Engine Questions!

[monmon030791]

1) 11:1 Compression ratio is that able to run on 91 octaine?
2) When you build an engine what was the breaking it in steps. Like when to change oil, re torque etc...?

Thanks!

[deuce_454]

depends... aluminum heads allow more compression, combustion chamber shape makes a diference... cam makes all the difference, its the DCR that determines what octane you need....

11:1 on an ls engine... yes..
11:1 on a mild cam, ironheaded SBC... not so much

[monmon030791]

hydraulic roller cam settup with a 4.80 lift roller rocker arms, i havnt decided what heads i want, for sure not aluminum.

[chevyIIpost]

One major factors effecting compression is the duration of your cam. It will directly effect your DCR(dynamic compression ratio) or accual pumping cylinder preasure not the mathamatically calculated compression ratio of say 11:1. The LS engines are the perfect example of efficient cylinder heads/combustion camber design that allows high compression with a short duration cam. This is a difficult balancing act to duplicate even for the best engine builders with all the knowlege of how each component effects this balance. Then even the vehicle and something like gearing and how the motor is loaded will also play into this package. When you start pushing the max limit of any SYSTEM or combination of parts the weak link or marginal item will show itself quickly. Pushing your compression ratio will require EVERYTHING else to be spot on or problems will show themselves quickly. But isn't that what Performance Hot Rods are basically all about. To take as many of the OEM compromises out of your car and improve it to meet your specific needs and not Joe Commuters.

You mentioned "not Aluminum"? The thermal properties of Aluminum heads will make the engine more tolerant of higher compresion by pulling heat out of the combustion chamber during daily driving or part throttle operation. This is a good thing if you are pushing your compression. The very basic rule of thumb is to reduce your mechanical or calculated compression a 1/2 point when running cast iron heads.

[monmon030791]

acctually im thinking of doing alluminum would that be good? i really didnt understand what you were saying in the last post.

[chevyIIpost]

If what I wrote was unclear or hard to understand I am sorry. There are many good books on engine dynamics and I would suggest reading and understanding this topic before you start building a high compression motor for street use. It is a delicate balance of componants and tolerances that need to be correctly selected inorder to obtain a engine package that is drivable and will not destroy itself due to detonation.

Mechanical compression is calculated from the relationship of total cylinder volumn from bottom dead center compared to the volumn remaining with the piston at top dead center. DCR is the actual functioning cylinder presure that will effect what octane will be required to keep the engine out of detonation. The duration of the cam effects this cylinder preasure because the valves are not closed as the piston starts rising during the compression stroke. The amount of time that the valves are open is directly controled by the cam shaft and it's duration. A higher duration cam holds the valves open longer which can reduce cylinder pumping preasure. This preasure can be directly read with a compression gauge and compared from one cam to the next. Higher cylinder preasure is good up to the point that the air fuel mixture detonates prematurely. Not only is the detonation happening early while the piston is trying to travel up but it is also a violent explosion that produces a very sharp preasure wave. This presure wave is damaging to numerous engine componants, especially pistons and bearings.

Compression ratio and DCR are only the basic items that effect an engines detonation tolerance. The design and arrangement of the combustion chamber, quench area, spark plug type and location, piston dome design, connecting rod length, cylinder head material, etc, etc.

It takes lots of reading and learning then even more trial and error to find that edge for a high compression street motor. GM's LS motor does a very very good job at this and is probably the best place to start and finish when trying to accomplish this task. Most of the design work is already done. All that is left is finding the perfect cam shaft for you vehicle package and presonal driving needs.