I'm working on a website for a friend of mine and I'm adding a bunch of automotive-related calculators for his customers to use. One calculator I have never seen online is a dynamic compression calculator...everyone just ends up using the downloadable one from here. I decided to give it a shot...can't be that hard, right?

Well, it's finished, and it seems to work well. I used the formulas from the page I linked to above to find the dynamic stroke, but he didn't list any formulas for how to do the cam math (specifically, how to figure out when the intake valve closes). I do, however, want to check a couple of the formulas that I ended up using to calculate the valve timing. Since you need to know the valve timing at the seat and not at 0.050", I had to find a formula to caculate that from the advertised intake/exhaust duration. I found these formulas in a newsgroup post:

AID = Advertised Intake Duration
AED = Advertised Exhaust Duration
ICA = Intake Centerline Angle
ECA = Exhaust Centerline Angle
LSA = Lobe Separation Angle
IO = Intake Open (BTDC)
IC = Intake Close (ABDC)
EO = Exhaust Open (BBDC)
EC = Exhaust Close (ATDC)

ECA = (2 * LSA) - ICA
IO = (AID / 2) - ICA
IC = (AID - IO - 180)
EC = (AED / 2) - ECA
EO = (AED - EC - 180)

When I compare these values to the values produced by the downloadable DCR calculator, they match up exactly. I'm assuming this means they're right, but I just wanted to double check and make sure. Just because it works for my cam doesn't necessarily mean it'll work every time.

And, while I have your attention (hopefully), do you all have any good links to other "car math" pages? I've found quite a few, but they all seem to repeat the same standard formulas over and over again (gear ratios, maximum MPH in a certain gear, guesstimate ET from HP and weight, blah blah blah). If there are any good ones out there, I'd like to hear about them.

 

That's a lot of checking but it appears to be accurate. You making a spreadsheet or something with a nice GUI?

I have a spreadsheet (not exactly elegant, just one a friend sent me) that calculates piston velocity and acceleration in relation to crankshaft angle. Also calculates load in pounds and % of critical load based on bobweight and rpm. Very simplistic, but makes for a nice quick check.

I also have....

Formulas for calculating frictional mean effective pressure, to frictional horsepower and ultimately brake horsepower. Gives you a decent idea of how much horsepower is consumed by friction. Shoot me a PM if interested.

Formulas for determing fueling needs (choosing pump capacity).

Fuel injector sizing formulas (these are all over the net).

Compression ratio formulas (all over the net but not all are specific enough for my tastes)

Formulas for determining intake manifold tuning lengths. I have a simple empirical formula (Vizard's) and a much more complicated formula I found in an SAE paper.

The formula for determining volumetric flow rate at different depressions.
Flow at new depression = sq rt (new pressure drop/old pressure drop) x flow rate at old pressure drop

Airflow demands (cfm)...
((rpm x displacement)/3456) x VE

or the one from Pro Systems...
(cid x rpm x VE)/2820

I also have quite a collection of formulas from various papers....

Let's just say I have quite a number. An ongoing project of mine is in writing a program with a very nice GUI, incorporating all these useful (to me at least) gearhead formulas. Maybe one of these days I'll actually get around to finishing it up.

-Mindgame

 

It's all going to be web based...unfortunately I don't have a host that supports ASP.NET at the moment, but here's a screen shot of the input form and results:

Dynamic compression ratio

Much, much simpler than the "tab through these pages out of order and do all of this blah blah blah" downloadable DCR program that's available. I also made up a page for cam math so you can view open/close events, overlap, etc:

Cam math

I also plan on adding a bunch of other simple calculators (the estimated HP, ET, etc I mentioned in my original post), a density altitude calculator (this one is tough!), and hopefully some more advanced calculators like you had mentioned. Basically, a one stop calculator shop that you don't have to pay for to use. It's too much of a pain keeping track of all of the links to all of these different ones I use at the moment.

 

Cool idea Type_O.

Your advertised duration figures.... I'm assuming these are from .006?
These methods are also assuming symmetric lobes. Just an observation, most are asymmetric but the difference is probably only a few degrees.

Looks good to me!

If you need any help just give me a yell.

-Mindgame

 

Yes, my advertised duration figures were taken straight from the cam card at 0.006" lift. It would be nice if Comp would include the seat timing along with the timing at 0.050". I'll probably add another field that will allow you to enter the actual intake closing angle instead of the guesstimated one. I guess the only way to actually figure out the "true" intake closing angle would be to toss the cam in the block and degree it, right?

When I finish up all of the "basic" calculators, I'll give you a yell about building the more "advanced" ones.

 

I will like a Pressure to CR. I read some one here have a program to do the calc and figure a 200 PSI are = 10.5 CR. Something like that.
I will search for the thread and post here maybe he can tell you how the calc are OR maybe Mindgame know the formula like allways.

the thread: http://web.camaross.com/forums/showt...hreadid=267387

 

You mean cranking pressure?

Don't know about the "always" part but yeah, I know a formula for that.
Have to find DCR first, but other than that... it's a piece of cake. Well, in comparison to the stuff I do every day at work it's easy.... and a bit more fun at times too.

-Mindgame

 

Great, Is easy to find a DCR in a 93 stock lt1 witout open de engine or degree de cam? my Z is stock only have CAI K&N but when I change the plugs made a pressure test and the result was very close to 300 PSI in all the plugs, maybe the last owner mill the heads and up the CR.
for this reason I like to know how could I find the CR and DCR. I only use 95 Oct. lead gas No cats.
Will be nice if you post this formulas.
Thanks.

 

Cool jeff

http://www.kb-silvolite.com/calc.php

Theres a bunch of good ones, including a DCR calculator.

Also...
http://www.smokemup.com/auto_math/index.php