Piston compression choice for turbo

[jakelandry]

Im beginning to take steps toward my turbo setup slowly and i was wondering what compression pistons i should go with. I know lower compression is better for turbo applications and whatnot but does it really make much of a difference? Ive seen plenty of people on here who run 10 comp and make fairly large numbers with the 420a. Im planning on getting howells long rod bottom end kit i just need help deciding on compression.

 

[necrotopsy]

several factors need to play in on your choice of compression.

are you going to run pump gas or e85?
how good of an intercooler are you going to run?
how much boost are you going to run?
what is the outsdie temp generally where you will be driving the car?

lower compression reduces the risk of knock, but also takes away power when not in boost. so answer those questions and i could probably help you more.

 

[joescool2008]

i'd say 8.5 to 1, my friend 1slow97 runs around that and has 450whp on his turbo 420a

 

[necrotopsy]

well i run 8.8:1 and got 410 hp on 22 psi.

 

[Tanro]

Well in short 8.5:1 is the gold standard. But a quick Google search and my bookmarks from when I was into to turbocharging saturns (same principal turbo charging a non turbo car) provided these links, All of them have very good info that apply to all forced induction applications in some form or another. If you have the time definitely check out the links on Garrett's website.

Turbocharging 101 - Garrett
Turbocharging 102 - Garrett

Adding Turbo - Google

Turbo Basics - My Bookmarks

I really suggest reading the last one, it helped me a great bit when I was doing the saturn thing, and the garrett reads are excellent for anyone.

It all really comes down to do I want to A) Just run a few lbs on stock internals, B) Want to Boost to high heaven but not have great lowend or fuel economy? (8.0:1 or lower) C) S0mething in the middle (8.5:1-89.0:1) Generally Something in middle is the preferred choice. However, you can run lots of boost on higher CR with forged internals, meth injection, full race gas etc. You just need to be aware that with higher CR and Higher PSI you have more chance for Knock/Detonation which is bad. The lower the CR the more boost you can run and vice versa without any need for a Meth/Water Kit, race gas etc.

 

[jakelandry]

Thank god. I was waiting for a few people who knew what they were talking about to help me. Im planning on runnning pump gas ( 93 octane) and itll be my dd. Probably going to keep it at 8 psi till i learn megasquirt then ill crank it up. Im shooting for 300hp with mega and around 225 without so pumo gas should be fine. So if i go low compression im going to want a setup that spools at a low rpm?

 

[bullettdsm]

Thank god. I was waiting for a few people who knew what they were talking about to help me. Im planning on runnning pump gas ( 93 octane) and itll be my dd. Probably going to keep it at 8 psi till i learn megasquirt then ill crank it up. Im shooting for 300hp with mega and around 225 without so pumo gas should be fine. So if i go low compression im going to want a setup that spools at a low rpm?

With those goals and MSnS, I would be comfortable 10.5 comp. Its all about the tune and MSnS is very helpful.

Now we do need to know what turbo you are going with (psi is not a measure of power 8psi on a 14b does not equal 8 psi on a GT42r) to be sure. But I was not knock limited until around 400whp (now that's on 112 oct and a BW366). I would be fully comfortable on street gas and high comp up to 300whp.

MB

 

[jakelandry]

With those goals and MSnS, I would be comfortable 10.5 comp. Its all about the tune and MSnS is very helpful.

Now we do need to know what turbo you are going with (psi is not a measure of power 8psi on a 14b does not equal 8 psi on a GT42r) to be sure. But I was not knock limited until around 400whp (now that's on 112 oct and a BW366). I would be fully comfortable on street gas and high comp up to 300whp.

MB

The only thing is i dont plan to get rid of my car anytime soon. Im sure i will hit 300 and want more from there, everyone does. Im ganna be running an evo 3 16g. What i dont understand is what is the advantage of running a higher comp? Lower comp has its advantages but i have yet to really notice and high comp advantages...

 

[necrotopsy]

higher compression = power power when the turbo isn't spooling. so like off the line power. actully i think it's just more power period(like some drag cars run 14:1 and higher). but like it's already been said, more compression = higher risk of knock.

to counteract knock you can do several things.

tune rich.
lower intake temps this could include:
--bigger intercooler
--water/meth injection
--etc.
use higher octane gas. octane rating is really an "anti-knock" rating.
go with e85 instead of regular gas.

 

[jakelandry]

higher compression = power power when the turbo isn't spooling. so like off the line power. actully i think it's just more power period(like some drag cars run 14:1 and higher). but like it's already been said, more compression = higher risk of knock.

to counteract knock you can do several things.

tune rich.
lower intake temps this could include:
--bigger intercooler
--water/meth injection
--etc.
use higher octane gas. octane rating is really an "anti-knock" rating.
go with e85 instead of regular gas.

Well im sure i will do fine with lower comp then cause im not trying to build a drag car here, just a fun dd. Thanks for all the info, maybe one of you 420a savey guys can help me with my next question also...?

http://www.dsmtuners.com/forums/newbie-forum/382841-420a-bearing-question.html

 

[Tanro]

If your shooting for 8 PSI you can do that on stock internals. I would go for 9.5:1 area if you end up buying forged internals you will get better gas mileage when off boost, and you will also be able to run more boost later if you so choose. Especially with forged internals. Max out that 16g on 9.5:1 and you could be putting down 350-400 WHP.

 

[jakelandry]

If your shooting for 8 PSI you can do that on stock internals. I would go for 9.5:1 area if you end up buying forged internals you will get better gas mileage when off boost, and you will also be able to run more boost later if you so choose. Especially with forged internals. Max out that 16g on 9.5:1 and you could be putting down 350-400 WHP.

Welll i want forged internals just for safety sake, but how would i get 9.5.1? The je's that come with that long rod kit are made specifically for long rods and are only offered in 8 10 and 12 i beleive... Could i get 10 and mill the head out more or something?

Ok plans could have possibly just changed, they just put an e85 station in 20 miles away and i saw its 2 bucks a gallon! How do i build my car to run off of e85? Does it require hellified tuning?

 

[jakelandry]

Well in short 8.5:1 is the gold standard. But a quick Google search and my bookmarks from when I was into to turbocharging saturns (same principal turbo charging a non turbo car) provided these links, All of them have very good info that apply to all forced induction applications in some form or another. If you have the time definitely check out the links on Garrett's website.

Turbocharging 101 - Garrett
Turbocharging 102 - Garrett

Adding Turbo - Google

Turbo Basics - My Bookmarks

I really suggest reading the last one, it helped me a great bit when I was doing the saturn thing, and the garrett reads are excellent for anyone.

It all really comes down to do I want to A) Just run a few lbs on stock internals, B) Want to Boost to high heaven but not have great lowend or fuel economy? (8.0:1 or lower) C) S0mething in the middle (8.5:1-89.0:1) Generally Something in middle is the preferred choice. However, you can run lots of boost on higher CR with forged internals, meth injection, full race gas etc. You just need to be aware that with higher CR and Higher PSI you have more chance for Knock/Detonation which is bad. The lower the CR the more boost you can run and vice versa without any need for a Meth/Water Kit, race gas etc.

Wow this is some good reading, i have one more question however. Ive read somewhere on here that dropping the compression to 8:6:1 from stock will only lose me 4% HP before turbo kicks in, 4% of 130HP is roughly 5.25 hp. So if i go low comp my car will basically pull like it does now before turbo kicks in?

 

[Carter_95RS]

It will be a little lower, yes. But I doubt it will be very noticeable with the fast spool of the 16g.

 

[necrotopsy]

eh... with 8.8:1 i'd say it's closer to 120(ish) HP with no turbo.
but even with that said, that is plenty enough to launch the car. the only thing i noticed is that i have to rev the car a little higher than stock compression engine to launch from stop (for daily driving). but that could be a combination of several things... idk.

thought i should mention that with forged internals you WILL get piston slap. its not necessarily bad thing, but it WILL happen and you WILL notice it (when the engine is cold). so don't freak out about it.

 

[jakelandry]

Alright cool well thanks for all the advice everyone, i think im going low comp i just hate to jump blindly into something i dont know much about, but ill learn as i go.

 

[Slo2gen]

Just because you have forged pistons doesn't mean you WILL have piston slap. A lot of that depends on the piston your choose(silicone content) and your machine shop's ability. And with stock compression and bolt on these cars make 117whp, thats about it. So if you use low compression pistons without boost, it WILL SUCK BALLS. Lowering compression and adding boost is a safe way to make power but you also sacrifice power. Do as you wish but I personally wouldn't go lower than 10.5:1, but if you don't have the ability to tune then stick with standard, 9.6:1. Take it for what is worth though, just my opinion.

 

[Tanro]

8.8:1 w/ a turbo will be fine.

 

[necrotopsy]

^^ huh?

for 1 forged internals are made out of different materials than stock block. so the 2 metals will heat up at different rates. metal expands as it heats up. different expansion rates = piston slap from not perfectly balanced expansion rates. i didn't say it would be loud as heck, but it is noticeable. sounds like light lifter tick in most cases.

for 2 he's building a TURBO 420a, and you're telling him to run 10.5:1? are you for real? that is a bad idea unless he planned on running e85, which he already said he plans on running pump gas. 10.5:1 + any sort of real boost = bad things.

for 3 117hp at the CRANK? maybe on a wore out engine. and a good engine with good compression should be close to 140HP. maybe 117 whp. and that would have to be an automatic.

now if he was staying NA then 10.5:1 i could agree with.

*note* maybe he was talking about OP building the engine, and adding a turbo later... which in that case i somewhat agree. don't run lower compression on a NA motor.either build it and turbo it at the same time, or don't build it IMO. you will want to beat your brains out over it if you do them separate.

 

[Locke]

^^ huh?

for 1 forged internals are made out of different materials than stock block. so the 2 metals will heat up at different rates. metal expands as it heats up. different expansion rates = piston slap from not perfectly balanced expansion rates. i didn't say it would be loud as heck, but it is noticeable. sounds like light lifter tick in most cases.

This is more dependant on the material used for the piston, rather than the fact that it is forged or not. Most forged pistons are hypoeutectic; low silicon content causes them to vary noticeably between cold and hot. 2618 pistons, such as those offered by JE and Wiseco, are very hypoeutectic. Wiseco's seem to slap less than the JE's, though.

4032 pistons, such as offered by Arias and Mahle, has much more silicon. These pistons are forged eutectic; typically not as strong as 2618 alloy pistons, but they expand less and are thus quieter.

The factory cast pistons are hypereutectic, and a different material than the block. It also suffers from expansion, but to a lesser degree than the others. These pistons could be forged for added strength, but the added benefits of forging are still limited by the alloy. Forging these is not very economical.

for 2 he's building a TURBO 420a, and you're telling him to run 10.5:1? are you for real? that is a bad idea unless he planned on running e85, which he already said he plans on running pump gas. 10.5:1 + any sort of real boost = bad things.

10.5:1 does require a bit more fine tuning, but it is very doable. Bullet made quite a bit of power on his boosted 10.5:1 420a. Zac's 10.5:1 turbo'd 420a made 400+whp recently. Both are (were) capable of much more, and I believe Zac is able to run pump gas.

Raising the CR of the pistons does the exact same thing increasing the boost does, except it does it all of the time. Less boost is required to make the same numbers, you spool faster, and make more power in vacuum, but the tuning window shortens.

for 3 117hp at the CRANK? maybe on a wore out engine. and a good engine with good compression should be close to 140HP. maybe 117 whp. and that would have to be an automatic.

He did say 117whp.

 

[TSIfreek]

8.5:1 Is what I would recomend.

 

[jakelandry]

Man its just like everytime i decide on it someone else chimes in with a good point . Im planning on ordering the kit then boosting my car as something to tide me over till the kit comes in in 2 months. With it being a long rod kit, i plan to get my crank balanced so i can hopefully turn a few more rpm's so that should give me a little more in the top end. I dont like the idea of being slow as balls when not in boost, but it would definitely be a plus to have people pull me off the line then just blow by them 100 feet down the road. Around what rpm do 16g's spool?

 

[necrotopsy]

He did say 117whp.

he didn't when that was first posted. that was edited. it said 117 at the crank, which is why is put crank in CAPITALS .

at 10.5:1 the risk of knocking is too high if you ask me, the tuning would be so hard, like you said.

 

[Locke]

I agree that for those wanting to make bigger numbers, but without the ability to tune well, should probably stick to a conventional low compression build, for safety's sake.

 

[Carter_95RS]

I agree on the lower compression. Its easier to tune for those who don't have the tuning expierence or knowledge. Op, what are your power goals?