Dynamic Compression and Octane Rating question(s)

hbps1978 · Post by **hbps1978** » Fri May 06, 2005 1:57 am

What is a good baseline dynamic compression ratio to run in an everyday driver engine with an intake valve closing at 33 to 38 degrees cam timing, 5.700" connecting rod and 87 octane unleaded gasoline in a naturally aspirated engine. I'm looking for comments, questions, answers from experienced engine builders on this one please. Thanks to all ahead of time.

let r eat · Post by **let r eat** » Fri May 06, 2005 10:13 am

Target is 8:1 for 91 pump gas. With good electronics you could go to 8.7 or so.

ozrace · Post by **ozrace** » Fri May 06, 2005 10:33 am

There are so many variables that can affect this:-
Cylinder head material
Charge temperature
Squish
Chamber design
Piston design
Mixture motion (swirl, tumble)
Residual heat of internal components i.e. piston dome, exhaust valve, etc

These things all need to be considered.

ChrisU · Post by **ChrisU** » Fri May 06, 2005 10:45 am

Excellent point ozrace!!!!!! That's the real deal there. There's not target number without taking into consideration the entire package. Chamber design plays a big role in this.

let r eat · Post by **let r eat** » Fri May 06, 2005 12:52 pm

I was simplifying for general rules. Not ellaborating on the things that you guys have talked about such as rod length, stroke, squish, chamber design, intake configurations, exhaust configurations, standalone ignitions, ect ect ect ect.

Just a general rule is 8:1 dynamic.

There are several posts on this topic that may help for your particular application.

Wolfplace · Post by **Wolfplace** » Fri May 06, 2005 1:29 pm

let r eat wrote:Target is 8:1 for 91 pump gas. With good electronics you could go to 8.7 or so.

=
I still have to disagree with such a blanket statement

I have posted at different times my thoughts on "DCR" as it is used in a few different places regarding intake valve closing & pump gas & here is a couple of those thoughts,,,,
Some of this was in response to a question asked on another forum.

DCR or more correctly "effective" compression ratio as Dynamic implies something in motion or transition as used in some of the DCR calculators is a very important part of the equation when it come to cylinder filing,, or lack of cylinder filling.
But,, there is a bit more to cylinder pressure than just when the intake valve closes.

At low RPM, DCR as it is being used here will suffice when trying to run crap gas with high compression.

But,,when you start getting up near peak torque everything changes & you better damn well have fuel with enough octane to support the cylinder pressures you are making.

This is not as simple as where the intake valve closes, it now becomes what I feel is truly Dynamic compression ratio which includes where the intake valve closes & how much cylinder filling you have done taking into account all the other factors like how efficient the intake & exhaust system is at a particular RPM, which is essentially what volumetric efficiency or VE is about.

VE is a ratio of how much you filled the cylinder verses what you could have filled the cylinder (100%) at atmospheric pressure & in a well tuned system (or forced induction) can reach well over 100%.
IE: "overfilling"

Here is the rest of the post:

"First let me say I think that Pat's DCR calculator is an excellent tool when used with a bit of common sense.

Here's some of my thoughts about DCR & high compression that I have posted before.

With a big cam you will see lower cranking compression but you also have to use a little common sense with the slam a big cam & lower the DCR syndrome
It may have low cylinder pressure at cranking & low rpm's but somewhere it is going to start getting efficient & the engine is going to see whatever compression you have.
If the fuel isn't good enough when this happens ugly things can & eventually will happen.

If the plan is to use & lean on your engine with pump gas most of the time I would prefer to limit the static compression with aluminum heads at 10.-10.5 for 91 octane & 10.5-11 for 93 & change the cam accordingly.

You are right in that cranking compression which is part of the "DCR" is determined from when the intake valve closes.
The higher the static compression the higher the cranking compression will be the sooner you close the intake valve.
This I think we all agree on.

Now there is a lot of other things that will effect cylinder pressure to a certain extent, like RPM, Barometric pressure, cam intensity, engine temp, air temp, humidity, intake & exhaust tuning,,,, & probably a lot of other little things I missed as well.

Here's another thought to screw up the works,,
You can build an engine with say 13.0 & overcam the crap out of it & on paper the DCR will tell you it will run on 91 octane.
The problem comes when that engine gets up in the rpm & becomes efficient & starts actually seeing that 13.0. Now that "DCR" as used in these calculators becomes less of an issue as you are starting to keep pressure in the cylinder & with enough load without proper fuel, it will detonate.

The load part is why lighter low gear cars will usually tolerate more compression than a heavier or higher geared car.

It also works the other way.
We do restricted engines you just could not put a load on at low rpm in most cases.
Small cam hi compression deals like 13.5+ with a cam of say 240 or so @ .050 & at low speeds they will detonate themselves to death if you ain't careful but from say 4000-4500 up they will never see that 13.5 compression as the intake is too small to let enough air in to fill the cylinders

Anyway, you are only compressing what is in the cylinder from the time the intake valve closes & this can be completely different depending on a number of things but most of them are listed above I think
,,, these are just a few of my uneducated thoughts on the merits of just using when the intake valve closes to calculate "DCR".

Someone else can pick up the VE, quench, swirl, chamber efficiency, timing issues & multitude of other things that effect cylinder filling "