I have been up and down and in and out and left and right with this. I've tried the calculators, read the books, web and magazine articles, read over posts here, tried to just work it out for myself by inputing variables, and been through every cam data providing website 5 times with a fine tooth comb, but after 3 days of hours on end work I just can't seem to riddle out exaaaactly how to do it, and it's so frustrating.

I am trying to calculate the dynamic compression ratio of my engine with the cams I have to choose from. I can of course just plug all the numbers into a calculator, but I really want to actually leeearn the math behind it all, because with the amount that the calculators can vary based on very subtle things like exactly what you consider closed for the intake, I'm never going to feel confident in one of those answers, unless I know every single bit of the equation.

Some of the calcs just say "intake closing point" and that of course leaves me open to the whole raaange (.050", .006", seated). Some of them seem to standardize it, by saying "add 15*" to .050" closing point, but after plugging in a sleeeeew of numbers, I've come to the conclusion that that 15* number is just so completely arbitrary it is almost worse (some cams drop right off after .050", some stretch on forever!).

Beyond just how to calculate the stuff reliably, I am getting my goal number from just a very limited number, of very unspecific sources. I am working with an 8:1 goal, because that's what I've heard an iron head can handle on "pump gas". Well, there are several different grades, and critically, several different costs, of pump gas, and also, is that 8:1 the max (you should shoot for 7.8:1, so as to NEVER top that 8:1 number)? Or the goal (some +/- is accounted for)? And I know me flooring it in my giant heavy old truck with a load on is alot different than what a Mustang can take. So what is the real safe number, say for 87 octane, and a heeeavy haulin truck?

If someone really knows alot about this (Bullitt390 you have seemed to in the past), I could really use an explanation here.

Thanks, AleX

 

The calclulators are usually off when it comes to the IVC°. You can use them accurately if you know how to calculate the IVC then plug it into their calculator. I use this calculatorUnited Engine & Machine Co. Incorporated
To figure actual IVC the formula goes like this:
Int. Adv.dur./2 + ICL(intake center line) - 180= IVC°
I'll use my cam for example.
274/2=137, 137+ 106=243, 243-180=an IVC of 63°
With 63° for my IVC that means that 9.8:1 is what SCR is needed to net a Dynamic CR of 8:1.

You can actually run about 8.25:1 or MAYBE a tiny bit higher but it'll get finicky if not tuned right. 8:1 gives it a little leeway. I wouldnt go over 8:1 in your case.

If you really want the math behind DCR I could post it but there's really no need as the error in the calculators is in the IVC. And the number you need is for seat to seat. Thats why some are off because a lot of cam manufacturers dont post seat to seat only .050 usually.

 

Ah, sleepy, reps for you! That is all of exactly what I've been wondering! I'd actually figured out that equation that you put up, but I just had no idea if I was right you know? And without any completed equation with a confirmed right answer, I was just throwing darts at a wall with no idea what end DCR to actually shoot for to figure out which potential equation was right.

Thanks alot! That fills a big void in my knowledge, fiiiinally I can move on!

AleX

 

Thanks for the reps! Glad to help. Just in case anyone still wants to know the trigonometry behind the calculators I'll show the math.

Calculating the DCR requires some basic information and several calculations. First off, the remaining stroke after the intake closes must be determined. This takes three inputs: intake valve closing point, rod length, and the actual crank stroke, plus a little trig. Here are the formulas:

Variables used:

RD = Rod horizontal Displacement in inches
ICA = advertised Intake Closing timing (Angle) in degrees ABDC
RR = Rod Distance in inches below crank CL
RL = Rod Length
PR1 = Piston Rise from RR in inches on crank CL.
PR2 = Piston Rise from crank CL
ST = STroke
1/2ST = one half the STroke
DST = Dynamic STroke length to use for DCR calcs

What's going on: First we need to find some of the above variables. We need to calculate RD and RR. Then, using these number, we find PR1 and PR2. Finally, we plug these number into a formula to find the Dynamic Stroke (DST).
Calcs:

RD = 1/2ST x (sine ICA)
RR = 1/2ST x (cosine ICA)
PR1 = sq root of ((RL x RL) - (RD x RD))
PR2 = PR1 - RR
DST = ST - ((PR2 + 1/2ST) - RL)
This result is what I call the Dynamic Stroke (DST) or corrected stroke as some call it, the distance remaining to TDC after the intake valve closes. This is the critical dimension needed to determine the Dynamic Compression Ratio. After calculating the DST, this dimension is used in place of the crankshaft stroke length for calculating the DCR.

So there ya go. If you want to figure all that go ahead. I learned it once, checked to make sure the calculator was right when the right numbers were input and havent used it since.

 

Well ya, I'm not even going to touch that until tomorrow haha, but ya I'm the same way, I need to prove everything myself to believe it, so I think I'll have fun trying to twist my brain around that tomorrow morning, test it against a few of the calcs (I've been using that United one, and a sort of simpler Wallace Racing one), and then move forward from there. Thanks again for all of this!

Beyond that though, as long as I have someone competent listening, and we're sort of on the subject, I do have a few more questions about cams (when you get a chance). One of the problems in my head when I came up with that sort of "split it in half" equation for calculating IVC based on duration and centerline, was that I wasn't at all sure how symetrical cam lobes were. Now that I know via you that you can pretty much go on splitting the duration evenly in half, I'll run with that, but are all cam lobes compleeeetely symetrical, half to half? Or are they all just within a few degrees, or does it differ roller to flat, or race to street, or what?

I'll start with that, thanks, I love this stuff!

 

No all cam lobes are not symmetrical. From what I understand the older style cam profiles are symmetrical and you can use the above formula for finding IVC. There is a error of 2-6° if you use that formula on an asymmetrical profile.
Cam design has progressed and an asymmetrical profile is better. It allows the intake ramp to open quicker while the closing ramp slows down the valve allowing more a/f to enter while setting the valve down easier so as not to "bounce". Asymmetrical cams are useful in HiPo applications where max power is desired while keeping valve train stress down and retaining engine vacuum. Lunati Voodoo cams are an example.
I wouldnt call myself an expert on the intricacies of cam selection. Getting the correct DCR is only part of the process in finding the right cam for your engine combination and desired use. As we have said engines with the same CR can and will have very different characteristics according to the profile of the cam as well as other factors.
If there was a school for this, I'd say I'm in 8th or 9th grade. I've been trying to learn all that goes into cam degreeing and matching profiles to engine combinations here recently. I'll get it all figured out soon enough, but until then I wont advise on a direct choice for a cam. I can only give part of the equation at the moment.

Check out Iskycams tech articles as well as hotrod.com and alot of others. Thats what I'm reading up on. One day we could be MASTER ENGINE BUILDERS!!! haha I'll be the next John Kaase just wait and see.

 

OK cool, I figured that it was preeetty likely there were some asymmetrical examples out there somewhere, but it's sort of comforting to know that at least they're only off by a handful of degrees at most.

Also ya, I've been puting myself through the same school recently, but I'd say I'm only still in elementary school so far, if that haha. Really interesting stuff though, I never realized before the last couple of weeks how much the cam reeeeally ties the whole rest of the engine together, but I guess in a single cam engine design, that's actually exactly what it does. It is theee link between the top and bottom end of the long block, and it determines sooo much. I love stumbling upon something like this where the complexities just run so deep, keeps me busy longer that way, outta trouble haha.

And ya I've been reading up on hotrod.com alot, it seems like every car term I google, one of their articles is near the top of the list. One thing that's thrown me off, is all the just straaaight no question BOGUS information that's out there on these really technical topics. I read an entire chapter on cam selection in "How to Rebuild the Small Block Ford", and after alooot of confusing internet research post read, I found out that a few of the books main points were just plain wrong, I couldn't believe it.

Like I've mentioned in a couple other recent threads, the engine that I'm building right now has really just got to be as good of a Frankenstien as I can come up with, with the parts I have on hand right now, and out of the 3 cams I have to choose from, I'm actually leaning towards the one I pulled out of my first stock 400 a couple years ago! It isn't very tall, only about .435" of lift I think? And only about a 250 seat to seat intake duration, but set straight up, it's going to give me a DCR of 7.97 with my current SCR of 9.2. My 2 other choices, even advanced to the max, can only offer me DCR's of 7.2, and 7.5, although that 7.5 one is a biiig roller, so it is tempting, but I don't know, I want my low end torque, and I need this build to work.

Anyway though, bedtime, but yup, dreams of that engine masters challenge will fill my night as always haha, and I guess as long as we're forming a line, I call spot number 3, I'm gunna be the next neeext John Kaase. I've got loyalty to the 400 on my side though, watch out, he likes those!