REASONS FOR COMPRESSION: PLEASE LOOK

General engine questions/issues that aren't specific to the other categories
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REASONS FOR COMPRESSION: PLEASE LOOK

By kingmorgan
- Wed Jan 14, 2009 2:13 pm #88905 I m just curious if anyone can explain to me the reasons for compression.like i kno low compression is good for boost.but why. and high compression is good for all motor and bottle? right?...im just curious as to why .. and on the compression calculator i pieced together a sohc that had like 13.2 or something.is that possible to run or would it knock or whatever. someone explain.
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By suspendedHatch
- Wed Jan 14, 2009 2:19 pm #88914 You lack a basic understanding of what compression is and how an engine works.

Air is sucked into the combustion chamber, compressed by the piston traveling upward, fuel is injected, the spark ignites the mixture, the piston is pushed downward, and the burned mixture is blown out through the exhaust.

Try "how stuff works" or wiki.

By kingmorgan
- Wed Jan 14, 2009 2:52 pm #88926 ive read that but, when the compressino gets so high whats start happenin? spark knock? or what
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By suspendedHatch
- Thu Jan 15, 2009 4:49 am #89157 Well we generally think of compression as static. In other words, it doesn't change. It's expressed in the specs as a ratio, ie 9.6:1. The only way to change that is to change to pistons that invade more of the combustion chamber, or shave the head to bring the combustion chamber down closer to the piston (not a good idea).

If you increase the compression of the engine by changing the pistons, you increase it's power and fuel efficiency. But you also run the risk that the fuel will ignite before the spark plug fires. That's how you blow your engine. To combat this, you use fuel that is harder to ignite. In other words, higher octane. Generally you want to reduce the ignition timing advance. Careful tuning is necessary.

People generally only increase the combustion ratio on naturally aspirated engines. Forced induction, ie turbo or supercharger, dramatically increases air intake temps which makes the engine more prone to knock. So you can see that both together is not a very safe combination. However, with proper, careful tuning, a big intercooler, and high octane fuel, you can unleash a world of hurt onto the pavement.

Anyways I was trying not to write this novel. But this could go on and on.
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By 95projektvic
- Thu Jan 15, 2009 5:06 am #89163 to answer the spark knock thing, correct me if i'm wrong but that generally happens when u use high octane on a low compression engine
like 91 on 8.1:1
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By teal_dx
- Thu Jan 15, 2009 2:19 pm #89219
95projektvic wrote:to answer the spark knock thing, correct me if i'm wrong but that generally happens when u use high octane on a low compression engine
like 91 on 8.1:1



opposite actually, using low octane fuel on a high compression motor will knock.
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By 95projektvic
- Thu Jan 15, 2009 6:25 pm #89285 ok so it doesn't do that both ways. cuz i'm going to bring that up in class.
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By zeppelin101
- Thu Jan 15, 2009 6:33 pm #89289 High Octane and low compression is the staple diet of big power turbo cars ;)
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By suspendedHatch
- Fri Jan 16, 2009 3:01 am #89377 Knock is preignition. Preignition means before the spark. You really don't want the mixture to ignite while the piston is traveling upwards in the bore; pushing down on it. Best case scenario: you lose power. Worst case scenario, the piston can't come up so it finds somewhere else to go, like out of the side of your block.

The lower the octane, the easier the fuel ignites. It can actually ignite from just pressure alone. Or it can ignite from the heat, or from a particular hot spot. Anything other than the spark and it's bad news.

Detonation is similar to knock except that it's parts of the mixture igniting at the same time as the spark. The multiple flame fronts collide wreaking all kinds of havoc. Nasty stuff.

Endyn Roller Wave pistons are pretty interesting. They shove the mixture toward the hot exhaust side enabling you to run very high compression w/out the associated problems.
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By 95projektvic
- Fri Jan 16, 2009 4:55 am #89391
suspendedHatch wrote:Knock is preignition. Preignition means before the spark. You really don't want the mixture to ignite while the piston is traveling upwards in the bore; pushing down on it. Best case scenario: you lose power. Worst case scenario, the piston can't come up so it finds somewhere else to go, like out of the side of your block.

The lower the octane, the easier the fuel ignites. It can actually ignite from just pressure alone. Or it can ignite from the heat, or from a particular hot spot. Anything other than the spark and it's bad news.

Detonation is similar to knock except that it's parts of the mixture igniting at the same time as the spark. The multiple flame fronts collide wreaking all kinds of havoc. Nasty stuff.

Endyn Roller Wave pistons are pretty interesting. They shove the mixture toward the hot exhaust side enabling you to run very high compression w/out the associated problems.


explains why desiel can pretty much start up with bad glowplugs
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By d-mon
- Fri Jan 16, 2009 3:52 pm #89474 yes because diesel fuel requires lots of pressure to burn. that why diesel engines have very high compression. glowplugs, unlike spark plugs don't 'fire'. they are on all the time (at least while the engine is running).
maybe you have been in a parking lot when some pos domestic sedan parks nearby. the driver turns off the engine and gets out of the car but the motor continues to run very badly for a minute or so. this called 'dieseling'. the spark plugs have long since stoped fireing but the fuel in the cylinders continues to combust from the residual heat.[quote
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By boosted94
- Tue Feb 03, 2009 4:44 pm #93975 High compression = high presures in the combustion chamber, no room for boost.
Low compression = lower unboosted presures in the combustion chamber = room for lots of boost.

9:1 ~ 130 psi n/a in combustion chamber.
9:1 + 10psi boost ~ 220psi in combustion chamber.

to run 130psi in CC at 10boost psi you would need roughly 5.2:1 compressiuon and gain no power.
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By hondajunkee
- Tue Feb 03, 2009 5:49 pm #93982 so a VX head on B7 block wouldnt be a good combo? there is a guy selling one here with a EX tranny for 200 and i thought about picking it up but my friend said it would run around 13:1 compression and that it would be a bad motor.
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By boosted94
- Tue Feb 03, 2009 10:06 pm #94017 that would make for a wiked n/a motor, as long as it was capable of handling the presures.. you would also need high octane fuel. so would not be practical for a DD. Secondly i could not imagin it producing that kind of compression ratio. I have heard, not saying i know for sure, "a d15z1 head on a d16z6 will show around 11:1. much more reeasonable.
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