Subaru EJ25 combustion chamber to do / not to do

[ptuomov]

A quick question about the popular high-hp Subaru turbo engines. This is along the lines of what not to do, and specifically interested in the combustion chamber and the piston. This is for a high-boost pump-gas motor with less than 8000 rpm redline, 100mm piston and stock stroke. What are the biggest mistakes one could make in choosing the pistons, piston dish shape, wrist pin, rings? What (head) chamber shape works the best (and worst) with high boost and pump gas? Specifically interested in preventing knock by keeping the end-gas temperatures and peak cylinder pressures down while still producing high power stroke average cylinder pressures.

(As background, we're doing another turbo conversion Porsche 928 S4 and this one will have aftermarket pistons. The engine shares almost all key design parameters with Subaru EJ25, and those that aren't identical are very close. Since Subarus make all sorts of crazy power out there, I'm just curious what I could learn for the Subaru tuners that are way, way further along in every possible direction than I am.)

[user-23911]

There's at least 5 different sorts of Subaru heads with different ports.

The biggest mistake would be thinking they're all the same.

[ptuomov]

There's at least 5 different sorts of Subaru heads with different ports. The biggest mistake would be thinking they're all the same.

I'm aware of at least some of the port differences, especially between the 2.0L Japanese and 2.5L US Sti versions. Let me be more specific, so we're all talking about the same thing: EJ257 as installed in the US 2004 Sti model, with cast pistons that have the "donut" dish and non-tapered pins. Would that specificity allow you to answer it with an answer other than "the question is stupid"?

[wyrmrider]

was the post about your question being stupid removed or are you paranoid.
not a way to get any help

[Carnut1]

I would love to see a picture of the chamber in question. Thanks, Charlie

[ptuomov]

I would love to see a picture of the chamber in question. Thanks, Charlie

Here are some IMO good shots:

https://forums.nasioc.com/forums/showpo ... ostcount=2

Scroll down a little and you'll see decent photos of the EJ257 B25 head. It has also other heads Subaru heads. (The non-Subaru head that I'm planning to modify probably looks most like the D25 head, also there in the photos.)

This link has the piston dish that the factory married with that head in the US:

https://forums.nasioc.com/forums/showpo ... ostcount=1

[KnightEngines]

What sort of HP are you shooting for?

For BIG Hp the 2L single AVCS head castings are the go - stronger castings than the EJ257 heads.

The 2L chamber is pretty good, better quench areas, you can open it up to unshroud the valves a little & blend in the top cuts of the valve job, but leave it more compact than the 2.5 head chambers & manipulate compression with piston dish volume.

Once you start getting over 800 or so HP the integrity of the valve job starts to become a problem, valve job will need regular touch ups as the chamber is shoved up into the casting & the seats distort around the plug hole.
You'll need to run at least 1.5mm exhaust seats (for the 45 deg) & Inconel valves.

Up to around the 800hp mark you can get away with 625+ head studs in stock 11mm size, above that you'll need 14mm head studs to stop the heads from lifting around peak TQ.

Above 1000HP the head castings become a consumable, methanol fuel can extend that a little - seems to be a little softer on the castings, but even then you'll still kill castings over time.

As you get into the higher power regions you actually need to reduce port sizing (not take as much out) to keep more metal in the heads & retain as much structural integrity as possible.

I've been using std size exhaust valves (Inconel) to leave more material in the exhaust seats.

For pistons I've been using CP's with a round dish & coated crown.

[ptuomov]

I am looking for 150hp/cylinder on pump gas. Peak power around 7000rpm with 100mm bore and 78.9mm stroke. That would translate to 600hp in 2.5L Subaru units?

I don't yet know what the structural integrity issues will be. So far none, but that's only at 2/3 of that target power level.

Valves are natively +1mm compared to Subaru valves, and there's room for very wide seats if desired.

To give it the best shot on pump gas, I was thinking of trying to make the head burn a little slower. The idea here would be to accept some inefficiency in order to keep down the peak cylinder pressures while still producing reasonably high average cylinder pressures for the power stroke. The method for making it burn slower would be to reduce the squish area and make the combustion chamber much more open. With race gas, or with pump gas and low boost, that would probably make things worse but maybe it might help with high boost and pump gas? I don't know if that's smart or dumb, or too clever by half...

What sort of HP are you shooting for?

For BIG Hp the 2L single AVCS head castings are the go - stronger castings than the EJ257 heads.

The 2L chamber is pretty good, better quench areas, you can open it up to unshroud the valves a little & blend in the top cuts of the valve job, but leave it more compact than the 2.5 head chambers & manipulate compression with piston dish volume.

Once you start getting over 800 or so HP the integrity of the valve job starts to become a problem, valve job will need regular touch ups as the chamber is shoved up into the casting & the seats distort around the plug hole.
You'll need to run at least 1.5mm exhaust seats (for the 45 deg) & Inconel valves.

Up to around the 800hp mark you can get away with 625+ head studs in stock 11mm size, above that you'll need 14mm head studs to stop the heads from lifting around peak TQ.

Above 1000HP the head castings become a consumable, methanol fuel can extend that a little - seems to be a little softer on the castings, but even then you'll still kill castings over time.

As you get into the higher power regions you actually need to reduce port sizing (not take as much out) to keep more metal in the heads & retain as much structural integrity as possible.

I've been using std size exhaust valves (Inconel) to leave more material in the exhaust seats.

[ptuomov]

was the post about your question being stupid removed or are you paranoid. not a way to get any help

Just tired and cranky after the weekend, not yet paranoid.

[wyrmrider]

good to know that was not U
were we reading stupid into Joe's post?
I never did figure out where stupid came from
anyway- back on track

[user-23911]

No, I didn't use the word stupid.

I had it pointed out to me some time ago about the different heads.
There's a horrible dog leg in one port of most of them. That's the bad bit.

The first step in modifying any factory part in anything is to open your eyes and check out all the different versions.
What's the point in modifying a 3/10 part when you could go to the junkyard and find a 9/10 part to modify instead.
That applies to all engines.


No,to the mention of US spec parts.........I only get to see the japan spec versions. Mainly EJ20s.


As above.....pictures are always good and far better than words.

[modok]

subaru ej25 " a high-boost pump-gas motor"
Wow, that's a really challenging game you intend to play

[ptuomov]

The first step in modifying any factory part in anything is to open your eyes and check out all the different versions.
What's the point in modifying a 3/10 part when you could go to the junkyard and find a 9/10 part to modify instead.
That applies to all engines.

In terms of the heads I am actually planning to modify, I know the casting versions pretty well and also the cost differences between them.

In terms of the Subaru heads, the US model D25 head has the most similar combustion chamber to what I've got. Here's a photo from the web that I think is of D25 head combustion chamber:

SubaruD25CombustionChamber.jpg

The question in my mind is that for high boost and pump gas, should I run the head as is or make some adjustments. In particular, should I reduce the squish?

The reason why I am asking about reducing squish is that the pump-gas 1000hp 5.0L Koenigsegg engine has basically all the squish taken out and the combustion chamber has almost like a four-valve hemi shape. The piston is also a doughnut dish shape that doesn't even have valve reliefs and no small-radius corners whatsoever. The designer writes that this is done intentionally to slow down the burn and limit the peak combustion pressure while still keeping the power-stroke average cylinder pressure high. I am thinking that if this works, then the Subaru tuners who build pump gas motors should know it.

[ptuomov]

Here's what I meant by the Koenigsegg reference of combustion chamber suited for high boost and pump gas:

http://koenigsegg.com/build128-the-koenigsegg-engine/

Our piston is designed with a ceramic coating on the face that helps to avoid hot spots and detonation when the engine runs at maximum power and efficiency. The curved top face of the piston acts together with the cylinder head to create a combustion chamber that reduces peak pressure but maintains high average pressure, which is necessary for high power output while minimizing risk for detonation (knock).

Peak pressure is what causes detonation whereas average pressure during the stroke is what gives you power. So in order to create the amount of power we create, you need to have a high average pressure during the power stroke but you also have to keep the peak pressure at TDC as low as possible.

We have a very unusual combustion chamber. We don’t have any ‘squish’ areas. We have created a kind of four-valve hemispherical combustion chamber that avoids speed differentiation to the flame propogation during the combustion process. This is another danger that can lead to knocking when you’ve got such high cylinder pressures. The chamber and the piston are both specially designed to maintain a very even volume in the chamber as combustion takes place.

[wyrmrider]

I did not see a pic
I'd try it with the squish shown then check the flame burn patterns
then remove some squish from the exhaust side and see what happens
On the Mopar EArly Hemi- they work better with squish (welded or special domes that come out to the sides of the chambers- PITA) on pump gas especially street or on a road course- may not be applicable due to big dome on the hemi FKs things up
What does the late Hemi chamber look like- they spent a ton of R&D on it and I suspect Subaru did also
Maybe the head in the write up has lots of tumble and then they can get away without the squish (which works in a perfect world- I've never been able to work in a perfect world reworking stock parts)
so just because it works for them (and their intake ports) it may not work for you and your heads
you could also bias your valve sizes
does Suburu make a DI head?- would solve a lot of problems