99 Ej25D w/ 251 pistons?

I have a good 99 EJ25D and a spare set of 251 pistons. Curious if the Rods from the 52mm Ej25D will work with the 251 pistons. Or if the stroke is different?

No the pistons have a different compression height, so your compression will be off.

Well, the compression height of 25D pistons is only 0.8 millimeters taller than 251 pistons. 25D rod length is actually 0.35 millimeters shorter than the 251 rods. So, there's only a total difference of 0.45 millimeters in length. That would give you a total of 0.50 mm piston deck height.

 

You're compression would be roughly 11.0:1 with a 0.045" thick head gasket. With a stock size gasket, you'd be looking at roughly 10.5:1 compression. Stock gasket thickness is about 0.06"

 

Now, whether or not you have piston to chamber clearance with a gasket that is any less than 0.05" thick, I can not tell you.

Essentially this 99 25D block would be turned into a 251 block..Or so I was hoping and running an STI HG.

 

This is the route I was going to go on a Full Ej251 with 22E heads.

So pretty much 99 EJ25D block, crank, rods w/ Ej251 Pistons.

I just typed out a fuucking novel to answer your question.... Then I pressed "backspace"

 

 

God effing dang it....

 

 

To keep it simple, the '99 25D motor is basically a 251 short block with 25D heads, pistons, etc... I believe that contains the EJ251 131.6 mm length rods and EJ25D pistons with stock 30.0 mm compression height (I have no sources whatsoever to confirm this, but it is the only thing that makes sense. I haven't had a chance to check the exact part numbers yet.). That would mean that it already has a PDH (pistons deck height) of 0.50 mm (this means that the piston at TDC would be 0.50 mm ABOVE the block deck).

 

If that's the case, running 251 pistons, STi head gaskets (0.50 mm thickness), and 22E heads will yield you roughly 12.0:1 compression (if the numbers I have are correct. I have found that the 22E head chamber volume is around 41cc. I incorrectly told you in the past that this is roughly what you're current motors compression is. I was wrong, as I used the wrong size gasket in my calculations. You're current motor is probably running about 11.0:1 compression. As I've told you before, the '96 had pistons that are very similar to the EJ251 in design and volume. But, given the specific design differences though, the 251 pistons should not require the use of anything higher than 93 octane.

I thought STi HG's were .56". But I could be very wrong. So are you saying I shouldn't have much of an issue running the full 99 EJ25D setup just with the 251 pistons?

 

I just did a quick search and came up with this.

 

"02-07 WRX/STI stock head gaskets are .6mm (.023 in) thick."

Hmmm TWE is where I've gotten all of my numbers from... That's what I've been using for the past few months. Some numbers I had previous found elsewhere on the internet didn't line up with what they said.

 

I don't think you'll have any issues as long as the piston clears the cylinder head chamber and you order custom sized head gaskets from Cometic. They make them from 0.027" and up. You can choose based on what you'd like your compression to be.

You're Stock gasket thickness is about 0.06"

 

Is that 0.06" with a crushed headgasket or a non crushed?

You cannot change rods.cranks between the two motors because 251 is #5 thrust and 52mm bearings while the 25D is #3 thrust and 48mm bearings. If you use 251 pistons with 25D rods/crank you would drop your compression from 9.41:1 to 9.28:1 with stock 25D headgaskets and heads.

Is that 0.06" with a crushed headgasket or a non crushed?

 

It should be the "compressed" thickness according to what I was told.

 

You cannot change rods.cranks between the two motors because 251 is #5 thrust and 52mm bearings while the 25D is #3 thrust and 48mm bearings.

 

This. I'm just saying that the '99 25D motor more than likely used rods that were the 131.6 length (251), and not the 131.25 length (25D) that was used from 96 to 99. Journal sizes, that I know, are specific to the 251 sizes. You know, why would Subaru design and use 131.25 length rods with 251 journals for one year alone... I also doubt that they would make the 25D pistons with a shorter compression height for one year either. So, that would give the '99 25D slightly higher compression (by only a mere 2/10th) than 97/98 motors given the overall piston deck height.

 

If you use 251 pistons with 25D rods/crank you would drop your compression from 9.41:1 to 9.28:1 with stock 25D headgaskets and heads.

 

I get around 9.4:1 with 0.05" piston deck height. With a 0.5" height I get 9.8:1 compression.

99 25D's are #5 thrust.

 

I should mention. My understanding is a 251 with STi HG's and 22E heads will yield around the 11.5:1 compression ratio, no?

 

I have a good 251 and then a torn apart 99 25D with a spare 251 (with a bad crank). So I was going to use pretty much just put together the 25D with the 251 pistons and yield the same compression.

 

I am building 2 frankenmotors. One to replace mine and one to go into the girls car. Would it be better to just leave the 25D as is or would it be worth the compression bump to use the 251 pistons...If they will fit without causing compression issues. The 25D would be using the sti HG's as well as long as it is using the 251 pistons.

 

Sorry if I just confused anyone. Extremely sick and head is spinning.

Baddog, bump the compression on the 25D/251 combo. There will be an increase all over the powerband, which is what you want since the 25D kinda' sucks on the low-end of things. The 25D cams will give you what you need on the middle/top end of the rpm once the compression is raised (of course, spark plug gap will have to be dropped to about .028 but that's no big deal.)

 

In comparison to the 22E/25D combo, the 25D/251 combo will run almost silky smooth & it will especially have a good midrange pull. It won't even NEED to have Delta cams because the compression rise will be enough (but if you want, those cams will be nasty once touched by Delta). Also, the 25D heads pull in air fantastically right from the factory, so you're all good in that department.

 

The 25D heads & their cams leave room for much improvement that will yield nice power gains one could only dream of with the 22E heads, provided that we're strictly speaking of only Phase 1 USDM heads here.

Both the motors I am making will run 22E heads with delta cams. I dont have any 251 heads unfortunately. Or even a decent set of 25D heads.

 

I was tossing around the idea of a 251 w/ 25D heads and an STi HG. But curious of my compression and if I would hit the valves. Also not sure if it is worth it over the 251 with 22E heads (with cams)

 

Hoping one of you who know hows to figure these things out will help me out. I have a few different options here.

I like the theory of the 251/22e head combo because of the big valves in the E heads, unfortunately not a lot of info on that...be a pioneer baddog..running 222 heads on the 251 block with the ultra thin 22e cometics...be a pioneer baddog.lol j/k but I'm subscribed to this since I still have my first gen torn down for suuuum type of build.

 

Hey baddog didn't your cousin do a compression build with an 18e/22e combo? I wanted to talk to that guy about that because of the 18e cam profile.

 

Threadjacker

I did the 251 with 25D heads and STi headgaskets, it worked great. Slightly more power than the stock 25D.

Subikid. Think it's worth it over a 251/22E build?

I was tossing around the idea of a 251 w/ 25D heads and an STi HG. But curious of my compression and if I would hit the valves. Also not sure if it is worth it over the 251 with 22E heads (with cams)

 

You're more likely for valve/valve contact than piston/valve contact. And any 1997-1999 25D heads > ANY P1 SOHC heads. I'd say that would go without question. I've had both & the 22E heads don't even come close to comparison.

 

Having them just makes maintenance easier but in terms of airflow/performance, they are a waste in comparison without head work & even then, the 25D heads STILL win out.

 

Even the 96 heads are better to work with than the 22E/T heads.

 

Sure you gain compression due to the combustion chambers on the 18-22E heads but it stops there.

 

DOHC by default runs smoother than SOHC & the 25D is no exception. Putting 97-99 25D heads on any EJ22 or EJ25 block is one of the best things anyone can do for the following reasons:

 

1. 25D heads bring in more air.

 

2. 25D heads allow for MUCH smoother operation, which leads to easier power delivery/less wasted energy.

 

3. Dedicated cams for intake & exhaust, and that means a wider adjustment of cam tuning (if adjustable cam gears are your sort of thing).

 

4. Believe it or not, 25D heads are not too much different from 22E heads in terms of maintenance. You just have to do a little more.

 

5. Dual port heads & compatibility with EJ20s (EJ20K, EJ20D, etc) means that you have a WIDE selection of head parts to choose from, if you wanted to. Also, dual port headers of any kind are much cheaper & easier to come by.

But if the heads are good and timed right. Shouldn't have any issue with valve to valve clearance, right?

 

I currently have a 25D with 22E cammed heads (dual port). I am for sure wanting more power than a stock 25d. So if its not worth me having a set of 25D heads gone over I won't do it. Since my 22E heads were rebuilt 16k miles ago.

Like I've said, everything you want to do should work as long as the piston doesn't hit the head/chamber itself since it is coming out of the block 0.5 mm

 

I don't think it will hit.

But if the heads are good and timed right. Shouldn't have any issue with valve to valve clearance, right?

 

I currently have a 25D with 22E cammed heads (dual port). I am for sure wanting more power than a stock 25d. So if its not worth me having a set of 25D heads gone over I won't do it. Since my 22E heads were rebuilt 16k miles ago.

 

If the heads are good & timed right, you will have no issues whatsoever with valve to valve clearance. If the belt pops, then the valves are history, unless you get lucky & the belt pops at idle. Even a 22E block with 25D heads at 22E stock compression is a fully interference engine.

 

If your 22E heads were built 16k miles ago, then keep on rocking them. Just open up the intake ports as wide as you can possibly get them. If you DO get 25D heads, most likely, they will have been overheated at some point & will need to be checked for warped spots.

 

But mod-for-money wise, the 25D heads definitely beat the 22E heads. The 22E heads are easier & cheaper to work with. There is also the powerband to consider. The low-end that you're used to, will be diminished & you will notice.

 

On the flipside, the midrange/top-end pull will increase & you will notice that also. The 25D cams take the power right to the 6500rpm limit, & then drop right off from there. Custom ground? I imagine a 7000+rpm limit from the cams is definitely possible.

If you have the 22E heads and cams for them then port them and run those. Like DOHCEJ22E1 said, the 22E heads have small ports, but if you open them up you should be in the similar power range of the 25D heads.

Hello from Germany,

 

I bump this Thread because I want to build a 1996 Legacy EJ25D up.

 

The Engine has the Oldschool Pistons and i want to swap the ej251 pistons in and want to use a 0.036 / 0.9mm Cometic Head Gasket the cr will be around 11.37:1 ist that right? And will i have enough clearance between the Valves and the Pistons?