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RB25/30 and quench?

StockyMcStock
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StockyMcStock Proficient

StockyMcStock

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okay so i today i bolted my crank and new pistons back in, and measured the deck clearance. by this i mean piston edge to block face clearance. it was zero, in other words the piston edges at TDC were exactly level with the top of the block.

i looked at my RB25 head, and noticed that the combustion chambers don't appear to have large squish areas. the only apparent squish areas i could see are the lateral (side to side, not front to back) edges of the combustion chamber, where it is exactly level with the head face. the closest recess after these (to the piston) is the lateral border of the combustion chamber but it is 2.5 mm or so from deck height, which is way too far for a conventional squish setup of 40 thou.

so my questions are:

1) do i set the quench (squish) up using just these areas as a reference guide? in other words, do i set up the clearance from piston edge deck height to the head face as 40 thou?

2) what thickness is a fully torqued down genuine nissan head gasket? (obviously need to know this so i can figure out the quench!)

cheers,

-Stocky

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Cubes Grand Master

Cubes

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Stock new pistons? Have you taken measurements for the comp ratio?

The quench area is the flat part of the combustion chamber that is level with the rest of the head plane.

I actually thought it was quite large for a 4valve head, there isn't really any room for more quench.

The RB20DET head has almost zero quench area.

StockyMcStock Proficient

StockyMcStock

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stock pistons with standard bore size will give me anywhere betwen 8.2 and 8.9 compression ratio, which i am happy with.

i thought as much with the squish area, but i thought it would have been larger. maybe i'm just optimistic and have looked at too many motorbike engines :cheers:

no worries, i can calculate the quench off the head clearance without any problems.

any ideas on RB30 head gasket thickness?

mjfawke Collaborator

mjfawke

Posted
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It's the area round the edge of the head where the pistons come close at TDC. The relatively cooler surfaces 'quench' the mixture to prevent detonation. The high HP GTR engines (according to an American engine builder living in Japan) remove both 'quench' blocks from the head, and for mid-power engines the one on the inlet side is removed and the exhaust side is radiused. He claims they are only for turbulence generation, not for quench. There are some decent head pics on Hybridz.org under the RB Engines forum.

Cubes Grand Master

Cubes

Posted
Posted

Thats what quench does.

Swirls or creates turbulence that cools hot spots, mixes the charge better and also squeezes it towards the central spark plug.

Quench is designed more so for low/mid range as it isn't as critical in the higher rpm's.

But as we all know average power is where its at.

If it wasn't why would we bother with the rb30det over the rb25det or even the rb20det. :)

ace_nz Apprentice

ace_nz

Posted
Posted

squish or quench is very important at all engine speeds, look at formula 1 heads and pistons, or any high reving bike engine, its mechanical octane. Removing the squish band from rb heads just increases the surface area of what is already a large deep chamber, id be adding in squish and biasing it to the exhaust side to increase the burn speed and reduce trapped end gases which cause detanation. (rb30 head gasket is 55 thou. will crush to 50)

  • 1 month later...
Abo Bob Grand Master

Abo Bob

Posted
Posted

This seems like a good thread to ask my question.

I have a stock NEO RB25 and want to run more boost. Tuner says put a thicker head gasket in to lower the compression ratio. I say, pull some timing from the tune and I don't need to.

Sydneykid says, "Personally I don't believe in thick head gaskets, they cause other problems."

I think he is referring to the change in quench/squish (Are these terms completely interchangable?).

Does that sound right?

What exactly will be the effect on quench if I put a thicker head gasket in? How is this bad?

Sorry for so many questions. As soon as you learn enough about turbos along comes quench and who knows what next! I've only just started looking at spark plugs...

Sydneykid Grand Master

Sydneykid

Posted
Posted
sweet, 50 thou, no worries mate! now i can deck the block and head by a combined 10 thou and get my flat 40 thou of quench.  

cheers!

The standard head gasket should give you the correct amount of squish, as designed by Nissan. ie; you should not have to deck the block or the cylinder head.

This seems like a good thread to ask my question.

I have a stock NEO RB25 and want to run more boost. Tuner says put a thicker head gasket in to lower the compression ratio. I say, pull some timing from the tune and I don't need to.

Sydneykid says, "Personally I don't believe in thick head gaskets, they cause other problems."

I think he is referring to the change in quench/squish (Are these terms completely interchangable?).

Does that sound right?

What exactly will be the effect on quench if I put a thicker head gasket in? How is this bad?

Sorry for so many questions. As soon as you learn enough about turbos along comes quench and who knows what next! I've only just started looking at spark plugs...

I have seen more than one engine develop more detonation after a thicker head gasket was installed. Squish is one reason for this, the other is the thick (metallic) head gasket itself creates hot spots. Potentially all around the combustion chamber circumference. Not a good idea.

I would much rather spend the same amount of money (as a thich head gasket costs) removing restrictions, rather than increase the boost to make more power. If you MUST reduce the compression ratio, then polishing the combustion chamber gives a better result for the same cost.

;)

StockyMcStock Proficient

StockyMcStock

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well i measured the head gasket from nissan with a micrometer and got 47 thou, assuming 5-10 thou crush the squish was just perfect with a deck height of zero. so i told the machinist to face the block, but take as little off as humanly possible. i think he took of 2 thou which is just about right for me ;)

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