9.0:1 Compression

7527 Views 20 Replies 6 Participants Last post by microsnook, Aug 16, 2013

microsnook

Discussion Starter · Aug 14, 2013

Right now my car has 78k miles on the engine and about 15k has been with my 20G set up. This includes supporting mods such as, 850cc injectors, FMIC, fuel pump, exhaust, EWG, uppipe and a tune.

My water pump is broken and Im thinking about replacing my timing belt, water pump and possibly throwing in some pistons for security since I'm running my 20G at 19PSI.

The pistons I were thinking about are the CP 9.0:1 99.75mm. This would give me low end power but not as much on the high end. I'm not sure how my car is going to handle the higher compression, so I figured I would get some input. I've read lots of opinions saying they are fine or they are no good, stay away, etc. Should I stick with stock compression and stock bore or what exactly should I be looking out for? Since I am only at 19 PSI, couldn't I increase boost since the turbo is capable but the higher compression won't allow me to boost as high?

I'm probably going to do the pistons, with a lightweight pulley, full gasket kit, a new tune and I would like to run a bit more aggressive since the pistons should provide security.

Opinions!
Thanks

1 - 20 of 21 Posts

rustler753

· #2 · Aug 14, 2013

Are you going to be running better fuel than just 91, like 91 + meth or E85?

If not I would stick to stock compression or lower than 9:1. If you are, hell I am going to be running 10:1 on an E85 only map.

Madcow

· #3 · Aug 14, 2013

Putting new pistons in is a lot more involving than just changing the water pump and timing belt. It would require the entire motor out. And because of the way the old pistons wear in the block it should be bored out for the size of the new pistons. So in essence you would be rebuilding the entire motor, which fiscally wouldn't make sense.

Also, i'm running my td06h 20g with a peak boost of 24.5 tapered down to 21.5 at redline on 93 octane with zero knock. So you have alot more to go on that 20g if you want.

microsnook

Discussion Starter · #4 · Aug 15, 2013

rustler753 said:
Are you going to be running better fuel than just 91, like 91 + meth or E85?

If not I would stick to stock compression or lower than 9:1. If you are, hell I am going to be running 10:1 on an E85 only map.

I run 93 pump gas and might move to CO where the elevation is higher (less compession) but octane is lower, 91. Not sure how that would make a difference.

Madcow said:
Putting new pistons in is a lot more involving than just changing the water pump and timing belt. It would require the entire motor out. And because of the way the old pistons wear in the block it should be bored out for the size of the new pistons. So in essence you would be rebuilding the entire motor, which fiscally wouldn't make sense.

Also, i'm running my td06h 20g with a peak boost of 24.5 tapered down to 21.5 at redline on 93 octane with zero knock. So you have alot more to go on that 20g if you want.

Correct sir, thanks for the response. I am planning on having it honed and bored to 99.75 size. The clutch will be replaced, lightweight pulley, gates kit, apr heads, replace the main and rod bearings and then get a tune.

Now what I'm unsure of is which route I should go with the pistons. I think I definitely want the 99.75 bore but now which compression should I do?
Will I need to change my fuel?
Like you said the 20G has a lot more potential and my injectors still have plenty of room as does the fuel pump...

Im looking to be around 400 ish....
Does this sound real?
Thanks !

Madcow

· #5 · Aug 15, 2013

Ah so you are basically talking about a full engine rebuild

You didn't mention that.

With 93 octane you wont want to raise the compression. More compression = less boost potential due to knock. And the trade off isn't worth it.

Another option you can consider if you want more power (because a 20g will only get you to high 300's hp on 93) is put a new cam in it when you get everything rebuilt.

BLKSTI05

· #6 · Aug 15, 2013

400ish sounds plenty real but at that power level your closing in on the stock rods limit. With that much wheel HP in mind you should consider upgrading rods also.

BLKSTI05

· #7 · Aug 15, 2013

Madcow said:
Ah so you are basically talking about a full engine rebuild You didn't mention that.

With 93 octane you wont want to raise the compression. More compression = less boost potential due to knock. And the trade off isn't worth it.

Another option you can consider if you want more power (because a 20g will only get you to high 300's hp on 93) is put a new cam in it when you get everything rebuilt.

Yes, what he said. Also a good header, port and polish the heads will help. Leave the compression alone. Safer that way.

microsnook

Discussion Starter · #8 · Aug 15, 2013

Madcow said:
Ah so you are basically talking about a full engine rebuild You didn't mention that.

With 93 octane you wont want to raise the compression. More compression = less boost potential due to knock. And the trade off isn't worth it.

Another option you can consider if you want more power (because a 20g will only get you to high 300's hp on 93) is put a new cam in it when you get everything rebuilt.

I wouldn't have said full but partial. Honing and pistons I guess would be a full build whatever you wanna call it

.
Ok so I will go with stock compression pistons, but should I go for the bigger bore, the 99.75 instead of stock? How do you feel on that?

What exactly will new cams benefit, Im still learning about this stuff...and Im already quoted at $4500 for all the parts and labor and cant afford to really go higher.

BLKSTI05 said:
400ish sounds plenty real but at that power level your closing in on the stock rods limit. With that much wheel HP in mind you should consider upgrading rods also.

Hmm. I guess thats true. What are they rated for around 500BHP or so?
Ill look into them thanks for the suggestion ,but probably wont make over 400 anyways. And I am on a budget...

BLKSTI05 said:
Yes, what he said. Also a good header, port and polish the heads will help. Leave the compression alone. Safer that way.

Thanks thats two suggestions on stock compression so Im glad I will go in that direction.
What about the bore size?
Ill consider the headers but again I am on a budget.

Thanks guys I appreciate the quick help

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rustler753

· #9 · Aug 15, 2013

microsnook said:
I run 93 pump gas and might move to CO where the elevation is higher (less compession) but octane is lower, 91. Not sure how that would make a difference.

If you do come to CO, you can swap to E85 depending where you live, we have it in quite a few places!

microsnook

Discussion Starter · #10 · Aug 15, 2013

rustler753 said:
If you do come to CO, you can swap to E85 depending where you live, we have it in quite a few places!

That might be a consideration

orestes85

· #11 · Aug 15, 2013

Madcow said:
Ah so you are basically talking about a full engine rebuild You didn't mention that.

With 93 octane you wont want to raise the compression. More compression = less boost potential due to knock. And the trade off isn't worth it.

Another option you can consider if you want more power (because a 20g will only get you to high 300's hp on 93) is put a new cam in it when you get everything rebuilt.

eh, I use 9:1 cp pistons and the car is tuned to 26 psi on 93 octane. Data logs are squeaky clean.

microsnook

Discussion Starter · #12 · Aug 15, 2013

orestes85 said:
eh, I use 9:1 cp pistons and the car is tuned to 26 psi on 93 octane. Data logs are squeaky clean.

That's awesome. I knew I would expect someone to say they were alright and others to say no. I ended up going with stock compression and bigger bore.

You are running 26 PSI, would you be able to push it a little more?
Whats your set up?

I'm curious to see how hard I would be able to push the turbo and what PSI I could run it at with the new set up. I run 93 too.

My car is going in monday so any advice helps.

orestes85

· #13 · Aug 15, 2013

Re: Re: 9.0:1 Compression

microsnook said:
That's awesome. I knew I would expect someone to say they were alright and others to say no. I ended up going with stock compression and bigger bore.

You are running 26 PSI, would you be able to push it a little more?
Whats your set up?

I'm curious to see how hard I would be able to push the turbo and what PSI I could run it at with the new set up. I run 93 too.

My car is going in monday so any advice helps.

I'm running 26PSI on a gtx3576 with a built longblock on gsc s2 cams and ID1000s. We ran out of fuel pump at 27psi when it started going lean, but it made 430whp.we could probably run 28-29psi with no issues with more fuel pump and injector to work with.

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microsnook

Discussion Starter · #14 · Aug 15, 2013

I'm running 19 psi now what do you think I will be able to increase it to if my injectors are only at 68%? It was boosting to 23psi before but I changed the wastegate spring to lower it. Now with the pistons I would like to increase the wastegate spring again...

Thanks

boatsnhos

· #15 · Aug 15, 2013

Madcow said:
Putting new pistons in is a lot more involving than just changing the water pump and timing belt. It would require the entire motor out. And because of the way the old pistons wear in the block it should be bored out for the size of the new pistons. So in essence you would be rebuilding the entire motor, which fiscally wouldn't make sense.

this is assuming he doesnt do the work himself. if you can rebuild the engine you can do it for much less than a new shortblock your looking at say 800-900 for pistons/rods, say 300 for a gasket kit, 120 for main and rod bearings, and at the most 400 to get the heads decked and hone the cylinders. thats about 1700. whats a built shortblock, at least 23 to 2500?? if you can do the work yourself , do it. if you have any doubts, dont even try

Madcow

· #16 · Aug 15, 2013

microsnook said:
I'm running 19 psi now what do you think I will be able to increase it to if my injectors are only at 68%? It was boosting to 23psi before but I changed the wastegate spring to lower it. Now with the pistons I would like to increase the wastegate spring again...

Thanks

You got plenty to go man i'm only hitting around 87% at 7k. There's some dispute as to how far you can push the injectors. I've read somewhere the subaru ECU's overvalue injector duty cycle by around 20% and people have gone over 100% while increasing richness.

Also why are you swapping out wastegate springs all the time? A proper tune will adjust your wastegate duty cycle for the boost you want.

BLKSTI05

· #17 · Aug 16, 2013 (Edited)

I have heard stock rods are good for aprox 450 depending on your tune. 4500 is a big price tag or at least i think. Dose this price include everything from engine R&R to dyno and tune? And dose this builder has a good rep?

microsnook

Discussion Starter · #18 · Aug 16, 2013

Madcow said:
You got plenty to go man i'm only hitting around 87% at 7k. There's some dispute as to how far you can push the injectors. I've read somewhere the subaru ECU's overvalue injector duty cycle by around 20% and people have gone over 100% while increasing richness.

Also why are you swapping out wastegate springs all the time? A proper tune will adjust your wastegate duty cycle for the boost you want.

I'm not swapping it out all the time its been how it is for a year now. I thought the EWG was spring related and needed the right pressure combo for desired boost.

BLKSTI05 said:
I have heard stock rods are good for aprox 450 depending on your tune. 4500 is a big price tag or at least i think. Dose this price include everything from engine R&R to dyno and tune? And dose this builder has a good rep?

My rods should be fine for my power level then I wont be able to make that much power unless e85.
This includes all the parts, maching, labor, dyno tune - yes.
I think they are a reliable company

Madcow

· #19 · Aug 16, 2013

You are partly correct. The wastegate spring, in conjunction with your boost control silinoid (and by extension wastegate duty cycle) controls when your waste gate opens. Changing the wastegate spring will definitely change your boost, but changing it without tuning the waste gate duty cycle could have some negative side effects (such as slower spool time, boost creep, boost spikes, etc).

So in short, just stick with your spring and get it tuned for the boost you want

Madcow

· #20 · Aug 16, 2013

Oh and remember the rule of thumb: You can't run less boost than what your wastegate spring is rated for. I.e. if you had 1 bar spring you can't run less than 14.5 psi. I think the highest I've seen is a 1.5 bar spring which if I recall is 21.75 psi.

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