Octane/Comp.Ratio -vs- Cam Selection(?)

[Kevin Shasteen]

Well; where to begin?

 

This process basically allows one to take any chouce of camshaft & determine one's "REQUIRED" Static Compression Ratio based on the Total Chamber Volume w/the octane of your choice; all this even w/out ever purchasing one component, w/the exception of the block/crank-you will need your block/crank with at least one piston in it to degree the vertical movement in the cylinder in relationship to the rotary movement in degrees of crank rotation. I thought it was pretty cool when I read it!

 

I found it in another book at B&N; the process was covered in two of the books pages, the rest of the book is pretty much like any other book. I purchased this book simply because of this process outlined.

 

I'll attempt to make it clear as I found it most interesting.

 

The theory was derived by David Vizard & Dema Elgin of Elgin Cams. The author was merely "regurgitating" their theory.

 

The theory once you understand it is like a "Light Bulb" going off & makes perfect sense and is another "Cog in the Latter" to determing which components to build around your block/cam choices. I'll attempt to explain the theory then give the example in the book as a viewable sample. In determing Intake Closing (IC) you need the "Seat to Seat" figure in Degrees & not the "@ .050" figure.

 

There are a few knowns you must determine prior to playing w/the numbers such as:

 

1)What is the Swept Volume of an individual Cylinder converted to cc's?

2)When does the intake valve close on a particular camshaft?

3)What type of fuel will be used/pump or high octane race fuel?

 

Once you've chosen your displacement the rest of the theory is based on the timing; specifically the closing of the intake as the piston rises on the compression stroke.

 

Vizard & Elgin had a graph w/the BBC crank in a 1*increments from 0* up to 90* & then backwards from 90* to 0* representing a full comleted 180* cycle. This way/when you obtain your IC* you merely look at the graph to determine where the cyl. is w/in the stroke & that tells you how much cyl.vol. remains w/in that particualr stroke.

 

Once you know the closing of the intake valve you then obtain the location of the piston w/in the cyl. as to how much volume remains between the top of the piston & TDC. Once you've obtained this "%" (Percentage) you multiply this "%" by the "Total Chamber Volume" & divide that figure by the gas type; the gas type is given as a number of "8.5" for pump gas and a "9" for higher race octane fuels. This will give you the Total Chamber Volume.

 

Then you can take the "Total Chamber Volume" to determine your required "Static Comp.Ratio" needed. In other words/you can determine which comp.ratio works best when this particular cam is chosen!!

 

The sample given is a BBC/the process remains the same as Comp.Ratio is a mechanical equation & is immaterial to the engine being a 4cyl,6cyl, V8 ect, ect/the process remains the same-the book chose a BBC as the book was about BBC's.

 

454 Big Block Chevy bored .030:

a)Bore x Stroke = 4.250 x 4.0

 

1)Swept Volume

460 divided by 8 (8cyl's in a V8) = 57.5ci per cyl

2)Convert C.I. per cyl. to cc's;

57.5 x 16.387 = 942.25cc's Total Swept Vol.per cyl

 

Example Cam = Intake closing (IC) of 48*ABDC

 

Now that we know the IC = 48* we now have to determine what "%" of the total cyl.volume remains in the comp.stroke. Upon degree'g the cam to find TDC & BDC/they determined 88% of the piston travel remains in the Comp.Stroke.

 

2)We know we have a "Total Piston Swept Volume" of 942.25 & 88% cyl.volume remaining & have chosen to use pump gas..."8.5" fuel type used.

 

Total Chamber Volume =

Total Swept Vol. x % of Cyl.Vol/Gas Type -1

 

942.25x88%/8.5 - 1 =

 

829.18/7.5 =

 

110.55733cc

 

Next we do the following in order to find the best static comp.ratio for the cam we've chosen:

 

Static Comp.Ratio Needed =

 

Total Swept Vol. + Total Chamber Vol/Ttl Chamber Volume

 

942.25 + 110.55733/110.5733 =

 

1052.8073/110.55733 =

 

9.5227272 or

 

9.5:1

 

THIS IS TOO COOL!

 

Another example he gave for that same eng.combo was a cam w/an IC = 78*ABDC; What Static Comp.Ratio would then be required?

 

The Remaining Cyl.Vol w/an IC of 78* turned out to be 68.4%. Working out the formula as previously exlained gives us the following:

 

Total Swept Volume =

 

942.25 x .684/9.0 - 1for race fuel =

 

644.499/8 =

 

80.56cc's

 

Now solve for the required static comp.ratio:

 

Static Comp.Ratio Needed =

 

942.25 + 80.56/80.56 =

 

1022.81/80.56 =

 

12.696251 or

 

12.7:1

 

I dont know about you but when I read this...I sighed a sigh of relief/as I'm always attempting to take the guess work out of determining the selection of a camshaft & its related/required components...hope I related this info intelligently.

 

BTW: The author also says-because different cam mfg's measure opening/closing rates at different degree's one should only compare a given cam to other cams w/in that mfg's selection of cams; also-dont compare a hyd.cam to a roller cam; or a hyd.roller cam to a mech.roller. This way you wont be comparing apples to oranges! If you dont know the "Seat to Seat" figures/simply call the cam's tech line & they'll have that info for you.

 

The author also claims he found that any cam offering an IC that leaves 88% cyl.vol will operate best at a static comp.rato of 9.5:1 on pump gas. If more than 88% cyl.vol remains then a lesser static comp.ratio of 9.5:1 may be used on pump gas of 89-93 octaines; 94% cyl.vol will work w/9.0:1 static comp.ratio's on pump gas. If there is less; say 84%/then the engine will require a 10.5:1 static comp.ratio & higher octaine fuel. Cams which leave 80% in the cyl. will require a statci comp.ratio of 11.0:1 while Cam's that leave 70% will need 12.4:1 static comp.ratio & high octane race fuels.

 

 

One last comment; once you've chosen your Bore x Stroke conmbo & required static comp.ratio; The actual "Figure" for the static comp.ratio is a figure that cant change. What you can change w/in the "Desired Static Comp.Ratio" are the Head Gskt compressed volume in cc's, the piston deck to block deck vol in cc's & the dome/dish vol in cc's; you can play around w/those variables to come up w/the "Desired Static Comp.Ratio"!

 

From there; you can choose a set of cyl.heads based on your required Airflow needs & an intake/carb to match...and the pen has never left the paper yet!

 

I know this isnt a quick fix for anyone-but w/a little patience on any given Saturday one could grab a degree wheel & dial indicator and map out your crank's location w/in your block; & the rest as they say is history! Really now-we spend so much time on our projects/what's one more saturday if it takes the guess work out of choosing a camshaft?

 

Doesn't anyone else understand this besides me? Once you've mapped out your crank in degrees.....you have it for the rest of eternity & can pick/choose cam's w/that graph!

 

Sorry if I rambled/but I know this will eleviate a lot of troubled minds when it comes to cam selection -vs- comp.ratio & fuel octanes!

 

Kevin,

(Yea,Still an Inliner)

 

 

 

[This message has been edited by Kevin Shasteen (edited April 15, 2001).]

[DavyZ]

Kevin, that sounds like a great way to take the guess work out of picking the right combo for what gas you want to run. It would be a nasty little mistake to have picked a combo that accidentally requires race gas! OOOPS!

 

What you DIDN'T give was the name of the book this information came from. Obviously you forgot after you wrote down the 3 page synopsis above. I understand that!

 

Seriously, thanks for bringing it up, because I'll use that information for my next engine, a 377 with a lightened Scat crank and roller camshaft etc. I can dream can't I?

 

Thanks, Kevin!

 

David

[Guest needwaymorespeed]

what about fuel atomization

and the ability of the engine to retain or dissipate heat?

Curtis

[Kevin Shasteen]

Yea David...did get kind of long-jsut didnt want to leave anything out in case there were those out there that didnt want to buy the book-they'ld at least get a good explanation of the process; besides if I only typed one phrase everyone here would've known it wasnt me & that I'ld been taken over by the Pod-People!

 

Yes, alluminum will usually allow an individual to up the Comp.Ratio "1" notch w/out worry of detonation; however, some folks feel that iron cyl.heads that retain their heat allows them to create more power because of iron's abilty to store that energy (I still prefer alluminum if I can afford it).

 

The book was, "How to Build Chevy Rat Motors"

the undercaption is Hot Rodding BBC's 90's Style & written by Ed Staffel/copywrite 96 & published by SADesign.

 

Again the book is about BBC's & the article on cam selection/octain & static comp.ratio is only two pages; there is a map/graph of the stock 454 stroke relationship in degrees -vs- location of cyl. & its remaining volume...but again only for the BBC & not for the SBC.

 

After having posted this initially I read it further & realized it really wouldnt be that hard to map the stroke of any engine's piston. I say this for this reason.

 

TDC is considered 0* & half way down the cyl is 90* while BDC is 180*; then half way back up the cyl is another 90* & all the way up to TDC is another 90*....voila 360*. At first I thought it would take at least a day but when you divide the cyl into 1/8th's it really probably wouldnt take that long to do; just my thoughts(?).

 

Kevin,

(Yea,Still a "3page Article" Inliner)

 

[This message has been edited by Kevin Shasteen (edited April 15, 2001).]

[pparaska]

In a book I have "Mechanics of Machines", the equation for the position of the piston in the cylinder is given as:

 

x = (R+L) - (R cos(theta) + L cos(phi))

 

R is the length of the crank throw (half the stroke)

 

L is the con rod length, center to center

 

theta is the angle of the crank, measured away from TDC (ATDC)

 

phi is the angle the rod center-to-center line makes with a line connecting the piston pin and the main bearing center.

 

Just geometry and trig.

 

I'm playing with a spreadsheet set up to calculate the % volume, etc. for different stroke/rod engines. The % volume versus crank rotation angle (theta) function really only depends on the rod length and stroke. theta and phi are running variables and phi can be expressed as a function of theta.

 

cos(phi) = SQRT( 1 - ((R/L)*sin(theta))^2 )

 

When I get the spreadsheet working, I'll let you know and post a link to it.

 

 

------------------

Pete Paraska - 73 540Z - Marathon Z Project

pparaska@home.com

Pete's V8 Datsun 240Z Pages

[Kevin Shasteen]

Alright Pete; you cant just dangle a formula in front of my face & expect me to just go away quietly! What does the {cos} in the first & second formula represent & what does the SQRT, *sin and ^2 represent in the second formula?

 

Rod/Stroke Ratio's definately change the data; results are only as good as their input. The crank throw is what kind of had me concerned when mapping out a graph-I didnt have any problem w/the vertical movement...but once the crank came up/down on its throw-that would've been a little more tedious attempting to map...other than the cyl.vol. merely maintaing the steady figure till the vertical movement began again.

 

Pete; if you can complete that spreadsheet-that would be a Godsend. It doesnt appear too many others realize the benefit that spreadsheet would offer. I think once they began playing w/the numbers & understand the underlying issues-their Light Bulb will go off!

 

When I read this in my newly acquired book-I didnt fully understand it at first...I understood/knew it had important significance-but I had to re-read it for about 30 minutes before my Light Bulb went off & my elevator bell rang anouncing it had finally reached the top floor! Ding-Ding-Ding!

 

PETE, I NEED THAT BOOK, "Mechanics of Machines" what is its copywrite year, who publisher it, who authored it & how much was the book?

 

Anxiously awaiting that spreadsheet Pete!

 

Kevin,

(Yea,Still an Inliner)

 

 

 

[This message has been edited by Kevin Shasteen (edited April 16, 2001).]

[pparaska]

Kevin, I'll work on the spreadsheet again tonight before 1am - it might go better if I'm awake fully .

 

cos = cosine of the angle in parentheses

 

sqrt = square root

 

*sin = times the Sine of the angle in parentheses

 

( * is a symbol for the multiplication operation in many programming languages and spreadsheets. I left that operator out of the first equation. My bad for bad consistency.)

 

^2 is "raise the thing to the left to the 2nd power" or "squared".

 

 

 

------------------

Pete Paraska - 73 540Z - Marathon Z Project

pparaska@home.com

Pete's V8 Datsun 240Z Pages

[Kevin Shasteen]

Pete,

 

I was pretty sure the * meant multiplication; but since I used it earlier as * (Degrees) I wanted to be sure what you intended it to be.

 

Believe it or not-your formula is beginning to sink in-I just need a little more prodding.

 

It appears to me the formula doesnt take that much "Cifering" (as Jethro from Beverly Hillbillies" would say; that is once you've determined the angles of the crank throw -vs- pitson pin angle. Could you mock up a hypothetical equation for me as a viewable sample like I did in my first post w/the formula I got from my book.

 

I'm dying to understand your forumula! What is the "cosine" of an angle? I think I pretty much understand the formula w/exception to not knowing what the "cosine" represents.

 

Just put some "close enough" digits where they belong & take me along each step of the formula/I'ld really appreciate it. I'ld owe you a steak dinner if/when we ever [meat], I mean meet. (haha)

 

For the spreadsheet-dont think you have to do it tonight/dont want to put you out or keep you up late; everyone needs their beauty sleep ya know.

 

BTW-who published that book, what's its copyrite & how much was it when you purchased it-if you dont mind me asking.

 

Kevin,

(Yea,Still an Inliner)

 

 

 

[This message has been edited by Kevin Shasteen (edited April 16, 2001).]

[pparaska]

Kevin,

 

If you're interested in the answer to "What's a cosine?", I suggest the following Short Course:

http://aleph0.clarku.edu/~djoyce/java/trig

 

To get straight to the answer:

http://aleph0.clarku.edu/~djoyce/java/trig/cosines.html

 

or http://www.ies.co.jp/math/java/trig/cosbox/cosbox.html

 

The book is:

"Mechanics of Machines, Advanced Theory and Examples," Hannah, J. and Stephens, R.C., Edward Arnold Publishers Ltd, 41 Bedford Square, London WC1B3DQ, ISBN: 0 7131 3254 X

 

Note that there are surely other books or places on the internet that have this equation. This is the kinematic equation for a slider-crank mechanism.

 

 

 

 

------------------

Pete Paraska - 73 540Z - Marathon Z Project

pparaska@home.com

Pete's V8 Datsun 240Z Pages

[Guest Anonymous]

Ah, of course, the old 'kinematic equation for a slider-crank mechanism' why didn't I think of that.

 

Man you guys continue to amaze. I usually just have to bolt it on, and ask the butt dyno if it feels faster.

 

Ahh, Math, I must have been studying horticulture ( ) when that was going on... Now, what were we talking about...

 

 

,

 

Lone

[pparaska]

Lone, others, sorry, I like math - especially when I can apply it to hotrodding! At least all we are talking about is Algebra and Trigonometry . We could be whipping out variational calculus and Reimann Space geometry (the stuff Einstein used in his theory for General Relativity).

 

But wait, if you guys keep upping the ante and building faster cars, the speed may be close enough to light speed that we need to start worrying about that

[Drax240z]

Hehe, I have my calculus III final exam tomorrow. Lets talk Lagrange multipliers, triple riemann sums, spherical coordinates, gradient vectors... I'm ready baby! Hehe.

 

I like math too pete, I've always had a knack for it from my early days of addition and multiplication.

 

The thing is, mechanical engineers are one of the few professions that actually get to APPLY this stuff, which is pretty cool IMHO.

 

------------------

"Nothing is fool proof to a sufficiently talented fool."

 

Richard Lewis - 1972 240z, Powered by a Nissan 2.8L Turbo Inline 6.

Drax240's Turbo Site

Beginners Turbo FAQ & Answers

[Guest Anonymous]

Hey I'm happy we have the people good with Formula's, Data Loggers, Fuel injection maps, its good info, and I can't think of a more well rounded site as far as peoples experiences go. I wish math were my thing, it never really was, Art and Music were more my thing. Pecularly enough, Music (from a strictly technical perspective) is about Math, at least in the timing of the notes. Ask Mike about that, drummers are amazing at it, often keeping four (one for each limb) rythymns at the same all within one time signature.

Keep on keepin on with the equations, heck, I'm getting (yeah right, Man, you ARE old) old, but not to old to learn. Thanks.

 

Lone

[Kevin Shasteen]

Thanks Pete for the site-should keep me busy for a few......YEARS! I'm definately gonna dive into it/curiosity is killing this cat!

 

Lone-I admire anyone who can carry a note & hold a beat. In my junior-high days I thought I wanted to be a drummer; took a few drum lessons/two lessons was all it took for me to realize I'm PURE-WHITE BOY. I couldnt carry a tune nor keep a beat; I couldnt dance my way out of a wet paper bag if my life depended on it, period!

 

Pete; keep working on your spreadsheet; This formula is an answer to the Hot Rodder's Prayer as it takes 90% of the guess work out of which cam should be used. David Vizard is a mechanical engineer-I'm sure he's devised some sort of spreadsheet/formula which your are working on, unfortunately-you know how stingy those Big-name shops can be w/their little secrets. I'm glad we-ve got our own resident engineer in you.

 

Pete, believe it or not I spent 4-5 hours at B&N yesturday evening reading their trig, calculus & some Geometry. Definately interesting books & mostly over my head. I was facinated w/all the graphs & flowcharts of metal's, composites, geometric figures & Trig theories. I've always been able to work math once someone has explained the principle issues/formula's process to me.

 

As for Einstein's Theory of Relativity; If it aint Relative-Then it Dont Matter! (Sorry-couldnt refuse that one); math has always intrigued me-I'm to impatient and am more of an instant gratification person-except when my attention has been sparked; thus, I've never really pursued the subject.

 

Kevin,

(Yea,Still an Inliner)

[Kevin Shasteen]

Originally posted by Kevin Shasteen:

[QB..,what type of fuel will be used/pump or high octane race fuel...,& divide that figure by the gas type; the gas type is given as a number of "8.5" for pump gas and a "9" for higher race octane fuels...,

 

Well-well-well-well, well; guess what? Remember this thread anyone? I have had a breakthrough.

 

Pete Paraska; remember when this originally came up you were not sure about using this process for picking a camshaft as you were not sure how Vizard & Elgin arrived at their 8.5-1 and 9.0-1 constants for pump gas and race gas; remember that dilemma?

 

Well, the breakthrough I experienced today is was exactly "Where those constants came from". I cant believe it took me a little over an entire year to figure out. If only any of you could imagine; I have been reacking my brains ever since I came upon this process in Staffel's book. At first, because the ref's towards pump gas and race gas were made, I [assumed] the constants were derived from some quality of some additive in the fuels(?). I even scoured thru the Hartman's "High Perf.Auto.Fuels & Fluids" looking for answers. I almost convinced myself the 8.5-1 & 9.0-1 constants refer'd to the R.O.N and M.O.N qualities of Methanol/Alcohol -vs- Gasoline; almost convinced but not quite. Something just kept pulling at me and therefore I didnt accept that rationalization.

 

The answer, what was the answer? Slightly over a year later, but it hit me today; after hours and hours of literally racking my brains out it finally hit me...its so simple you guys are gonna laugh. The answer was right in front of us all the times.

 

I didnt notice the answer till I began playing with the Static Compression and Dynamic Compression Ratio's backwards; you know-using Math Substition; then it hit me, yea, I know it took over a year: we'll just call it a "Slow Hit".

 

The 8.5-1 and the 9.0-1 constants: Get This(!), represents what the 'Powers That Be' have determined a perfect Dynamic Compression Ratio for a street engine...thus 8.5:1, and the 9.0:1 Dynamic Compression Ratio for a race engine! Can you believe it was that easy.

 

If you didnt come across this article; and you decided to work the math backwards, using the displacement of one cylinder, also using the Slider Crank Mechanism to determine when the IVC (Intake Valve Closes) which tells us how much cyl.volume remains after IVC then you would see the 8.5-1; specifically the "-1" within the substituion process. As the Chamber Volume represents "1" when you cancel out that "1" by subtracting that "1", you now have to subtract that "1" from the other side...thus the "-1".

 

You guys have no Idea how much this has troubled me this entire past year . Ever since Pete sugested that a longer duration camshaft could be used other than the cam this process suggested; I have been solmewhat "challenged" to find out why. The answer is that the 8.5:1 for pump gas and 9.0:1 for Race Gas is not an [absolute] but a somewhat hallowed unwritten rule.

 

If one wanted that rumpity rump-then you would want to build your engine with a 7.5:1 Dynamic Compression Ratio..but then you sacrifice low rpm torque; but you would get that lopey idle (that you like so much Pete).

 

Anyway, just had to share this with ya'll. I really was pleased when the light bulb came on; it really was a light bulb. I have gone thru so many pieces of paper attempting to figure this thing out.

 

Sorry if I rambled; just had to pass it along to those whom I thought would appreciate this process as much as I.

 

Now, if I could just get a few books on Tire Friction characteristics(?). Its always something, isnt it?

 

Kevin,

(Yea,Still an Inliner)

[DAW]

This is a great thread! I'm going to have to dust off some old analytical geometry, trig and calculus books to follow it all but it's a great discussion. DAW