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Forged Piston Options for Draw-Thru L28


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Hi all, 

 

I don't make it on here enough and probably should have consulted this great knowledge-base more thoroughly when I was building my setup. My car had an L26 originally, I sourced an L28E and rebuilt it. It's an N42 block and N42 head also I think? The bores were corroded so I sourced some .030" (I believe) oversized hypereutectic pistons, Hastings chrome rings, and the motor was mostly standard apart from some cooling system alterations, standard camshaft & RPM range, etc.

 

post-29836-0-78536000-1471438926_thumb.jpg

 

After driving it part time for a year, I pulled it off the road and set up a Draw-Thru Turbo system (I sourced a used kit that I modified to suit my engine bay) and when I got that running, I struggled to get the tune and boost-retard function working well, I babied it around a little and tried to get it better but ultimately I had big oil consumption issues (blowing out the tail pipe on boost). 

 

post-29836-0-35882800-1471439615_thumb.jpg

 

I investigated further with a compression test:

1 - 185psi

2 - 185psi

3 - 185psi

4 - 185psi

5 - 170psi

6 - 145psi

The leakage is into the crank-case not out the ports.

 

I believe my compression was 8.7:1 or close, and the turbo system runs up to 10psi and I have certainly been able to run it at 10psi with no detectable detonation - but in various states of tune (different timing/fuel mixture) who knows what it experienced momentarily.

 

I am pulling this motor out tomorrow night to assess the damage. My hunch is that forged pistons would be a suitable upgrade, but I am not sure how necessary they are.

 

I feel compelled to frame this right too, I don't really have an appetite for more than 200 and a bit horsepower - whatever I can run with the standard fuel pump and two HS6 SU's. Just to clarify this car is a noise-maker, not something that will be doing motorsport or ever see sustained boost.

 

The only pistons that I have found available are custom so far ('Ross' seem the most cost-effective but still a very pricey upgrade), but perhaps someone on here has alternative suggestions? I'd rather avoid the price and lead time of custom pistons if possible. Then again, my bores may be in good condition and if so, I don't necessarily want to machine the block.

 

For those interested, this is the vehicle powered by the L-series in discussion:

post-29836-0-55791300-1471439549_thumb.jpg

 

Look forward to hearing your feedback.

Edited by Hezath
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I've used ross in the past. My last L I built was in my zx in NA form except the last 5k miles before I sold it. I had it turned up to 10psi on a stock efi turbo setup.

 

But, the general consensus is that forged are only need in extreme cases or when RPM are going to exceed "elevated" levels. If you're staying below 6500 you shouldn't have a problem. Also it sounds like the rings got toasted, not the pistons, from uneven detonation. 

 

Have you torn the engine apart to inspect?

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Forged piston are used in Turbo cars because they are much more stronger than cast pistons. The pin boss take a lot of load with Forced Induction engines and forged pistons are much stronger in this critical area and also in the dome. They are also able to handle detonation, Torque and shock loadings and high combustion temperatures much better than cast. Forged pistons can withstand loads that will shatter cast pistons.

 

RPM is not the really big problem on Turbo motors. Torque load is. On VW.Audi 1.8T engines guys can bend stock rods at 3,000 rpm. New technology Turbo's ( BB and Hybrid ) with quick spooling can easily build 20plus lbs boost by 2,500 RPM these days.

 

Forged pistons are a very good investment on any Turbo engine boosting over 10 psi.

Edited by Chickenman
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Thanks for the feedback so far.

 

I've used ross in the past. My last L I built was in my zx in NA form except the last 5k miles before I sold it. I had it turned up to 10psi on a stock efi turbo setup.

Cool, were they custom or somehow 'off-the-shelf'?

 

Motor is coming out and apart tonight. Just don't want to have the car apart for long so trying to be prepared.

 

Are you from Oz? You guys have Pure Performance motorsport in Australia.

Is your car a Cedric?

I'm from Aus, I've never dealt with that company. I should be in touch? Yep it's a H230 Cedric.

 

Forged pistons are a very good investment on any Turbo engine boosting over 10 psi.

This certainly is not in elevated RPM ranges - no more than 6000rpm really, and 10psi on 98 octane fuel, carbureted with no water/meth injection (at this stage).

 

I will assess the damage shortly.

Edited by Hezath
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Curious if the chrome rings never seated properly. Even your low number is on the acceptable side if my memory is correct.

 

Are you sure it is the bottom end? 

 

For no more then 200hp, I'm not sure it is needed. I am curious how you got two SU carbs to feed a draw through turbo setup. I would think there wouldn't be adequate fuel.

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Hey Fletch! If I anticipated this issue and general complexity of getting this stuff to work, I probably wouldn't have done anything to the motor apart from twin carbs. If I had better performance & reliability, I might have blown up the box, but not yet.

 

seattlejeseter, whilst ~140psi may be ultimately acceptable, having that huge difference between the pots - and the motor's oil retention issues - tells me that something is very wrong.

 

I don't know how much/little power the motor was making, it has 'Grose' high flow needle/seats, and the fuel pressure was consistent even coming onto full boost - the wideband was telling me the right things but there may be more to it. I am going to pay someone to tune it when it's back together.

 

JelmerPatrol I spoke with PPM and they were reasonably helpful. Price is no more extreme than anywhere else.

Edited by Hezath
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Well tonight the motor came out and I pulled #3 (a known good one) and #6 out. #6 has a scratch up the bore... no damaged ring lands, no cracked rings. The only other thing I'm seeing is that it looks like the rings haven't bedded in properly on #6 but I'm no expert on this stuff.

 

Needless to say, I didn't expect this. I also am not seeing the merit in investing in forged pistons if nothing has been damaged. Not entirely sure what the best next move is. I guess I'll clean the shit up and take it to an engine builder/machinist on Monday.

Edited by Hezath
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I've been making 350+ hp on cast nissan pistons since 1985. They are not required for a turbo engine not revving beyond 6,500 or making more than 400HP.

Detonation kills EVERYTHING. Including Forged Pistons. You put forged pistons in an engine that's detonating all you are doing is increasing your parts costs.

 

Remove the detonation. Don't mask it for three more RPM's before something breaks.

Same goes for MLS Head Gaskets. Stock Fel Pro will hold 650+ HP if you aren't detonating. 

 

If you are, then it blows. What do you want, a blown head gasket, or broken pistons and rings. Those are your choices if you run an engine that is detonating.

 

Fix the problem, not the symptom. The failure will repeat, only be more costly.

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Yeah totally Tony. Thanks for the input.

 

FWIW, my intention was to get this thing running right but that seemed like a bit of a lost cause with the oil consumption I was experiencing.

 

I actually pulled it apart to change other stuff (alter carb position, die grind some aggressive surfaces in the piping to stop fuel pooling, fit a wall covering 80% of the CSA in the log manifold, etc.)

 

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It was only ONCE I had the thing apart that I wanted to be thorough and do a compression test, and the discussed results came in.

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