Headgaskets, why they affect CR?

[geno8r]

Hi

Since joining the forums i've learnt a huge amount of information about cars & engines but need some extra information to which i havent found answers for.

I've read about people using difference headgaskets 2.0mm, 1.6, 1.2. When talking about these they refer to Compression Ratio.

I'd just like to know what is compression ratio & why do the headgaskets affect it, all i know about headgaskets is they sit at the top of the engine block just above the end of the pistons i assume. Do different pistons affect CR too?

Anyone who would like to help your info would be much appreciated.

Cheers,

PS: Since getting so into skylines i've fell in love with them & engines really interest me now, I'm thinkin i might study to get into it one day, but for the moment i dont want to leave my IT job since its easy good money, plus i enjoy it. Working at a performacne shop building up engines sounds pretty good tho.

[siksII]

Compresson ratio is related to the combustion space in the whole cylinder when the piston is at the bottom of its travel compared to when it is at its highest point, so when the piston is at the top of its stroke it sits inline with the top of the block.

there is the headgasket between the block and head, which raises the head and therefore increases the space of the combustion chamber. (compared to if there was not head gasket)

So if you use a thinner than standard headgasket you have decreased the amount of space in the combustion chamber, which will increase the compression ratio, because the piston is going up in the same space but being compressed into a smaller compression chamber.

Therefore increasing the compression ratio... if you know what i mean

[GTR-Ben]

Not only that it will mess around with the engines "Squish".. so may have negative effects..

[geno8r]

Not only that it will mess around with the engines "Squish".. so may have negative effects..

Go on then... the effects

bringing down the CR sounds good as in a tighter more powerful bang?

Start to get technical on me... owww the difference in 1.2mm & 2mm doesnt sound like much too!

[GTR-Ben]

I'm no expert on squish, so won't go into it to much. Best to go and read up on it, do a search.

It's basically the shape made by the crown of the piston and the compression chamber in the head. This effects the way the flame front happens.. which can directly effect how likely your engine is to detonate and how effective the "explosion" is

Sometimes you may find that by just adding a thicker gasket to lower compression, you actually get worse detonation even though this is what you were trying to combat.

To do it properly head work must be done.. as I find quite a few GTR's with a larger chamber.. although i'm still not sure about how it's done..

[BBQ]

From what I know about Cr and Head Gaskets, Thinner the gasket, eg 1.2 mm will give you better response and drivability, but not able to run as much boost and compared to a gasket that is 1.8mm thick, where you would be able to run more boost but response won't be quiet as good.

But the advantage of a larger gasket is it will keep the cylinders cooler then what a thin one would, reducing detonation. But rembering the only reason most people go an after market head gasket is if your doing a piston replacement, changing the bore size using Forged Oversized pistons to, eg 87mm, 88mm which a standard gasket won't fit. so thats how you get most of your benifits, out of an aftermarket gasket is increasing your cylinder capacity.

Other wise its not worth it.

[Remix-]

What are some recommended and proven head gasket sizes for lowering the compression of an RB25DET without results to drastic?

[BBQ]

What are some recommended and proven head gasket sizes for lowering the compression of an RB25DET without results to drastic?

1.2mm, 87mm bore, Metal head gasket. from my experience.

[nmcmahon83]

By lowering compression I assume you mean raising compression ratio??

[siksII]

no not correct..

when the thicker the headgasket, the larger the combustion chamber is so the lower the compression ratio is..

the larger the bore is (say if it is 87.5 instead of 87) the higher the compression would be with the same thickness gasket, as there is more volume in the bore but the combustion chamber is the same.

[siksII]

From what I know about Cr and Head Gaskets, Thinner the gasket, eg 1.2 mm will give you better response and drivability, but not able to run as much boost and compared to a gasket that is 1.8mm thick, where you would be able to run more boost but response won't be quiet as good.  

But the advantage of a larger gasket is it will keep the cylinders cooler then what a thin one would, reducing detonation. But rembering the only reason most people go an after market head gasket is if your doing a piston replacement, changing the bore size using Forged Oversized pistons to, eg 87mm, 88mm which a standard gasket won't fit. so thats how you get most of your benifits, out of an aftermarket gasket is increasing your cylinder capacity.  

Other wise its not worth it.

You can still use a standard head gasket if you have an oversize bore, that is not the reason people use metal gaskets.

usually rb head gaskets are not the source of problems so most people with oversize bore/pretty modified motors use the standard gasket.

[blind_elk]

By lowering compression I assume you mean raising compression ratio??

In discussions such as this, "compression" and "compression ratio" are the same thing.

The hole in the head gasket effectively becomes an extension of the combustion chamber. So, the thicker the head gasket, the larger the combustion chamber volume. And since

C/R = (cylinder volume + combustion chamber ) / combustion chamber

then a larger combustion chamber (for a given cylinder volume) LOWERS the C/R.

The reason for running the thick head gaskets is to reduce the C/R where a high boost level is being run. This brings into play the difference between "static C/R" and "Dynamic C/R". "Static C/R" is what you measure with a compression meter. It is basically the measure described above. "Dynamic C/R" is a result of ramming additional air (compressed by the turbo) into the cylinder.

Hope I haven't got too technical for you.

[geno8r]

Ok thats good info. So if you were wanting to make some large hp out of your rb25det, say 500 bhp+ you might want to do pistons & headgaskets just to make it easier on the engine, you could then cope for much larger power like 550 bhp with cams & a dirty big turbocharger right?

[Mik]

You can still use a standard head gasket if you have an oversize bore, that is not the reason people use metal gaskets.

usually rb head gaskets are not the source of problems so most people with oversize bore/pretty modified motors use the standard gasket.

The standrad nissan Gasket is only good for up to 87mm bore though. Any bigger, you need an aftermarket gasket.

[Remix-]

Can't altering the cam timing have effects on your dynamic compression ratio? Thus is you wanted a lower compression you could take the short cut and alter your timing (more overlap) rather than change a head gasket?

Another point maybe is that people think running such a high boost level is what makes power, but boost is just pressure, not cfm. Cfm is what makes the power and torque and should not be confused with boost. Concentrate on moving large volumes of air at a lower boost level whilst still retaining a decent (higher) compression ratio so you dont loose to much down below. I guess at the end of the day what we really want is a nice linear power curve. But if your a drag racer i guess low end doesnt matter.

Sorry if this is hard to understand and please correct me if im wrong

[DoughBoy]

Ok thats good info.  So if you were wanting to make some large hp out of your rb25det, say 500 bhp+ you might want to do pistons & headgaskets just to make it easier on the engine,  you could then cope for much larger power like 550 bhp with cams & a dirty big turbocharger right?

Pistons are a recommended upgrade as the cylinder pressures and temperatures at higher power levels are beyond what the standard cast pistons can handle, they melt/crack/call it quits as soon as the going gets tough.

Headgaskets are normally changed to reduce compression without changing pistons, also since the headgasket resides in the combustion chambers it's one of the first parts in line to cop the increased pressures from higher outputs and detonation. The aftermarket multi-layer metal gaskets are pretty strong, best thing is you can resuse them after cleaning.. provided they aren't damaged.

Static compression is a basic mathematical equation of swept volume + combustion size / combustion size. Dynamic compression takes into account camshafts bleeding out pressure due to overlap + increased duration. Also note that in the higher RPMs when there is less physical time for the valvetrain events to happen combustion chamber filling is greater with the larger camshafts, and so you're going to get higher combustion pressures than smaller camshafts. You really need to take into account head flow stats, as well as manifolds (in + exh) as well as turbine backpressure to accurately calculate cylinder filling, otherwise just suck it and see.

[blind_elk]

There are a whole bunch of different things you can do to alter the power delivery. The Nissan Group A GT-Rs used to run about a 9.2 C/R and 1.8 bar boost, and delivered something in the order of 450hp. The Sierras probably delivered the same level of power, but did it by running C/R of 6, and 3bar boost (which explains why they were such dogs off-boost, and diabolical in the wet when the power suddenly cut in).