IW STi Forum banner

1 - 8 of 8 Posts

·
Registered
Joined
·
3 Posts
Discussion Starter #1
I managed to find a set of lightly used 99.5mm forged 4032 mahle power pack pistons (WRX209917I22). The car will be used exclusively for track and driven very hard. Plans are stock turbo, larger injectors (ID), full 3 inch turboback, Flexfuel - most likely e85 on track but would like the option of running on 98 octane if it's not available. Tuning will be based upon intended use with a margin of safety on both e85 and 98 octane

My questions are as :

1.What is an appropriate ring gaps for such a setup - seems like plenty of people are running stock gaps on e85 with a stock turbo, does the fact that these are forged pistons make a difference?

2. Can I use OEM rings on aftermarket pistons such as these ( I have a set lying around)

Thanks for the help!
 

·
Premium Member
Joined
·
5,330 Posts
No idea! But I would only use the mfgs rings and gap according to their spec.
 

·
Premium Member
Joined
·
746 Posts
Don't know about E85. What car, which turbo (can figure that out if I know Y/M) and what boost are you aiming for? Have you purchased the pistons yet? You should use 2618 alloy. I can help you with gap on that alloy.
 

·
Registered
Joined
·
3 Posts
Discussion Starter #4 (Edited)
Thanks for the reply,


2006 WRX STI EUDM. Pistons are already purchased. I don't have a specific boost target but I'm aiming for around 300-320 @ the wheels on e85 and a fair bit less on pump gas. I'm just looking for max reliable (track) power on stock turbo and e85 and 98 octane. Stinks about the alloy difference- i guess I could talk with mahle regarding gaps, couldn't find any application specific gapping info on their site. '

Can confirm that the stock oil rings don't fit on the Mahle pistons as I checked today.
 

·
Premium Member
Joined
·
746 Posts
I'd go by Mahle's instructions. If the pistons are coated on top that could be one variable to consider.
I used the CP and JE recommendations. Since I run 105 octane and 24 PSI, I went to the highest "nitrous turbo/supercharged" ratio. That means, for the 2618 alloy (and stock bore) I shot for .021-.022" .
 

·
Premium Member
Joined
·
5,330 Posts
I'm aiming for around 300-320 @ the wheels on e85 and a fair bit less on pump gas. I'm just looking for max reliable (track) power on stock turbo and e85 and 98 octane
. . . . Can confirm that the stock oil rings don't fit on the Mahle pistons as I checked today.
You be fine w 4032 and if done properly they last longer.

Didn't think rings would be inter-changable.
 

·
Registered
Joined
·
1,616 Posts
I would recommend using what ever rings that piston comes with along with the specs and size according to manufactures instruction.
 

·
Registered
Joined
·
3 Posts
Discussion Starter #8 (Edited)
Thanks for the tips all.

I did a bit more digging and found the MAHLE guidelines ( they are nonspecific as to alloy ) I did the calculations, as well as called mahle.

Ended up with two different results:

Calculated ring gap in inchestop ringsecond ring
charged gas0.021545275570.02350393698
Charged EtOH0.017627952740.02154527557


I called John at Mahle motorsports technical help. He said alloy makes no difference in ring gaps (I'm guessing this is a simplification and that what he meant was that alloy has little real world impact on gap).

I told him i was going to bash the car on the track and that:
8.2:1 CR
I was aiming for about 20psi of boost on e85, and a bit more conservative on pump gas. This is what he came up with:

According to mahle motorsports - John 20psitop ringsecond ring
0.026-0.0280.024-0.026


Having a look between my calcs, Scargos gaps at 0.021-0.022 at significantly higher boost the tech help gaps seem a bit conservative. I might just go for my calculated charged gas gap of 0.021 and 0.023

Scargo - How is the blow by on your gap?

I checked the gap on all the used rings that came with the "slightly used" pistons and they seem to be in spec. Am I crazy for thinking I could fine tune them and reuse them?
 
1 - 8 of 8 Posts
Top