Jump to content
SAU Community

Recommended Posts

Ive been reading alot on the internet about how the guys in the states and japan modify the combustion chambers on the RB26 a company in the US www.theoldone.com has gone one step further and had pistons made to suit the modified chamber.

I just wanted to know what people do here in Australia for their HIPO RB26 heads assuming ever other aspect of the head has been addressed

I was thinking of having the quench/squish pads removed to allow a circluar combustion chamber and then have a piston made that has the same shape as the chamber the size cronm of the piston would obviously be determined by the piston manafacturer to aquire the determined compression ratio.

Am I on the wrong track here?

CRACKD

BOOSTD: That is sort of what I aim to do but I want my combustion chamber to blen into where the squish/quensh pads are. I assume the reason your lag increased was due to a drop in compression.

What was your final compression?

Here is a rough graphical representation of what I aim to do once the chamber has been modified, another thing that i would like to look into is biasing the crown of the piston towards either the inlet or exhaust valves to increase efficency any comments are welcome.

post-29853-1150992568.jpg

Here is a rough graphical representation of what I aim to do once the chamber has been modified, another thing that i would like to look into is biasing the crown of the piston towards either the inlet or exhaust valves to increase efficency any comments are welcome.

Ummm, i dont know what squish zones etc are, and this topic certainly goes over my head, but i was just wondering something about the above diagram.

When the piston is coming back up after firing, the inlet valve is closed and the exhaust open; the shape of the piston in the diagram doesnt really seem promote efficient scavenging of waste gasses through the exhaust ports. The gas on the inlet side of the chamber looks like it has to work really hard to squeese past the spark plug and out the exhaust. Does this matter?

Again, i know nothing about this topic. Just from a laymans point of view.

PS. What are squish/Quench zones by the way?

Thanks,

Shaun.

Thats okay, but after speaking to some of the big tuners over in the States and Japan they all state that they remove the squish zones and use a custom piston to suit hence the purpose of this post since they arent reluctant to release what combustion chamber/piston design they use that allows high compression (9.0-9.5:1) with a fair amount of boost (1.5Bar) on pump fuel. And before anyone says Japan has better fuel the US doesnt and they still seem able to do it

From what I was told by two reputable workshops, the squish zone is removed from high power RB26's mainly for 'safety' reasons. With the squish zones in place carbon can build up under them and randomly ignite causing detonation.

This is namely mentioned with the high price stroker kits (read, JUN) designed to run very high power levels.

Going off your picture with the centre of the piston dome moved toward the intake side looks along the lines of the rollerwave pistons that Larry from Endyne has made for the Honda's and Mitsi's. It does make sense to move the charge toward the exhaust valves before it fires, you have most of the charge in the same area to save the flame having to move all the way to the intake side of the chamber when the plug fires,it's closer to the valve to get out and it also means that on overlap there is less burnt gas on the intake side so theoretically you will have less reversion of burnt gas moving back into the intake ports when the intake valves first open.

I still think that the quench pads are a very important part of the combustion process, as the flats on the piston run up close to the pads it creates a shockwave that forces the charge towards the sparkplug. If I was going to modify the pads at all I would make the exhaust pad slightly smaller so the charge ends up slightly towards that side. Another trick that I have seen recently, this is on wedge chambers on V8's is to machine tapered grooves in the quench area to direct the charge toward the exhaust valve, the idea is to ensure as much of the charge is burnt as possible.

Anything that reduces the chance of detonation is a good idea, when you see most RB engines that have had detonation problems it is on the intake side of the chamber and piston as they get a pocket of fuel and air that is stuck there and preignites. In theory, if you could have the piston just kiss the head on the quench pads it would be the best you could get combustion wise as all the charge is forced in toward the plug.

The biggest problem with and RB, as well as quite a few other japanese engines is that they run a domed piston which will never burn as well as a flat top piston as the flame front doesnt have a clear path. The problem is that with such a short stroke engine it's not possible to get the compression without the dome. I've just spent quite a bit of time on an EJ25 Subaru which run a dished piston along with quench pads, the chamber is a much better design, it's a proper pent roof chamber and even at 9:1 compression it has no signs of detonation with 30psi boost. It would be great to have the time and money to make a new head for an RB with a similar type of design.

Just to finish this novel off, I think you'll find that Jun remove the quench pads on their "customer engines" which makes you wonder if they do the same on their own engines or if it is done as a bit of a safety feature rather than for outright performance gain.

Interesting thoughts Pro engines , I was told that propper pent roof compact combustion chambers with large quench pads as you say tend to work best . I'll have to dig up some pics of Cosworths designs ie DFV's and BDA's to refresh my memory . I do remember thinking FJ's have a nice chamber design .

Have you seen pics of 5 valve chambers and some 4 valve types where the chambers close in from all sides to minimise the volume and as you say compress the mixture close to the plug for short flame paths and minimul end gas to detonate .

When I fist looked at RB 4 valve heads I thought the step from the quench zone was pretty deep and wondered if decking them and using deeper dish pistons could help that low lift scavange phase .

Cost aside I suppose the best way is mill the head and custom pistons though I'm not sure how thick the deck is on RB's and what is a reliable minimum . SK mentioned an 80 thou gasket and depending on if or by how much the crown came up in the gasket the quench zone gap (between pistons and head) may be reasonably small . The person whos views I follow said work on 25 to 40 thou which sounds the part to me . I would be interested to know what sort of stretch you allow for in a reasonably high reving RB ie 8000 so that the piston never gets to meet the head !

Cheers A .

I work on a minimum of .030" (.75mm) piston to head clearance for a customers engine, usually closer to .040". On RB engines I run the pistons .008" (.2mm) out the top of the block with a .048" (1.2mm) head gasket. You can get away with a little less if you want but you do need to allow for expansion, rod stretch, piston pin flex, crank flex and the piston rocking. Some pistons that are made for a lot larger bores than 86mm actually have a slight angle milled on the top running down to each side to provide clearance with the head when the piston rocks but this is getting very close to the limit.

One thing that needs to be considered as well is bad tuning, I've seen pistons that have touched the area above the top ring against the bore and transferred metal across. Considering that on any piston this area is about .030" or so smaller in diameter than the bore that is a lot of rocking that the piston is doing to touch the bore.

I've run an engine with .025" piston to head clearance and the piston has touched the head very slightly and not damaged anything but would never do it on a customers engine. This was on a friends speedway engine and we were chasing every last bit but the engine came apart regularly so engine life wasn't an issue.

Proengines: Thanks very much for your input. I have drawn up some other chambers as you mentioned, I didnt quite understand the one about the V8 Wedge engines so I will wait for a clarification. If you can let me know which of the combustion chambers will work the best.

My engine specs are RB26, high comp (9.0-10.0:1) running 1.5 bar and pump fuel to make 500awkw which is approx 800bhp.

Chamber images:

1. Totally removed squish inlet and exhaust

2. Reduced squish inlet and exhaust allowing flat top piston due to piston entering the chamber where the squish was removed

3. Reduced exhaust squish with a Edyne style piston

I greatly appreciate your's and everyon elses opinion

post-29853-1151164165.jpg

post-29853-1151164182.jpg

post-29853-1151164195.jpg

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


  • Similar Content

  • Latest Posts

    • Had I known the diff between R32 and R33 suspension I would have R33 suspension. That ship has sailed so I'm doing my best to replicate a drop spindle without spending $4k on a Billet one.
    • OEM suspension starts to bind as soon as the car gets away from stock height. I locked in the caster and camber before cutting off the kingpin. I then let the upright down in a natural (unbound) state before re-attaching it. Now it moves freely in bump and droop relative to the new ride height. My plan is to add GKTech arms before the car is finished so I can dial camber and caster further. It will be fine. This isn't rocket science. Caster looks good, camber is good, upper arm doesn't cause crazy gain and it is now closer to the stock angle and bump steer checks out. Send it.
    • Pay careful attention to the kinematics of that upper arm. The bloody things don't work properly even on a normal stock height R32. Nissan really screwed the pooch on that one. The fixes have included changing the hole locations on the bracket to change the angle of the inner pivot (which was fairly successful but usually makes it impossible to install or remove the arm without unbolting the bracket from the tower, which sucks) and various swivelling upper arm designs. ALL the swivelling upper arm designs that look like a capital I (with serifs) suck. All of them. Some of them are in fact terribly unsafe. Even the best one of them (the old UAS design) shat itself in short order on my car. The only upper arm that works as advertised and is pretty safe is the GKTech one. But it is high maintenance on a street car. I'm guessing that a 600HP car as (stupidly, IMO) low as you are going is not going to be a regular driver. So the maintenance issues on suspension parts are probably not going to be a problem. But you really must make sure that however your fairly drastically modded suspension ends up, that the upper arms swing through an arc that wants to keep the inner and outer bolts parallel. If the outer end travels through an arc that makes that end's bolt want to skew away from parallel with the inner bolt, you will build up enormous binding and compressing forces in the bushes, chew them out and hate life. The suspension compliance can actually be dominated by the bush binding, not the spring rate! It may be the case that even something like the GKTech arm won't work if your suspension kinematics become too weird, courtesy of all the cut and shut going on. Although you at least say there's no binding now, so maybe you're OK. Seeing as you're in the build phase, you could consider using R33/4 type upper arms (either that actual arm, OEM or aftermarket) or any similar wishbone designed to suit your available space, so alleviate the silliness of the R32 design. Then you can locate your inner pivots to provide the correct kinematics (camber gain on compression, etc).
    • The frontend wouldn't go low enough because the coilover was max low and the upper control arm would collapse into itself and potentially bottom out in the strut tower. I made a brace and cut off the kingpin and then moved the upright down 1.25" and welded. i still have to finish but this gives an idea. Now I can have a normal 3.25" of shock travel and things aren't binding. I'm also dropping the lower arm and tie rod 1.25".
    • Motor and body mockup. Wheel fitment and ride height not set. Last pic shows front ride height after modifying the front uprights to make a 1.25" drop spindle.
×
×
  • Create New...