boost Vs Compression (long read)

Compression Ratio

As the name indicates, the compression ration of an engine indicates how much air-fuel charge is compressed during the compression stroke of the four-stroke process. A 10 to 1 ratio means that the air-fuel mixture gets squeezed down from the full volume of the cylinder to a volume that is just roughly one tenth of the cylinders size. So how does an engine's compression ratio affect performance? all other factors being equal, an engine with a higher compression ratio will deliver a higher thermal efficiency. This means that the engine is able to turn more of the heat generated from the combustion process to horsepower instead of wasted heat. In basic terms, higher thermal efficiencies translate into additional horsepower and better millage.

How much additional power can be expected with a raise in compression ratio? The old-school rule of thumb is that each additional point that the compression ratio is raised will deliver an additional 4% in Power.

Boost Pressure

In dealing with naturally-aspirated applications, high compression ratios are the key to serious power levels. In dealing with Forced-induction applications, it's well known that increasing the boost pressures of a properly sized turbocharger will increase power production ( at least, to the point when the capacity of the turbo or fuel system is exceeded). Of course the big downside to high boost pressure is that the likelihood of encountering engine-damaging detonation also increases.

the balance of Boost Vs Compression ratio has been an engine builder's and tuner's challenge for Years. Picking up a copy of one of those 60's technology forced-induction manuals will highlight their solution. The higher the boost pressure the lower the compression ratio of the engine. For "serious" race forced-induction setups compression ratios of 7.0:1 were not common

Fortunately , poor manifold and fuel delivery designs, as well as low-efficiency "blowers," are not found on too many of today's popular performance vehicles. Today, the average high-performance street or strip turbocharged four-cylinder race engine sports a compression ratio of 9.5:1 with some even running compression ratio's of 11.5:1. Modern technology allows our race generation to get the best of both worlds/ High boost pressure with High Compression ratios.

Fuel and Detonation
Octane & Knock

the octane rating indicates the likelihood of the fuel to experience "knock." Knock the audible sound given to the condition. also goes by the name of detonation or autoignition. Whatever you call it, the important thing to remember is that knock is dertimental to performance and reliability and it needs to be avoided.

Knock occurs when the fuel-air mixture in the cylinder doesn't experience an ideal burn. An Ideal burn allows the mixture to combust evenly initiating from the spark plug until all the air-fuel mixture occurs. In a laboratory environment, the ideal burn will occur at about 100ft/second in a vacuum. In the turbulence of an engine's combustion chamber, good flame speeds may be up to 250ft/second. During detonation or knock the burn will see a violent 500ft/second explosion instead of a burn. Burn rates are crucial to how pressure builds in the cylinder.

The burning of the air-fuel mixture results in a pressure increase. Ideally pressure builds in the cylinder at the optimum time reaching peak pressure somewhere between 17 to 29 degrees after top dead center. This allows the cylinder pressure to produce the most horsepower at the crank. The pressure cycle within the cylinder doesn't occur as desired. in fact when knock occurs , the original flame front and pressure wave from the desired spark-ignited front meet an undesired auto-ignited flame front. When these 2 pressures meet, the pressure oscillations produce "knocking" sound. When knock occurs power is reduced, while rod bearings, connecting rods, head gaskets and pistons may suffer slight damage or catastrophic failure depending on the severity of the knock. Elevated temperatures generally result from knock and this can lead to pre-ignition problems that cause air-fuel mixture to ignite even before the spark fires.
Knock or detonation is not the same as pre-ignition. Pre-ignition occurs when the air-fuel mixture becomes ignited before the spark plug fires. Sometimes elevated temperatures or a hot spot in the cylinder can cause pre-ignition. While both knock and pre-ignition cause undesired burns of the air-fuel mixture, the difference between the 2 are simple. Knock or detonation occurs after the air-fuel mixture has started it's burn, pre-igintion occurs before . Both produce undesirable performance and can translate into engine damage.

Need for Higher Octane

If you engine is experiencing knock you'll need to run a higher fuel or retard ignition timing. The need for fuels with a higher octane rating generally occurs as peak cylinder pressures rise. Peak cylinder pressures tend to rise as compression ratio, volumetric efficiency, ignition advance and boost pressure rise.

The general rules are simple naturally-aspirated engines will need a higher-octane fuel as either compression ratio is increased or ignition timing is advanced. Forced induction engines are the same. but will also need a higher octane as boost pressure increase.

You may have heard the following: "don't use to high of an octane fuel or you will lose power." This is a half truth. Having a fuel with too high of an octane will not make your engine lose power. However, having a fuel with to high of an octane will not make your engine lose power. However, having a fuel with a burn rate that is too slow can make your engine lose power. In general, the popular components used to make the octane of a higher also slow the burn rate. Of course, that is just a generality and it doesn't hold true for all fuels.

As an example, VP racing fuels worked with Scranton Brothers and their turbocharged Pro Class Celica throughout the 2002 season to develop a new fuel blend. This blend was designed to provide 120+ motor octane with the quickest burn rate possible. The result was the VP Import blend which produced six percent more power in the Celica than VP Racing C16.

decent read if you have time,
But where did you find it?

article that i wrote about 2-3 years ago i done alot of reading back then on boost alot of what you read is where i get my information. the example was from a magazine.

So you can read and copy and paste?? im pretty sure if you could write like that you wouldnt be driving an accent

alexward wrote:

So you can read and copy and paste?? im pretty sure if you could write like that you wouldnt be driving an accent

Writing has nothing to do with what you drive. I could write that if i took the time..

Who are you anyway (accent)

caleb

red accent?? yellow fogs??/ at all the shows???

alexward wrote:

red accent?? yellow fogs??/ at all the shows???

its more of a copper...but yah i think that'd be him...

I do beleive that he does have the capability of writing that article...

never meet him but him trying to tell me bout my nissan was kinda pissing me off .....anyway sry for jaking the thread

Well the color is called tango red and i have been dealing with boost and all that for a couple years now, and i am more then capable of writing that and if you want to burn shit on what people write then don't bother to post thanks alex

kthnxbye

but yes that is my car and the fogs were not yellow till after all the shows last year just for the record feel free to flag me over to have a chat sometime hotdog

will do Mang.

ok so did u write the article or read it in a magazine???
youve benn dealing with boost for awhile now....in what?
i had nothing against you til you started telling me that i didn't know what i was talking about. i know more about cars then 90% of the people on this site. not trying to brag its just a fact.

actually alot of people on this site do know alot about cars so you take 54 people and you say you know more then 90 % of that.... so that makes you smarter about cars then everyone registered at this time besides 1 person if that... seriously alex you want facts you will get them and the article that i read was about 10 pages long so if you think that it was copy and paste good luck as it came out of a magazine.

Caleb, didn't you know that once you buy a turbocharged car, you automatically know everything there is to know about all internal combustion engines???

Barricade wrote:

Caleb, didn't you know that once you buy a turbocharged car, you automatically know everything there is to know about all internal combustion engines???

It musta slipped his mind....

actaully all i do is read i dont do much else so up yours. if i had the money to pay someone to do all the thinking i wouldnt have to read lol but i dont so i do know what im talking bou i dont know everything but i do know wat my z31 is cappable of

-hotdog wrote:

Barricade wrote:

Caleb, didn't you know that once you buy a turbocharged car, you automatically know everything there is to know about all internal combustion engines???

It musta slipped his mind....

I forgot it was a fact...

PS any mod that reads this can we make a section called FYI just for some good technical reads I will feel glad to post in there what articles i have written and find that others have written that may be of benefit.

You can read?