The idea of forced induction is to raise the compression ratio to give more power, correct???
Then why do we have to install low compression pistons?
why not just use lower boost

I know their is a reason but I car'nt figger it out

Please explain...

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The idea of forced induction is to cram MORE fuel and air in there, not just to increase the compression ratio.
The lower the C/R is to start with, the more boost you can apply and the more mumbo you get. After turbo is actually spooled up... below that point the low C/R is detrimental to performance.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

you make more power out of boost than you do out of compression

you dont want a lot of boost with high compression, because the air fuel mix is already compressed quite a bit, then you are trying to compress it again and somethings gotta give.

Docs on the ball...

More air and fuel mean more power, when a standard mini 1275cc revolves it sucks in 1275cc* of air and fuel for each revolution and its from burning this it produces power. Now if we could buy a 2550cc mini engine we would assume even more power as it sucks in more airfuel to burn.

By Turbocharging a 1275 when it revolves we dont let it only suck in 1275cc* we try to push more in. The engine could have 2550cc of air and fuel pushed into it and to do this into the same space of 1275cc we have to double the pressure*. You then let the engine compress it with the piston moving up the bore and then ignite it with the spark plug. Problem is the air fuel will burn correctly upto a specific point of pressure and temperature, after which it just explodes.

So we need to be careful when squashing it with the piston on the compression stroke not to squash it to far or it explodes. Correct burn is what we call a flame front burn and its normal for Internal combustion engines, when it explodes it's called knocking or pinging or detonation and equals bad juju and loss of power.

To get max power from an engine we need the most pressure change (burning fuel and air expand) and the same expansion for a higher pressure means more power. Remember the piston travel and bore diameter are always the same regardless of how much airfuel you jam in*.

In turbocharging (read forced induction) we need to lower the ratio of volume when the chamber is being filled to the squashed volume when the piston is fully up and the air fuel is ready for ignition as we are avoiding explosions while working with more air and fuel in the same small space. So we decompress the small space to give it a little more volume to fit the extra airfuel, but not much.

*volumetric efficiency, ignition timing and a whole lot of tech crap has just been generalised.

thanks I understand a bit better now

This is refered to as turbo lag, correct?

also a s/c dosn't suffer from this (or nowhere near as much) from what I can work out so, what is better turboed or supercharged? or is that a personal choice?

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because turbo's are driven by exhaust, you have to create the exhaust first to spin the turbo to create boost.
you can feel it take a moment from when you plant the accelerator.

superchargers dont suffer from this being driven by the speed of the engine, however superchargers draw power to turn like an alternator, whereas a turbo uses waste energy.

basically turbochargers are more efficient, and better at high rpms.
superchargers are more responsive and better at low rpms

As others have said, the idea is to cram more air/auel mix into the cylinder. The most powerful F1 engines of the 80's had CR's between 6.7-7.5:1. Most street cars are better off higher as the drivabilty of a low comp engine suffers and you get most of the power in an uncontrollable rush. This is fun to drive but doesn't really get you anywhere any faster.

If you get the chance, take the new Mini for a test drive and you'll feel how a good street turbo should, relatively high comp with a small turbine is much bettr than a supercharger.

Turbo lag is best described as the guy following (the one you just beat on the racetrack), they always lag once boost hits!

Daniel

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Videos - Turbocharged A-Series on Dyno & at Wakefield
Mafs waz not my strong subgeckt at skkol

so how would a high comp motor with low boost run

and because your ment to run low comp pistons, why is this the case for everything else, such as big port / small port, cam size, lift and valve sizes,

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http://www.ausmini.com/forums/viewtopic.php?t=44717

more down low and in the middle of the range, stuff all difference up top.

if make the c/r standard and the boost is 1 atmo you have a carbie, as you decreae c/r you can increase boost and the power increase moves up the rev band.

A turbocharger only gives you as much power as the theoretical power limit of the engine. So a really good na setup will give you more power than a poor blown setup.

Best way to feel the effect of a high comp low boosted car is to drive any turbo SAAB they are experts at this and they fixed turbo lag decades ago!

Rally cars and some other aplications tackle lag with tricky use of allowing airfuel unburnt to access the manifold just prior to the turbine side of the turbo. It is then ignited and expands as well as building pressure rapidly. This spins the Turbo very quickly producing Boost from the compressor side of the turbo. Using this technique Lag time between right foot application and max power are reduced.

Some inject fuel direct into the manifold, some cut spark to a cylinder and others just tell me but the result is the same.

The Saab that I used to drive on the racetracks was a good example of how Saab beats turbo lag, they don't! You had to hit the throttle well before the apex of the corner (a heck of a lot earlier that a N/A car) to get it to drive out of the corner. Every lap you would hit the throttle a bit earlier to see what would happen. Eventually you have to resort to braking with the left foot and throttling with the right foot.
I know Saab makes claims about reducing turbo lag but it is all hype (like most advertising claims by car manufacturers)

I am with you on that the 900 Aero TUrbo I had - has awful boost lag - I reckon about 6-7 seconds after planting the throttle the power would start to come one all at once and then it was over, change gears and wait again.

Fun for a while but the throttle lag pissed me off so much I sold it.

The Turbo Citroen Activa I had was far better - little blower with stuff all lag. You would get a nice torquey shove all the way to the redline