What are the benefits of reduced compression?

[orsum]

Although I am so close to my S2 to RS2 conversion I cannot get the remark made to me when I was planning the upgrade. I was told the you can reduce compression to compensate for the low octane petrol we get in SA.

I was also advised I could compensate by running a water meth system which I did not do.

If I decide to change the head gasket is fitting two gaskets acceptable?

How much perfformance will be gained by reducing the compression ratio?

Will I have to have the software changed again to suit a lower compression?

orsum

[quattro-nick]

with lower compression you can run safe moor boost pressure if the internals can handle it but this give also a little bit more lag
i woud not take 2 gaskets,risk of leakage!

[PRY4SNO]

Corky Bell wrote an interesting book called Maximum Boost that I highly recommend. Within it, he says, "The two largest influences on compression ratio are boost pressure desired and intercooler efficiency. Fuel octane certainly plays a big part, but we are usually limited to using commercially available pump gasoline."

Later, he goes on to explain that, "A variety of methods exist to change a compression ratio. Almost all are unacceptable. The crux of the matter is upsetting the 'squish volume' around the rim of the chamber. ... Consider 'squish volume' sacred and do not tamper." I couldn't find the spot where he advises against stacking head gaskets but I believe it's in there somewhere.

You can find the volume here if you're unable to come up with a copy of your own:

http://www.fusca.net/wp-content/uploads/Maximum_boost.pdf

Yes, people have had success stacking head gaskets. Is it the proper way to do things? No. Whether you want to do so is a personal choice.


Some interesting reading:

http://turbobygarrett.com/turbobygarrett/compression_ratio_with_boost

Your tune should be based on intake air volume, fuel quantity used and spark timing. In other words, compression ratio won't directly affect your tune.

[PRY4SNO]

PS

Reducing your compression ratio will effectively lower your combustion chamber heat and allow you to add more timing/boost at the cost of lag/throttle response.

[newsh]

On an RS2 tune stay with stock comp ratio. Especially if it's a road car.

[220bhp]

I reduced my compression by using the centre section of one metal gasket inside a complete metal gasket.
I did this for two reasons. I had the head ,and the block faced, which obviously removed a small amount of material, but also to reduce the compression.
It is quite easy to do, just drill out the brass rivets and they fall apart.
I am running rs2 setup with 3" down pipe and exhaust. Wagner evo intercooler, PRJ live mapped.
The car has a fare bit of get up and go, but it is not good at low revs .
Doug at MRC has this head gasket on his coupe, which is around 600bhp, He said his car was hard to drive of boost.

[orsum]

Looks like I have quite a bit of reading to do.

Seeing the responces so far I am concerned.

Just got a responce for the importers of the VR Head Gasket people.

With regard to installing two head gaskets on this engine I would not recommend that this get done under any circumstances. Firstly this is a multi layer steel (mls) head gasket which is coated with a very special elastomer coating which is used for micro sealing between the head and block. By placing two head gasket together you would have a issue with the elastomer coatings not functioning in the way they were designed to work. Besides this your torque settings would be altered and the sealing of the gaskets will fail.

orsum

[Voodoo]

I don't know what the benefits, if any, are on dropping the compression, however, I do know that 034 sells a compression dropping headgasket.

http://store.034motorsport.com/head-gasket-compression-dropping-multi-layer-steel-audi-5-cyl.html

[Error404]

On an RS2 tune stay with stock comp ratio. Especially if it's a road car.

As above, with lower CR you can run more boost and more aggressive timing but both pointless on an otherwise stock RS2 tune. You would need a custom map to hope to see any benefit, which at RS2 power are probably nil anyway. The ECU will adapt for the poorer quality fuel anyway, I guess by pulling timing.

[diesel des]

What octane fuel do you get in sa. A small drop wouldn't be a problem in terms of off boost power but going to 8:1 without a suitable map would reduce low down power. Prj is the guy to ask. He often recommends a drop in cr as at stock cr timing is excessively retarded. Poor fuel will make this much worse.

Sent from my GT-I9195 using Tapatalk

[orsum]

95 Octane.

Prj did my map and he also spoke about a reduced compression. Did not take him up on that at the time, one issue was not being able to get him to do a live map in South Africa.

orsum

[PRY4SNO]

95 RON is equivalent to the 91 (R+M/2) we get here.

You'll be fine with the standard MLS gasket at RS2 power levels. If you want to go beyond that at some point, you may well consider w/m injection and/or the compression dropping MLS head gasket. But still, there are plenty of people who run a mid size turbo on stock blocks with a good tune.