Dynamic Compression Ratio CHART
 

NEW: see Post # 28 on page two of this thread for the LIVE SPREADSHEET. You don't have to use the lousy .JPG at the bottom of this post.
___________________________________

This C6 Tech Forum is like a graduate education in performance piston engines. For example, I have learned that Dynamic Compression Ratio (DCR) is the way to look at the interplay between cylinder heads and camshafts. The consensus (from SpinMonster, of course!) is that with a tight quench (0.030" or so) the heads for the LS2 (and LS3) can run up to 8.7 to 1 DCR on pump (93 octane) gasoline.

The major variables in computing DCR are the position of the piston in the cylinder at intake valve CLOSURE (defined as 0.004" tappet lift or 0.006" valve lift), expressed in degrees after bottom dead center (ABDC), and the total combustion chamber volume, which can be computed from the static compression ratio (SCR). The position of the piston as a function of crank angle is influenced by the ratio of connecting rod length to crankpin stroke.

Once I got this all figured out (with just high school algebra and trigonometry ;) ) I put it into a spreadsheet with the major variables being Theta (intake valve closure ABDC) and static compression ratio. I also included cells to put in the stroke and rod length for the proper trigonometric correction factor.

The stock static compression ratio for the stock LS2 is 10.9 to 1, and the stock LS2 cam closes the intake valve (0.006" off the seat) at 68 degrees ABDC. The attached chart (click on it to see it full size, I doubt you can read the thumbnail :toetap:) shows that the stock DCR is 8.45 to 1.

The pink diagonal cells show where you need to be to stay near or just below a DCR of 8.7. Theta is not always listed on cam cards, but if you have the duration at 0.050" and the duration at 0.006" (valve), subtract them, and add one-half the difference to the listed (or computed) Intake Valve Closing at 0.050". For the stock LS2, the intake duration (at 0.050") is 204 degrees, and the intake closing is at 36 deg. ABDC. The duration at 0.006" is 268 degrees, so 1/2 the difference is 32 deg., so the "real" intake valve closure is 34 deg. + 32 deg. = 68 deg. ABDC.

If you want to put this chart into your own spreadsheet, the basic equation is shown at the top of the chart for the first cell, E11. Once you get it typed in, and get the right result in the cell, just copy that formula to all the other cells. The formula references cell A1, which is just PI/180, or 0.0174533, and cell V4, which is the square of the ratio of rod length to one-half the stroke. The example I used, 3.6" stroke and 6" rod length, is not exact for the LS2, but is close enough for making these calculations. Input your own specifications for absolute accuracy.

Feel free to PM if you have any questions, or find any inaccuracies that would be significant to using this chart.

Very interesting...I always hated the lack of TQ off idle with more cam so bumping the compression up to the max safe level is really good to be able to quantify what is generally known common sense wise, but is generally somewhat more of a general trial and error sort of thing to guess how much cam to run.

volumetric eff and bmep may still get too high at tq peak, but that's what tuning is for I guess.

can you explain the trigonometry of the rod length ratio and the stroke - I'm trying to understand the rotational relationship.

Let's see what I can do. Refer to the attached thumbnail (click on it to open in a new tab): the red circle and line at the bottom represent the crankpin, Theta is the crank angle at which the intake valve closes (0.006" off its seat), the blue line is the connecting rod, and the green shape is the piston in a red cylinder.

If the connecting rod did NOT lean over during the stroke, (see dotted blue line to the right of the cylinder) the piston would actually be somewhat higher in the cylinder (dotted green line) by a value I call omega. The formula in the spreadsheet in my OP takes this into account and computes omega as a function of theta, and of the ratio of the rod length to the stroke of the crankpin.

Note that if we call the angle between the actual connecting rod and the straight line between the main bearing and the wrist pin beta, then

L*sin(beta) = (stroke/2)*sin(theta)

and you can use Pythagoras' Theorem to find omega as a function of theta.

Since omega always lowers the piston compared to the straight-rod scenario, the DCR is increased due to this affect. Ignoring it would underestimate the TRUE DCR and could lead to detonation and all its nasty consequences.

There will be a quiz on Monday :willy: .

Great info! Haven't seen that kind of math since I moved to Miami, you must not be from around here! LOL.

Would you mind sending me the spreadsheet to Joegut at Gmail dot com?

By the way we are at Countyline Dragway most Saturday nights and will be at the new Moroso on Nov 9. Hope to meet you one of these days.

Oops - - -
 

OK, I found the first typo in the chart - in shrinking it down I left out the row for Theta = 80 degrees. Here is the corrected chart.

And you guessed right, I did not learn math and physics in Miami . . . . .

Anyone else who wants a copy of the live EXCEL spreadsheet, just drop me a PM

Here a good equation that helps to understand the rod length and crank offset relationship.

s = o cos a + SQRT(r^2 - o^2 sin^2 a)

s = distance between crank axis and wrist pin
o = crankhaft offset
a = crankhaft angle
r = connecting rod length

This is good stuff. I was wondering how to define dynamic compression because my textbook doesn't cover it.

Compression ratio is define as the ratio of volume between TDC and BDC or

Rc = Vbdc / Vtdc

I'm trying to wrap my head around your definition. Did I read it right that the stock valve timing closes the intake valve after the piston reaches the bottom and is on the way up during the compression stroke? Is that right?

Correct, to take advantage of the inertia in the incoming air column at speed, the intake valve closes AFTER bottom dead center, during the compression stroke, by quite a bit.

The definition of Dynamic Compression Ratio is similar to the definition for static compression ratio:

DCR = (Volume when intake valve "closes") / Vtdc

And "closes" means when the valve is 0.006" off its seat, which is what most cam grinders use for "Advertised duration".

Like I said in the OP, this Forum is like a graduate course in engine performance, so it is more advanced than your textbook.

If you deconvolute the EXCEL formula at the top of my figure, you will see that it contains the mathematical equivalent of the formula from your textbook. I remembered seeing this equation years ago, but it was more fun to sit beside my swimming pool with a cup of coffee and a tablet and derive the equation for myself. Yes, I am a NERD, and proud of it . . . . .

OK, got it. This book is a real treat. It's generally used as part of a masters program in engineering. I haven't done this kind of math in 20 years so it's kicking my butt all over the place. Even then, my major was PolSci.

You're a nerd, I'm a geek. Sounds like the only difference between us is that I'd be drinking beer instead of coffee.

:cheers:

Keep in mind that cylinder pressures fall when you go up in altitude and you will likely have no issue with a given set-up tuned to 93 octane at sea level then run 91 at 1 mile up.

This is really good info presented in a good format. The only thing I have a comment on is the above. All duration numbers whether it's .050" or "advertised" timing is given as tappet lift...different rocker arm ratios can alter valve lift duration numbers. For example, the 1.8 rocker arms on my 467 boat engines take my .050" duration numbers from 238 degrees with the stock 1.7 rockers to 241 degrees. I think you're getting confused on the advertised numbers because there are three different numbers used by various manufacturers which is why they went to a .050" standard. GM uses .002", Comp Cams uses .006", and most other cam manufacturers use .004"...but they're all at the tappet.

The bolded part is a very important consideration when building an engine. If you follow just the DCR method, you may end up with a SCR so high that while it won't knock/detonate at low RPM, it will at higher RPMs. With the right cam and intake/exhaust tuning, volumetric efficiency can exceed 100% around the torque peak and cause detonation that even tuning can't get rid of. DCR exists because the "effective" compression ratio is a "dynamic" number that is always lower than or equal to SCR with DCR lowest at cranking RPM and highest at peak torque RPM where DCR=SCR. If your cam wants 15:1 SCR compression to achieve 8.7:1 DCR to make torque at low RPM, you're going to have some serious detonation issues in the upper RPMs.

Good job here guys!:thumbs:

Excellent information and spreadsheet, Lee! Thanks for taking the time to share it. I wish I was as fluent in math as you guys are. I am beginning to think that if we could put Lee, Guy, Richard, and Ben all in one place, we might have a new version of nucleosynthesis.

Well shoot. Here I am on a Sunny Sunday morn, kids watching Cars (Lightning McQueen, for the 10,000 time) and a fresh cup of coffee..ready to calc the DCR of my G5x3.

As per Clevite's text above, Theta is not always listed on cam cards, but if you have the duration at 0.050" and the duration at 0.006" (valve), subtract them, and add one-half the difference to the listed (or computed) Intake Valve Closing at 0.050"

.

But I don't know my cam's duration at .050, or .006, let alone the valve closing at .050! :(

Sounds like a good excuse to put a degree wheel on your crankshaft and a dial indicator on your #1 intake rocker arm - after all you have already seen Lighting McQueen 9,999 times. :lol:

I think someone you know has the specs on the cam, and that person might be willing to tell you the intake valve closing point at 0.006" tappet lift (since your cam is ground by COMP to LG specs). If you ask really politely, LG might be willing to part with that ONE piece of data on their proprietary cam.

You could also check LS1TECH.com - someone may have put this cam on a Cam Doctor.

LOL right you are! I will do a bit of searching and perhaps someone will chime in and help me.

Though I coincidentally DID buy a dial indicator yesterday (on sale at Harbor Freight, picked up a digital micrometer too), pulling a valve cover isn't my idea of fun in the morning. That needs to be done in the evening...I don't like to get oily without beer nearby. :lol:

So I'll search and wait before cracking a valve cover.

BTW and sorry for the OT, but been doing a lot of research on the Livernois Stg. 2 heads you are getting...they are really looking promising from a performance/$ standpoint. I think they will be a great compliment to my SPINFAST. I'm saving my nickels.

A picture is worth a 1000 words...Thanks for the diagram.

You are welcome. Having a scanner hooked to the computer comes in handy for putting a quick sketch in a post.