Camshafts and compression ratio

Looking for technical explanation of aggressive camshaft profiles lowering compression. I have a basic understanding but am wondering if there's a way to calculate compression loss from camshaft profiles to determine optimal static compression ratio? Essentially, can you pick an appropriate cam for a desired powerband, figure out how it will change compression and THEN design your piston?

Ignoring inertia effects of the intake charge and stuff, the engine only starts building compression once the valve closes. If the cam were closed at BDC, dynamic compression ratio (what this is called commonly) would be equal to static compression ratio. If the cam were closed with the piston halfway up the cylinder, dynamic comrpession ratio would be half static compression ratio, so on and so forth. The long and short of it though is that as there's more duration, the valve is closed later in the compression stroke which means it doesn't get squeezed as much. Translating that kind of thing into crank degrees is a little more annoying in terms of math, so here's a handy calculator I found in 20 seconds of googling: http://www.wallaceracing.com/dynamic-cr.php You can look up governming equations if you want to also; it's basically just looking at piston motion.

Note that all of this crap is, of course, an approximation, as are any estimates of how much compression something can take on whatever gas and so on. Still, it's a more useful approximation than static compression ratio.

more later Aaron with some examples, but I gotta make some top mounts.

Whatcher up to? More details.

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John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

Thanks Mr. DeRosia. Great info.

DCR.

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Pete Remner
Cleveland, Ohio

1984 RX-7 (rallycross thing)
1978
Silence is golden, but duct tape is silver.

Ok everybody talks about "I haz 11:1 compression" and they refer to service book that says 10:1

and then maybe they go and measure cylinder volume by running the piston to BDC, the running it up and saying
"I has 500cc cylinder volume + 7 cc gasket volume + 43cc head volume so 500+7+43= 550cc at BDC (Bottom Dead Center) and at the top at TDC theere's gasket + head volume of 50cc
so 550/50= 11. I'm bad"

But look at this chart:


There's a big hole that is open from BDC up to 51.5 degrees

The piston only goes up 180 degrees of crank rotation to move the piston from the bottom to the top...and 51.5 degrees of crank rotation before the hole is close means you don't have 11:1

You have less.
(Incoming charge migh be whistling down the ports at anywhere from 200-320 mph so it has a lotta inertia, it WILL keep on coming in past BDC--that is why they leave the valve open, but how much depends on a number of factors but you can't ignore that....but that wasn't the question)

Look at this chart


Now in this one the intake is left open all the way to 72 degrees after BDC-ABDC so the piston has gone even higher up the bore before the gaping hole is close...

Even less compression...

So you have to compensate..

Wonder if the link is any good?
Yep except the fucking inches that nobody works in

First cam, boring street thing

Quote

Static compression ratio of 11.:1.
Effective stroke is 2.60 inches.
Your dynamic compression ratio is 9.58:1 .
Your dynamic cranking pressure is 200.42 PSI.

Second cam, a little hotter

Quote

Static compression ratio of 11.:1.
Effective stroke is 2.19 inches.
Your dynamic compression ratio is 8.23:1 .
Your dynamic cranking pressure is 164.92 PSI.

I used Ford YB dimensions inluding a stumpy short rod 5.06" aka 128.5mm c-c

Just for fun lets try a longer rod say 139mm

Quote

Static compression ratio of 11.:1.
Effective stroke is 2.18 inches.
Your dynamic compression ratio is 8.19:1 .
Your dynamic cranking pressure is 163.88 PSI.

Even rod length effects compression...

So you thought you had a manly 11:1 but you have a limp-dick 8.2 really---plus maybe 4% more from the charge inertia.

So you see if there is going to be any pop when using a warmer cam there has to compensation and a fair bit of it if you want any yank.

---

John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

john thats the best entry level explanation to overlap and scavenging i've ever seen explained. good job dude, i'm gonna go see my second year teach from auto and give him the ole L forehead and pass him a printout of this so he doesn't f up future generations of young techs. rocky mountsin rally is pretty good,wish you were here
Sincerely
Jeremy

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All bridges burnt, island life is beauty



Edited 1 time(s). Last edit at 11/01/2014 12:14AM by Snidewhips.

Thanks Professor JVL! Your charts also brings to light the effects of cam timing. Awesome info that is very educational to a mere mush brain like myself.