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.