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Compression

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Compression

Post by mrbeige on Tue Nov 24, 2009 2:58 pm

Ok, so we all know that if you stick a turbot on an engine, you need to lower the compression. My question is....why do you need to lower the compression? You can fit a Vortech charger to a VR without lowering compression, so why not a turbot?

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Re: Compression

Post by dirtytorque on Tue Nov 24, 2009 3:21 pm

dunno..depends on boost levels I guess.One thing to consider is that Turbo's are less progressive by their nature....
I guess it's not just the amount of pressure generated but the change in cylinder pressure in relation to time that breaks things...
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Re: Compression

Post by mrbeige on Wed Nov 25, 2009 1:52 am

Yeah, I was thinking that it might be that turbos come 'on-boost' in a fairly aggressive manner.

So, technically why do we need to lower compression? If you didn't, what would you expect to happen? Would you get detonation, as the air/fuel mixture would auto-ignite due to the extreme pressure and thus heat generated on the compression stroke?

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Re: Compression

Post by kevhaywire on Wed Nov 25, 2009 2:47 am

Yep, heat is the main reason.

Turbo engines run very rich, not because that is the best Air fuel ratio for power, but purely to suppress knock. Very rich mixtures aid in-cylinder cooling (where it's needed most) and therefore reduces the chance of knock. Low compression also reduces the chance of knock as it prevents the air being compressed too much and exploding uncontrollably, as you said.

Well all that /\ is now a legacy of the 80s. You may have read in various magazines that 10:1 compression is now common place with modern turbo engines when using direct injection. The fuel being injected right into the cylinder acts as a coolant and it works well because it enables the TT RS's 5 pot turbo lump to run 1.2 bar boost with a 10:1 static CR!. And not only that, it maintains 14.7 AFR on boost aswell - until EGTs hit 850 deg and then more fuel is added.

You'd never be able to do that on a port injected engine like the VR6. The fuel would be too hot by the time it reaches the rear cylinders and common to all port injected engines, a lot of the fuel leaving the injector tip just gets blasted onto the port walls.

Speaking of the TT RS engine, Evo made it their "Engine of the year" in last month's eCoty special. I reckon it will make a damn fine conversion engine as it fits transversely no worries.

They managed that by making the engine very under square (bore much smaller than stroke) so that they could squeeze the bores closer to each other, resulting in a very compact 5 pot engine. It's made of vermicular graphite cast iron, which is very strong, so wall thickness isn't an issue.

So, roll on pranged TT RSs Twisted Evil

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Re: Compression

Post by dirtytorque on Wed Nov 25, 2009 3:45 am

When I was reasearching and umming and rring about compression I saw the trend in mordern engines to run more compression under boost.

I saw the audi TT figures of 10:1,although this is fsi as said.

I notice The 1.8T's run compression levels that before would have been considered as aggresive to say the least.
Even the audi s2,a car from 1992 runs 9.2:1 i think at 1 bar in a 4 valve/cylinder setup.

That car would have been at 8.0;1 in the 80's m'thinks.

As an aside I noticed that most 16vT boys run 9:1 ish on a 16V head @1 bar.
As I knew the glarder would never put those boost levels out I gave very very serious thought to running a stock 16v 9a engine.(10:1).
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Re: Compression

Post by JNLRacing on Mon May 17, 2010 2:11 am

Basically ramming more boost into your engine is the equivalent of increasing compression. So the more boost the lower the compression. However there is a trade off.

The lower the compression the more sluggish the throttle response, however at the top end you can throw more ignition timing into the map for more top end power without the fear of det.

The higher the compression the better the throttle response but the less ignition timing you can run for top end power. So it all comes to a trade of as to how the car is going to be used.

I'm running 2.2Bar at 8.7:1 when racing but I had to buy £150 worth of spark plugs to cop with the insane cylinder pressures so it wouldn't missfire.

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Re: Compression

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