Hey fellas, Ive got some formulas you can use to find out your actual final compression ratio or DYNAMIC EFFECTIVE COMPRESSION RATIO. It can be a real shocker! Use Keith Black's website and there formula: http://www.kb-silvolite.com/calc.php?action=comp So ill start with mine.



Now with that said, for you guys using forced induction like me, use this formula to get your final compression ratio under boost.

((Boost/ 14.7) +1) x DECR.

I will be running about 12lbs of boost so:

((12/14.7) +1) x 6.444 = 11.7

This should be very steetable on pump gas!

Whats yours?

 

Mine is real close to what I've gotten from other sources. It's nice to have both formulas on the same sheet.

ENTER YOUR DATA CALCULATED DATA
Cylinder Head Volume (cc) Cylinder Head Vol
(cubic in.) 04.270
Piston Head Volume (cc) Piston Head Vol
(cubic in.) 00.427
Gasket Thickness (in.) Swept Volume
(cubic in.) 50.847
Gasket Bore (in.) T.D.C. Volume
(cubic in.) 05.133
Cylinder Bore Diameter (in.) Gasket Volume
(cubic in.) 00.436
Deck Clearance (in.)
Note: Neg. nubmer above deck, Pos. number below deck Deck Volume
(cubic in.) 00.000
Stroke (in.) STATIC COMPRESSION RATIO 10.906

OPTIONAL DATA
Rod Length (in.) Adjusted Stroke (in.) 02.768
Intake Closing Point (degrees)
ABDC @ 0.050 lift plus 15 degrees DYNAMIC EFFECTIVE COMPRESSION RATIO 08.312

 

Cylinder Head Volume (cc) Cylinder Head Vol
(cubic in.) 04.148
Piston Head Volume (cc) Piston Head Vol
(cubic in.) 01.281
Gasket Thickness (in.) Swept Volume
(cubic in.) 50.847
Gasket Bore (in.) T.D.C. Volume
(cubic in.) 05.833
Cylinder Bore Diameter (in.) Gasket Volume
(cubic in.) 00.540
Deck Clearance (in.)
Note: Neg. nubmer above deck, Pos. number below deck Deck Volume
(cubic in.) -0.136
Stroke (in.) STATIC COMPRESSION RATIO 09.717

OPTIONAL DATA
Rod Length (in.) Adjusted Stroke (in.)
03.072
Intake Closing Point (degrees)
ABDC @ 0.050 lift plus 15 degrees DYNAMIC EFFECTIVE COMPRESSION RATIO 08.141

 

Cylinder Head Volume (cc) 64 Cylinder Head Vol (cubic in.) 03.904
Piston Head Volume (cc) 5 Piston Head Vol (cubic in.) 00.305
Gasket Thickness (in.) .038 Swept Volume (cubic in.) 48.548
Gasket Bore (in.) 4.100 T.D.C. Volume (cubic in.) 04.711
Cylinder Bore Diameter (in.) 4.060 Gasket Volume (cubic in.) 00.502
Deck Clearance (in.) .000 (cubic in.) 00.000
Stroke (in.) 3.75 STATIC COMPRESSION RATIO 11.305


Rod Length (in.) 5.7 Adjusted Stroke (in.) 03.264
Intake Closing Point (degrees) ABDC @ 0.050 lift plus 15 degrees 50*
DYNAMIC EFFECTIVE COMPRESSION RATIO 09.969


Engine is not gas pump friendly. Will not run on 95 octane either. I have new .062" head gaskets and will be going with a different camshaft. It should drop the SCR to 10.66 anf the DCR to 9.08. I hoping it will then be able to run on 93 octane.

 

"Your engine summary is as follows: Bore 4.030 inches, stroke 3.48 inches, rod c-c length 5.7 inches, with a static compression ratio of 7.54 :1. Your camshaft specifications call for an inlet valve closing of 57 degrees ABDC (after bottom dead center).

Your chamber volume is 111.22 cc's. With this camshaft your dynamic, or effective stroke is 2.88 inches. Your dynamic compression ratio is 6.41 :1 corrected for cam timing, altitude, and rod length. Your dynamic cranking pressure, corrected for cam timing, rod length and altitude is 118.79 PSI. Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and 12 PSI is 11.64 :1. "

I don't know anything about boost or supercharging or turbocharging but it seems to me you might have to rethink this. When you do your boost calculations for compression ratio this is still boosted dynamic compression, not boosted static compression when you get the ~ 11.64 final compression ratio. Anythng over 9:1 DCR is going to ping like crazy. Like I said I haven't really gotton into boost and compression ratios so i could be wrong here.

 

Nevermind. I used Google.

 

Where are you guys finding hard reputable reference data that correlates max DCR to min Octane?

Same for SCR?

I'm not disputing anyone's take on it ... but what's your reference? You're welcome to take to email.

 

Good Question. I never seen anything hard data. Just a general range of what would seem to be work or not work. I went over the line and can say from personal experience that my setup does not work.

 

No speculation you shoot for 8-8.5 DCR anything higher and you are looking for trouble. You can go lower but I would not go any lower than 7.5 DCR. Do a search on google. THis is the whole point of matching components, you match your compression ratio to the cam you are using and try to get the DCR in that range. This is why big cams in low compression motors run like crap and high compression motors with small cams ping. No magic no mirrors it just works that way

 

Here's mine, 11:1 compression ratio and it doesn't ping and this is the reason it doesn't, it is under 8.5 DCR. If you don't make these calculations before you build a motor then you shouldn't be building motors

 

MotorHead:

I looked at your numbers and I am curious as to how you came up with the IVC at 70*for the calculation.
Comp doesn't show the IVC spec @ .050" on their web site. Do they list the spec on their cam card? On their web site they show the cam with a closing point of 66* with 280* duration @ .015". The also show the cam has 242* duration @ .050". I believe cams have a longer ramp on the backside of the lobe, but for sake of discussion lets assume they are equal. The difference between 280 and 242 is 38. Half of that is 19*. That would have the intake valve at 47* ABDC @ .050".
Adding 15 to that for the calculation would be 62* instead of 70*. It looks like you are running 1.6 ratio rocker arm that would another degree or two.

The biggest reason I am looking at this thread is because I have to make a change. I already have new head gaskets and trying to get a good feel for a camshaft that will work. With 3.08 gears and 2400 stall converter I don't want to go overboard on duration.

 

I don't have the calculations handy right at the moment but this is the way I worked it out. First to get the correct DCR the intake valve must be closed. Second I can't remember at the moment the exact calculations I made but using 66deg at 0.015" and using the total duration and knowing there is 180deg between BDC and TDC I figured there was 4 degrees more to be added to the 66 degrees and the valve was completly closed. THis makes sense and is inline with what they tell you to do if you have a degree measurement at 0.050", they tell you to add 15degrees. So basically at 0.015 I add 4 degrees to get 70 degrees after bpttom dead center and the valve it completly closed. It can't be less than 66 because the valve is still open then

I am assuming the 15 degrees they are adding is for general purposes and to get you in the ballpark for a mild cam. A big cam like mine will take more than 15 degrees to close after the .050" measurement

My advice is to pay close attention to DCR when you select a cam or you will be back in the same boat

 

There are different calculators to figure out DCR. Here is a page that allows one to be downloaded. http://members.uia.net/pkelley2/DynamicCR.html
The idea is the same; however, this calculator use the point where the intake valve is closed. The Keith Black uses the point at .050" plus 15*. The DCR will be different. When trying to make comparisons, the same calculator has to be used. Both will calculate the SCR correctly, but since they use a different point where they say the valve is closed (enough) to start making compression the final result will be different. One calculator wants the valve completely closed and the other is saying 15* after .050" is when compression starts even though the valve is still partially open. DCR is sort of Voodoo. It has some merit and is only a guideline.
The cam I currently have is 284* duration and 218* @ .050". Even though it has about as much duration as your cam, it (according to Crane and Lunati) is too short and is not bleeding off enough compression to run pump gas. The intake valve is closing at about 68* ABDC. The biggest difference is the effective duration. Crane recommended a cam at 284* duration, same as I have but with 228* @ .050". Lunati recommend a cam with slighly more duration; 287* but with a duration of 230* @ .050".
Part of the point I am trying to make is that when using the Keith Black calculator, the correct data has to be inputed not what you want it to be. You chose a point where the valve was closed and they wanted a different point (Intake Closing Point (degrees) ABDC @ 0.050 lift plus 15 degrees).

 

Probably a good motor; hope it does a good job for you. But I have no idea how good a job you do sir, nor do you of me.

Moreover, I wasn't looking for an arguement, an opinion or leastly a shot across my bow ... when I asked where reputable reference data comes from I clearly stated I wasn't disputing anyone's take on DCR. I fully understand how DCR works & have run the calcs myself. I fully understand it has much potential to help; but I can't see the validity of it if there's no reputable reference range that might take into account head alloy, octane etc. I do NOT discount validity of one's own use of DCR if they've learned their feel for ranges via personal seat of the pants experience; as I have w/ SCR.

I do know this; use of DCR does not a builder make, while not using DCR does not a fool make. True a generation ago and today.

Again ... anyone ... where are you finding hard reputable reference data that correlates max DCR to min Octane?

 

Wasn't even thinking of you when I typed that. Just making a general statement, sometimes my opinions can sound harsh, but I assure you I am only trying to help, if I get someone's attention that is contemplating building a motor then maybe they will be happy with the motor they built

I probably should have said, you should not build a motor for the street without making DCR calculations. Professional engine builders can do this more by experiece, over time they know what cams to use with what compression ratio

 

I’m not sure where this thread went, but maybe compression is voodoo like some think. I too could only find that you don’t want to go over 8.5:1 or 9:1 DCR on pump gas. But here is a conflict in data. Use this link to another site that has a calculator that takes boost into account. http://www.rbracing-rsr.com/comprAdvHD.htm It gets a real close DCR to what I first posted. So let’s do an example that is really proven and reliable. Take a stock LS1 and put a MagnaCharger boosting 6 psi on it. (I had to fudge the LS1 cam a bit because I couldn’t find exact specs on the centerline. It should be real close though)

Bore in Inches : 3.898
Stroke in Inches :3.622
Rod Length in Inches :6.098
Static Compression Ratio :10.1:1
Inlet Valve Closes ABDC :29 (0 to 90 Degrees @ .050" Lift)
Boost Pressure in PSI :6
Target Altitude :250 (Feet)

The result is 13.57:1. Now you could jump in that car, drive across the country, on 91 octane, with out one sign of detonation.

So what gives? I’m all ears if someone has a solution to this black magic!

 

knock sensors, computers. Technology has come a long way since 1970. I use 1970 because in my opinion, 1970 was the peak year of performance for stock chevy engines in corvettes. (excluding the L-88 and ZL-1 engines)