Subaru EJ25 combustion chamber to do / not to do

[ptuomov]

I don't know if you're still doing your research on this or came to any conclusions, but i've recently been looking at Duratec engines and their crown designs mirror the what I called 'satellite dish' type bowl which Saab were using back in the day (see pic back there). I'm quite confident that is the shape to go for if you can achieve it. Aftermarket forged manufacturers won't want to do it as it'll mean spending more than 10 minutes doing the programming so it'll cost more if they agree to it.

In other news I found that the guys tuning the RB26 engine at high power levels are milling off the squish pads in the heads to stave off det', a bit of Googling will reveal more. I also found with wet flow testing on some 4vpc heads that there was some interference between the two inlet valves in the combustion chamber and that what I was using got plastered onto that squish pad. On taking engines apart if they'd been running close to det or even detting lightly that same pad by the inlet valves was the one which had a sandblasted finish to it. On these engines i'm thinking it's a bit of an overreaction to remove both pads, but just the one by the inlet valves or mill it down at an angle
This was also backed up by browsing the latest copy of 'Four stroke performance tuning' where the author said he had seen power increases on some engines with them removed and came up with the theory that the mixture was being trapped in this area, almost to the point of being like hydraulic lock. To sum up I think they have their uses for mild to medium power outputs, but when you've gone beyond that and nothing else matters, bar ultimate power then remove at least the one by the inlet valves.

It seems to me that a shallow spherical dish seems to work very well with high boost. That is, the dish is a section of a sphere with a very long radius that intersects with the piston crown, leaving about 5mm squish pads around. I don't know why this seems to work, one possibility is that spherical dish with a very small squish band keeps the tumble motion going very efficiently as the piston gets further down. The twin squish pad pistons aren't nearly as popular among car factories as they were in the 1990's.

[4vpc]

I don't know if you're still doing your research on this or came to any conclusions, but i've recently been looking at Duratec engines and their crown designs mirror the what I called 'satellite dish' type bowl which Saab were using back in the day (see pic back there). I'm quite confident that is the shape to go for if you can achieve it. Aftermarket forged manufacturers won't want to do it as it'll mean spending more than 10 minutes doing the programming so it'll cost more if they agree to it.

In other news I found that the guys tuning the RB26 engine at high power levels are milling off the squish pads in the heads to stave off det', a bit of Googling will reveal more. I also found with wet flow testing on some 4vpc heads that there was some interference between the two inlet valves in the combustion chamber and that what I was using got plastered onto that squish pad. On taking engines apart if they'd been running close to det or even detting lightly that same pad by the inlet valves was the one which had a sandblasted finish to it. On these engines i'm thinking it's a bit of an overreaction to remove both pads, but just the one by the inlet valves or mill it down at an angle
This was also backed up by browsing the latest copy of 'Four stroke performance tuning' where the author said he had seen power increases on some engines with them removed and came up with the theory that the mixture was being trapped in this area, almost to the point of being like hydraulic lock. To sum up I think they have their uses for mild to medium power outputs, but when you've gone beyond that and nothing else matters, bar ultimate power then remove at least the one by the inlet valves.

It seems to me that a shallow spherical dish seems to work very well with high boost. That is, the dish is a section of a sphere with a very long radius that intersects with the piston crown, leaving about 5mm squish pads around. I don't know why this seems to work, one possibility is that spherical dish with a very small squish band keeps the tumble motion going very efficiently as the piston gets further down. The twin squish pad pistons aren't nearly as popular among car factories as they were in the 1990's.

It's pretty much the perfect design for a port injected 4vpc N/A or boosted* IMO. The first time I saw it I knew it looked right, but had to back track to why I thought that. Yes to small squish pads, but also It has no hiding place for the mixture and reflects fuel, heat and energy towards the centre.
* Difficult to implement for high power boosted engines on pump gas, as the lower CR needed means you can't get the piston pin high enough because the centre of the dish is too low. This (and lazy machining) is why we see so many bathtub type recesses in aftermarket pistons.

[ptuomov]

Here’s the piston dish shape from the new S58 2020 M3 engine:

6BE354B9-9D7F-42A2-BB12-442782EE2612.jpeg

E44DA332-2774-4337-8032-CBB23C6A9375.jpeg

7080937E-E068-46D7-A4F3-A2049019A8AA.jpeg

[hoffman900]

Here is supposedly a piston from the new turbo era for Formula One Ferrari engine. Not sure when or how current it would be since they've been around for 5 years now...

[ptuomov]

That F1 piston is from a compression ignition engine, so I would read too much into it for regular engines.

[hoffman900]

That F1 piston is from a compression ignition engine, so I would read too much into it for regular engines.

Could be. They now use prechamber ignitions so not sure what that piston would look like, but yeah, unlike anything else.

[ptuomov]

In contrast, the BMW piston is from a conventional pump
gas engine that can take a stupid amount of boost with 9.5:1 compression ratio.

[ptuomov]

This piston mockup is starting to look like a modern production engine turbo piston:

0255F0C6-AB12-4163-B521-6EDD2329CDC6.jpeg

606107B4-D7FF-401C-9A9A-1D65A6E2ECEE.jpeg

4F9907B7-F7FD-4FFB-8931-D328E523B126.jpeg

[Calypso]

Been years and going by the memory, but I faintly recall rollerwave pistons having slightly different quench clearances in the intake and exhaust side, which was part of the idea of creating a "rollerwave" combustion.