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Gtir Motorsport club » General Discusion » Gtir related Discussion » Stupid pistons!

Stupid pistons!

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1 Stupid pistons! on 15th March 2017, 12:58 am

Ryaneg6freak


Had low compression in a few cylinders and determined it was a bottom end problem.

Finally got the pistons out and found this.  Every single one of them looks like this, the upper part of the ring land is ate away in exactly the same spot on all of them, on the firewall side piston skirt.

dodgy tune maybe?  I never heard any spark knock, always run 91 octane fuel and it's tuned rather conservatively with Nistune at 1 bar of boost with 400cc injectors and a 340lph fuel pump.

I never have AFR's higher than 13.1 even under full boost.

What gives? Are the stock pistons that super weak? I mean, I know they're cast, but damn.


2 Re: Stupid pistons! on 15th March 2017, 1:51 am

splmum

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91 octane fuel,
Unless you use some different scale to the rest of the world you are using the WRONG fuel.
These in standard trim use 98 octane and in an emergency you can use 95 to get you to a good fuel station.
That will probably be your problem, poor fuel.

3 Re: Stupid pistons! on 15th March 2017, 1:58 am

Ryaneg6freak


91 is all you can get my friend, SOMETIMES, 93 in certain states.
The US doesn't have much in the way of high octane fuel unless you buy it by the can from special petroleum dealers. You can't find anything over 93 at the pumps.

The tune is set up and scaled FOR 91. I thought about making the switch to E-85 but not before I get soem good forged pistons in there.

4 Re: Stupid pistons! on 15th March 2017, 4:38 am

Mr B

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gtir technician
gtir technician
Ringlands are known issue when got det present, would imagine your fuel and tune killed them, standards do okay if fuel and tune ideal for them . Bottom end was not going give you low compression . Good thing in USA is forged pistons and general rebuild parts pretty cheap :-)

5 Re: Stupid pistons! on 15th March 2017, 7:30 am

johnny gtir

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moderator
moderator
Is 91 not what you use sure my mate said it was all that was availible they score there's different and all imports run on it over there ? Built wrong depending on age and map I would say

6 Re: Stupid pistons! on 15th March 2017, 8:56 am

Mr B

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gtir technician
gtir technician
We got 91 and 95 here . 91 okay if set up well for it and everything working A1 . standard piston ringlands soon fail when lean/det . With potential low prices of forged pistons in US it easy resolved .

7 Re: Stupid pistons! on 15th March 2017, 9:01 am

ducie54

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In most countries, including Australia, New Zealand and all of those in Europe,[citation needed] the "headline" octane rating shown on the pump is the RON, but in Canada, the United States, Brazil, and some other countries, the headline number is the average of the RON and the MON, called the Anti-Knock Index (AKI), and often written on pumps as (R+M)/2. It may also sometimes be called the Posted Octane Number (PON).

8 Re: Stupid pistons! on 15th March 2017, 1:28 pm

Mr B

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gtir technician
gtir technician
So the US grade is equal to the 95ron then .

9 Re: Stupid pistons! on 16th March 2017, 1:45 am

Ryaneg6freak


@Mr B wrote:Ringlands are known issue when got det present, would imagine your fuel and tune killed them, standards do okay if fuel and tune ideal for them . Bottom end was not going give you low compression . Good thing in USA is forged pistons and general rebuild parts pretty cheap :-)

Bottom end will give you low compression if your whole ring land is gone! lol.

We initially thought it was the head because compression did not go up when oil was added to the cylinder. Turns out that little trick doesn't work when there's a damn hole in the piston hahaha

10 Re: Stupid pistons! on 16th March 2017, 1:47 am

Ryaneg6freak


@Mr B wrote:We got 91 and 95 here . 91 okay if set up well for it and everything working A1 . standard piston ringlands soon fail when lean/det . With potential low prices of forged pistons in US it easy resolved .

I can get a set of forged CP's for around $600usd. Not particularly cheap, but not Skyline expensive.

Good news though, the rod bearings look like brand new, for an engine with 119,000kms on it.

It's never been opened up before, so other than this, it appears it was well taken care of.

11 Re: Stupid pistons! on 16th March 2017, 3:55 am

ADSgtir


It is not the pistons fault , all pistons have the same melting point , regardless of being cast or forged , as they are the same material , before the melting point is reached the pistons get weaker , the closer to the melting point the weaker they get by a large amount.
The heat path from combustion is across the piston top then through the rings to the cylinder wall and into the water system ,it also passes through the rings to the oil system .
The piston top is cooled by oil sprayed underneath it .
any carbon build up creates hotspots and can cause detonation, so :
There are a few things that can cause problems , water system must be working properly , pump rad flow ,stat etc , oil system must be working properly, pump ,flow, quantity pressure , oil squirters etc ,carbon build up cannot be excessive , worn rings or very rich running and the mixture/timing and fuel quality be correct , lastly the plugs and valves play their part ,they pull heat out to the head and onto the water system.
Basically it must all work properly together or your new pistons will do exactly the same fairly quickly

12 Re: Stupid pistons! on 16th March 2017, 6:29 am

Mr B

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gtir technician
gtir technician
Quite a variety of piston alloy all with differing benefits . Forged constructed is far less brittle which huge benefit on serviving some level of det, modern designs also have better heat quenching helping temp control.  would of thought you could find some forged pistons around 480usd to 550usd direct in usa. Standard pistons do well when fueling ignition an temps ideal but det and fatigue of these old parts soon ends in ringland failure. As mentioned above will need full check after rebuild as a issue would still potentially damage forged pistons given enough time .

13 Re: Stupid pistons! on 16th March 2017, 5:27 pm

ADSgtir


You would be best served to fit a water injection after you have rectified any/all the issues and rebuilt the engine , it serves many purposes , but the main one is it saves your engine ,time and time again.
you dont need to tune specifically for it , use minimum amounts of water on boost ,a very simple system is all you need .
I would say it adds around 100000 miles of service life and keeps internals spotless clean and the best thing you can add to any turbo car , I have one car coming up on 300000 miles ,has factory compression and running 600 hp , I ran WI for around a year before I tuned or altered anything simply to clean the internals .
You will need new bearings , I would go with Mahle motorsport , I expect they do a set for the GTIR and are well priced for the exceptional quality .

14 Re: Stupid pistons! on 19th March 2017, 10:06 pm

Ryaneg6freak


@ADSgtir wrote:You would be best served to fit a water injection after you have rectified any/all the issues and rebuilt the engine , it serves many purposes , but the main one is it saves your engine ,time and time again.
you dont need to tune specifically for it , use minimum amounts of water on boost ,a very simple system is all you need .
I would say it adds around 100000 miles of service life and keeps internals spotless clean and the best thing you can add to any turbo car , I have one car coming up on 300000 miles ,has factory compression and running 600 hp , I ran WI for around a year before I tuned or altered anything simply to clean the internals .
You will need new bearings , I would go with Mahle motorsport , I expect they do a set for the GTIR and are well priced for the exceptional quality .

I already have a full set of ACL race mains and rods with thrust bearings. They were very inexpensive and I've had good results with them in the past.

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