high comp/turbo ?

[number77]

i can't figure this out, i know how both work, but i still don't understand what the difference is in having a high compression engine or a turbo charge engine, both cause the air to be compressed and to me it seems like a high comp build should make just as much power as a turbo, can anyone explain this to me?

[zturbo]

A turbo/supercharged motor can generate more hp because it forces more air into the cyl. With a NA motor you can only use the engines pieces to generate vacume to suck air into the motor.

Steven

[TimbrSS]

A motor is a big air pump, the more air you can put thru it, the more power you're gonna make.

The N/A motor can only suck in so much air on it's own. A Turbo'd/Supercharger will force more air into the cylinder than would naturaly be sucked in.

Static compression ratio doesn't have nearly as much effect on horsepower as the amount of air going thru the motor. For example:

LT1 300 horse na motor on 10.5:1

Option 1. Lets assume we had as much boost as we can to this motor, without detonation- 6#. We now have 400 hp.

Option 2. Lets put in new pistons to lower the compression ratio to 9:1. In NA form the motor would be down to 275hp.
Lets assume we had as much boost as we can to this motor, without detonation- 12#. We now have 475 hp.

These numbers aren't accurate, but give you an idea of what happens when you mess around with compression and boost.


Here's a decent "Compression 101" if you would like to know more.

http://www.hondalife.com/articles/compressionratios.htm

Heck, I'll even quote the last part which kinda answers your question, but read it all for more understanding.

For maximum power output you want to make the biggest fire possible in the combustion chamber, and to do that you need to get as much air and fuel into it as possible. And that's where increasing VE comes in, the more you pack into the cylinder, the more power you will naturally make. This is why increasing VE through turbocharging and supercharging is usually more effective at increasing engine power output than simply by increasing the static CR on an NA motor, even with the idea that for best power you need to lower static CR in an FI motor. The point is this... for more efficiency, push the fuel closer to it's limits, for more power, put more fuel and air into the chamber. And for the absolute maximum power output for any engine setup, maximize both. Hope this helps explain things, peace

[89ProchargedROC]

also realize that timbrss is giving you examples with pump gas....you can keep high compression and high boost...you just need the octane and the computer management to handle it

[number77]

sorry, maybe i wasn't specific enough. what it was is that i got to think that if you have a turbo powered engine with a low compression (like 6.5 or something), and you take a NA engine and give it an extremely high dynamic comp ratio (like 25:1; something unrealisticly high), then shouldn't the NA and turbo motor make about the same hp/tq asuming that the NA engine is able to get the entire cylinder filled with air/fuel

[TimbrSS]

Originally posted by number77
sorry, maybe i wasn't specific enough. what it was is that i got to think that if you have a turbo powered engine with a low compression (like 6.5 or something), and you take a NA engine and give it an extremely high dynamic comp ratio (like 25:1; something unrealisticly high), then shouldn't the NA and turbo motor make about the same hp/tq asuming that the NA engine is able to get the entire cylinder filled with air/fuel

Have you been huffin gas lately?

[97SS ragtop #077]

LMAO!!!!

[Brandy]

If you can get the same cylinder pressure and timing in the same size cylinder then yes. Unfortunately though there is alot less room for the a/f mixture to burn in an extremely high compression motor (domed pistons and milled heads vs dished pistons and hogged out combustion chambers) so there is no possible way to do what you are asking.

Another factor is the flame travel. A low compression boosted motor can get pretty close to a hemispherical combustion area so the cylinder pressure and heat are applied all over the area. In high compression motors there are always inherent hot spots from the valve reliefs in the pistons and the spark plug notch, so they are prone to detonation.

[rskrause]

Some of the ideas expressed have been a bit off target. Think of it this way. The maximum power you can get is limited by the mass of air in the cylinders. Raising CR does not change this, all it does is increase the efficiency of the combustion process. And the effect is not as pronounced as you might think. Within the range we are talking about there is a couple of percent or less increase in power output for each point the CR is increased. Ultimately, max CR is limited by the octane of the fuel used. An additional practical problem is achieving an efficient combustion chamber shape with a high CR.

Lower CR combined with more boost produces more power compared to higher CR with less boost, with the product of boost and compression limited again by octane. IOW, more boost/less compression as compared to low boost/high compression makes more power because of the larger mass of air and fuel. The cylinder pressure prior to ignition will be the same in both setups if they are optimized (limited by octane) but the high boost setup will be igniting a larger air:fuel charge.

Rich Krause

[number77]

i understand it better now, the way i was thinking of it as was that a high compression ratio would be similar to a tightly wrapped fire-cracker, and a loosly wrapped firecracker with more powder would be like a an engine with boost, (if the tightness was the compression ration and the powder was the air going into the engine), but now i have realized that the objective is to get a long net force (explosion)to move the piston/crank, instead of a quik burst that isn't as powerful, this stuff just keeps me thinking (which i like)