Custom Pistons And Modified Combustion Chamber

[CRACKD]

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

[bogcock]

wat hp u chasing???

[BOOSTD]

I modified the squish zones on my Rb25, I found it reduced exhaust port velocity and increased lag. I did not however run pistons to suit.

[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?

[BOOSTD]

It may have been, comp was 8.7 which shouldn't change the lag as much as it did. I also had quite extensive porting

Edited June 22, 2006 by BOOSTD

[BOOSTD]

pic

[CRACKD]

Was your compression 8.7:1 after the chambers where modified?

[BOOSTD]

yes, we cc'd them to get an accurate measurement. I used a metal shim gasket of stock thickness, 1.3mm or 1.2mm I think it was.

Edited June 22, 2006 by BOOSTD

[rob82]

pic

Thats heavily polished for an intake port was it a street car??

[CRACKD]

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.

[Shaun]

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.

[CRACKD]

Shaun: That was my next questions and you beat me to it LOL.

[Sydneykid]

We do flat topped pistons and keep the standard squish zones with 9 to 1 compression ratio with a 0.8 mm head gasket.

cheers

Edited June 23, 2006 by Sydneykid

[CRACKD]

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

[Amaru]

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.

[CRACKD]

Amaru: i was never told that thanks, another tuner in the US Edyne reduces the size of the squish areas by around 150thou on both inlet and exhaust side and makes it up in the pistons

[proengines]

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.

[discopotato03]

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 .

[proengines]

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.

[CRACKD]

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