Hey guys,

ive found out that my 2.5ltr hybrid (2.0ltr 03 sti heads and cams) is running a competition cometic head gasket which brings the overall compression to 8:2.5:1..

Ive been having problems making any power with my GT35R (0.82) power severly drops off at 5000rpm..

Could this be a reason for this??

What compression do u guys normally run on built 2.5ltr engines?

I have a simliar engine setup to your's: 8.2:1.

I am strubbling through IC plumbing leaks so I am attributing my power deficiency on too much fuel, plus I am not pushing real hard right now until I seal my intake plumbing up (don't want to overspin my turbo).

Let me ask you something. On the cometic gaskets, is oil leaking out of the bottom of your heads where they meet the block? Right where the oil return is for the heads? just curious.

First off it could be bad tuning but if not then:

Your compression ratio is fine but if you're running standard 2.0 STI heads and cams then that could be one of the problems (not sure on the lift of those cams). The 2.5 needs lots of room and cam to breath on the top end. What works for the 2.0 does not work on the 2.5. A set of Spec C heads would work wonders or even putting a set of cams such as JUN 272 will bring back the top end.

The US STI heads are also nice with larger valves and higher lift cams. These heads will make great power to 7500 rpm flowing up to 550 whp. Over that power level cams and head work are in order.

Also make sure your cam timing is correct. If you are experiencing exceptional spoolup this could be a sign you have your intake cam advanced which will kill high rpm power.

Good luck.

Phil
www.elementtuning.com

Hey guys,

ive found out that my 2.5ltr hybrid (2.0ltr 03 sti heads and cams) is running a competition cometic head gasket which brings the overall compression to 8:2.5:1..

Ive been having problems making any power with my GT35R (0.82) power severly drops off at 5000rpm..

Could this be a reason for this??

What compression do u guys normally run on built 2.5ltr engines?

The components you are using will mechanically lead to the issues you are having with your motor. Using a thicker head gasket to lower your compression ratio will cause your motor to be very det prone because of the increase in the squish volume. The un burnt fuel from having to large of a squish area causes the detonation.
Another mechanical issue you have is cam timing. The thicker head gaskets will advance your cam timing, and due to the design of the boxer motor, the left cams will be advanced twice as much as the right cams :eek:

Back to your original question, We run 8.1 to 1 on our 2.5L pump gas motors.

Quirt Crawford
www.crawfordperformance.com

Quirt- Do you have these "problems" on your S2 / S3L engines combined with stock wrx heads?

OEM compression on EDM WRX/STI are 8 to 1
OEM compression on EDM -06 version are 8.4 to 1 according to norwegian subaru website. (First time we get a 2.5L engine in a impreza, interresting to see what engine the new EDM sti will have since we always have had 2.0L here)

Quirt- Do you have thees "problems" on your S2 / S3L engines combined with stock wrx heads?

OEM compression on EDM WRX/STI are 8 to 1
OEM compression on EDM -06 version are 8.4 to 1 according to norwegian subaru website. (First time we get a 2.5L engine in a impreza, interresting to see what engine the new EDM sti will have since we always have had 2.0L here)

No, We supply different pistons to mate to the type of head you will be using on the short block you order from us. This comes at no extra cost to you. We will also make the short block to the compression ratio of your choice.

You will be very pleased with your new EDM 2.5L motors :)

Quirt Crawford
www.crawfordperformance.com

The components you are using will mechanically lead to the issues you are having with your motor. Using a thicker head gasket to lower your compression ratio will cause your motor to be very det prone because of the increase in the squish volume. The un burnt fuel from having to large of a squish area causes the detonation.
Another mechanical issue you have is cam timing. The thicker head gaskets will advance your cam timing, and due to the design of the boxer motor, the left cams will be advanced twice as much as the right cams :eek:

Back to your original question, We run 8.1 to 1 on our 2.5L pump gas motors.

Quirt Crawford
www.crawfordperformance.com

That will do it. Thanks for your mechanical expertise on this combination. I bet you are correct on the cam timing which is causing the lack of power.

I know many are afraid of higher compression but most on these boards has an option for some form of programmable engine management (Hydra, Cobb, UTEC, etc). Thicker heads gaskets are a bad idea due to effect it has on quench and you are simply better off tuning your EMS to handle the higher compression if you cannot have custom pistons made. Higher compression in moderation will also allow you to make more power at the same boost pressure. Often this requires less ignition advance which has a positive effect on your rod and rod bearings.

If you are not tuning your own EMS then let your tuner know you have a high compression setup and if they are experienced the will know what to do.

Thanks,
Phil
www.elementtuning.com

I wish there were comments from the other engine builders that recommend and sell the thicker head gaskets as a solution to smaller combustion chambers of the wrx heads.

I wish there were comments from the other engine builders that recommend and sell the thicker head gaskets as a solution to smaller combustion chambers of the wrx heads.

+123456789

I wish there were comments from the other engine builders that recommend and sell the thicker head gaskets as a solution to smaller combustion chambers of the wrx heads.


There have been, just evidently not the folks you wanted to hear it from. ;)

S.

There have been, just evidently not the folks you wanted to hear it from. ;)

S.
in this thread? must have missed them somehow...

The components you are using will mechanically lead to the issues you are having with your motor. Using a thicker head gasket to lower your compression ratio will cause your motor to be very det prone because of the increase in the squish volume. The un burnt fuel from having to large of a squish area causes the detonation.
Another mechanical issue you have is cam timing. The thicker head gaskets will advance your cam timing, and due to the design of the boxer motor, the left cams will be advanced twice as much as the right cams :eek:

Back to your original question, We run 8.1 to 1 on our 2.5L pump gas motors.

Quirt Crawford
www.crawfordperformance.com


Thanks for ur insight guys...

I naively bought the long block without looking into all the components :o, this has lead to me goin back to the orginal engine builder and asking wot was done to the engine...

What compression do u guys then recomend for my setup?? and wot type of head gasket would u guys use? This is my full setup to my knowledge

Would i need to know the brand of pistons that im currently using?? As this would also affect the ratio?


USDM STI 2.5ltr built block (forged pistons, rods, balanced crank, machined
head, ARP head stud kit)
standard 03 STI heads and cams with AVCS (Australian spec)
GT35R with 0.82 A/R rotated mount
Perrin FMIC with R/T custom aluminum piping
Custom 4" aluminum intake with pod
reversed manifold (keeps A/C)
Tial 44mm external wastegate
Tial 50mm BOV
X-force titanium exhaust
X-force equal length headers (coated and wrapped)
S/S R/T custom downpipe
Tilton twin plate clutch
PE800cc injectors with fuel rails
Dual walbro inline pumps
MOTEC M800 plug and play

First off it could be bad tuning but if not then:

Your compression ratio is fine but if you're running standard 2.0 STI heads and cams then that could be one of the problems (not sure on the lift of those cams). The 2.5 needs lots of room and cam to breath on the top end. What works for the 2.0 does not work on the 2.5. A set of Spec C heads would work wonders or even putting a set of cams such as JUN 272 will bring back the top end.

The US STI heads are also nice with larger valves and higher lift cams. These heads will make great power to 7500 rpm flowing up to 550 whp. Over that power level cams and head work are in order.

Also make sure your cam timing is correct. If you are experiencing exceptional spoolup this could be a sign you have your intake cam advanced which will kill high rpm power.

Good luck.

Phil
www.elementtuning.com


Im using motec M800 for ecu tuning (which a respected tuner is tuning) Spoolup is around 4500rpm atm which is a lil bit laggy for my setup with reversed manifold and shorter piping.. however not all the timing etc has been put into the tune.

The motec is reading the cam timing correctly apparently

I have a simliar engine setup to your's: 8.2:1.

I am strubbling through IC plumbing leaks so I am attributing my power deficiency on too much fuel, plus I am not pushing real hard right now until I seal my intake plumbing up (don't want to overspin my turbo).

Let me ask you something. On the cometic gaskets, is oil leaking out of the bottom of your heads where they meet the block? Right where the oil return is for the heads? just curious.

i think i have seen some deposits near the block on the right hand side of the car from the drivers position (my car is right hand). The car dosent seem to be using much oil though

The components you are using will mechanically lead to the issues you are having with your motor. Using a thicker head gasket to lower your compression ratio will cause your motor to be very det prone because of the increase in the squish volume. The un burnt fuel from having to large of a squish area causes the detonation.
Another mechanical issue you have is cam timing. The thicker head gaskets will advance your cam timing, and due to the design of the boxer motor, the left cams will be advanced twice as much as the right cams :eek:

Back to your original question, We run 8.1 to 1 on our 2.5L pump gas motors.

Quirt Crawford
www.crawfordperformance.com

I somewhat disagree with what you are saying here Quirt. (but in a polite and cheerful way! )

In a rare swap of roles, I lend greater accuracy to the practical measured results in deference to the theory.

The theory goes something like this (I should add that in concept I agree with Quirts observations on this theory):

Using a thicker head gasket will result in two potential problems.

First, since the thicker gasket pushes the heads out, it will offset the cams. Based on a previous post, the additional 1mm of gasket spacing will offset the passenger cams by approximately a degree, and the drivers cams by approximately 2 degrees. As I mentioned in that previous thread, I am not fully convinced that this offset will be significant in terms of measured power delivery. Furthermore, on a car with AVCS, the AVCS system will detect the offset, and can correct for it based on the called for CAM angles. (Note: There would be a limit to this, as the cam angle can’t be retarded beyond to zero pressure setting). None the less, while there is a theoretical cam advance issue, I do not see it as largely significant.

Second, since the larger head gasket sits between the head and the top of the piston, it logically extends the quench height. Theoretical discussions about quench height have not occurred here on Nasioc with any great success (primarily due to the lack of engineering data about real effects with the EJ motors and EJ heads).

However, we can look a bit closer at some of the different motor/head combinations:

Let us start by looking at some older tables that I have posted:

http://www.pdxtuning.com/nasioc/EJ257Info.gif

Looking at this chart, there is an obvious deviation in the quench height on the stock Subaru Motors. Notice that the EJ207 motor has a MUCH larger quench height then the US counterparts. This was a surprise to me when I first made this chart. For those used to working in English units, the US quench is .038”, while the EJ207’s is .081”. .081” is a VERY large quench by my limited understanding.

Now let us look at what might happen with a thicker head gasket:

http://www.pdxtuning.com/nasioc/DysonCalcs.jpg

Notice that the WRX heads WITH the gasket more closely matches the EJ207 configuration. I fully agree that the theoretical application of quench says this would be too much, however practical results seem to show otherwise.

For example, I have tuned a number of both Gasket and Non Gasket WRX Head/EJ257 Block combos. Both configurations have made great power, but in general I was a bit more successful with the gasketed ones. My personal WRX runs the EJ257, Spec C heads, and Cometic gasket, and makes ample power (421whp on pump gas, on our dyno). Given that I have posted significant data results from this tune, it is easy to see how the motor responds to the timing I could run.

Other tuners could probably also comment on their experience with the EJ207 platform compared to the EJ205. ( Clark has made comments on this several times, as have many others). I have often suspected that the additional quench height combined with the piston design of the EJ207 made it so much less knock prone then the EJ205s.. However my guesses are just that, and I have to rely on practical experience to determine what steps will work best.

I would love to hear others comments.

Jeff Sponaugle
PDXTuning.com

+123456789

That's me!
Just kidding Quirt!
Nice meeting you at Sema btw!
Jeff

For the folks that are likely to only build one engine up, what's good for me to know is:

1) gasket thickness and quench can be a problem
2) if you can avoid a problem (say by using stock gaskets) that might be a good thing to do
3) reducing your CR with a thicker gasket could cause a problem with quench, and is better solved by lowering CR with different sized pistons (larger dish)

1) gasket thickness and quench can be a problem
2) if you can avoid a problem (say by using stock gaskets) that might be a good thing to do
3) reducing your CR with a thicker gasket could cause a problem with quench, and is better solved by lowering CR with different sized pistons (larger dish)


While I agree in theory, I am much suprised to find the thicker gasket does not seem to be a problem.. In fact in some cases it seems to improve knock resistance.

These observations, combined with the data from Subaru on the EJ207 make this a very interesting and compelling topic! I would love to hear from other tuners who have tuned with and without the gaskets to comment as well.

Jeff

I have a crawford s3l with 8.7:1 pistons, V7 sti heads, and V8 cams that should be all together this weekend. I'm using stock head gaskets. I'll probably be taking the car to TXS to tune. I'll try to get nathan to post what he notices from my car in this thread. Just another data point...

While I'm no SAE engineer or tuner, I'll add that it's not "good science" to speculate on the effects of quench using three different piston crowns (205, 207 257), two different R/S ratios( 205, 257), and different camshafts (205,207, 257).

But...keeping within the spirit of the topic, I'll add:

I like the minimum amount of quench, especially in a detonation prone engine. Anything to speed up the burn. :)

S.

Jeff, is your messenger broken?

I'll add that it's not "good science" to speculate on the effects of quench using three different piston crowns (205, 207 257), two different R/S ratios( 205, 257), and different camshafts (205,207, 257).

But...keeping within the spirit of the topic, I'll add:

I like the minimum amount of quench, especially in a detonation prone engine. Anything to speed up the burn. :)




Yeah, from what I've learned from various sources this seems to be right on (of course I'm no SAE Engineer either ;) )

Well, here's a good way for one of you "pro" engine builders to broaden your product line and offerings. Which ever company that you are having manufacture your pistons, why not go a step further and have a couple of more pistons designs made, i.e changing the crown to reflect the higher CR of the various head options.

On mine, I had Wiseco make pistons with a lower CR design piston top. I think they made them -24cc IIRC. It didn't really cost that much more for them to do it either (design reflect their off the shelf pistons) and with WRX heads, EJ257 gaskets, I'm at 8.4:1 by choice, no guessing. Something like this would definitely win some customers doing hybrid setups and there are a lot of them.

Just thinking out loud :D

My CP pistons were custom made by cp to crawford's specs to mate up with my V7 heads. The tops of the pistons are actually different shape to the pistons crawford uses with regular ej257's. They are also different from the off the shelf CP pistons (which I have a brand new set laying around if anyone wants to buy them ;) )

Here is an apples to apples comparison on an 04 STI with a Greddy T67 8cm:

Customer A flew me in to tune his car which made around 390 whp on pump fuel.
Customer A decides to install oem replacement forged pistons (along with a thicker head gasket so he can run more boost reliably and make more power)
Customer A flew me in to tune his car with new pistons and thicker head gasket (no other changes).
Customer A’s engine was indeed less knock prone at the same boost, AFR and timing values.
Customer A’s setup with thicker head gasket made substantially less than 390 whp.
Customer A’s new engine gets more boost, more ignition timing, and a leaner AFR to hit 390 whp.
Customer A’s engine was just as knock prone at 390 whp as it always has been.
Customer A removed the thicker head gasket and lived happily ever after with less boost, timing, and more conservative AFR.

Thanks,
Phil
www.elementtuning.com

That's me!
Just kidding Quirt!
Nice meeting you at Sema btw!
Jeff

Thanks Jeff, I enjoyed meeting you and all the others at sema this year :)

I am sad to see you listening to your engine builder instead of your dyno when it comes to building a motor with the proper specifications / components. :(
In my opinion, cutting corners is not the correct approach when building a high performance engine.

Quirt Crawford
www.crawfordperformance.com

I am sad to see you listening to your engine builder instead of your dyno when it comes to building a motor with the proper specifications / components. :(
In my opinion, cutting corners is not the correct approach when building a high performance engine.

Quirt Crawford
www.crawfordperformance.com


hmmm. am I missing something? I even re-read the thread....

What effect does the area/volume, not the height, play in all of this? These Subaru heads have a tiny quench area as far as I can tell. And the STi pistons make this area even smaller. It appears the CP pistons take advantage of the head's quench area a bit more than the stock pistons - but it still appears to be rather small.

http://i25.photobucket.com/albums/c67/b0nza1/IM000390.gif

Can I get some info on this engine ?

How far is the piston down in the hole.
Compressed Head gasket thickness.
Chamber Volume in cc's.
Cylinder diamter.
stroke length.

Thanks Dave

This post should answer all of your questions regarding specifications.

In a rare swap of roles, I lend greater accuracy to the practical measured results in deference to the theory.

The theory goes something like this (I should add that in concept I agree with Quirts observations on this theory):

Using a thicker head gasket will result in two potential problems.

First, since the thicker gasket pushes the heads out, it will offset the cams. Based on a previous post, the additional 1mm of gasket spacing will offset the passenger cams by approximately a degree, and the drivers cams by approximately 2 degrees. As I mentioned in that previous thread, I am not fully convinced that this offset will be significant in terms of measured power delivery. Furthermore, on a car with AVCS, the AVCS system will detect the offset, and can correct for it based on the called for CAM angles. (Note: There would be a limit to this, as the cam angle can’t be retarded beyond to zero pressure setting). None the less, while there is a theoretical cam advance issue, I do not see it as largely significant.

Second, since the larger head gasket sits between the head and the top of the piston, it logically extends the quench height. Theoretical discussions about quench height have not occurred here on Nasioc with any great success (primarily due to the lack of engineering data about real effects with the EJ motors and EJ heads).

However, we can look a bit closer at some of the different motor/head combinations:

Let us start by looking at some older tables that I have posted:

http://www.pdxtuning.com/nasioc/EJ257Info.gif

Looking at this chart, there is an obvious deviation in the quench height on the stock Subaru Motors. Notice that the EJ207 motor has a MUCH larger quench height then the US counterparts. This was a surprise to me when I first made this chart. For those used to working in English units, the US quench is .038”, while the EJ207’s is .081”. .081” is a VERY large quench by my limited understanding.

Now let us look at what might happen with a thicker head gasket:

http://www.pdxtuning.com/nasioc/DysonCalcs.jpg

Notice that the WRX heads WITH the gasket more closely matches the EJ207 configuration. I fully agree that the theoretical application of quench says this would be too much, however practical results seem to show otherwise.

For example, I have tuned a number of both Gasket and Non Gasket WRX Head/EJ257 Block combos. Both configurations have made great power, but in general I was a bit more successful with the gasketed ones. My personal WRX runs the EJ257, Spec C heads, and Cometic gasket, and makes ample power (421whp on pump gas, on our dyno). Given that I have posted significant data results from this tune, it is easy to see how the motor responds to the timing I could run.

Other tuners could probably also comment on their experience with the EJ207 platform compared to the EJ205. ( Clark has made comments on this several times, as have many others). I have often suspected that the additional quench height combined with the piston design of the EJ207 made it so much less knock prone then the EJ205s.. However my guesses are just that, and I have to rely on practical experience to determine what steps will work best.

I would love to hear others comments.

Jeff Sponaugle
PDXTuning.com

With the limited amount of info on the web regarding quench distance.. all info says .81" is way too much.. none seem to address forced induction engines, though. Also, there are many people on these boards running the ej257/Cometic gasket and WRX head combo quite successfully.

Quirt's pistons look like they reduce the quench potential of the pistons more than the stock STi pistons.

http://www.we-todd-did-racing.com/wetoddimage.wtdr/wMTE0NjYzNDZzNDEzZGZkMzF5NTQx.jpg

And the CP pistons used by Axis seems to increase the quench area to it's maximum.

http://axispowerracing.net/25sti/Picture004.jpg

Since I'm using the Cometic gaskets.. I'm assuming the added area of the Axis CP pistons may cause me some additional headaches.

Is this a pump gas engine ? What kind of power output should something like this make ? And what kind of power are you making now ? Sorry I am somewhat of a newb when it comes to subaru engine setup's I just ordered Dynomation programs and going to start trying to understand more of this motor.. From what I am used to on our ford 4.6 4v engines on a max race engine where pump gas is not used we run as high as 11.7:1 Cp with over 35 p.s.i on a t 76 twins... I am am also a fan of Higher compression Fi engines when pump gas is not being used...

http://www.pdxtuning.com/nasioc/EJ257Info.gif

Now let us look at what might happen with a thicker head gasket:

http://www.pdxtuning.com/nasioc/DysonCalcs.jpg

I would love to hear others comments.

Jeff Sponaugle
PDXTuning.com

Jeff, looking at the above, your two charts contradict each other. For instance, you show the std WRX/STi hybrid (stock .6mm gasket) with a CR of 8.6:1 in the first, but then 8.8:1 in the second. Also, stock WRX CR went from 8.17:1 to 8.38:1.

All of this resulted from your WRX head vol changing from 51 to 49. Why is that? The first batch of numbers seemed more in line with everyone's reasonings? Was this a typo?

-C

bump this thread for helpfull info