Jump to content
SAU Community

RB25/30 and quench?

StockyMcStock
 Share

Recommended Posts

StockyMcStock Proficient

StockyMcStock

Posted
Posted

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

Link to comment
https://www.sau.com.au/forums/topic/72033-rb2530-and-quench/
Share on other sites
Cubes Grand Master

Cubes

Posted
Posted

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

Posted
  • Author
Posted

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
Posted

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

Posted
  • Author
Posted

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 ;)

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share
Go to topic listing


  • Similar Content

  • Latest Posts

    • Milkmun
      The FD RX7 is the best looking 90's JDM car

      By Milkmun · Posted

      Jdm DC2R is also nice for a FF car compared to the regular hatches of the time.
    • PranK
      The FD RX7 is the best looking 90's JDM car

      By PranK · Posted

      Gotta admit, I love the 20b Cosmo.
    • joshuaho96
      Nismo Super Coppermix Twin and R33 GTR transmission replacement

      By joshuaho96 · Posted

      Now that the break-in period for both clutch and transmission is nearly over I'd like to give some tips before I forget about everything that happened, also for anyone searching up how to do this job in the future: You will need at least 6 ton jack stands at full extension. I would go as far as to say maybe consider 12 ton jack stands because the height of the transmission + the Harbor Freight hydraulic platform-style transmission jack was enough that it was an absolute PITA getting the transmission out from under the car and back in. The top edge of the bellhousing wants to contact the subframe and oil pan and if you're doing this on the floor forget about trying to lift this transmission off the ground and onto a transmission jack from under the car. Also do not try to use a scissor jack transmission lift. You have to rotate the damn thing in-place on the transmission jack which is hard enough with an adjustable platform and a transmission cradle that will mostly keep the transmission from rolling off the jack but on a scissor lift with a tiny non-adjustable platform? Forget it. Use penetrating oil on the driveshaft bolts. I highly recommend getting a thin 6 point combination (box end + open end) wrench for both the rear driveshaft and front driveshaft and a wrench extension. These bolts are on tight with very little space to work with and those two things together made a massive difference. Even a high torque impact wrench is just the wrong tool for the job here and didn't do what I needed it to do. If your starter bolts aren't seized in place for whatever reason you can in fact snake in a 3/8 inch ratchet + 6 point standard chrome socket up in there and "just" remove the bolts for the starter. Or at least I could. It is entirely by feel, you can barely fit it in, you can barely turn the stupid ratchet, but it is possible. Pull the front pipe/downpipe before you attempt to remove the transmission. In theory you don't have to, in practice just do it.  When pulling the transmission on the way out you don't have to undo all the bolts holding the rear driveshaft to the chassis like the center support bearing and the rear tunnel reinforcement bar but putting the transmission back in I highly recommend doing this because it will let you raise the transmission without constantly dealing with the driveshaft interfering in one way or another. I undid the bottom of the engine mount but I honestly don't know that it helped anything. If you do this make sure you put a towel on the back of the valve cover to keep the engine from smashing all the pipes on the firewall. Once the transmission has been pulled back far enough to clear the dowels you need to twist it in place clockwise if you're sitting behind the transmission. This will rotate the starter down towards the ground. The starter bump seems like it might clear if you twist the transmission the other way but it definitely won't. I have scraped the shit out of my transmission tunnel trying so learn from my mistake. You will need a center punch and an appropriate size drill bit and screw to pull the rear main seal. Then use vice grips and preferably a slide hammer attachment for those vice grips to yank the seal out. Do not let the drill or screw contact any part of the crank and clean the engine carefully after removing the seal to avoid getting metal fragments into the engine. I used a Slide Hammer and Bearing Puller Set, 5 Piece from Harbor Freight to pull the old pilot bearing. The "wet paper towel" trick sucked and just got dirty clutch water everywhere. Buy the tool or borrow it from a friend and save yourself the pain. It comes right out. Mine was very worn compared to the new one and it was starting to show cracks. Soak it in engine oil for a day in case yours has lost all of the oil to the plastic bag it comes in. You may be tempted to get the Nismo aftermarket pilot bearing but local mechanics have told me that they fail prematurely and if they do fail they do far more damage than a failed OEM pilot bushing. I mentioned this before but the Super Coppermix Twin clutch friction disks are in fact directional. The subtle coning of the fingers in both cases should be facing towards the center of the hub. So the coning on the rearmost disk closest to the pressure plate should go towards the engine, and the one closest to the flywheel should be flipped the other way. Otherwise when you torque down the pressure plate it will be warped and if you attempt to drive it like this it will make a very nasty grinding noise. Also, there is in fact an orientation to the washers for the pressure plate if you don't want to damage the anodizing. Rounded side of the washer faces the pressure plate. The flat side faces the bolt head. Pulling the transmission from the transfer case you need to be extremely careful with the shift cover plate. This part is discontinued. Try your best to avoid damaging the mating surfaces or breaking the pry points. I used a dead blow rubber hammer after removing the bolts to smack it sideways to slide it off the RTV the previous mechanic applied. I recommend using gasket dressing on the OEM paper gasket to try and keep the ATF from leaking out of that surface which seems to be a perpetual problem. Undoing the shifter rod end is an absolute PITA. Get a set of roll pin punches. Those are mandatory for this. Also I strongly, strongly recommend getting a palm nailer that will fit your roll pin punch. Also, put a clean (emphasis on clean) towel wrapped around the back end of the roll pin to keep it from shooting into the transfer case so you can spend a good hour or two with a magnet on a stick getting it out. Do not damage the shifter rod end either because those are discontinued as well. Do not use aftermarket flywheel bolts. Or if you do, make sure they are exactly the same dimensions as OEM before you go to install them. I have seen people mention that they got the wrong bolts and it meant having to do the job again. High torque impact wrench makes removal easy. I used some combination of a pry bar and flathead screwdriver to keep the flywheel from turning but consider just buying a proper flywheel lock instead. Just buy the OS Giken clutch alignment tool from RHDJapan. I hated the plastic alignment tool and you will never be confident this thing will work as intended. Don't forget to install the Nismo provided clutch fork boot. Otherwise it will make unearthly noises when you press the clutch pedal as it says on the little installation sheet in Japanese. Also, on both initial disassembly and assembly you must follow torque sequence for the pressure plate bolts. For some reason the Nismo directions tell you to put in the smaller 3 bolts last. I would not do this. Fully insert and thread those bolts to the end first, then tighten the other larger pressure plate bolts according to torque sequence. Then at the end you can also torque these 3 smaller bolts. Doing it the other way can cause these bolts to bind and the whole thing won't fit as it should. Hope this helps someone out there.
    • PLYNX
      The FD RX7 is the best looking 90's JDM car

      By PLYNX · Posted

      Of the 90s ! !
    • PLYNX
      The FD RX7 is the best looking 90's JDM car

      By PLYNX · Posted

      Every one has seemed to of have missed . . . . . . . The Mazda Cosmo . . . . . . what a MACHINE ! !
×
×
  • Create New...