# lowering a N.a compression



## mattysupra (Oct 31, 2008)

how do you lower the compression of a engine? If i has a N.A can i lower the compression to run turbos?


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## Boosted (Jun 5, 2008)

Couple of ways. Thicker head gasket, or different pistons.


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## mattysupra (Oct 31, 2008)

Boosted said:


> Couple of ways. Thicker head gasket, or different pistons.


Thanks boosted, can you explain how a thicker head gasket allows me to lower the compression on a N.A engine please. And how do pistons help lower compression.


Acn i simply use a thicker headgasket and leave pistons standard and then run maybe 1.5 bar boost through a N.A engine? Maybe the ringlands would fail if they cant take it?


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## Boosted (Jun 5, 2008)

A thicker head gasket will raise the height of the head slightly, enlarging the combustion chamber. So there will be less compression because the combustion chamber is bigger and the fuel/air isn't been compressed as hard as it was when it was being compressed into a smaller area.

Different pistons will lower compression because of the crown shape. Dished pistons will be concave crowns, which has the same effect as using a thicker head gasket, ie; it enlarges the combustion chamber. 

As for what your engine will take, I'll be very surprised if if it takes that much boost on standard rods and pistons when it has been turbo'd. It might be worth getting forged pistons for it, or even rods as well. But it all depends on the particular engine and how strong it is


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## mattysupra (Oct 31, 2008)

Boosted said:


> A thicker head gasket will raise the height of the head slightly, enlarging the combustion chamber. So there will be less compression because the combustion chamber is bigger and the fuel/air isn't been compressed as hard as it was when it was being compressed into a smaller area.
> 
> Different pistons will lower compression because of the crown shape. Dished pistons will be concave crowns, which has the same effect as using a thicker head gasket, ie; it enlarges the combustion chamber.
> 
> As for what your engine will take, I'll be very surprised if if it takes that much boost on standard rods and pistons when it has been turbo'd. It might be worth getting forged pistons for it, or even rods as well. But it all depends on the particular engine and how strong it is



I dont understand! If i make the cylinder size larger i would get more cc would i not? 

Or is it the fact i have increased the size of the cylinder but the piston is still the same size? 


By the way im talking about fitting a set of turbos to a V8 Rs4 Audi. There 414 bhp as standard, i wonder how much power they would make with a set of 2530's , Hks fcon pro, injectors, 'headgasket' and a good map at maybe 1.4 bar?


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## Boosted (Jun 5, 2008)

No, the capacity stays the same, you are not affecting the stroke or the bore. All you are doing is enlarging the combustion area.



I can't say for sure. The engine might be ok, or it might break. On standard internals you might have to run less boost though, but in all honesty it might be worth asking Mark or Garth tomorrow. I don't know how tough those Audi lumps are


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## M SKinner (Feb 19, 2007)

decompression plates are another option. Bit like a thicker head gasket but a solid alloy plate about 2mm thick. Not the ideal way to do things though as it can blow or increase the chance of det compared to running low comp pistons.


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## TAZZMAXX (Mar 9, 2010)

mattysupra said:


> By the way im talking about fitting a set of turbos to a V8 Rs4 Audi. There 414 bhp as standard, i wonder how much power they would make with a set of 2530's , Hks fcon pro, injectors, 'headgasket' and a good map at maybe 1.4 bar?


Might pay to do a bit of digging about the Gumpert Apollo engine setup. They run at 600-800BHP and are the twin turbo 4.2 V8

Weren't the early RS6 twin turbo V8 before they went to V10? It may possibly be cheaper to get some of the parts or complete engine out of one.

If it were me, I think I'd be tempted to get another engine complete with turbos, swap with what you've got now and then sell the N/A engine.

Hope this helps.


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## GT-R Glenn (Nov 9, 2002)

Matt
Put simplistically
Its compression "ratio" which basically means 
(A) the total area above the piston including the cylinder head bowl volumn, when the engines at the top of its stroke 
(B) the total volumn in the cylinder when the piston is at the bottom of its stroke to the top of its stroke.(usually the engine capacity divided by the # cylinders)

= ratio


Therefore
If 10 x the amount of the above volumn A is the same as the area in the engines stroke (B) the engine has a 10:1 compression ratio

Or you divide the area in the head (A) into the cylinders capacity (B)

A thicker headgasket increases the volumn above the piston at TDC (top dead centre / the top of its stroke) so the ratio drops ....

Lets pretend RB26 = 2600 cc 
Each lung actally has 433 cc (in its actual moving stroke)
Lets pretend theres 43 cc in the area above the piston including the bowl in the head (combustion chamber)

so 433cc/43cc = 10 ....so the cr is 10:1 (10 cylinder volumns fits into 1 cylinder volumn)
If you lift the head ( a thicker headgasket) that will increase the head volumn by spacing the head furthur away from the top of the piston (so that area will be bigger)
Lets pretend its now 53 cc
So 433cc / 53cc = 8.1:1 RATIO ...

Clear ?

Theres at least 3 ways to lower compression
Modify the combustion chamber 
Pistons which either have a larger bowl in the top OR shorter distance from the wrist pin to the crown
A thicker headgasket

OR a spacer plate


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## Marlon88 (Sep 20, 2008)

If it is a V8 you can forget about thicker headgaskets or plates. You will run into problems with the intake manifold fitting. You will lose the geometry of both heads and the intake manifold wont seal properly.

Do you know the compression ratio of the RS4 V8?


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## mattysupra (Oct 31, 2008)

Marlon88 said:


> If it is a V8 you can forget about thicker headgaskets or plates. You will run into problems with the intake manifold fitting. You will lose the geometry of both heads and the intake manifold wont seal properly.
> 
> Do you know the compression ratio of the RS4 V8?


Think the compression ratio is 12.5:1


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## Boosted (Jun 5, 2008)

Just buy a turbo car Matty


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## mattysupra (Oct 31, 2008)

Boosted said:


> Just buy a turbo car Matty




I used to have a RS6 that was V8 twin turbo. But i did not like the 4 wheel drive split on it (not enough power to th rear wheels) 

I got it upto 512 bhp with 513 ftlb of torque but struggled for more power as there was not enough room for bigger turbos (or so the tuners led me to belive, now i would say that they was full of shit and scared to try something other than a remap) 

Also i hated the paddle crappy flappy gearbox and would like a manual. 


So that points me towards the RS4 that is lighter with manual gearbox, better 4x4 split and make a nice noise. Only problem is that its only 414 bhp and that my friends is not enough!


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## mattysupra (Oct 31, 2008)

thanks glen for all that info :bowdown1:


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## Boosted (Jun 5, 2008)

mattysupra said:


> I used to have a RS6 that was V8 twin turbo. But i did not like the 4 wheel drive split on it (not enough power to th rear wheels)
> 
> I got it upto 512 bhp with 513 ftlb of torque but struggled for more power as there was not enough room for bigger turbos (or so the tuners led me to belive, now i would say that they was full of shit and scared to try something other than a remap)
> 
> ...




Furry muff :thumbsup:


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## Bucky (Mar 21, 2003)

When I turbo'd my citroen vts engine the first time I used 2 head gaskets. They were multi layer steel gaskets (mls) and they were fine upto 1.2 bar. Never leaked IR lost compression. Rest of the internal's were standard. 
Later i used low comp forged pistons. 

1bar will be enough boost, trust me once you boost an Na engine it's unreal. 
The cost of doing it is massive aswell, manifolds, turbos, sump mods, intercoolers, piping, clutch ect


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## bigkev (Mar 2, 2008)

its also not uncommon to remove the 'squish pad' area of the bowl. this will of course also lower compression, however i am unsure as to how this effects hot spots etc.

k


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## M SKinner (Feb 19, 2007)

Marlon88 said:


> If it is a V8 you can forget about thicker headgaskets or plates. You will run into problems with the intake manifold fitting. You will lose the geometry of both heads and the intake manifold wont seal properly.


Can always use 2 seperate plenums or modify the stock setup!


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## GT-R Glenn (Nov 9, 2002)

How do you know theres a squish area to remove on an Audi v8 ?
The first thing you need to know is what is the comp ratio now ?
Do you even need to lower it ?
Theres lots of examples of turbo engines with slightly higher than what is desirable compression ...
It will limit the amout of boost you can run ...
Theres plenty of chev engines with hairdryers still running mid 9:1 comp ...
You can turbo a factory GSXR1300R ... they have a fairly high factory comp ratio ...
Just cant run more than 8lb boost....on pump gas


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## mrjolly (Aug 1, 2009)

or lower your sights and aim for 0.4-0.5 bar of boost and run std compression. i would even consider a supercharger as there is far less plumbing and heat generated, probably the easier way to go really. 
1.2 bar through a 400+ bhp car to start with and you should be aiming for a 800bhp+ monster and cr will be the least of your worries. 
regards sid.


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## mattysupra (Oct 31, 2008)

GT-R Glenn said:


> The first thing you need to know is what is the comp ratio now ?



compression ratio is 12.5:1


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## bigkev (Mar 2, 2008)

GT-R Glenn said:


> How do you know theres a squish area to remove on an Audi v8 ?


not sure if this question was aimed at me glenn, but i wasnt refering to the audi v8 in specific. the thread question was about lowering Na compresssion. i was simply giving an example of a way this can be done. not audi v8 specific.

cheers

kev


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## GT-R Glenn (Nov 9, 2002)

It was directed at you, I read the title too, and thought RB, but there were 11 post's refering to the Audi engine prior to your post.
But you are right, its a good way to lower the RB.
12.5:1 is getting up there a bit ...
Its either run it on CNG or lower the comp I think ...


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## mattysupra (Oct 31, 2008)

whats 'CNG' glen?


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## TAZZMAXX (Mar 9, 2010)

Compressed Natural Gas


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## Huskyman (Feb 9, 2010)

Here's another option... In the states they do a turbo conversion for the Porsche Cayman where they leave the internals alone but fit a large light pressure turbo.
This could be done, but IIRC space is pretty tight in the RS4 engine bay, but you could always fit remote turbos, where they are mounted at the rear of the car where the silencers go. Seen this done on a couple of M5's.


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