasket(.043")....................................................8.85cc
CC from flat top of piston and deck suffice (.020) ................3.90cc
Piston dish............................................................17.00cc
Total...................................................................86.75cc

Now 756.618(stroke volume)+86.75=843.368 Divided by volume above piston (86.75)= 9.7218213
This is a example of a 258 rod 4.0 piston 258 crank stroker........................
Note: If we would have used the stock head gasket (.051") the compression ratio would be a little bit closer to runs on pump gas........But our quench height would be less, require even less compression ratio to keep the ping out of you new stroker.
All tho, raising the piston height will increase compression as well as tighten quench height.........THEY ARE NOT THE SAME THING!!!!!!!!!!!! More on that in a minute.

Dynamic Compression Ratio(DCR) Is the compression ratio that the eng See's when it is running. after the SCR has been decided, the only way you can lower compression, do to a ping in the eng, is a cam with a longer intake open time.
A eng, or a cylinder, can not begin to compress the air, until the both valve are closed and the piston is going in a upward direction.
There is not a cam shaft out there, that closes the intake valve at BDC( Bottom Dead Center) so, DCR will always be lower then SCR. There is no set exact number, that will or will not ping on a certainty octane of fuel. But it is said(from what i have read and learned about DCR,) You want a 6 to 8:1 DCR compression ratio for the street.

So using the spec above let figure DCR with a couple cam's and there intake close spec.............
Lets start with a Comp cam 68-231-4 cam with a Intake Valve Closed (IVC) Spec on 56*........that means that the the piston will travel from BDC up .612" or just over a 1/2 inch, before the valve close..... So the compressible stroke of a 3.875" stroke is 3.26" with this cam
THE MATH .7854 X Bore 3.895 X 3.895 X stroke 3.26=38.84 x16.387=636.53+(Above piston volume)86.75=723.28 Divided by 86.75 =8.33:1..........A little bit above the 8:1 DCR recommend for the street.

Now lets look at a Crane cam #753901....the IVC is at 61* which equates to 3.14" stroke with this cam(these numbers are figure from a DCR program or calculator, that converts IVC spec of a cam, to distance the piston has traveled up)
.7854 X Bore 3.895 X 3.895 X stroke 3.14=37.41=613.10 + 86.75=699.85 Divided by 86.75=8.06:1
This is why you here people that claim to run 11.5:1 compression on street with out racing fuel :o with out ping and then, some ones else 8.1:1 has to run 91 octane to stay out of the ping............BUT THERE IS EVEN MORE TO THIS EQUATION, THAT WILL, OR WILL NOT, CAUSE A KNOCK OR PING.

QUENCH

Quench is the distance of the Flat surface of the piston, at TDC( Top Dead Center) and, the same flat surface of the head.

So quench, is Part of the combustion at TDC but not all.
As the piston enters the top of it's travel, the air in this area is much closer to the head then the bowl of the piston or the head.
as the piston hits TDC the air is Squish out of the thin area of the piston to the bowl..............This will do two things.

1st the better the fuel and air is mixed, the less chance of a knock do the uneven burn

2nt the Squeezing of the air from the tight side of the combustion chamber to the bowl actually causes a cooling affect of the air as the piston is causing heat by the compressing of the air. The cooler the air is, the less chance of a hot spot in the combustion chamber, will fire the fuel be for the spark plug does
If you have .100 at TDC there is no affect or quench...........the closer you bring the piston to the head the better quench affect you will get.
but will also raise you compression ratio, Defeating you hole purpose of quenching or cooling the intake charge or air.

The answer??????????

If you enlarge the head bowl chamber you will not affect the quench height but will lower you static compression ratio!
By the same note you could have you piston dished deeper and you would lower you compression ratio with out affecting Quench.
If you shave you head you will raise you compression ratio but Quench will be unchanged......
If you deck your block, you piston is now higher up in the combustion chamber causing compression ratio to go up(bad idea) but will also make a smaller quench area(good idea)
If you have .100" of quench (piston to head clearance) there is little to no affect
If you had .000 head to piston clearance you would have the best quench Possible........However with piston expansion and rod stretch(AT HIGH RPM) the minimum clearance is .045"(absolute minimum is .040")
The strokers Quench above^^ is .063.......If i would have installed the stock .051"(stock) head gasket, my SCR would have been less but my quench would have been .071 Also i thing the piston to deck height is more in the .030 then the .020 that i choose for our example. Which would change that .071 to .081............

Now you have the facts, all you have to do is decide how much compression you can live with, and how much more Cash you are willing to spend, to get closer to or get that .045 Quench
89 XJ with 300,000 on the original eng

"I've also never completed a motor, yet. My mouth (fingers) is also writing checks my ass can't cash."

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Muad'Dib
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Re: Compression Ratio FAQ

Post by Muad'Dib » February 25th, 2008, 11:15 am

Split / Stickied .. to be able to be found easier.
If it feels right, then STROKE it!
You're lucky that hundred shot of CAPS LOCK didn't blow the welds on the forum!!

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