Subaru EJ25 combustion chamber to do / not to do

[xr4x4ti]

ptuomov,

I really think you should make it more obvious that you are not actually working on a Subaru, but a 5.0l Porsche 928 motor. The Subaru motor is just an example of the chamber shape.

Every body is replying to you like you are working on a Subaru, suggesting different heads, etc.

Just trying to help!
Tim

[ptuomov]

ptuomov, I really think you should make it more obvious that you are not actually working on a Subaru, but a 5.0l Porsche 928 motor. The Subaru motor is just an example of the chamber shape. Every body is replying to you like you are working on a Subaru, suggesting different heads, etc.
Just trying to help! Tim

Thanks. Yes, this is for turbocharging a Porsche 928 S4 engine, specifically the next version. The previous version run great without any internal engine modifications at low boost settings such as 10 psi. The current version is being tested, and I don't yet have the high-boost data. Even when we have the high-boost test data, we don't really know whether its running as well as it could. So I wanted to ask Subaru tuners about how they get their 2.5L four banger engines to make 600hp or 240hp/l or 150hp/cylinder on 93 pump gas (semi)reliably. The 928 S4 shares a lot of design parameters and has a combustion chamber that looks very much like the D25 Subaru WRX head from the EJ25 engine.

To be specific, I am mostly interested in combustion quality, not filling the cylinder. In particular, how do the best Subaru tuners control the cycle-to-cycle variability of combustion with high boost? Are you using plugs that project deeper into the combustion chamber? Do you reduce or increase the squish area and/or squish clearance when going really high boost on pump gas? Does a tumble port or dump port style porting work better at high boost and pump gas, thinking that tumble motion is one of the main determinants of the combustion speed in a four valve head and influences the cycle-to-cycle variability? Which piston dish shape works best with pump gas and high boost? More generally, how do you keep the peak cylinder pressures of worst cycles in check while still producing high average power stroke cylinder pressures?

I am basically trying to tap into the vast empirical turbocharging knowledge of the Subaru community, which dwarfs in comparison anything that I currently have.

[Carnut1]

I would look into D.V.'s polyquad design. Might be just what you are looking for. Thanks, Charlie

[ptuomov]

I would look into D.V.'s polyquad design. Might be just what you are looking for. Thanks, Charlie

Is that popular among Subaru tuners?

[wyrmrider]

pointing ptuomov at the best suburu head is a good place to start
I suggested different sized intake and exhaust sizes and porting to match but DV polyquad is a better explanation

[ptuomov]

pointing ptuomov at the best suburu head is a good place to start

I agree. Which one of the heads burns the best at high boost and pump gas? D25 or B25?

[wyrmrider]

which head used here?
viewtopic.php?f=1&t=47889

[KnightEngines]

2L dual AVCS castings, not sure what they come on in the US.
They are much the same as the single AVCS 2L castings, just with dual AVCS which we wanted.

I've been setting up the last few billet engines with 2L castings with the smaller chambers & reworking the chambers a little.
But that is mostly a durability thing, the later 2.5 head castings are weaker & don't cop the punishment as well.

Once you get above around 20-25psi boost chamber seems to be far less important.

[ptuomov]

2L dual AVCS castings, not sure what they come on in the US.
They are much the same as the single AVCS 2L castings, just with dual AVCS which we wanted.

I've been setting up the last few billet engines with 2L castings with the smaller chambers & reworking the chambers a little.
But that is mostly a durability thing, the later 2.5 head castings are weaker & don't cop the punishment as well.

Once you get above around 20-25psi boost chamber seems to be far less important.

This Porsche 928 engine is proving to be quite durable. We tried to do some destructive testing with a completely stock engine and smaller turbos. We force closed the wastegate and let it rip on the dyno. The stock engine just yawned at 750 re ft-lb of torque and let the compressors and compressor inlet piping run head on to the mass flow limit.

IMG_3775.JPG

So far, the only thing we know is not good for this engine is knocks. We think that excessive knocks over time kill the rod bearings. So I'm trying to figure out how Subaru people run their huge power outputs on pump gas without knocks.

[KnightEngines]

Limit ignition timing around Peak TQ, ramp the boost in a little slower & keep boost conservative around peak TQ.
Most of the bigger HP subi's are on E85, which helps a lot.
The only issue with E85 is the boost response can be a bit too fast & they'll take a lot more ignition timing - can lift the heads around peak TQ. We've been using 14mm head studs & 'stopper' type head gaskets to keep the heads sealed.

The billet engines don't need to be so conservative with timing & boost curve, head clamping is much better thanks to block rigidity & they have all been on E85 or meth, so the det threshold is much higher.

[modok]

ptuomov,

I really think you should make it more obvious that you are not actually working on a Subaru, but a 5.0l Porsche 928 motor.
Tim

no kidding, I was wondering why you'd put a subaru motor in a porsche.

Yes it has been found highly boosted engines like a big hemi chamber and in many cases don't need quench, but it varies depending on app.
Roots blower, drawthrough turbo with no intercooler.........cut out the quench pads. You don't need them,

If it has lazy ports and an intercooler, reasonable boost, not super high rpm, leave the quench pads.

I suppose if you are somewhere in the middle then do something mid-way

[n2xlr8n]

Limit ignition timing around Peak TQ, ramp the boost in a little slower & keep boost conservative around peak TQ.
Most of the bigger HP subi's are on E85, which helps a lot.
The only issue with E85 is the boost response can be a bit too fast & they'll take a lot more ignition timing - can lift the heads around peak TQ. We've been using 14mm head studs & 'stopper' type head gaskets to keep the heads sealed.

^^This. Agree 100%.

[ptuomov]

"Once you get above around 20-25psi boost chamber seems to be far less important."

Even on pump gas?

"Tumble port" or "dump port" porting style for high boost?

[wyrmrider]

you did read this?
http://www.motortecmagazine.net/the-fut ... e-engines/

we found some squish helped even at 50 lbs on alky 2valve motor

[modok]

I don't think it has to do with the gas.
More to do with the heat, density, and turbulence generated in the charge. It will show up in how much ignition advance you need. Since you did a test engine already, you should have some idea of how much timing it needed, and how that compares to other engines.