http://cochise.uia.net/pkelley2/

some stuff from vizard i think


This is a useful little VBasic program I wrote to calculate Dynamic Compression Ratios (DCR). Knowing the DCR is very useful for cam selection, since compression in the cylinder cannot start until the intake valve is closed. Knowing the remaining amount of crank stroke left after the intake valve is closed is needed to calculate the DCR. According To David Vizard, a street engine running on pump gas should have around 7.5:1 DCR, a race engine should shoot for 9:1. These ratios will produce the best torque output. All these calculations must be done with seat timings. Since compression doesn't start until the intake valve closes, using .050" tappet lift gives an incorrect result. At .050" the valve is still open .050" times the rocker ratio. In the case of a SBC, the valve is still open .075" (that's around twice what a plug is gapped at) and a BBC's intake valve is open .085". That's a big leak as far as compression is concerned.


The DCR Calculator provides three separate calculators. One for calculating the valve closing points, another for calculating the remaining stroke length after the intake valve closing, and a third for calculating compression ratios. There are various way the DCR Calculator can be used.

If you don't know the valve event times, use the "Cam Timing and Overlap Calculator." Enter the advertised duration for both the intake and exhaust, the Lobe Separation Angle, and the installed Intake Lobe Centerline Angle. The calculator will return the valve events for that cam.

Next, enter the results along with the rod length and crank stroke for your engine in the "Dynamic Stroke Length Calculator." This calculator returns the amount of the crank stroke remaining after the intake closes. This is the Dynamic Stroke Length (DSL). The distance the piston is from BDC is also shown.

After getting the DSL, go to the "Compression Ratio Calculator." Enter the required information using the DSL for the "Stroke" input. The calculator returns the Compression Ratio for that combo, in this case the Dynamic CR. You can also use the crank stroke to get the Static CR.

Where this info is most helpful is in selecting a cam and setting the proper CR. Since a pump gas engine wants around 7.5:1 CR, you can work with the chamber size and piston dome/dish to get this for a chosen cam. Going the other way, if you already have the heads and pistons, it is easy to try different cams to find one that give the desired results. Of course, this works for race engines, too. Just shoot for 9:1 DCR.

After getting the DCR where you want it, enter the crank stroke to find out what the static CR needs to be. Order the parts accordingly. As for the cam, the intake lobe center, and LSA don't really matter as far as the DCR goes. The only thing that affects the DCR is the intake closing time. Get a cam with the correct intake closing point and you will have the desired dynamic CR. The rest of the specs are up to you.

David Vizard never specifies if these numbers, 7.5 and 9:1, are for iron or aluminum heads. Since almost all he talks about in his books are iron heads, I'm assuming these numbers are for iron. I know of a fellow running at 8.12 DCR with aluminum heads and 92 octane without any problems. It appears that aluminum heads can tolerate (and probably need, due to thermal loss) another 1/2 point of DCR.

This is my first Visual Basic program. I got some help from a friend of mine, Blair Legate, on the coding for the "Dynamic Stroke Calculator" module. I did the design of the interface and wrote the code for the remaining modules. The CR calculator formula was written by Ray Forceno and adapted to this program by me. I hope it proves useful.

 

Don't be too fooled by this therory and formulas.
Although usefull for designing an engine, you should take it with a grain of salt. Especially when designing a street engine
for street pump gas.

The true "Running Duration" of a camshaft in the motor
is not the "Advertized duration".
The REAL WORLD octane requirement of a motor is closely tied
to the mechanical compression ratio reguardless or the cam timing. The cam timing affects the running cylinder pressure
but basic gas laws (heat of compression) remain fairly constant.
It is the heat of the charge after compression not the pressure
that determinres the auto ignition point and octane requirement.
"Other wise inter-cooling wouldn't work.)
Among other engine operating conditions.

Exhaust gas dilution from overlap will help supress detonation at low rpm, but at peak torque the egr should be minimal if the engine has the "right cam" for maximine perf on that motor.
So if you're stuck with a certain octane gas you will only be able
to run a certain compression ratio +/- a small variable amount
for any engine design. We're talking Small bock chevys here
not some exotic Fueling design.


Ways that this max octane requirement can be fudged
are rich airfuel ratios, over camming ( egr, and charge density reduction from over scavageing and reversion), induction
anti-tuning and restriction, retarded ignition timing, humidity.
You may fudge the compression ratio a little using one of these methods but all these reduce power from what it could be.

High compression ratio engines, tuned correctly always perform best with high octane fuel.