Cams and Compression

[Guest butlersZ]

Does anyone know if there is a ratio or formula to figure out how high of compression ratio you can have still running pump gas per cam duration? Does that make sense? Not sure if I worded my question right. Anyway, I heard that you can run a higher compression ratio if a cams duration and overlap are enough to bleed off low-speed cylinder pressure, 11-1 11.5-1 , and still run pump gas. Is there any way to figure out how high of comp. ratio you can go with a certain cam given it's duration and LSA? Example: with a cam that has (seat-seat) 292 In, 300Ex, on 108 LSA, what would be an acceptable comp. ratio?

[Kevin Shasteen]

Excellant Question!

 

The answer you are looking for is found in the relationship between Airflow Velocity (Port Pressure) and Cylinder Pressure at a specific rpm range.

 

The rpm range can be broad or narrow depending on the intent of the engine-car combo.

 

The quick answer to your question would be to understand the "Dynamic Compession Ratio" basics yet the DCR alone wont get you the peppy neck snapping engine you desire. You will also need to determine what level of performance you expect/desire/need from your engine-car combo.

 

Different Levels of Performance

1) Pure Street - Supercharged, Turbo, Nitrous

2) Pure Street Engine - Normally Aspirated

3) H/O Street Engine

4) Weekend Warrior

5) Dedicated Racer - Supercharged, Turbo, Nitrous

6) Dedicated Racer - Normally Aspirated

 

Each level listed above comes with its own RPM ranges where peak Tq/Hp will surface as well as its own rate at which the intensity of that desired power is reached within the specific RPM range.

 

So the questions you need to ask and grasp are:

 

1) What is the intention of the engine-car combo

2) What Displacement will I choose

3) What RPM Range is expected with the engine-car combo I have chosen

4) What DCR to SCR relationship gives me the expected RPM range needed

5) What Intake Port Sizing is required to meet the power output expectations within the RPM range my level of performance demands...(this includes the Intake Manifold Runners as well as the Cylnder Head's Intake Ports)

 

Not to toot my own horn but these issues are exactly what I wrote about in my CD-Book I announced a few months ago:

 

htt://hybridz.org/nuke/index.php?name=PNphpBB2&file=viewforum&f=2&&topicdays=0&start=225

 

If this link doesnt work you will find my CD-Book advertisement on the 10th page of the "Non-Tech Forum".

 

If you wish to know what the DCR is and how to match that DCR to your SCR (Static Compression Ratio) then you can check out the book:

 

How To Build Max Performance Chevy Rat Motors: Hot Rodding Big Block Chevys 90's Style

 

By Ed Staffel

Published by Cartech

1996 Copyright

 

Look on page 112-113 for the instructions and page 108 for the Crank Angle Chart their example on page 112-113 utilizes to come up with their answers.

 

To come up with your own Crank Angle Chart you can either make your own charts (with the use of trigonometry) or you can buy my CD-Book, or you can go find Jack Kelly's DCR Calculator.

 

I dont have Jack Kelly's DCR Calculator on hand...,

 

ANYONE ELSE HAVE A REFERENCE TO HIS FREE DCR CALCULATOR; please post it so butlersz can begin playing w/the numbers-thanx.

 

Kevin,

(Yea,Still an Inliner)

[Kevin Shasteen]

sorry, it wasnt "Jack" Kelley...it was Pat Kelley.

 

Here is his free DCR Calculator,

 

http://members.uia.net/pkelley2/DynamicCR.html

 

Remember, you have to match your engine-car combo's intent to the rpm range. This requires matching the DCR to SCR relationship to the Intake Port Volumes: if you mis-match components then your engine will not be as efficient as it should be.

 

This requires understanding the peak powers: Tq peaks first (Cylinder Pressure) with HP peaking secondly (Port Pressure).

 

Understanding DCR is the foundation on which your engine is built. If your DCR to SCR foundation is inefficient so to will the performance of that engine throughout the entire rpm range.

 

Kevin,

(Yea,Still an Inliner)